00 SVE 2Eng Auxiliary Equipment 1 Notes PDF

Engineer Officer Small Vessel Certificate of Competence Second Engineer – Less than 9,000 kW, less than 3,000 GT, unlimited area, III/2 Auxiliary Equipment – 1 Page 1 Introduction Page 5 Valves Page 21 Pumps Page 41 Compressed Air Systems Page 53 Hydraulic & Pneumatic Control Principles Page 81 Steering Gear Page 99 Propulsion Page 125 Transmission & Shaft Arrangement Page 153 Electrical Plant Revised Nov 2019 Second Engineer Introduction Welcome to the Auxiliary Equipment 1 course here at Solent University and Warsash Superyacht Academy. Our new facilities here at the main University Campus are part of a £43 million investment by the University in maritime skills The new “Engineer Officer Small Vessel Certificate of Competency” route was launched by the MCA under MIN 524 (M+F). A full list of the course syllabus (written & oral) can be located on the MCA’s website via the link below. A copy of the syllabus for the written examinations is copied below for clarity. Please also refer to MIN 594. https://www.gov.uk/guidance/uk-seafarer-careers-training-provision-and- information These notes are intended as a study guide only and should not be considered as a textbook on the subject. Additional information will be given to the candidate during the preparation week before the examination. Students are reminded that this is a revision course. It is designed to prepare the candidate for the forthcoming examination. The week with us at Solent University does not provide sufficient time for you to learn everything from scratch, this is why we press home the need for you to start learning at least 6 weeks before your course. The week’s course brings everything together for you as a revision week with the lecturers on hand to assist with any questions you may have. Included with these notes are the past SQA exam questions arranged by topic. Students should use these examples as “guidelines” for their study prior to attending Warsash Maritime Academy. We hope that the course delivers or refreshes your engineering skills and if you do decide to progress with us through the remaining modules of the Small Vessel Engineer route, we hope this course will give you a taste of how we delivery and how we aim to help you achieve your goals and grow as an engineer, both with an industry recognised qualification and a University teaching processes behind it. Please feel free to give your feedback to the staff at the end of the course as it is extremely important to me to enable changes and improvements to be made. The feedback is anonymous, and I am happy to see both the good and the bad, so please give us as much feedback as you can. John Gouch CEng CMarEng FIMarEST FHEA Course Leader – Small Vessel Engineering Senior Lecturer – Marine Engineering (Merchant & Superyachts) Warsash School of Maritime Science and Engineering T: 02382 014081 E: [email protected] Introduction 1 Small Vessel 2nd Engineer Syllabus: Auxiliary Equipment 1 1. Valves 1.1 Types of valves 1.2 Construction and application of following valves: a) Construction and application different types of valves: b) Simple plug cock c) Ball valve d) Screw-lift valve e) Screw-down non-return (SDNR) valve f) Butterfly valve g) Gate valve h) Diaphragm valve i) Quick closing valves j) 3-way valves k) Valve chests l) Basic valve symbols m) Pressure relief valve - differentiate between a safety valve and a relief valve 1.3 Materials used in valve construction (cast iron/ steel, bronze, brass, stainless steel, etc.) 1.4 Compatibility of the materials used in construction with fluids flowing through the valve, aspects of corrosion and erosion. 2. Pumps a) Types of pumps b) Basic classification of pumps, positive displacement and centrifugal c) Typical pump applications in systems d) Basic pump symbols e) Positive displacement pumps, construction and theory of operation f) Types of positive displacement pumps: i) electric motor driven reciprocating pump ii) lobe pump; iii) gear pump iv) rotating piston pump v) screw pump vi) vane pump g) Positive displacement pump relief valves and system flow pulsation damping h) Centrifugal pumps, construction and theory of operation i) Types of centrifugal pump, volute casing and diffuser j) Problems associated with priming centrifugal pumps and methods of priming Introduction 2 3.0 Compressed air systems a) Knowledge that the compressed air system commonly has two pressure levels, 30-40 bar for starting air, and 7-10 bar for control air b) Basic compressed air system, layout and components (air compressors, air receivers) c) Compressed Air Safety and Safety devices associated with the compressed air systems: Isolation and draining of system, safety valves, fusible plugs, compressor bursting discs, compressor lifting heads, pressure cut outs 4.0 Hydraulic and Pneumatic control principles a) Basic hydraulic and pneumatic symbols (interpretation of system drawings) b) Hydraulic system safety: safety devices fitted to the system and equipment, safe working procedures on hydraulic system and equipment. c) Basic electro-hydraulic and electro-pneumatic control d) Hydraulic fluids, types, uses, characteristics e) The importance of clean air supplies for pneumatic control systems f) Basic control air supply system, layout and components (reducing valve, driers, filters, lubricators) 5.0 Steering gears a) Basic functions of a steering gear b) Types of steering gear construction, 2-ram and rotary vane c) Knowledge of signal transmission from remote steering positions. d) Feedback control within steering system e) Constant and Variable pressure hydraulic system for a steering gear system f) Steering gear protection and system redundancy (split hydraulics, dual pumping arrangements, isolating valves, by-pass valves, movement limiters, and shock valves) 6.0 Propulsion a) Basic theory of propellers (number of blades, skewed etc.) b) Methods of securing the propeller to the propeller shaft c) Fixed and variable pitch configurations including water jet, Azipod and Voith Schneider propulsions d) Controllable Pitch Propellers (CPP), advantages/disadvantages, construction, operation, control systems e) Configurations and construction of transverse thrusters (fixed drive, fluid drive, water jet) f) Safety devices and manual control (fail safe / fail set) Introduction 3 7.0 Transmission and Shaft arrangement a) Shaft support bearing types and construction (plain bearing, roller bearing) b) Thrust bearings (tapered roller bearing and thrust collar) c) Bearing lubrication d) Stern tube bearings (linings, oil and water lubricated) e) Stern seals (Lip seals, mechanical seals) f) Rigid and flexible shaft connections (fitted coupling bolts, flange, hydraulic muff, steel diaphragm) g) Basic shaft alignment and Shaft coupling alignment (lateral and angular) 8.0 Electrical Plant a) Basic construction and operational principles of AC alternators (production of voltage and current, pole/speed relationship, exciters, speed control, auto- voltage regulators (AVR), response to change in load) b) Synchronising and paralleling alternators manually and automatically (in phase, check synchroniser, synchroscope, synchronising lamps, load sharing, speed droop and voltage droop) c) Switchboard layout (main alternators, main switchboard, emergency alternator, Emergency switchboard d) Switchboard safety devices list (alternator main breaker trip devices, earth leakage detection, preferential tripping, sequential starting and discrimination protection) and their functions e) Main breakers and their safety protection devices f) Fuse types and applications g) Shore power connection and safe operation h) Neutral point insulated distribution i) Essential and non-essential consumers j) Earth lamps k) Earth fault tracing l) Basic construction and operational principles of 3-phase and single-phase motors (direct on-line starting, alternative starting arrangements, torque and current characteristics, safety protection, fault finding, single phasing) m) Basic construction and operational principles of batteries (lead-acid, alkaline, Ni-ion, charging circuits, inspection and maintenance, safety) n) Emergency power supplies (SOLAS legislation, typical installations, testing, and recovery from blackout) o) Electrical safety (legislation - COSWP, safe isolation, safe testing procedures) p) Electrical maintenance q) Understanding, maintenance, troubleshooting of Electrical Lighting Circuits including Navigation lights r) Basic transformer theory, safe working practice, routine maintenance s) Intrinsically safe circuits and equipment Introduction 4 VALVES Revised May 2020 Valves 5 APPLICATIONS OF VALVES Valves are inserted into pipeline systems to control, throttle and shut off fluid mass flow whenever necessary. Typical applications are: Isolation of a system Stop valve Drain valve Filling valve Control of Process Flow Control valves Feed check valves Control of Process Pressure Reducing valves Safety valves, relief valves. VALVE TYPES To achieve the above functions valves come in many types and designs, each manufacturer will have a different style for each type but all valves are generally made to standard ISO dimensions and specifications Screw down non- return valve Screw lift valve. Non- return or check valve Gate valve. Globe valve Butterfly valve Taper cock. Sleeve packed cock Ball valve Simple Relief Valve Quick closing Valve. Pneumatic, diaphragm operated automatic valve Solenoid valve The material, strength and size of a valve selected will depend on the intended application, taking into account the system pressure, the system temperature and whether the system media, or its boundaries and surroundings, are of an aggressive nature. The type and size of the valve selected will have to be in accordance with requirements of the appropriate Classification Society. (Lloyds, ABS etc.) This can be verified from Classification Society rules and is of the utmost importance when a replacement valve is being fitted. A test certificate and type approval certificate must accompany any replacement valve used or stocked. It is important that all parts of the valve are of non- corrosive material since the formation of corrosion (rust) or scale in service could result in a jamming or malfunctioning valve. Revised May 2020 Valves 6 Valves are supplied in a variety of forms for installation. Most common is the flanged attachment using 4, 6 or 8 bolts clamping for attaining a pressure tight joint. Valves for lighter duties and in smaller sizes are produced and used at times with screwed connections. These tend to be used on auxiliary systems for air, drains and domestic fresh water. In the smaller sizes valves are sometimes used with compression fittings for such applications as gauge lines, pressure switch lines and lubrication oil sampling lines. These compression fittings are normally made in steel using steel olives. In some systems, small bore copper lines will be found to have been fitted for convenience and these require careful monitoring due to their work hardening in use, which can lead to cracking and failure if suitable precautions are not taken. SAFETY - RELEIF VALVES It will be noted that all safety/relief valves are provided with a means for locking the set adjustment. When these valves are overhauled and inspected at Classification Society Surveys the final test is the satisfactory operation of the valve on a calibrated test rig. Once the adjustment has been made and satisfactory operation demonstrated at the correct pressure, the valve is locked with an anti- tamper seal A Safety Valve is designed such that when it operates the spindle will lift ¼ of the valve diameter, giving a large and immediate relief of pressure, it will close at a pressure lower than its set point to prevent it “feathering”. It may also have a mechanism so that it can be remotely operated from a safe distance. Typical uses for gasses under pressure. A Relief Valve will lift at its set pressure, but only by a distance dictated by the strength of the spring. The greater the pressure the more it will lift, it does not have a definite closing action and as such is susceptible to damage by feathering. Cover / Locking Nut Setting screw for lift pressure Anti-Tamper Seal Revised May 2020 Valves 7 One final test is carried out on the test rig to ensure that the operating pressure is still correct and the valve may then be mounted in its designated position and any discharge pipes, drains or remote operating wire pulls reconnected. The reason for locking the bonnet and covering the adjustment nut is to prevent unauthorised tampering. Most safety valves and relief valves will have stamped on the body in a prominent position the set and tested pressure at which the valve operates. It is important that all parts of the valve are of non- corrosive material since the formation of corrosion (rust) or scale in service could result in a jamming or malfunctioning valve. Common System Applications: Steam Compressed air Fire-main Fuel oil Lubrication oil All positive displacement pumps should be fitted with a relief valve! GLOBE VALVES The globe valve has a bulbous body, housing a valve seat and disc or plug arranged at right angles to the axis of the pipe. The seat and plug faces may be made of stellite (a very hard wearing alloy), which makes them almost indestructible; alternatively, the seat may be renewable and screwed into the body or given a light interference fit and secured by a grub screw. The seat may be flat or more commonly mitred. The spindle or stem may have a V or square thread and may be fitted to the valve plug by a locknut and button; others will be found in which the button locates in a simple horseshoe. Leakage along the valve spindle is prevented by a stuffing box, packed with a suitable material and a gland. When the body is so formed as to change direction, e.g. in a bilge suction or on the side of a pressure vessel the valve is referred to as an angle valve. The flow is from below the seat, so that the gland is always on the low pressure side of the seat and plug. Revised May 2020 Valves 8 Some types of globe valve have the plug unattached to the spindle and are known as screw-down non-return (SDNR) valves. The plug must be guided by wings or a stem on the underside so that it locates properly in the seating bore. Such valves are used in bilge suction lines, feed check and boiler stops to prevent back flooding if the valve is left open. The greatest lift required for the valve to be fully open is one-quarter of the bore of the seat. The main disadvantage with valves of this type is that they restrict or offer more resistance to flow and may give rise to large pressure drops. The above diagrams illustrate how a standard valve body can be arranged in either Screw Lift or Screw Down Non-Return configurations Revised May 2020 Valves 9 Below is a sequence of operations that will allow you to produce a good drawing of a valve in under 10 minutes. Start off with some simple constructional lines, develop the shape & rub out the bits that are no longer needed. All that is left now is to insert the valve plug as either a screw lift or screw down non-return, depending on how the question is worded. Revised May 2020 Valves 10 NON-RETURN OR CHECK VALVES Non return valves are commonly found in all systems where a back flow needs to be prevented, e.g. Bilge Injection, air compressor discharge, lubrication oil pump discharge on standby unit. In a conventional globe type non-return valve with a winged valve lid, the valve lid is not connected to the valve spindle. The valve spindle when wound down holds the valve closed. This type of valve is known as a Screw down Non Return Valve. The valve when wound back, opens due to the pressure imbalance across the valve. For sea water duties the valve body is commonly made from cast steel or bronze. the spindle may be made from bronze or stainless steel, the valve seat and lid would normally be made from bronze, though for low pressure applications some may have rubber or neoprene seats. A non-return valve for an air compressor would utilise a cast steel body and often a stainless steel valve and seat. A spindle and hand-wheel are not always used with a non-return valve. Non return valves are available in many forms, the simplest of all being the hinged flap of rubber attached to a valve plate over a port. This is used in some hand operated small boat bilge pumps and in the emergency smoke helmet air pump. The dual plate check valve below is common in Sea Water / High Temp / Low Temp pump discharge lines Revised May 2020 Valves 11 GATE VALVES The gate valve is commonly used in overboard discharges and ballast systems. Its main advantage is the minimal pressure drop when open and its ability to seal on closing. It is important that this type of valve is regularly operated to prevent scale forming on the sealing faces when open or silt building up in the bottom under the gate; this would seriously impair the ability for smooth operation or tight sealing. Only hand pressure should be used when operating these valves in particular as the action of the wedge shaped gate can give rise to very high loads being applied. The body of the valve is usually of cast steel or sometimes bronze in smaller valves. The valve spindle is usually either stainless steel or bronze. The gate is usually cast steel. Both the gate or wedge and the body have either pressed in or screwed in ring shaped inserts that form the sealing faces. Very often, particularly on larger gate valves, a screwed plug will be found in the base of the valve. This is used to ensure that the valve is sealing tightly. With the valve in the closed position, the plug is removed and a head pressure applied to either or both sides of the valve. Once the valve has been either fully opened or fully closed the hand-wheel should be eased back fractionally to help prevent jamming. Note: Gate valves can easily become jammed when closed, due to a large change in temperature such as experienced when sailing from tropics to a more temperate climate, particularly when over-tightened. Revised May 2020 Valves 12 BUTTERFLY VALVE The butterfly valve is used only in low pressure applications. The main advantage of this type of valve is its free flowing characteristics and the minimal pressure drop across the valve. The body of the valve is normally cast steel with a neoprene rubber coating in the bore of the valve, on the side sealing faces and on the butterfly itself. A mechanical lock on the operating handle, if it is of the simple tiller type is required to ensure positive closing or holding in any intermediate position. In the butterfly valve, the flow control element is a circular disc and is always in the fluid path. When the valve is open it splits the flow into two separate paths around it. Because the butterfly valves essentially symmetrical, either end can be the inlet, and thus flow can be in either direction. The form of control for which butterfly valves are best suited is regulated flow volume (called throttling). They can also be used as stop valves in which case this has to be specified when ordering to ensure the correct liner/seat configuration is supplied Butterfly valves are now being fitted in preference to gate valves for many applications because they are: cheap, inherently simple, have an economic design that consists of fewer parts so are easy to repair and maintain. Wafer designs do not transfer the weight of the piping system directly through the valve body, they are easy to operate or fit an actuator and can control flow without wire drawing. On the downside they are very hard to remove if the valve has failed in the open position. Common problems are the taper pins for securing the disc to the shaft, shear and booted seats are known to trap the valve in the closed position. Revised May 2020 Valves 13 DIAPHRAGM VALVES Diaphragm valves (or membrane valves) consists of a valve body with a diaphragm, and a "saddle" or seat upon which the diaphragm closes the valve. The valve can be constructed from Plastic, Steel or Bronze (yellow metal) however for the shipping industry Steel or Bronze would be the materials used Diaphragm valves can be manual or automated. The older generation of these valves are not suited for regulating and controlling process flows, however newer developments in this area have successfully tackled this problem. Diaphragm valves are commonly used as equipment isolating valves on sea water systems Revised May 2020 Valves 14 THREE-WAY VALVES Three-way valves or diverter valves are used in flow control circuits, particularly where flow is used to regulate temperature. The drawing shows a typical three-way valve where the position of the valve plug determines whether the flow from the inlet port at the left passes through the downwards outlet port or the right outlet port. If the valve plug is in an intermediate position then flow will be equally divided between each port. This type of valve will be driven by either an air or motor actuator and requires an external control circuit to regulate the position of the valve plug Three way mixing valves can be used on the following systems of a diesel Engine Lubricating Oil Temperature control Jacket water temperature control Central Cooling Temperature control Wax Element Valves On smaller more simple installations, internally wax operated 3-way valves are generally utilised. As the liquid passes through the elements the wax will expand and contract depend on the temperature, as it expands and contracts it operates a sliding valve which in turn can open and close two outlet ports on the element. As one port opens the other closes and vice versa. Revised May 2020 Valves 15 BALL VALVES The ball valve consists of a bored sphere that can rotate in a spherical chamber. In the closed position the hole in the sphere is at right angles to the liquid flow, in the open position the hole is in line with the liquid flow. The valve is quick acting but cannot be used for adjusting flow because of the risk of wire-drawing. Used mainly for low level flow operations and can be easily damaged by grit in the flow medium. COCKS Cocks may be straight-through, right- angled or open-bottomed as required by its situation in a pipe system, the plug may be tapered or parallel; tightness may be achieved by grinding the plug to the body or by resilient packing material packed tightly into suitably placed grooves in the body (for low or high pressures respectively). If under pressure, they may be double glanded, so that the plug cannot blowout. They are also so constructed that the handle can be removed only when the cock is closed. The top of the plug is provided with a slit to indicate whether the cock is open or shut. Revised May 2020 Valves 16 QUICK CLOSING VALVES Quick closing valves are typically used for remote operated tank valves where they can be tripped to shut in an emergency. The drawing shows an arrangement of a lever operated quick closing valve that can be fully opened or shut with a single movement of the lever. For remote operation the valve stem would be spring loaded and the lever held open by a latching mechanism. If the latch is released the valve will spring shut. In some applications a fusible link is included in the latching design, so it will close automatically in case a local fire. Various methods exist to operate the latch, the commonest being a simple wire and pulley mechanism, pneumatically or hydraulically operated cylinder are also used. Revised May 2020 Valves 17 Valve Chests Some pumping systems are arranged so that one pump can be used on a number of different systems. e.g. bilge, ballast and fire main systems. So that the various systems can be accessed by the pump the pipelines are combined and controlled in valve chests. Sea Overboard suction discharge Bilge Fire main suction Ballast Ballast suction delivery The drawing shows a typical arrangement that allows a single pump to serve the all systems shown. Note that the valves fitted in the suction chest are all screw down non-return valves to prevent back flooding and also that the pump is fitted with its own isolating gate valves. Obviously, regulations dictate that the bilge suction would only be for emergency use Graphitic corrosion: or graphitization, occurs when the metallic constituents of cast iron are selectively removed or converted into corrosion products. This process leaves behind the graphite matrix of the cast iron, in the shape of the original casting. While pipes or valves undergoing graphitization may appear sound and may conduct water adequately, the metallic portion of the pipe wall may, in places, be significantly thinner than the apparent thickness of the wall. Graphitized regions of pipe wall will be brittle and subject to failure under load as the result of temperature variation, heavy traffic, or shock. Graphitic corrosion is one example of the de-alloying of a metal. During de- alloying, one component of an alloy is selectively dissolved, leaving other components behind. The relative position of two metals in the galvanic activity series determines which will most readily participate in electrochemical reactions, such as corrosion. Revised May 2020 Valves 18 Valve Materials When looking at the material selection for valves there are generally two key aspects that we need to consider; Corrosion Erosion In addition to this we also need to look at the system as a whole to ensure that materials are compatible with both the fluid they are handling, and also other materials used in the system. Classification societies offer guidance and also insist that ship side valves are all fully class approved, whilst also forming part of the vessels survey routines. Any valve needs to be adequately designed for the task in hand whilst also being capable of handling the system and environmental conditions such as stress, shock loading, temperature etc. The following key materials identify some of the advantages and disadvantages with their inclusion in valve construction. Mild Steel: A relatively cheap material that is surprisingly ductile at room temperatures. However to cast a valve in mild steel it is not easy to achieve good characteristics, resulting with poor hardness levels and a rough surface finish. It is also very susceptible to corrosion and therefore not suited to sea water systems. Cast Iron: There are a number of types of cast iron but in general they can be classed as susceptible to corrosion and being very brittle, therefore open to failure. The casting process is generally more intense and therefore costly. Cast iron can however be improved upon, with certain variants having better resistance to corrosion, erosion and with improved strength. Stainless Steel: Again, there are numerous types of this material. The key three types with regards to the yacht curriculum are; Ferritic – Good stress resistance against cracking Austenitic – can be hardened, is ductile and tough. This material can however be susceptible to stress corrosion cracking Martensitic – High strength with moderate corrosion resistance Overall they are highly resistant to corrosion Revised May 2020 Valves 19 Bronze: An alloy of copper and tin which is hard, strong and corrosion resistant. Easily cast for valves. A variant known as “Gun Metal” is very tough, hard wearing and very resistant to corrosion – ideally suited to sea water applications. High costs however can become prohibitive in some applications. A good choice for quality ship side valves. Brass: An alloy of copper and zinc, variants with aluminium have increased corrosion resistance. Brass is sadly susceptible to dezincification where seawater leaves the material spongy and with reduced strength i.e. open to failure unless a dezincification resistant form is used. Generally perceived as an overall soft material and must be selected wisely with regards to its application. Composite: Composite materials are becoming more popular with classification societies gradually accepting them more for specific applications. They can be thermally stable with very good fire resistant properties. Resistance to erosion is very good due to the materials employed along with the smooth finished that can be achieved. Corrosion resistance is also very good and individual materials can be selected to ensure compatibility with various oils and chemicals. Their composite material properties also ensure that they do not interact with system materials and essentially offering an inert component. Revised May 2020 Valves 20 PUMPS Revised May 2020 Pumps 21 A pump can be described as a machine that is used to draw in fluid at its suction and accelerate it out into the system by increasing the fluid’s kinetic energy. The resistance encountered by the flowing fluid causes a pressure to build up in the system. The level of pressure corresponds to the total resistance that is a result of the internal and external resistances in the system. The relationship between flow and pressure varies depending upon the pump type, but in general, resistance to flow creates an increase in pressure, so we can relate reduced flows to higher pressures and vice-versa. Ensure you understand these processes. Certain pumps have a wide range of uses and others have more specialised applications, but all pumps can be split into two basic categories; Dynamic Positive displacement PUMPS Positive Eductors Rotodynamic Displacement Centrifugal Axial Flow Reciprocating Rotary Volute Diffuser Mono Vane Lobe Gear Screw Dynamic Pumps With this type of pump, the fluid is accelerated through a rotating impeller either axially or radially by the use of centrifugal force. The increase in velocity or kinetic energy is then partially converted by diffusers and volute (snail shell) casings into pressure or potential energy. These types of pump are usually non-self-priming and require either a positive suction head or a separate priming mechanism. This priming mechanism is often integral with the pump, e.g. liquid ring primer, or may be an air driven eductor controlled by discharge pressure or a float chamber on the suction side of the pump. High flow rates and relatively low pressures are normally obtained, although high pressures are possible with suitable arrangements of impellers and casings and also by multi-staging the pump (in which case a pressure relief valve may be required) Revised May 2020 Pumps 22 Axial Flow Pumps In this type of pump, the rotor typically takes the form of a multi-bladed propeller, which accelerates the fluid axially through the blades. This velocity increase is partially converted to pressure by the shape of the blades and by diffuser vanes arranged in the pump casing. They are most suited to low pressure/high volumetric throughput and as such are often used for large cooling circulating systems. The axial flow principle is also used in water jet propulsion thrusters, jet skis and engine room ventilation supply fans. Centrifugal Pumps Centrifugal pumps fall into two main classes; those using volute casings and those using diffusers. It is also common to find pumps, which use a combination of the two. As the load on these pumps is a function of the mass flow it is normal to start them with the discharge valve closed or partly open to minimise the motor starting current. Revised May 2020 Pumps 23 Volute Type This is the commonest pump type found in marine use. It usually consists of a closed impeller having a number of curved vanes which is connected to the drive shaft. The impeller rotates inside a casing whose sectional area increases as it nears the discharge outlet i.e. a volute. This forms a diverging nozzle, which acts as a type of diffuser around the impeller. Rotation of a centrifugal pump impeller causes the liquid it contains to move outwards from the centre to beyond the circumference of the impeller. The revolving liquid is impelled by centrifugal effect. It can only be projected into the casing around the periphery of the impeller if other liquid in the casing can be displaced. Displaced liquid in moving from the casing to the delivery pipe causes flow in the discharge side of the system. The liquid in the impeller and casing of a centrifugal pump is essential to its operation. In moving out under the influence of the centrifugal effect, it drops the pressure at the centre, to which the suction or supply pipe delivers the liquid to be pumped. The moving liquid acts in the same way as a reciprocating pump piston on its suction stroke. Provided that a centrifugal pump is filled initially with liquid and that flow is maintained, the suction stroke action will continue. If such a pump contains no liquid initially, it is as though an essential part is missing. Revised May 2020 Pumps 24 The ratio between the velocity, pressure and volume is determined by the size of the impeller and the configuration of the volute casing. A thin wide diameter impeller will produce a high velocity and pressure but low volume. A thick small diameter impeller will produce a large volume flow but relatively low velocity and pressure. Diffuser Type With this type a static vane ring is placed around the impeller. The vanes form a series of diffusers, which act to convert the high velocity of the pumped fluid to pressure energy. The design of the casing further assists with this conversion and by forming the casing into a volute then the diffuser ring acts in conjunction with the volute to complete the transformation into pressure energy. Note: Some pumps will have a combination of diffuser and volute designs built into the same casing. The characteristics of a Centrifugal pump are such that when the discharge valve is shut on the pump, the flow will be zero and the head pressure high. It is also noted that since the flow is zero the power required to run the pump is low. That is why when starting a centrifugal pump, it is recommended that the discharge valve should be closed, so as to keep the starting current low. Electrical Motors on Centrifugal Pumps have been known to trip off on overload due to the discharge valve being open. Common Uses for Centrifugal Pumps Simple single stage single entry volute types are used for both fresh and seawater cooling systems where there is normally a positive suction head. This will be low pressure/high volume duty. Double entry types are used where greater flow rates are required still with low discharge pressures. Where higher pressures are required it is common to use multi-staging with two or more impellers on the same shaft with suitably designed diffusers and casings. Each impeller discharge is then used as a feed for the next stage impeller. Two stages are commonly used for fire pumps where a substantial head pressure may be required to reach the extremities of the vessel. By using several stages, the pressures required for boiler feed pump duties can be obtained. As a dynamic pump cannot create its own suction, if used for bilge or ballast water, where the level of the fluid may be below the pump, some form of priming arrangement is necessary to create the necessary suction to draw the fluid into the pump. Revised May 2020 Pumps 25 Priming Aids Eductors One of the commonest forms of Air supply Venturi priming devices is the solenoid operated air driven eductor. At Solenoid start up, or with the pump air valve discharge pressure below a pre-set Vacuum level during pump operation, a pipe pressure switch activates the solenoid. This allows compressed air at about 7 bar to pass through the eductor nozzle. This causes a Pump pressure drop in the vacuum line to suction the pump suction pipe. As the pump picks up suction and the discharge pressure rises, the pressure switch operates and closes the solenoid valve shutting of the Air driven eductor air to the eductor. Liquid Ring Priming Units The water ring or liquid ring primer can be arranged as an individual unit mounted on the pump and driven by it, or as a motor driven unit mounted separately and serving several pumps. The primer consists of an elliptical casing in which a vaned rotor revolves. The rotor vanes revolve and force a ring of liquid to take up the elliptical shape of the casing. The water ring, being elliptical, advances and recedes from the central hub, causing a pumping action to occur. The suction piping system is connected to the air inlet ports and the suction line is thus primed by the removal of air. A reservoir of water is provided to replenish the water ring when necessary. Revised May 2020 Pumps 26 Cavitation – an important process to understand Cavitation occurs when bubbles form and implode in pump systems around the impellors. Pumps put liquid under pressure, but if the pressure of the substance drops or its temperature increases, it begins to vaporize, just like boiling water. Yet in such a small, sensitive system, the bubbles can't escape so they implode, causing physical damage to parts of the pump or propeller. A combination of temperature and pressure constraints will result in cavitation in any system. No manufacturer wants to run pumps that keep getting affected by cavitation, as it will permanently damage the impellors and casing of the device. The vaporization actually causes a loud, rocky noise because the bubbles are imploding and making the liquid move faster than the speed of sound! Inside every pump, the impellor draws liquid from one side of the chamber to the other. Even though the total chamber stays under the same pressure, and the materials are temperature regulated, cavitation manages to occur right next to the surface of the impeller. An impellor rotates through a liquid and actually creates localized differences in pressure along the trailing edge of the impellor vanes. The bubbles of cavitation appear in these low-pressure areas but then immediately want to implode with such force that they make dings and pits in metal. An impellor exposed to cavitation resembles the surface of the moon, with tiny, scattered craters. Cavitation will also happen in a centrifugal pump due to the speed of the impellor being so fast that small areas of vacuum are created behind the back side of the impellor vanes and as these areas of vacuum move out into the pump they implode causing damage to the end (tips) of the impellor Revised May 2020 Pumps 27 Impeller Clearances The Clearances between the Casing and Impellor wearing rings are critical because if they are to great the liquid will leak from the high-pressure side to the low- pressure side causing a drop in efficiency For determining the clearances between wearing rings the manufacturer’s manual should be consulted however for a rule of thumb clearances between impeller wear ring and case wear ring should be about 1mm per 100mm diameter up to 300 mm diameter and 1.5 mm per 100mm above 300mm diameter Pumps intended for seawater circulation duty can be subjected to erosion damage from sand & other debris; this often shows up as excessive wear between the impeller & pump casing, affecting the pumps efficiency. Manufacturers of large pumps will design the casings & impellers with replaceable wear rings so the pump, efficiency can be maintained in service without resorting to expensive spares and off site machining. Revised May 2020 Pumps 28 When designing centrifugal pumps careful consideration has to be given to balancing the forces on either side of the impeller. The rear, hub cavity usually extends to the radius of the pump shaft, which has a much smaller radius than that of the wear ring. The pressure force on the rear side of the impeller would then be greater than on the front, and without some special provision, their difference might represent an excessive thrust bearing load. Therefore, on larger pumps, a sealing sleeve is made on the hub also, which has the same radius as the wear ring, as shown below. Balancing holes are drilled from the pump inlet through the hub, near the shaft, to allow for leakage past the rear sealing sleeve. The balancing holes must be large enough to establish pump inlet pressure in the rear clearance space near the shaft, in the balance chamber, and to assure that the major pressure drop occurs across the sealing sleeve Each manufacturer will have their own design of impellers, from fully open to totally enclosed. Each pump will be selected for its characteristic when matched to a particular pressure & duty requirement. Revised May 2020 Pumps 29 Positive Displacement Pumps Although there are many different types of positive displacement pump they all have two important features in common; They are normally self-priming, although in some cases it is necessary to fill the pump prior to start up to ensure adequate lubrication. Since, theoretically, they will develop a discharge pressure high enough to overcome whatever discharge resistance is present in the system; To prevent any damage due to overpressure these pumps must always be fitted with a relief device connecting the discharge to the suction side. Reciprocating Pumps At one time these units would make up the bulk of the pumps found in marine use, usually steam driven. With the advent of cheap and simple electrical motors however, they are now much less common, having been superseded by rotary pumps. The various arrangements found included single acting, double acting and duplex (twin cylinder) arrangements of both. Reciprocating pumps are still found in use today but are normally restricted to bilge pumping. These pumps are normally duplex and driven by an electric motor through a worm and Electric gearwheel. The rotary movement of motor the gearwheel is translated into a reciprocating movement by a connecting rod and crosshead. Worm & wheel Pulsation Damper Cross head Valve chest Cylinder and piston Suction & (bucket) discharge pipes Revised May 2020 Pumps 30 The pump end consisted of a piston (or bucket) running in a cylinder with an arrangement of suction and delivery valves usually of a spring-loaded plate type. On the double acting type as shown the downward stroke creates a vacuum above the piston thus drawing fluid through the suction valve into the upper part of the cylinder while at the same time the fluid under the piston is compressed, raising its pressure high enough to lift the discharge valve and allow flow out into the system. It should be noted that the flow from these units is intermittent. Duplex arrangements with Accumulator (Pulsation Damper) in the discharge lines give smoother flow conditions due to the more frequent delivery stroke and the damping effect of the air cushion in the accumulator. The accumulator uses the principle that a fluid is incompressible, and a gas is compressible. The accumulator is pre-charged with air to a certain pressure and acts as a cushion to absorb pressure fluctuations in the fluid leaving the pump. As mentioned before the pulsation damper is used to dampen out the pressure variations caused by a reciprocating pump. However, for a rotary positive displacement pump such as a screw pump, gear pump or vane pump etc the flow is smooth, without pressure variation. However, no matter what type of positive displacement pump is used a relief valve must be fitted Revised May 2020 Pumps 31 Rotary Pumps All of the following types of pump are prone to leakage from the high to low pressure areas of the casing due to the need for clearance between the rotating elements and the casing. This leads to a reduction in the efficiency of the fluid transfer. Some indication of the relative efficiencies are given in the examples. Gear Pumps The commonest type of small hydraulic pump is the gear pump. This consists of two contra-rotating gears, one driven and one idler. As they rotate, fluid is drawn from the inlet, passes round the OUTSIDE of the gears between teeth and casing. The movement of the gears accelerates the hydraulic fluid and increases the kinetic energy. NOTE this is referred to as an “External Gear Pump” The thrust produced by the interaction of the gear teeth produces a side loading on the casing and this limits the physical size of this type of pump. Because of the running clearances this type of pump suffers from back leakage which reduces its efficiency. Typically used for pressures up to about 150 bar and flow capacities of 0.7 m3/min. Efficiency 80 – 90% Revised May 2020 Pumps 32 Lobe Pump The lobe pump is very similar in operation to a gear pump, the simplest type is illustrated on the previous page. The lobe pump comes in many different formats, some of these being complex rotor shapes where the driven lobe is arranged inside the driver lobe. NOTE: this type of pump is also referred to as an “Internal gear Pump” The type of lobe pump chosen for any application will depend upon the required discharge rate and the viscosity of the fluid. A typical use for these type pumps is for lubricating and fuel oil transfer. Vane Pumps The vane pump consists of a casing, containing an offset rotor, in which are inserted spring-loaded strips or vanes. The springs cause the vanes to be pressed against the inner wall of the casing. Due to the offset nature of the rotor, the vanes will be pressed into the rotor on one side and extended on the other side. As the rotor turns, the fluid will be entrapped and drawn through the pump. The vanes are typically constructed from a hardwearing self-lubricating plastic material to reduce contact wear on the casing. Back leakage is minimal with this type of pump and, therefore, efficiency is high. Pressures up to about 320 bar and flow capacities of 0.5 m3/min. Efficiency above 90% Revised May 2020 Pumps 33 Screw/Scroll Pumps Screw pumps are used for higher capacity systems. The screw pump comprises two shafts, one driven and one idler, The two shafts are held in synchronisation by gear wheels mounted at the drive end of the spindles. Fluid is drawn from the inlet, passes up the OUTSIDE of the screws Outlet between scrolls and casing. The movement of the screws accelerates the hydraulic fluid and increases the kinetic energy. The problem with single screws is one of end thrust. The drawing below shows a double scrolled pump that balances the thrust. Note the relief valve between the suction and discharge of the pump. Again, because of the running clearances this type Inlet of pump suffers from back-leakage which reduces its efficiency. Typically used for pressures up to about 150 bar and flow capacities of 1.7 m3/min. Efficiency 70 – 85% Revised May 2020 Pumps 34 Variable Delivery Pumps These pumps take the form of reciprocating pumps, in which the relative stroke of the pistons and therefore the capacity of the pump can be adjusted during operation. The two commonest types are the radial piston (Hele-Shaw) and the axial piston (VSG). They are commonly used for steering gear systems, stabiliser systems and other constant running hydraulic equipment. Radial Piston Pump The drawing shows a typical variable delivery radial piston pump in which a number of pistons (in this case 6) are fitted in liners within a rotating cylinder block. The pistons are spring-loaded and press down on an off-centre inner stationary hub. As the cylinder block rotates within the fixed casing, the pistons move in and out as they move round the hub. This creates a pumping action, drawing fluid into each cylinder as each piston moves towards the hub and expelling fluid as each piston moves outwards away from the hub. Fluid is therefore drawn from the inlet side of the fixed casing and pumped to the outlet side of the casing. If the hub is moved so that it is no longer offset in relationship to the rotating cylinder then there will be no relative movement of the pistons and no pumping action. If the hub is moved to an offset position in the other direction then the pumping action and the fluid flow will be reversed. The position of the hub therefore determines the direction and pumping capacity of the pump on a continuously variable basis. Revised May 2020 Pumps 35 Hele-Shaw Radial Variable Delivery Pump This is an alternative arrangement to the variable delivery pump shown on the previous page. In this pump the fluid is drawn from and delivered to ports in the hub instead of the periphery. Both the pumping stroke and direction are varied by moving the floating ring. The floating ring guides the slipper plates, to which is attached the pistons. The piston stroke can therefore be varied relative to the cylinder spider. This is shown in the following diagrams. Revised May 2020 Pumps 36 Axial Piston Pump This type of pump, widely used for steering gears etc., comprises a fixed block (usually integral with the casing) containing the inlet and outlet ports. Running in close contact with the fixed block is a rotating cylinder block containing several pistons that are free to move in and out. The rotating cylinder block is driven at a constant speed and direction by a shaft passing through a static swash plate that can be tilted at different angles from the vertical. The piston rods are attached to a sliding shoe plate that, in contact with the swash plate causes the pistons to move in and out as the drive shaft rotates. A pumping action is thus set up that can be varied in magnitude and direction depending on the angle of the swash plate. The drawing shows the swash plate angle set up so that the pistons are drawing in fluid from the lower port and delivering it to the upper port Fixed suction Rotating & delivery cylinder block block Non-rotating swash plate Delivery port Drive shaft Suction port Swash plate operating lever Delivery port End on view of suction & delivery block Suction port Revised May 2020 Pumps 37 The next drawing shows the swash plate moved into the neutral position. The pistons remain in the mid position as they rotate round the swash plate and no pumping action is set up. The last drawing shows the swash plate moved full over in the opposite direction. The pistons are now drawing in fluid from the top port and discharging it to the lower port, thus the pumping direction has been reversed. Suction port Delivery port The pump is therefore fully adjustable and can be set to pump from zero delivery to full delivery in either direction by adjusting the angle of the swash plate Revised May 2020 Pumps 38 A typical bilge pumping system for a vessel over 400 GT Salient points to note: The normal method for pumping bilged is by a small POSOTIVE DISPLACEMENT pump into the bilge tank (avoids emulsifying oil/water mixture) The OWS pump is also a positive displacement pump. The OWS has a 15ppm monitoring meter & recirculation line There is a self-priming Emergency Bilge Pump located outside of the engine room discharging directly overboard. Can be centrifugal or piston type) There is an additional General Service pump that can be connected to the bilge main discharging to the bilge tank (normally centrifugal for high volume of liquid) This pump has direct suctions from the engine room that can go directly overboard in case of emergency. There may be isolation valves fitted for fault finding Revised May 2020 Pumps 39 Generic pump Centrifugal pump Generic positive displacement pump Gear pump Screw pump Reciprocating pump Diaphragm pump Bi-directional pump Typical pump symbols Revised May 2020 Pumps 40 Compressed Air Systems Revised May 2020 Compressed Air Systems 41 Air compressors are basically positive displacement air pumps and can be split into two main types:; Reciprocating compressors Rotary compressors A compressor is selected by the pressure and the volume of air it has to deliver to the air receiver. The storage pressure at the air receiver is normally kept higher than the pressure that is needed for the system it is supplying. It is this pressure, known as the working pressure, which the air compressor is expected to deliver. This pressure is then reduced to the various pressures required by the system, when it then becomes the operating pressure. Two pressure ranges may be maintained on board, 30 – 40 bar for engine starting purposes and 7 –10 bar for control and working air Why 30 Bar for Engine Start, 7 Bar for GS Air and Control Air 30 Bar for Engine Start Most Large Slow Speed Engines can start quite happily on 16 bar air but the volume of air required to meet the 12 start requirements would mean that the receivers would be very large, as such using 3O bar allows us to meet the requirements and have smaller receivers. In fact, some vessels are now being built with receivers at 45 bar, whereas on small generators we are only using 10 bar air pressure to start them 7 Bar for General Service and Control Air For general service air one has to consider the safety side of things, air hoses rated at 30 bar would be very expensive and if they did burst the “whip” would be lethal, we have enough air pressure related injuries as it is at 6 to 7 bar, plus the fact all the pipe work around the vessel would have to be rated for 30 bar, it just wouldn’t be a viable method For control Air, most controllers are working on a 0.2 to 1.0 bar with some positioners and actuators working up to 2 bar. At present at each controller you will find small regulators reducing from 6.5/7.0 to 2.0 bar it would be a very expensive regulator to reduce from 30 bar to 2.0 bar and on top of that we still have the expensive pipework to route around the Engine Room, again not a viable method Revised May 2020 Compressed Air Systems 42 The drawing above shows a typical arrangement of a single cylinder air compressor with details of plate type suction and delivery valves. Plate valve are chosen because they open and shut rapidly and have a relatively large area. These valve types are only used on small compressors & are also quite common on refrigeration compressors. The most usual valve arrangement is shown to the right and a breakdown view on the following page. Revised May 2020 Compressed Air Systems 43 The next drawing shows a graph of a single suction and delivery cycle of an air compressor piston. The work done during the cycle is given by the area enclosed by the graph. Revised May 2020 Compressed Air Systems 44 The two-stage compressor uses the thermodynamic principle that by compressing the air in two stages and by cooling the air between the first and second stages, the efficiency of the compressor is increased. Inter-stage cooling can be achieved by air or water, both can be equally as efficient. Air coolers can take up more room and are prone to damage, they save on plumbing costs, but absorb energy from the compressor in driving the cooling fan. Water coolers can be configured either as water passing through tubes surrounded by air, or by air passing through tubes surrounded by water. The second method will generally withstand higher air pressures. In both cases, the water side of the cooler must be fitted with a relief valve as a protection in the event of a tube failure Some Definitions Volumetric efficiency. The ratio between volume of air drawn in per stroke to the actual stroke volume. Clearance volume. The volume of air left in the cylinder at the end of the compression stroke. Valve loss. The pumping efficiency lost as a result in the delay in the suction and delivery valves opening and closing. Revised May 2020 Compressed Air Systems 45 Cross section through a 30 bar, 2 stage, water cooled air compressor Compressor safety devices Protection against overpressure Relief valves are to be fitted after each stage in a compressor to protect against overpressure due to valve seizure, or blockage. Relief valves can only be tested by an approved service agent using calibrated equipment, they will then issue a certificate. They must NEVER be tested by shutting in the discharge valve. As an alternative to relief valves, some smaller compressors may be fitted with bursting disks. These are thin sheets of metal (usually monel metal) that are designed to rupture at a set pressure. Protection against overheating Fusible plugs MAY be fitted in the air delivery casings after each stage of the compressor and are designed to melt and release the pressure if the compressor loses its cooling medium and overheats. The final stage of the compressor MUST have either a fusible plug or an alarm device that operates at a temperature of 121oC, the emergency air compressor is exempt from this regulation Revised May 2020 Compressed Air Systems 46 Water cooled air compressors must also be fitted with a pressure relief valve in the water jacket. This protects the compressor in the event of a cooling pipe bursting and the cooling water jacket being exposed to the full delivery air pressure from the compressor. A bursting disc will perform the same function A compressor will also be fitted with an automatic non-return valve in the delivery line to prevent the contents of the air storage bottle passing back through the compressor when it is stopped. The normal practice is for the relief valves on the compressor to lift before the one on the starting air receiver. The following criteria have been taken from Lloyds rules The test pressure of the receiver will be 1.5 x the design pressure The Relief Valve must start to lift at a pressure no more than the design pressure The relief valve should be of such a size, and so set, that the maximum accumulation pressure will not exceed the working pressure by no more than 10%, should all the discharge valves be shut, and all compressors are filling the receiver at maximum efficiency. For example: Test Pressure = 45 Bar Design Pressure = 30 Bar Working pressure = 27 Bar Problems with Relief Valves Reasons why a Relief valve may lift is Service Excessive pressure due to malfunction in the system Broken Spring Spring adjuster not locked; vibration has caused the tension on spring to be reduced Incorrectly adjusted Reasons why a relief valve may not open Poor maintenance and the she shaft is seized in it sleeve Poor maintenance an there is a build up of scale above the valve plug Drain hole on the waste pipe blocked causing a head of water to act on top of the plug Incorrectly Adjusted Revised May 2020 Compressed Air Systems 47 How to overcome Problems with Relief Valve Most issues with relief valves can be avoided with the following Having a Good Plan Maintenance process in use, inspections and testing, maintenance can be carried out without adjusting the settings, such as Drop the pressure of the system and lift the hand operating lever to check that the valve moving freely With pressure completely off the system (locke and tagged out) remove the discharge pipework and check condition above the valve, again operate by hand for visual inspection, check condition of seating. Check all drain lines and drain holes are clear If possible, check the spring for signs of wastage. Ensure you have sufficient spares onboard in case an overhaul is required Remember only strip the valve down if you have to, if possible, wait until the next port and have it overhauled by an accredited company Compressed Air Storage Bottle The drawing shows a typical layout of a compressed air storage bottle. Mountings include: Compressor delivery isolating valve Supply isolating valve (slow opening design) Calibrated pressure gauge Drain, can be automatic Pressure relief valve Inspection manhole door Maker’s nameplate Storage bottles may also be fitted with a fusible plug to protect against overheating. The valves on the air receiver are designed so that they can be “Hammered shut” and opened SLOWLY to prevent shock waves that could damage the system and pipework. All high-pressure pipes in the system must be made from solid drawn steel and also have a pressure test certificate. Revised May 2020 Compressed Air Systems 48 External/ Internal Inspection of Air Receivers External Inspection of Air Receiver If possible open up inlet valve for Internal Inspection (Normally carried out during periodic shipyard) If Possible open Discharge Valve for Inspection (Normally carried out during periodic shipyard) Inspect relief valve, remove discharge pipe work for internal inspection, open the valve for the manual handle, is it free to move, does it snap shut (do not adjust any pressure setting unless sending to accredited workshop for overhaul) Calibrate Pressure gauge If possible, Calibrate pressor sensors (Auto Start/ Stop, Lagg/Lead) Open and inspect Auto drain Valve and drain bypass valve Do a complete check of the Air Receiver Body for damaged paint work and rust Internal Inspection of Air Receiver Check Manhole door seating (Flanges for damage) Check for a “tide” mark which will give a good indication that the Auto drain hasn’t been working at some time and watchkeepers are not draining. Check the surface for the presence of oil. Check the varnish coating (it should be clear varnish) for cracking, is there any corrosion that can be seen through the varnish. Any areas with a break down in coating should be removed, the surface underneath being inspected and then recoated. The areas to be noted in the work report for routine follow up inspections Do a complete check for corrosion, especially any areas that may be a little cooler where blowers have been directed at the receivers Check the area around the high-pressure outlet pipes which could be subject to erosion due to high velocity air Check all penetrations Have some one hand lift the relief valve, does anything flow out of it, what does the condition of the valve plug look like Revised May 2020 Compressed Air Systems 49 Schematic of a 3-stage air compressor with all safety devices Typical Compressed Air System – A drawing that you need to be aware of Revised May 2020 Compressed Air Systems 50 Removal of Contamination Filters are required at the compressor suction and at specific locations in the air delivery system in order to remove any particles from the air and avoid damage to the equipment. Air also contains water vapour. The amount of water vapour in the air is referred to as the relative humidity. For a given volume, the amount of water vapour that can be held is dependent on the temperature and the pressure. If the pressure of a specific volume of air is increased, the volume will decrease (Boyles’ law) As the volume decreases, the water vapour in the air will reach saturation point. If the temperature of the high-pressure saturated air is then dropped, then the volume of the air will decrease even more (Charles’ law) and the vapour will condense out as free water. Air passing through a compressor will therefore contain large quantities of free moisture, which, if allowed to remain in the system, will corrode the various components. The water will also mix with any oil carried over from the compressor and form a sticky emulsion. This, if allowed to remain in the system, will gum up components and prevent free movement. Free moisture and emulsions can be removed from the system at the air compressor drains, the air storage bottle and at the various filter/drains situated at various points in the air distribution system. It is quite common to fit a refrigerated dryer system to remove water vapour between the compressor & air reservoir. Extract from Lloyd's Rules - Part 10, Chapter 1 7.1 Initial Starting Arrangements 7.1.1 Equipment for starting the main and auxiliary engines is to be provided so that the necessary initial charge of starting air or initial electric power can be developed on board the craft without external aid. If for this purpose an emergency air compressor or electric generator is required, these units are to be power driven by hand starting, or oil engine except in the case of small installations where a hand operated compressor of approved capacity may be accepted. Alternatively, other devices of approved type may be accepted as a means of providing the initial start. 7.2 Starting Arrangements. Air Compressors 7.2.1 Two or more air compressors are to be fitted having a total capacity, together with a topping-up compressor where fitted, capable of charging the air receivers within one hour from atmospheric pressure, to the pressure sufficient for the number of starts required by 7.3. At least one of the air compressors is to be independent of the main propulsion unit and the capacity of the main air compressors is to be approximately equally divided between them. Revised May 2020 Compressed Air Systems 51 7.3 Air Receivers 7.3. 1 Where the main engine is arranged for air starting the total air receiver capacity is to be sufficient to provide without replenishment, not less than 12 consecutive starts of the main engine, alternating between ahead and astern if of the reversible type and not less than six consecutive starts if of the 'non-reversible type. At least two air receivers of approximately equal capacity are to be provided. 7.3.2 For multi-engine installations, where more than one engine is driving each propulsion shaft line, the following requirements apply: Twin engine installations driving fixed pitch propeller, where one of the engines can be reversed, six consecutive starts per engine are required. For all other types of multi-engine installations, three consecutive starts per engine are required Revised May 2020 Compressed Air Systems 52 Hydraulic & Pneumatic Control Principles Revised April 2020 Hydraulic & Pneumatic Control Principles 53 Revised April 2020 Hydraulic & Pneumatic Control Principles 54 Hydraulic Systems Marine hydraulic applications are widely used on board many modern yachts, they mainly consist of relatively simple, slow moving power systems such as; Steering gear. Controllable pitch propeller. Bow/stern thrusters. Stabilisers. Deck cranes. Deck winches/windlasses. Hatch covers. Trim tabs Remote operated valves. Side & stern door openings Swimming platforms Sail & mast tensioning devices Hydraulic machines use liquid pressure to transfer a force from one place to another employing the following properties possessed by all liquids. A liquid cannot be compressed. Any change in pressure within the liquid is transmitted instantly to all parts of it. Hydraulic Pressure = Force/Area or Force = Pressure x Area Mechanical Analogy The drawing above illustrates the principles used in all hydraulic power transmission systems. The small diameter cylinder on the left creates a pressure in the fluid, this pressure is also acting upon the area of the large piston on the right. By this means a relatively small power unit can exert a large force which can be transmitted over a long distance from the source to the place where it is needed to do the work. The relative areas of the pistons means that to move the large piston a small distance the small piston will have to move a much larger distance. Revised April 2020 Hydraulic & Pneumatic Control Principles 55 Hydraulic oil is the fluid used to power hydraulic systems in applications as diverse as automobile brakes, garbage truck lifts, and aircraft flight controls. There are many different fluids used for this purpose, including mineral oil, water, synthetic compounds, and water- based mixtures to name a few. The basic oil stock could be glycol, castor oil, ethers, esters, organophosphate ester, mineral oil, polyalphaolefin, and silicone or propylene glucol. Mixtures are created with chemicals and synthetic products according to what the oil will be used for For marine purposes Hydraulic Fluid is normally oil Hydraulic oil differs from lubricating oil, since it should be able to transmit power to remote positions accurately, quickly, and efficiently. It should be able to provide the following functions: Transmission power Hydraulic oil is employed as a method of transferring power in a number of applications. The selected fluid must be capable of flowing easily through lines provided. In addition, the fluid must be as incompressible so that the action is instantaneous. Lubrication In most hydraulic systems the internal lubrication is provided by the hydraulic fluid. Moving parts slide against each other on a film of fluid. The oil must contain additives to ensure anti-wear characteristics. Sealing In many cases the fluid is the only seal against pressure in a component. The degree of fit determines leakage rate. Cooling Heat is generated in hydraulic systems due to friction between metal surfaces of the system components and friction within the fluid generated by travelling through orifices etc. Cooling is essential to ensure that the oil is maintained at its correct viscosity for correct system operation and to prevent it from deteriorating, forming sludge and destroying its properties. In extreme situations it may reach its flashpoint. Cleaning Hydraulic oil transfers particle contamination to filters for retention. Several factors have to be taken into consideration when choosing a hydraulic fluid for a particular application. Some important factors are; Stable over the full range of expected operating conditions. Non-corrosive to the various materials used in the system. Good lubricating qualities to reduce friction. Leakages should have no impact on the surrounding area. Low compressibility – can transmit pressure signals without delay. If the fluid comes into contact with air then the fluid’s characteristics should not change (oxidation) High flashpoint to reduce the possibility of fire. Revised April 2020 Hydraulic & Pneumatic Control Principles 56 Viscosity The viscosity of the oil should be carefully matched to the temperature to ensure that the fluid’s viscosity is not too low (thin) nor too high (thick). If the fluid viscosity is too low then leakages will take place through the clearances of the mechanical components of the system, leading to loss of power, control and efficiency. If the fluid viscosity is too high then the system will be slow to respond, and high pressures will tend to build up. The viscosity of the oil will tend to change as it heats and cools in service and this change is indicated by the Viscosity Index. The V.I. is an arbitrary figure and ranges from below 10 to above 110. The higher the number the more resistant an oil will be to a change viscosity due to temperature. A typical hydraulic oil will have a viscosity index of 90 so will be relatively highly resistant to changes in temperature. For continuous low temperature operation where the oil is unlikely to overheat (deck hydraulics) a low viscosity oil is chosen with a typical viscosity of ISO 22 For continuous high temperature operation or where the oil is likely to heat-up in operation (heavy duty machinery spaces) a standard high viscosity oil of ISO 68 is chosen. Hydraulic oils that are exposed to very wide temperature variations are formulated with viscosity index improvers, which help to regulate the change in viscosity relative to temperature. Revised April 2020 Hydraulic & Pneumatic Control Principles 57 Hydraulic Hygiene Hydraulic hygiene is the prevention and cure of contamination of hydraulic fluids by dirt, water and air. Contamination is often the result of insufficient attention being paid to hydraulic hygiene. The consequences of dirt in a hydraulic system is not always immediately apparent, but it can cause malfunction of components and loss of efficiency. The long term effects are far more serious leading to premature wear of components and increased unreliability and possible catastrophic failure. The amount of dirt in any re-circulating fluid system is a balance between the quantity being induced or created and the quantity being removed by the system filters. When the system is first assembled original dirt becomes “built in” despite rigorous precautions. It is common for sections of pipework to be connected to a mobile filtration system that recirculates oil through very fine filters until samples of the oil reaches a point where it meets or exceeds the minimum contamination standards. With the full system finally connected the fluid is at or near maximum contamination. As the process continues dirt is removed by the filters, and as a consequence, the filter element is rapidly degraded, and it may be found necessary to clean or change the filter several times before the system has settled down to a steady cleanliness level. Providing the system is not opened up for repairs, and any fluid added is filtered correctly, the contamination will drop until it consists of wear products which include attrition wear from pipework, particles of rust, and some organic constituents such as gums and resins from the fluid, this is controlled by the filtration system. Oil will dissolve small quantities of water in the order of hundreds of ppm whilst still remaining clear and bright. The nature of the oil will define the maximum concentration, and water solubility will increase with temperature. Whilst it remains dissolved the water will not cause problems within the system, but upon being cooled it may come out of the solution as suspended water. In the case of saturated oil, the water is extremely finely dispersed, as a result upon cooling the water separates very slowly. This finely dispersed water renders the oil cloudy but provided that separation of free water does not take place it does not present a great threat to the efficiency of the system. Free water in the system is particularly undesirable, as it offers no lubrication and can be the cause of corrosion if the system has been left idle for a period of time. The main sources of water are: Condensation Leaking coolers Contaminated top up Revised April 2020 Hydraulic & Pneumatic Control Principles 58 Sampling A check for water content should become part of the routine system checks, you can use the same test kit that would normally be used for checking engine oil. The maximum recommended level is 250ppm or 0.025%. In addition to this it is strongly advised that a “representative sample” of oil from all critical systems be sent for laboratory analysis as part of a regular planned Maintenance system. Filters It is essential to ensure that all traces of contaminates and wear particles are removed as they will cause damage to system components, to achieved this, a number of filters may be present in the system. It is normal to have high pressure fine filter on the pump discharge to protect valve blocks & spools from damage or seizure. There may be a magnetic filter incorporated in this unit. It is normal practice to filter the return lines at a common point before they enter the tank, there is usually a filter built into the filling connection. There is quite often a spin on cartridge type filter fitted to the oil tank air vent to prevent airborne contamination. If the system is using a Heli Shaw (radial piston) or swashplate (axial piston) pump where the direction of flow reverses through the pump then an in-line filter is pointless, these system will have a separate pump and filtration system fitted. Pumps used for Hydraulic Power When a hydraulic pump operates, it performs two functions. First, its mechanical action creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system. A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load. All pumps may be classified as either positive-displacement or rotodynamic (non-positive- displacement) a centrifugal or propeller pump produces a continuous flow. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. If the output port of a non-positive-displacement pump were blocked off, the pressure would rise, and output would decrease to zero. Although the pumping element would continue moving, flow would stop because of slippage inside the pump, these properties make it unsuitable for shipboard hydraulic operations In a positive-displacement pump, slippage is negligible compared to the pump's volumetric output flow. If the output port were plugged, pressure would increase instantaneously to the point that the pump's pumping element or its case would fail, hence the requirement for a relief valve or unloading mechanism. Revised April 2020 Hydraulic & Pneumatic Control Principles 59 External-gear pumps A gear pump produces flow by carrying fluid around the periphery of two meshing gears. External-gear pumps are comparatively immune to wear particles in the oil, which will increase wear rates and lower efficiency. They are therefore very reliable and are able to produce high pressure. Internal-gear pumps These have an internal gear and an external gear. Because these pumps have one or two less teeth in the inner gear than the outer, relative speeds of the inner and outer gears in these designs are low. For example, if the number of teeth in the inner and outer gears were 10 and 11 respectively, the inner gear would turn 11 revolutions, while the outer would turn 10. This low relative speed means a low wear rate. These pumps are small, compact units able to produce higher pressures than an external gear pump Screw Pump A screw pump is an axial-flow gear pump, similar in operation to a rotary screw compressor. The inlet hydraulic fluid that surrounds the rotors is trapped as the rotors rotate. This fluid is pushed uniformly with the rotation of the rotors along the axis and is forced out the other end. The fluid delivered by a screw pump does not rotate but moves linearly. The rotors work like endless pistons, which continuously move forward. There are no pulsations even at higher speed, this results in very quiet operation. These are commonly used on rotary vane steering gears. Revised April 2020 Hydraulic & Pneumatic Control Principles 60 Piston pumps The piston pump is a rotary unit which uses the principle of the reciprocating pump to produce fluid flow. Instead of using a single piston, these pumps have many piston- cylinder combinations. Part of the pump mechanism rotates about a drive shaft to generate the reciprocating motions, which draw fluid into each cylinder and then expels it, producing flow. Axial piston pumps are the most common type and are available as fixed and variable displacement pumps, the swash plate pump as it is commonly termed can pump in both directions and is very common on steering gear systems. If oil is fed into this pump it will change function and become a motor. Vane Pump In vane pumps, a number of vanes slide in slots in a rotor which rotates in a housing or ring. The housing may be eccentric with the centre of the rotor, or its shape may be oval. In some designs, centrifugal force holds the vanes in contact with the housing, others utilise light springs, while the vanes are forced in and out of the slots by the eccentricity of the housing. Pulsation Dampers The use of pulsation dampeners and a captive acceleration tube in hydraulic pump systems can significantly smooth the flow of the pumped liquid and extend both pump and system life. The most common design of a bladder type pulsation dampener has a single liquid connection at one end of the metal housing and a gas-charging valve at the other end with the bladder between. The gas volume acts as a spring, compressing and expanding to meet liquid system pressure changes. When the pressure is doubled, the volume is halved. The more the gas is compressed, the more potential energy it contains to give back when needed. The poppet valve prevents the bladder from being extruded into the system. Revised April 2020 Hydraulic & Pneumatic Control Principles 61 Dry nitrogen is the gas of choice. It is readily available and, being oxygen free, it prolongs bladder life. The pulsation dampener housing must be made of a material that meets the corrosion requirements of the pumped liquid and the installed environment. The bladder must be an elastomer that is compatible with the liquid and permits the necessary expansion and retraction over time. The poppet valve is there to prevent the bladder from extruding into the system if the pressure falls below normal. Servicing the Dampener The charge must be checked periodically for optimum performance. The charging must be made with atmospheric pressure at the fluid port. The pre-charge should be set between 50 percent and 70 percent of the system pressure. When running at set pressure the average volume of gas will be 50 percent to 30 percent, respectively, of the pulsation dampener volume. The manufacturer will advise the appropriate pre-charge to match the pump type, accumulator size, bladder and liquid characteristics. Like electrical systems, hydraulic and pneumatic systems are portrayed in diagrammatical form by using symbols to represent components. These symbols may bear little relationship to the construction of the actual component but give a very good indication of what it does within the system. The following four pages give a useful selection of the symbols that are most often used. Revised April 2020 Hydraulic & Pneumatic Control Principles 62 Constant pressure hydraulic system (Also referred to as an open loop system) In an open loop system, the fluid is drawn from and returned to a storage tank The drawing shows an open loop hydraulic system used to power a winch motor. The system as drawn uses symbols from the charts on the preceding pages and gives a good idea how they are linked together to form a functioning circuit. The system has pressure, direction and independent flow control in both directions. The directional control valve is solenoid operated with spring centring and is configured to re-circulate the hydraulic fluid to tank in the neutral position. Revised April 2020 Hydraulic & Pneumatic Control Principles 63 Variable pressure hydraulic system (Also referred to as a closed loop system) In a closed loop system, the fluid is constantly circulated by a variable delivery pump. Typically used for steering gears but also for cranes and some winches. The drawing shows a closed loop hydraulic system used to power two cylinders that could be used to operate a rudder. When not in use the system is maintained at a constant pressure by a header tank supplying through two check valves. The system as drawn again uses symbols from the charts on the preceding pages. The system operates by using a variable delivery pump. The system is maintained under a static head pressure by either a tank, as shown, or a small hydraulic supply pump running at constant pressure. This ensures that any leakage will be out of the system and that air cannot be drawn into the system. The variable delivery pump controls direction, speed and load of the cylinders by changing the amount of fluid circulating within the system. The pump is protected against overpressure by two relief valves set to operate in the direction that the pump is pumping. The non-return valves prevent fluid passing back into the header tank when the variable delivery pump is pumping at a pressure higher than the head pressure of the tank. Revised April 2020 Hydraulic & Pneumatic Control Principles 64 Basic purely pneumatic circuit Marine pneumatic applications, however, are linked to fast acting multiple step systems using more complex control logic functions. Logic control allows multiple choice systems to be built, such as in diesel engine management. The components used in pneumatic systems are very similar to those used in hydraulics, the main difference being that waste air is vented to atmosphere rather than being returned to a storage tank. This shows the operation of a pneumatic cylinder that could be used for remote operation of a fuel/ballast/bilge valve and consists of two operating 3/2 valves that are used to supply pilot operating air (dashed lines) to the 5/2 cylinder directional control valve. The 5/2 valve has its own direct air supply, and this allows pressure to be maintained on the cylinder when the 3/2 operating valves are released. Air flow control valves are also fitted to enable speed control of the cylinder. The operating force of the cylinder is purely a function of the supplied air pressure. Revised April 2020 Hydraulic & Pneumatic Control Principles 65 Electro-pneumatic & electro-hydraulic control Direct pneumatic/hydraulic logic control is generally inefficient, particularly over long distances. It is often easier and cheaper to interface the pneumatic/hydraulic valves with electrical sensors and operating devices. By using electrical control, long runs of pneumatic/hydraulic piping can be replaced with electrical cabling. High speed electronic logic controllers can be used, and the number of pneumatic/hydraulic components required can be reduced. To achieve electrical control the first requirement is to sense the condition of the process. Three main types of electrical process sensor are used; Pressure sensors Flow sensors Proximity or motion sensors Pressure sensors: These sense the pneumatic/hydraulic pressure in a system and can be used to limit the force generated by a cylinder. As the cylinder extends against a resistance the pressure will increase. The sensor can then generate a signal to reduce the fluid flow to the cylinder, or even cause the cylinder to retract. Flow sensors: Not often used and then only in hydraulic systems. These sense the hydraulic flow in a system and can be used to limit the speed of operation of an actuator. Could be used as part of a closed loop speed control system for a hydraulic motor. Probably easier to sense the actual speed of the motor. Proximity sensors: Used to detect position and movement. Can be of four main types Roller cam switch: The moving object contacts a roller arm. This in turn makes or breaks an electrical contact. Often termed a micro-switch. Magnetic reed switch: A magnet fixed to the moving object comes into proximity with two thin contacts and the induced magnetic field causes them to either make or break. Can be made very small. Inductive sensors: These are proximity switches that work on the principle that a magnetic field will be changed if a ferrous object moves within the field. The sensor produces an electromagnetic field which, if disturbed will trigger a switch to make or break. Capacitive sensors: In this case the sensor detects the changes in the dielectric strength of the air surrounding the sensor, if disturbed it will trigger a switch to make or break. Revised April 2020 Hydraulic & Pneumatic Control Principles 66 Electrical actuators Electrical solenoids are widely used to control the position of pneumatic/hydraulic valves and a typical configuration is shown. The movement of the ferrous core is used to move directional control valves or change pressure settings on regulating valves. Electrical circuits The above drawing shows two

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