Auxiliary Machinery: Pumps PDF

Chapter 5 ©D Auxiliary Machinery Pumps Of the various kinds of onboard auxiliary machinery, pumps by far outnumber other kinds of equipment, both in the number of types and installed units. Except for certain types, pumps are usually provided in multiple units - several units are used for contin...

Chapter 5 ©D Auxiliary Machinery Pumps Of the various kinds of onboard auxiliary machinery, pumps by far outnumber other kinds of equipment, both in the number of types and installed units. Except for certain types, pumps are usually provided in multiple units - several units are used for contin­ uous running while the ship is under way, and the remaining units are on standby as backups. Technically speaking, the pump is designed to continuously transfer a liquid at a low level to a high level with the help of external motive power. Used to transfer any kinds of liquid, such as fresh water, sea water or oil, pumps come in various types and sizes. Onboard pumps are largely classified into the following two types according to their working principles and structures: Type 1: the non-positive displacement turbo pump. With this pump, the impeller is rotated at a high speed in the casing that contains a liquid. The centrifugal force generated by this rotation gives kinetic energy to the liquid and discharges it. Type 2: the positive displacement pump. This pump locks up a liquid within a certain space, then pushes it out little by little. Turbo pumps Centrifugal pump Volute pump — Mixed flow pump — Axial flow pump Positive displacement pumps Reciprocating pump Piston pump Bucket pump — Plunger pump — Rotary pump — Gear pump — Screw pump — Vane pump — Axial piston pump — Radial piston pump Special pumps Jet pump Friction pump Fig. 5-1: Pump head 063 Elementary Marine Engineer Textbook Chapter hapter The vertical distance between a pump's suction and discharge surfaces is called net actual head. Net positive suction head (NPSH) values are used when evaluating suction per­ formance of pumps. NPSH values depend on pump performance capabilities, shape of the suction-side pipe, etc. The term “Hydraulic head” refers to potential energy of water translated as height of a water column. If the pump is operated under conditions beyond its designed suction perfor­ mance, cavitation may be generated within the pump, making it impossible for the pump to realize its designed performance or causing damage to pump compo­ nents. Therefore, sufficient attention should be paid to this problem. Each type of pump has its own performance characteristics which are shown in a pump characteristic curve. With this curve, it is customary to plot discharge values on the horizontal axis. Changes in net actual head, pump efficiency and required pump input power relative to discharge values are shown. Ensure highly efficient, energy-saving and trouble-free pump operations by confirming the state of pump operation by referring to such a characteristic curve. In the following sections, we will take up representative types of pumps one by one and outline their features. MOVIE 5-1-1. Centrifugal pumps lEl The centrifugal pump gives a centrifugal force to a liquid by means of high-speed rotation of the impeller. The pump has a structure to convert the resultant kinetic energy into potential energy. According to difference in such structure, centrifugal pumps can be classified as follows: hapter Chapter V... Chapter 5 =5 <T> " Chapter Chapter 7 Chapter 8 (1) Single- and multi-stage pumps Shown in Fig. 5-2 is an example of a hori­ zontal-shaft, single-suction, single-stage vo­ lute pump, which is the most basic type. With this pump, the liquid is sucked in from the center of the fast-rotating impeller and dis­ charged from its periphery. Its speed energy is converted into pressure in the spiral casing and is outputted. But a head available from one impeller can be insufficient. If that’s the case, a multi-stage pump is used, in which two or more impellers are arranged on the shaft according to the re­ quired discharge head. Impellers are connect­ ed in series by inter-stage passages. Heads are added one after another so that the re­ quired high head can be obtained when the liquid comes out of the final stage. 9 Chapter Spiral casing 10 Chapter Impeller Fig. 5-2: Single-suction, single-stage volute pump Elementary Marine Engineer Textbook 064 Chapter 5 Auxiliary Machinery (2) Two-stage volute pumps An example of a two-stage pump impeller and its casing is shown in Photo 5-1. It incorporates two impellers, the upper and lower impellers. The liquid is sucked into the lower impeller at the bottom of the casing and is pressurized. After going through the passage inside the casing, the liquid is sucked into the upper impeller from the uppermost part of the casing where it is pressurized even further be­ fore being pushed out from the center of the casing. (3) Single- and double-suction volute pumps The single-suction pump sucks in the liquid from one side of the impeller while the double-suction pump sucks the liquid in from both sides of the impeller. Compared with the single-suction pump, the double-suction pump allows a much greater amount of discharge to be obtained with one impeller. Shown in Photo 5-2 is an example of the double-suction pump. As it travels through the passage in the casing, the liquid, which has been sucked into the pump, branches out and is sucked into the impeller from above and below. After joining together again in­ side the impeller, the liquid is finally pushed out of the pump from the center of the casing. Photo 5-1: Two-stage volute pump Photo 5-2: Double-suction volute pump (4) Horizontal- and vertical-shaft pumps With the horizontal-shaft pump, the pump’s main shaft is arranged horizontally. The vertical-shaft pump has a vertically arranged main shaft, which means a benefit of smaller space for installation. Photo 5-3 shows the exterior of a vertical-shaft pump including an electric motor. Though centrifugal pumps come in a wide variety of types, all are unable to work unless the cas­ ing is filled with a liquid prior to starting to pump. Before starting a pump, the inside of the pump casing must be filled with a liquid in an operation called “priming.” If a pump’s suction head is at a higher level than the liquid inlet, priming must be conducted by opening the air vent valve. The valve is closed after air and water (fluid) has come out. 065 Elementary Marine Engineer Textbook Chapter hapter Chapter V... Chapter 5 =5 I'D " Chapter pump head (m) Photo 5-3: Volute pump (%) input power Discharge amount m3/min (kw) Fig. 5-3: Volute pump characteristic curve Next, we will explain precautions to be taken when starting or operating pumps. Fig. 5-3 is an example of a volute pump characteristic curve. Discharge values are plotted on the horizontal axis. So, once a discharge value is fixed, then the corresponding total head, pump input power and efficiency can be found. With volute pumps, the required pump input power is lowest when the flow rate is at its mini­ mum. Therefore, this is when the pump is started with the suction valve open and the discharge valve closed. Typically, discharge amount is adjusted by changing the valve opening position. Now, here are points you should remember when handling centrifugal pumps: (1) 6 Before starting the pump, hand-check the bearing to confirm that there is no abnormality. Then, conduct priming to fill the pump casing with liquid. (2) With the volute pump, start the pump with the suction valve fully open and the discharge valve closed. Gradually open the discharge valve only after confirming that the discharge pressure has increased with no problem re­ garding the impeller rotation. (3) Confirm that there is no leakage from the shaft-sealing device’s mechani­ cal seal. As for the gland packing-type shaft-sealing device, adjust the gland by tightening it equally on the right and left so as to allow slight leakage. (4) Pay attention to temperature rises in the bearing and shaft-sealing device as well as their vibration sound. (5) Pay close attention to pressure gauge and ammeter readings. Elementary Marine Engineer Textbook 7 'hapter 3 9 Chapter 10 Chapter 066 Chapter 5 Auxiliary Machinery MOVIE 5-1-2. Rotary pumps 6 With the rotary pump, the liquid is pushed out as the pump’s main part rotates. Its excellent feature is its simple structure with a relatively small volume per capacity, thereby mak­ ing it suitable for high-speed operation. Also, very few problems occur. As such, rotary pumps are often used to transfer oil and other vis­ cous liquids while also being suitable for use for hydraulic systems. Because the rotary pump is a displacement type, it is necessary to keep the bypass line open by opening both the inlet and outlet valves when starting. To adjust discharge amounts, it is customary to adjust flow rate of the bypass line which connects the outlet and inlet ports. (1) Gear pumps Two gears of an identical shape (driving and driven gears) are meshed with each other and rotate in an enclosed casing. The liquid, which flows into the tooth space from the suction side, is carried to the discharge side along the casing’s inner circumference. To ensure smooth pumping, double-helical gears are typically used to enable multiple teeth to be meshed simultaneously and the meshing points to move smoothly. Figs. 5-4 and 5-5 and photo 5-4 show an external gear pump, in which double-helical gears’ ex­ ternal teeth mesh with each other. Other types of gear pump include: the internal gear pump, in which a toothed gear is made on the rotor internal surface; and the trochoid pump, which is a kind of internal gear pump having teeth of a special shape. 067 Elementary Marine Engineer Textbook Chapter hapter While the pump’s discharge and suction sides are divided by the gears’ meshing part, liquid can leak through a gap between both sides of the gears and the casing. So, such a gap must be adjusted and managed properly. (2) Screw pumps The screw pump is designed to push out the liquid by means of groove movement of screw rotors housed in the casing. Typical­ ly, the number of screw rotors is one to three, of which three-shaft screw pumps are most commonly used. Fig. 5-6 is an example of a three-shaft screw pump. When the driving screw in the center, which is directly coupled with the motor, is turned, the two driven screws on both sides of the driving screw mesh to­ gether, rotating at the same speed but in the reverse direction. The liquid, which flows in from the suction inlet, is sent out in the shaft direction thanks to the screw movement. Chapter V... Chapter Driving screw Driven screw 5 =5 I'D " Chapter Fig. 5-6: 3-shaft screw pump With screw pumps, liquid discharge is continuous and free of pulsation, thus generating mini­ mal noise and vibration. Very little liquid leakage means they are suitable for high-pressure appli­ cations. They can also accommodate high-speed rotation. Therefore, a great liquid flow rate is available despite their relatively small sizes. As such, screw pumps are commonly used as fuel oil transfer and lubricating oil pumps. This pump’s driven screws rotate on their own using a pressure difference without power from the driving screw. Unlike gear pumps, there is no wear on tooth surfaces, giving a longer life span for the pump. Vane Rotor 7 (3) Vane pumps 9 Shown in Fig. 5-7 is a balanced vane pump. Its cylindrical rotor has grooves cut in the radial direction, into which rectangular plate-like vanes are inserted so that they can slide freely. The rotor turning produces a centrifugal force, allowing the vanes to rotate while being pushed against the cam ring inner Fig. 5-7: Vane pump surface. Because the rotor driving shaft is eccentric to the cam ring center, the volume between vanes gradually increases toward the suction side, making it easy to suck in the liquid. Conversely, the volume between vanes gradually decreases toward the discharge side, raising the liquid pressure and promoting discharge. This pump is applied to many hydraulic systems. Elementary Marine Engineer Textbook hapter Chapter 10 Chapter 068 Chapter 5 Auxiliary Machinery 5-1-3. Pump trouble Bearing and mechanical seal problems account for approximately 40% of the total cases of trouble with pumps for fresh/feed water, sea water, FO and LO uses. In the following, our explanation will focus on bearings and mechanical seals, the two important components that require special attention during inspection. (1) Pump ball bearings Ball bearings are commonly used as bearings for pump-rotating shafts. Because of their limited service life, these ball bearings need to be replaced after being used for a prescribed period of time. But you should remember that using them under the following conditions is likely to shorten their service life or cause damage to your pumps even if the bearings you are using are within the allow­ able time: ® Increase in pump shaft thrust ® Degradation of lubricant @ Increase in pump shaft vibrations When it comes to volute pumps, increase in shaft thrust is caused by excessively widened gap due to wear of the mouth ring. Oil or grease is used to lubricate ball bearings. Regarding the use of lubricating oil, it is important to check the oil amount then replenish con­ sumption or replace it at regular intervals. Regarding the use of grease, there are two types of ball bearings: the enclosed-type ball bearing requires periodical replacement of ball bearings; the open-type ball bearing requires periodical re­ plenishment of grease. Also pay sufficient attention to liquid leakage due to poor maintenance of the shaft-sealing device. Photo 5-5: Ball bearing Fig. 5-8: Horizontal-shaft pump ball bearing 069 Elementary Marine Engineer Textbook Chapter hapter (2) Probable causes of increase in pump shaft vibrations There are several probable causes of pump shaft vibrations, one of them being an excessive deviation in drive shaft and pump shaft alignment. Here, we will explain measurement of shaft alignment deviation and its allowable range. Overhaul a pump equipped with a flexible coupling. After the restoration assembly is complete, check the shaft center, compare its values with al­ lowable values, and make adjustments in case the values exceed the allowable level. At this time, if the pump in question is a type dealing with high-temperature water like a boiler water circulating pump and a feed water pump, be sure to perform checks and adjustments in a high-temperature state. Shown in Fig. 5-9 © and ® are two types of shaft center measuring methods and Table 5-1 Chapter V... Chapter 5 =5 I'D " Chapter 6 7 Shaft center measuring method I Shaft center measuring method II Motor output (kw) Coupling side surface 1 Coupling end surface (mm) 37 or less 0.05 or less 0.10 or less 40 or more 0.07 or less 0.16 or less 'hapter 3 9 Table 5-1: Pump shaft center allowable error margins Chapter 10 In the shaft center measuring method I, the side surface is measured with a dial gauge and the end surface with a clearance gauge. The measuring method II is an easy method that uses a ruler and a clearance gauge to measure the side surface. Bearing trouble is usually detected from symptoms such as vibration, abnormal sound and heating. So, detect and deal with any such symptoms at an early stage by carrying out patrol inspection and so on to prevent damage which may affect various equipment. hapter Elementary Marine Engineer Textbook 070 Chapter 5 Auxiliary Machinery (4) Mechanical seal Leakage accounts for almost all mechanical seal problems. Leakage is caused by defective sliding surfaces or damage to O-rings for attaching seal rings. Because mechanical seal performance depends largely on whether it is properly mounted or not, close attention must be paid to its assembly. Fig. 5-10 is an example of a mechanical seal. Fig. 5-10: Mechanical seal; mechanical seal in place The following are precautions to be taken when attaching a mechanical seal: ® The turning direction of a mechanical seal is specified. So, prior to use, confirm that its turning direction is correct. @ Cleanse the sliding surface, packing ring and its ring groove using clean washing oil, taking care not to scratch or bruise them. ® When attaching the packing ring, remember to apply lubricating oil to it. ® When attaching the floating seat, insert it together with the packing ring and crimp them to the bottom so as not to scratch the sliding surface. ® To attach the rotary part, insert it into the sleeve, push the stopper ring into the setting base line, then set the sleeve in place after securely fixing it with a setscrew. ® Following assembly of the rotary part, push the seal ring by hand and confirm that it moves freely on the sleeve with a load of the spring. ® When using a mechanical seat, conduct flushing by injecting a liquid ex­ tracted from the high-pressure part into the sealing part to keep the slid­ ing surface in a favorably lubricated condition and to prevent accumula­ tion of impurities in the stuffing box. 071 Elementary Marine Engineer Textbook