Fire Protection Systems PDF

Fire Protection Systems • Chapter 15 ^ OBJECTIVE 5 Describe the layout, components, and operation of a typical firewater system with fire pumps and hydrants. Explain seasonal considerations fora firewater system. FIRE PROTECTION FOR INDUSTRIAL SITES Figure 19 shows a typical water piping system f...

Fire Protection Systems • Chapter 15 ^ OBJECTIVE 5 Describe the layout, components, and operation of a typical firewater system with fire pumps and hydrants. Explain seasonal considerations fora firewater system. FIRE PROTECTION FOR INDUSTRIAL SITES Figure 19 shows a typical water piping system for fire protection of an industrial site. Typical details shown are the connections to public mains and supplies for a private fire pump, main water piping loops, sectional control valves, and hydrants. The NFPA-24 covers fire protection for industrial sites in the standard for the installation of private fire service mains. A few well-established principles are illustrated in Figure 19, including the following: • The facility contains a fire loop system that makes firewater available to every area of the facility. The fire loop supplies several fire hydrants, fire hose stations, and standpipes located strategically within the facility. • The fire loop has two sources offirewater. One is a connection to the external, public (city) water system (this option is not available to all facilities due to location). The other is a large firewater storage tank within the facility from which firewater pumps can supply the fire loops. The latter is required of all large industrial facilities, even if the facilities are also connected to an external source. • Several isolating valves are located within the system, allowing equipment and sections of the fire loop to be isolated for repairs and other maintenance. Often these valves are located below ground level, so they must have handles above ground to operate the valve. The handle must also clearly indicate the position (open or closed) of the valve. • If there is a city supply, there will be a check valve and a flow meter, which has isolating and bypass valves to accommodate repair in such a way that does not interfere with the city and plant supply sources. 3rd Class Edition 3' Part A2 795 Chapter 15 • Fire Protection Systems Figure 19 - Industrial Site Firewater Protection System Hydrants E ^_ 8-in. Jw^ ?5^-———— CB _s. Cap or plug Pump house (see detail below) ^ Hose 250,000 gal. (946m3) <s> Main facility mains suction tank As 6-in. Stations 6-in. Future Future growth of facility 6-in. ^ ,»—fl As 6-in. As 6-in. ^ Office bldg. As 6-in. As 6-in. As 6-in. ^ ^ 8-in. > TH 1^1 6jn. £L ^ ^ See detail of pit 1 12-in. public main ^ F-^™ L_| ^_ Cap or 6-in. ^ 3-irL_ P3 ^ •TH To yard system 8-outlet hose header ^/ from city water supply From suction tank 1000-gpm (3785 L/min) fire pumps Detail of pit 1 Detail of pump house (Courtesy of the National Fire Protection Association) Figure 19 illustrates several isolation valves, which can be described as follows. Three sectional isolation valves in pit 1 allow city water to be sent to three sections of the loop. Valves G and H supply the loop for the main facility, while valve J supplies a short branch that covers the office building. Note that this may ultimately supply an additional section in the future. Other sectional isolation valves (shown by indicator posts C, E, and F) may be used to cut the loop into four sections (in conjunction with valves G and H). In large or complicated underground systems, it is recommended that indicator posts that control risers to sprinklers or standpipes be painted a different colour from sectional control valves. Generally, no more than six hydrants or indicator posts should be located between sectional valves. Isolating (gate) valves are provided on each hydrant lateral to allow the hydrant to be isolated in the event it malfunctions or requires repair. 3rd Class Edition 3 • Part A2 Fire Protection Systems • Chapter 15 Hydrant Location Hydrant spacing is usually determined by the fire flow demand, which is established on the basis of the type, size, occupancy, and exposure of structures. When hydrants are located on a private water system and hose lines are intended to be used directly from the hydrants, the hydrants should be located so as to keep hose lines short, preferably not over 75 m. At a minimum, there should be enough hydrants to ensure that two streams are available at every part of the interior of each building that is not covered by a standpipe system protection. The hydrants should also be able to provide hose stream protection for exterior parts of each building while using only the lengths of hose normally attached to the hydrants. It is desirable to have a sufficient number ofhydrants so no hose line exceeds 150 m in length. Hydrants are preferably placed about 12.2 m from buildings. Sometimes that is not possible, since they must also be placed where the chance of injury by falling walls or debris is small and where firefighters are not likely to be driven away by smoke and heat. In crowded industrial yards, hydrants can usually be placed beside low buildings, near substantial stair towers, or at corners formed by masonry walls that are not likely to fall. Hydrants located in areas of heavy traffic require protection from collision. Hydrants located in parking lots of shopping centres and mill yards are good examples of hydrants that require this protection. Seasonal Considerations The depth of ground cover to protect a hydrant against freezing varies with climate. This depth can be anywhere from about 0.76 m in the southern United States to about 3.05 m in northern Canada. Because there is normally no water flowing in firewater mains, they require greater depth of ground cover than public water mains. A minimum cover should always be maintained to prevent mechanical damage in addition to freeze damage. The depth of covering should be measured from the top of the pipe to ground level, and consideration should always be given to the future or final grade of the soil and to the nature of the soil. A greater depth is required in a loose, gravelly soil (or in rock) than in compact or clay soil. A safe rule to follow, regardless of soil type, is to bury the top of the pipe no less than 0.3 m below the lowest frost line of the locality. FlREWATER PUMPS The size and complexity of the fire protection system at a particular industrial site is generally determined by the nature of the process, the potential for fire, and the ultimate effects of a fire. Some industries have much greater fire potential than others. In general, besides a companys own determination to protect life and property, it is the underwriting insurance companies that determine the system's size and design. Included in that design is the type, size, and number of firewater pumps required for that particular location. Note: The main code that governs the design, installation, and testing of firewater pumps in North America is the National Fire Protection Association (NFPA) Code 20, Standard for the Installation of Stationary Fire Pumps for Fire Protection. Insurance companies in Canada generally require that this code be followed. Fire Pump Types and Installations The type of fire pump selected depends on the intended source of firewater for the pump. The firewater source determines whether the pump will have water flowing to the pump suction by gravity (suction head) or if the pump will have to create lift to raise the water to the pump suction (suction lift). One water source may be a large, above-ground storage tank, in which case the pump will have suction head. A second water source may be an in-ground reservoir, in which case, depending on the relative elevation of the fire pump, the suction may have head or lift. A third water source may be a lake, river, or ocean where the pump (such as an offshore platform) normally operates with suction lift. 3rd Class Edition 3 • Part A2 797 ?& Chapter 15 ' Fire Protection Systems Where suction head is involved, the usual fire pump design is the horizontal, split-casing, centrifugal pump. An alternative, in some cases, is a vertical, inline, centrifagal pump. The water level above the suction of the pump must be high enough to ensure the pump is always flooded to allow good priming and no loss ofsuction. Figure 20 shows a typical installation in which a centrifugal pump takes suction from a large firewater storage tank. Only the major components are shown, which maybe described as follows: • Firewater storage tank: This is a large capacity tank, between 2000 m3 and 4000 m3 (500 000 gallons and 1 000 000 gallons), with a level that is constantly maintained from an outside source. It should be dedicated solely for firewater to ensure it always has the maximum level available. • Pump suction: An inverted suction inside the tank with a screen to prevent solids entering the pump. The suction block valve is a full-opening gate valve with a rising stem for immediate recognition of valve position. In cold weather climates, the suction line must be protected from freezing by a heated enclosure. • Pump: The horizontal, centrifugal pump is located inside a heated pump house. An automatic air vent on top of the pump casing helps prevent vapour locking of the pump. Suction and discharge pressure gauges and a casing pressure relief valve are also located on or near the pump. The discharge line contains a check valve. • System relief valve: NFPA-20 aUows only three situations in which a relief valve shall be installed on the discharge of a fire pump. 1. When the pump is driven by an electric motor that has a variable speed controller to limit the discharge pressure, and the total discharge head (when the pump is operating at rated speed with the discharge closed) exceeds the pressure rating of the system components 2. When the pump is driven by a diesel engine that has a pressure-limiting control, and the total discharge head (when the pump is operating at rated speed with the discharge closed) exceeds the pressure rating of the system components 3. When the pump is driven by a diesel engine where 121% of the net rated shutoff (churn) pressure added to the maximum static suction pressure exceeds the pressure for which the system components are rated In all cases, the relief valve should be sized to discharge sufficient water to prevent the pump discharge pressure from exceeding the pressure rating of the system components. • Control panel: This panel contains gauges, alarms, switches, and other components that allow the operator to interact with the firewater system. • Fire loop supply: In cold climates the fire loop supply line is directed below ground to a depth that is below the normal frost line. In warm climates this line could remain above ground. The line is kept pressurized with firewater so it is always ready for immediate service. • Test line: This line contains a flow meter and is used only when testing the pump capacity. The line usuaUy discharges back to the storage tank. • Hose station supply: This is a smaller, additional supply line that feeds fire hose stations in the facility. It is for emergency only and is drained when not in use. 798 3rd Class Edition 3 • Part A2 Fire Protection Systems • Chapter 15 T® Figure 20 - Typical Fire Pump with Suction Head System Air casin9 relief vent .relief valve (if necessary) To fire loops ^-(hydrants, deluge, sprinklers) Note: Not to scale; pump driver not shown 3rd Class Edition 3 • Part A2 799 ^ Chapter 15 • Fire Protection Systems Figure 21 shows a very simplified sketch of a typical fire pump installation with suction lift. In this case, a multi-stage, vertical turbine pump is driven by an electric motor and takes suction from near the bottom of a deep, concrete sump. The sump is located at an appropriate elevation and is in close proximity to the water source, which may be a lake, river, or in-ground reservoir. The water flows freely into the sump from the source and the sump level corresponds to the level of the source. Some key features of this installation are as follows. • Intake screens: These are vertical screens on the intake side of the sump that collect and prevent debris from reaching the pump. These screens are particularly important when the source is a river or lake. The screens can be individually lifted and cleaned without interrupting the water supply. • Pump suction: The intake of the pump is protected from debris and sludge by a suction screen. The lower section of the pump must be at an elevation that is below the lowest expected level of the source to ensure the impellers are flooded and to ensure proper priming. With river and lake sources, the level can vary considerably. • Air vent and relief valve: The piping configuration has potential for air in the system, so an automatic air vent is located at the pump discharge. A full-capacity relief valve discharges back into the sump. Figure 21 - Typical Fire Pump with Suction Lift Air Circulating Test vent relief ^e valve Intake from lake, river or reservoir Multi-stage vertical turbine pump Suction screen 800 3rd Class Edition 3 • Part A2 To fire loops Fire Protection Systems • Chapter 15 Fire Pump Drivers The most common drivers of industrial fire pumps are electric motors and diesel engines (or other internal combustion engines). Another acceptable driver, though not very common, is the steam turbine. The choice of drivers depends largely on the driver's reliability in a fire situation. It is important to realize that electrical power and steam may become unavailable during a major fire. An electric motor is useless if there is no power and a steam turbine is useless if there is no steam. However, diesel engines will operate when power and steam are not available. Most industrial facilities require more than one source offirewater and more than one fire pump, with at least two different drivers. Some special considerations are as follows: • Wlien a facility is supplied with firewater solely by pumps, there should be at least two fire pumps. The pumps may both be diesel engine driven, one diesel driven and one electric driven, or both electric driven. • Diesel engines must have at least two independent batteries and chargers. The engines should have low oil pressure and high water temperature alarms, but not trips (per NFPA-20). It is better to risk the pump running to destruction than to have a nuisance shutdown during a fire. However, an overspeed trip is allowed at 120% of normal speed. The engine may be started manually or automatically but must be shut down manually. • Electric motors that drive fire pumps must have two dedicated, independent power supplies. One may be the normal power grid and the other an emergency grid, supplied by an emergency generator. There must be an automatic switchover device to transfer the power source and ensure there is always power to the motor. • The fire protection control system, which controls the pump drivers and the system monitoring devices, must have an alternate source of power to ensure controls remain available during a fire. Jockey Pumps Most industrial fire systems, regardless of the number of main fire pumps, employ an additional, smaller pump in the system. This pump is called a jockey pump and its purpose is to maintain a constant pressure in the fire loop when the main fire pumps are not running. This accounts for any leaks in the system and minimizes shock when the main pumps start. The jockey pump relieves the main fire pump from needing to start and stop to maintain system pressure. The jockey pump must be of small enough capacity that it cannot sustain system pressure if any one of the sprinklers or deluge valves open. The pressure reduction will trigger the startup of the main fire pumps for the probable fire situation. Figure 22 shows a simple fire loop diagram, which shows one main pump and a jockey pump. The following conditions apply to the operation of the jockey pump: • The jockey pump takes suction from the same source as the main fire pump (or, as in Figure 22, the tank fill line) and discharges directly into the fire loop. • The pump does not run continuously, but is started and stopped automatically by pressure switches mounted on the fire loop. • WT-ien the fire loop pressure drops to a predetermined low setting, the low-pressure switch (PSL) will start the pump. • When the pressure then increases to a predetermined high setting, the high-pressure switch (PSH) will stop the pump. • This on/ofF cycle will continue indefinitely as long as the main fire pump is not running. Note: The discharge valves of the fire pump and the jockey pump should be kept locked in the open position during normal operation. 3rd Class Edition 3 ' Part A2 801 ^ Chapter 15 ' Fire Protection Systems Figure 22 - Fire System with Jockey Pump Xl Outside screw and Start PSL yoke (OS&Y) gate valve OS&Y Gate valve or indicating butterfly To hose headers To yard hydrants, deluge & sprinklers PSL PSH Tank fill Fire Pump Control The number of pumps, the operating and control pressures, and the complexity of control will vary between facilities. However, for automated control systems, the following points are fairly common. Any figures quoted are examples only. • Provided the start switch is in "auto," the start circuit of the main fire pump is initiated automatically by a low-pressure switch on the fire loop. If a deluge valve or sprinkler opens automatically, or if a hydrant is opened, the pressure in the loop will quickly drop. When it reaches the setpoint of the PSL (say, 600 kPa) the pump will start (see Figure 22). • If there is more than one fire pump, the second pump (if selected to auto ) will start automatically if the pressure continues to fall and reaches the setting of a PSLL (pressure switch low low). In such systems, the operator can usually select which pump is the primary and which is the secondary. • If the driver is a diesel engine, the starter will crank for a short time (e.g., 30 seconds) with power from one set of batteries. If the start fails, it will attempt again using power from the other set of batteries. • WTiile the main fire pumps start automatically (with manual start if necessary), they remain running until they are shut down manually. The operator must stop the pump, ensure its condition, and then reset the start switch to auto. • The jockey pump starts and stops betiveen the settings of two pressure switches. It must keep the pressure above the "start" setting of the main fire pump. For example, if the main pump is set to start when the pressure drops to 600 kPa, the jockey pump should be set to start at a minimum of 90 kPa higher (690 kPa). The jockey pump should then stop at about 800 kPa. NFPA-20 gives some specific setting relationships, but allows for some flexibility. 802 3rd Class Edition 3 - Part A2 Fire Protection Systems • Chapter 15 Fire Pump Testing Fire pumps must be tested after manufacture and (as a minimum) annually after installation. The testing is required by insurance companies, which rely on the standards set by the Underwriters Laboratories of Canada (ULC). Pump manufacturers must be approved by the ULC. A new fire pump, after successful inspection and testing, has a ULC label attached. The manufacturer is required to conduct an operation test and a hydrostatic pressure test on each fire pump. The operation test determines the pump speed, horsepower input, and suction and discharge pressure at rated capacity and at 150% of rated capacity. The hydrostatic pressure test of the pump casing requires that twice the maximum design working pressure (but in no case less than 1724 kPa) be maintained, without leakage, for at least 5 minutes. Annual testing of the pump involves using the test line (see Figure 22) on the pump discharge, which directs the flow through an orifice, through a flow meter, and back to the storage tank. The pump speed or the valves are manipulated to achieve a pressure that is 65% of design pressure. At this pressure, the pump must deliver 150% of the design flow. For example, if the design flow is 15 000 L/m at a pressure of 600 kPa, the test must achieve 22 500 L/m at 390 kPa. Annual testing is also covered in NFPA 25 Requirements for Fire Pump Tests. While annual capacity testing is considered the minimum, normal practice is to also test run the fire pumps (not a capacity test) for a short time on a regular schedule (usually weekly) to always ensure they are ready and able to start in an emergency. 3rd Class Edition 3 ' Part A2 803