Fire Pump Design
NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection protects life and property by providing requirements for the installation of fire pumps to ensure that systems will work as intended to deliver adequate and reliable water supplies in a fire emergency.
A fire sprinkler system is a critical component of life safety in a building. The International Building Code grants a number of exceptions when a building is “fully sprinklered,” such as reductions in rated separations, reductions in fire hydrant flow demands, increased egress travel distances and increased building heights and areas. These exceptions are permitted with an expectation that, in the event of a fire, the sprinkler system will suppress the fire to a sufficient degree that occupants can safely evacuate the building and the growth of the fire will be controlled until the fire department arrives to fully extinguish it.
Often, the municipal water system has sufficient pressure to operate the sprinkler system. A fire pump is required when the available water source does not have adequate pressure. When a sprinkler system relies on a fire pump, the performance of the system is dependent on the pressure created by the pump.
Because of the critical importance of the fire pump, careful consideration should be employed when selecting and designing a fire pump. Fire pumps are a critical component of a building's sprinkler system in settings where the water supply is insufficient to provide the pressure needed to keep the water flowing to all the sprinkler heads. Fire pump intakes are connected to underground public water supply piping, or a tank or reservoir located onsite, to provide water flow at a higher pressure to the sprinkler system risers and hose standpipes.
Sizing a Fire Pump
A fire pump’s size is dictated by the most hydraulically demanding area of the fire protection system. In many high-rise buildings, this can be the automatic fire standpipe system demand which requires 500 gallons per minute at 100 pounds per square inch at the top of the most remote standpipe, plus 250 gpm for each additional standpipe, up to a maximum of 1,000 gpm for wet systems or 1,250 gpm for dry systems.
For example, a new sprinkler system might be installed in a five-story medical office building with a partial basement (overall building height of 69 feet). The building construction is noncombustible, Type II-B and each floor is approximately 18,000 square feet. The basement level contains electrical rooms, general storage rooms, a small oxygen storage room (250 square feet) enclosed by a two-hour fire rating and a covered exterior loading dock.
The most hydraulically demanding area in this example is the level five mechanical room. Though the density for this remote area is only 0.15 gpm (ordinary hazard group 1), the top floor location requires additional pressure to overcome the head loss from elevation. The remote area size is increased to 1,950 square feet due to a 30% increase for slopes exceeding 2:12 (NFPA 13-2013, Section 184.108.40.206.4). The estimated flow demand for this area is approximately 380 gpm (0.15 gpm x 1,950 square feet = 292.5 gpm + 30% for sprinkler head overflow = 380 gpm). A preliminary hydraulic calculation indicates a required system pressure of 90 psi.
Selecting the Fire Pump
Once you have calculated the gpm and psi requirements for the pump, you need to determine the type of pump that works best for the job. The three most widely used pumps are horizontal split case, inline and vertical turbine.
Horizontal split case pumps are also called double-suction fire pumps, because the water pathways direct water to both sides of the impeller. They are the most common type of fire pump on the market, partly because of the ratings available in this style of pump, typically 250 through 5,000 gpm. This was the first type of pump used for fire protection systems.
Vertical turbine pumps are used in situations where the water supply is below the suction flange of the fire pump, because NFPA 20 requires a positive suction pressure to a fire pump.
The other item that needs to be determined is the type of drive: diesel or electric. Once that is determined, you can find the appropriate pump model and horsepower in a manufacturer’s catalog. I don’t recommend using pump curves to select fire pumps, as every selection must be UL approved, which might lead to picking the wrong horsepower for a particular selection.
One other note on fire pump selection is that selecting pumps that have a higher rpm is not necessarily a misstep, because fire pumps only run once a week for a limited amount of time, so the length of life will be about the same for a 1,750 rpm pump as for a 3,500 rpm pump.
Any source of water that is adequate in quality, quantity and pressure shall be permitted to provide the supply for a fire pump.The available flow at the fire pump discharge at the lowest permissible suction pressure shall be a minimum of 100 percent of rated flow.
Maximum Pressure for Centrifugal Pumps
The net pump shutoff(churn) pressure plus the maximum static pressure, adjusted for elevation, shall not exceed the pressure for the system components are rated. Pressure relief valves and pressure regulating devices in the fire pump installation shall not be used as a means to meet the requirements.
Centrifugal Fire Pump Capacities
A centrifugal fire pump for fire protection shall be selected so that the greatest single demand for any fire protection system connected to the pump is less than or equal to 150 percent of the rated capacity (flow) of the pump.
Centrifugal fire pumps shall have one of the rated capacities in gpm (L/min) identified in Table 4.10.2 and shall be rated at the net pressures of 40 psi (2.7 bar) or more.
A pressure gauge having a dial not less than 3.5 inch (89 mm) in diameter shall be connected near the discharge and suction side with a nominal 0.25 inch (6 mm) gauge valve. The dial shall indicate pressure to at least twice the rated working pressure of the pump but not less than 200 psi (13.8 bar).
Circulation Relief Valve
Excessive heat in the pump can cause severe damage in a short period of time causing pump bearings to fail - as the bearings' lubricants typically degrade twice as fast for each 10 degrees Celsius rise in temperature - while seals and packing begin to leak. In more extreme cases, the rise in temperature can cause the water to flash to vapor and cause cavitation damage to the impeller and pump internals.
To avoid this damage, we must lower the temperature of the water being churned in the pump casing or housing. This can be achieved by allowing a small amount of cooler water into the pump casing.
For all fire pump systems, except those using a cooling line to a diesel engine driven pump, a circulation relief valve should be fitted in order to extract heated water from the pump's discharge. This valve should be fitted between the pump discharge side and the outlet control valve. This circulation relief valve should be set to open at the pump unit shutoff pressure plus the minimum pump suction pressure. It should allow a relatively small amount of hot water to constantly flow out of the pump casing, which is then replaced by an equal amount of cool unheated water entering the pump through the suction side, thus cooling the pump and its casing.
The automatic relief valve have a nominal size of 3/4 inch (19 mm) for pumps with a rated capacity not exceeding 2500 gpm and have a nominal size of 1 inch (25 mm) for pumps with a rated capacity of 3000 gpm to 5000 gpm.
Circulating relief valve requirements shall not apply to engine-driven pumps for which engine cooling water is taken from the pump discharge.
Fire Pump Buildings or Rooms with Electric Drivers
Fire pump buildings or rooms enclosing diesel engine pump drivers and day tanks shall be protected with an automatic sprinkler system installed in accordance with NFPA 13 as an "Extra Hazard Group 2 occupancy".
Fire Pump Buildings or Rooms with Diesel Engine
For buildings that are required to be sprinklered, fire pump buildings or rooms enclosing electric fire pump drivers shall be protected with an automatic sprinkler system installed in accordance with NFPA 13 as an "ordinary hazard Group 1 occupancy".
Suction Pipe and Fittings
Suction pipe shall be galvanized or painted on the inside prior to installation with a paint recommended for submerged surfaces.
Sections of steel piping shall be joined by means of screwed, flanged mechanical grooved joints or other approved fittings.
The piping around check valves, orifice unions, orifice plates, flowmeters and other devices that have restricting orifices shall have a means to perform an internal inspection or a means to disassemble the piping to allow for the internal inspections.
Suction pipe size shall be such that, with all pumps operating at maximum flow (150 percent of rated capacity or the maximum flow available from the water supply at the lowest permissible suction pressure), the gauge pressure at the pump suction flanges shall be 0 psi or higher.
Where supply is from a suction tank with its base at or above the same elevation as the pump, the gauge pressure at the pump suction flange shall be permitted to drop to -3 psi at 150 percent of rated flow with the lowest water level after the maximum system demand and duration have been supplied.
Where a tank is used as the suction source for a fire pump, the discharge outlet of the tank shall be equipped with an assembly that controls vortex flow in accordance with NFPA 22.
ANTI-VORTEX PLATE IS INSTALLED IN THE SUCTION LINE OF FIRE PUMPS TO CONTROL THE TURBULENCE IN A FLOWING WATER, THEY ARE SIMPLE IN DESIGN AND VERY EFFECTIVELY CONTROLLING THE VELOCITY OF THE FLUID THUS PREVENTING CAVITATION TO FIRE PUMPS AND DAMAGE TO IMPELLERS.
Pressure Relief Valve
Pressure relief valve (Fire pump relief valve / safety valve) is used in the discharge of fire pump to automatically relieve excessive pressure, so to maintain constant pressure. It allows the fire pump to be stopped without causing surging.
A Pressure Relief Valve is defined by NFPA 20 (220.127.116.11 Relief Valve) as “A device that allows the diversion of liquid to limit excess pressure in a system.”
In general, a PRV is a safety device, designed to protect a pressurized system during an overpressured event. An overpressured event refers to any condition which would cause pressure in a system to increase beyond the specified design pressure or maximum allowable working pressure. Since Pressure Relief Valves are safety devices, there are many codes and standards written to control their design and application.
For Centrifugal Pumps, the NFPA 20 (18.104.22.168) requires that “a Pressure Relief Valve shall be installed. where a diesel engine fire pump is installed and where a total of 121 percent of the net rated shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, exceeds the pressure for which the system components are rated.”
A Pressure Relief Valve is required by the standard to be installed when the diesel engine is turning faster than normal, because the pressure is proportional to the square of the speed that the pump is turned. This is a relatively rare event; if pumps create pressures less than the pressure rating of the fire protection system components [typically 175 psi (12.1 bar)] at 110 percent of rated speed, a Pressure Relief Valve is not required.
The standard specifically does not permit the use of a main pressure relief valve on an electric fire pump, except where a variable speed driver is used. Variable speed drivers are required to default to constant rated speed operation. In the event the variable speed driver fails, the rated speed can result in system over-pressurization. In this case a pressure relief valve is required.
When designing a fire pump, it is highly important that the designer match the pump to the system demands, in order to avoid overpressurizing the system and then using pressure regulating devices to compensate.
Pressure Maintenance (Jockey or Make-Up) Pumps
A jockey pump is a small pump connected to a fire sprinkler system to maintain pressure in the sprinkler pipes. This is to ensure that if a fire-sprinkler is activated, there will be a pressure drop, which will be sensed by the fire pumps automatic controller, which will cause the fire pump to start.
A jockey pump is sized for a flow less than the flow to one sprinkler in order to ensure a system pressure drop. The function of jockey pumps is an important part of the fire pumps control system.
Jockey pumps are typically small multi-stage centrifugal pumps, and do not have to be listed or certified for fire system application. The control equipment for jockey pumps may however carry approvals.
Summary of Centrifugal Fire Pump Data
A packaged fire pump assembly, with or without an enclosure, shall meet all of the following requirements:
The components shall be assembled and affixed onto a steel framing structure.
Welders shall be qualified in accordance with the Section 9 of ASME Boiler and Pressure Vessel Code or with the American Welding Society AWS D1.1, Structural Welding Code — Steel.
The total assembly shall be engineered and designed by a system designer as referenced in 4.3.2 (NFPA 20,2019).
Fire Pump Test Arrangement
Where the water supply to a fire pump is a tank, a listed flowmeter or test header discharging back into the tank with a calibrated nozzle(s) arranged for the attachment of a pressure gauge to determine pitot pressure shall be required.
Automatic Air Release Valve
Air release valve is used in wet pipe sprinkler systems and on casing of horizontal split case pump, so to to automatically release small pockets of accumulated air while the system operates under pressure exceeding atmospheric pressure.
A nominal 1.5 in (38 mm) pipe size or larger automatic air release valve shall be provided to vent air from the column and discharge head upon the starting of the pump.
This valve shall also admit air to the column to dissipate the vacuum upon stopping of the pump.
This valve shall be located at the highest point in the discharge line between the fire pump and discharge check valve.
Pressure Sensing Line
The sensing line is connected to the fire protection system between its respective pump discharge check valve and the discharge control valve (E and F). The size of the sensing lines shall be a minimum of ½-inch and must be of non-corrosive metallic pipe or tube (brass, copper, stainless steel).
Both fire pump and jockey pump sensing lines are piped exactly the same with separate connections.
Non-ferrous material is used for the sensing line (bronze, stainless, or copper).
Both lines have 2 orifices drilled into check valves at least 5 feet apart
Size of the orifice in check valves is 3/32 inch
Arrows on check valves point away from the control panel
Size of sensing lines are ½”
Jockey pump is installed on the high pressure side of the fire pump piping.
Fire pump is installed on the high pressure side of the fire pump piping.
A ¾” casing relief valve has been installed on the discharge side of the fire pump before the fire pump check valve.
The direction of the arrow on the casing relief valve is pointing towards the drain.
Diesel Engine-Driven Fire Pump Systems
A fire pump can be driven by an electric motor, diesel engine or steam drive. While electric motors are most common, steam drives are infrequently utilized. Diesel engines are often used when the electrical supply to the property is unreliable or has insufficient capacity. They also are used by clients who desire a primary electric motor-driven fire pump system and a redundant diesel engine-driven fire pump system or used in a property requiring a redundant system due to its seismic zone or height.
For all fire pump applications, diesel engines must be “listed” for the fire pump system application. Typical listing agencies for diesel drivers are UL and FM Global. Listed fire pump engines are provided by companies such as Caterpillar, Clarke Fire Protection Products and Cummins. These companies are responsible for ensuring the diesel engine is suitable for fire protection services.
Fuel Supply and Arrangement
NFPA 20 requires a dedicated listed fuel tank located aboveground under municipal or other ordinances, and per the requirements of the authority having jurisdiction. In areas subject to freezing, the tank must be located in the fire pump room. It is required to have a dedicated fill, vent(s), and a visual and monitored fuel level gauge installed.
In the marketplace today, two types of fuel tanks are available: single- and double-wall tanks. While a single-wall tank is initially less expensive, a full tank capacity spill-containment system must be provided. A double-wall tank, by design, includes the spill containment device. Its containment space must be monitored for leakage from the inner wall. Vents from interstitial spaces of double-wall tanks may not be manifolded together with a vent from the primary compartment of the tank.
The minimum capacity of the fuel tank is driven by a simple equation: 1 gallon per rated horsepower plus 5 percent for sump and 5 percent for expansion. While you should install a fuel tank of at least minimum size, providing too large of a tank is not necessarily a better idea. Diesel fuel has a shelf life and there are requirements for periodic fuel-quality testing.
A proper exhaust system is required to dispose of all combustion gasses safely. It must be routed to a safe place of discharge; design and installation must be in accordance with the engine’s manufacturer’s installation manual, NFPA 20, municipal or other ordinances, and per the requirements of the authority having jurisdiction.
Sizing of the exhaust system needs to be completed by a calculation program to ensure the back pressure on the diesel engine complies with the listing. The type of silencer selected, the size and length of the pipe run, the number and type of fittings, and other factors will all impact the minimum pipe size required.
Fire Pump Controller
In simple terms, a fire pump controller is a device to reliably start and stop a fire pump, as well as monitor conditions in an ongoing manner that could hinder or prevent the proper operation of a fire pump. They act the same way any motor controller operates, except they are built with more strict standards to ensure that the priority is protecting a property and its occupants over the fire pump motor (or engine) itself.
UL and FM both “list” or “approve” fire pump controllers. This listing ensures that all fire pump controllers, regardless of manufacturer, all contain specific elements and design to ensure reliable operation in the event of a fire. The minimum required components of fire pump controllers are governed by NFPA 20
Fire pump controllers can be manually used to start fire pumps, either through use of a button switch, or lever on the controller. But most commonly, fire pump controllers are set up to operate automatically, either by a drop in system pressure in a sprinkler system, or via a signal from a remote device, such as a flow switch or deluge valve. Each controller also can be set up to run continuously until manually shut off, or a timer can be used to automatically stop the fire pump after a set amount of time has transpired (assuming the pressure in the system has stabilized).
Once started, a fire pump controller can be set to automatically reset, or (more commonly) require a manual stop, whereby someone has to manually push a button on the controller to stop it.
Fire Pump start and stop settings
Fire Pump Settings. The fire pump system, when started by pressure drop, should be arranged as follows:
(1) The jockey pump stop point should equal the pump churn pressure plus the minimum static supply pressure.
(2) The jockey pump start point should be at least 10 psi (0.68-bar) less than the jockey pump stop point.
(3) The fire pump start point should be 5 psi (0.34 bar) less than the jockey pump start point. Use 10-psi (0.68-bar) increments for each additional pump. (Also note that NFPA 20, section 10.5.2.5 also has a provision for multiple pumps. The second pump must start 5-10 seconds behind the primary pump starting to prevent water hammer.)
(4) Fire Pump shall be shut off manually.
(5) Where the operating differential of pressure switches does not permit these settings, the settings should be as close as equipment will permit. Pressures observed on test gauges should establish the settings.
Pump: 1000-gpm, 100-psi pump with churn pressure of 115 psi.
Suction Supply: 50 psi from city — minimum static. 60 psi from city — maximum static.
Jockey pump stop = 115 + 50 = 165 psi.
Jockey pump start = 165 - 10 = 155 psi.
Fire pump stop = 115 + 50 = 165 psi.
Fire pump start = 155 - 5 = 150 psi.
Fire pump maximum churn = 115 + 60 = 175 psi.
Hydrostatic Tests and Flushing
Suction piping shall be flushed at a flow rate not less than indicated in Table 22.214.171.124 or at the hydraulically calculated water demand rate of the system, whichever is greater.
Flushing shall occur prior to hydrostatic test.
Where the maximum flow available from the water supply cannot provide the flow rate provided in Table 126.96.36.199, the flushing flow rate shall be equal to or greater than 150 percent of rated flow of the connected fire pump.
Where the maximum flow available from the water supply cannot provide a flow of 150 percent of the rated flow of the pump, flushing flow rate shall be greater of 100 percent of the rated flow of the connected fire pump or the maximum flow demand of the fire protection system.
Suction and discharge piping shall be hydrostatically tested at not less than 200 psi (13.8 bar) pressure or rated 50 psi (3.4 bar) in excess of the maximum pressure to be maintained in the system, whichever is greater.
The pressure shall be maintained for 2 hours.
The installing contractor shall furnish a certificate for flushing and hydrostatic test prior to the start of the fire pump field acceptance test.
Field Acceptance Tests
The pump manufacturer, the engine manufacturer (when supplied), the controller manufacturer, and transfer switch manufacturer (when supplied) or their factory authorized representatives shall be present for the field acceptance test.