Understanding fire pumps, their applications and sizing

Understanding fire pumps, their applications and sizing

Fire pumps are an essential part of most fire protection systems, as they allow for taller buildings, smaller piping and higher pressure and flow rates in systems. Incorrectly sized or selected pumps can result in not enough pressure or flow being created, which can cause several problems:

• Systems function incorrectly.
• Spray patterns from sprinklers don’t develop properly.
• Not give enough pressure to hose valves at the top of a high-rise building.
• Too much pressure, causing components to burst and break open during use.

To size and select the correct pump, it is important to have an understanding of the different operating principles, types, drivers and applications for fire pumps.

There are two main principles of operation of pumping water, centrifugal and positive displacement.

Centrifugal Pump

Centrifugal pumps use the principle of a spinning action to generate centrifugal forces — water starts off in the center of an impeller and as the impeller rotates it is thrown to the outer parts of the impeller. This is similar to a spinning ride at a carnival; as the ride speeds up the people inside are pushed against the wall. The faster the spin, the more force is applied to the people inside.

Positive Displacement

Positive displacement uses the principle of capturing a select amount of liquid in a single revolution and using a mechanical process to displace that liquid. Think of an air compressor, with an outside energy source and pistons to compress the air. A defined volume is allowed into the chamber when the piston at the bottom and the intake are open. As the outside motor spins, the shaft the intake is closed and the piston is forced up. In the case of water and water-based fluids, they can’t be compressed, so they build up pressure as the piston moves up. Once the outlet valve is opened, that water can flow out of the chamber.

Each one of the operating principles have its advantages and disadvantages. Centrifugal pumps are better at higher flow rates when compared to positive displacement, which provide constant flow at a range of pressures and typically have low flow rates and higher pressures. Popular applications of centrifugal pumps are low- and high-rise buildings and utility/campus central pumping stations. Positive displacement pumps are used to pump fluids that are not always water, such as foam concentrate or in systems that require high water pressures, such as water mist systems.

Fire pump types

Positive displacement pumps come in two main types, reciprocating and rotary. Reciprocating is described above and consists of plunger-style pumps. Rotary-style pumps use a spinning internal mechanism, such as lobe pumps, to capture water and move it through the chamber of the pump. This is similar to rotary engines in some automobiles — inside a specially designed chamber, a center lobe or rotor with vanes rotates and forces water through the pump body. A select volume of water is captured in each rotation by the lobes and the vanes and pressure is added to the system.

Centrifugal pumps come in several types: horizontal split-case, vertical in-line, vertical line shaft and end suction pumps. Horizontal split case is one of the most popular pumps used in building application due to its range of flow and pressures that falls in line with most buildings. Horizontal split-case pumps are characterized by a housing that is split in half and bolted together. The driving motor or engine and the impeller are parallel to each other and are connected to each other with a shaft. The motor/engine and impeller are mounted on a pump skid, with the motor/engine and impeller to rotate around their horizontal axis.

Vertical in-line pumps are similar to horizontal split-cases in that they use a motor in parallel to the impeller, but in the case of in-line pumps, the impeller and motor are mounted to rotate around their vertical axis, with the motor most commonly being located on top of the impeller. This creates a solution for tight pump rooms as they will take up less floor space.

Vertical line shaft pumps are similar to the vertical in-line pump, with the motor and impeller mounted to rotate around the vertical axis. They differ in that a vertical line shaft pump uses a long vertical shaft to connect to either an impeller or set of impellers remotely located. Typically, these pumps are used to pull water from sources below the pump, such as an underground well, lake/pond or a below-grade water tank where the net positive suction head is too great for other kinds of pumps.

End suction pumps are unique in that water comes into the pump typically in the horizontal direction into the center of the pump and leaves in the vertical direction.

Fire pump drivers

There are three driver types outlined in NFPA 20: Standard for Installation of Stationary Pumps for Fire Protection: electrical motor, diesel engine and steam turbine systems. The motors/engines are what drive the impellers and spin the shafts that provide water to the systems.

Electrical motors are the most common type of driver, and are outlined in NFPA 20 Chapter 9. This is due to their ease of use, the limited number of additional items required for operation and their cost-effectiveness. An electrical motor takes electrical power provided from either a utility connection, generator or other approved power source. As the motor turns, it spins a shaft that is connected to the impeller.

The second most commonly used driver is a diesel engine, outlined in Chapter 11. Diesel engines are good choices in places were the power gird is unreliable, not sized to handle the load or a lack of of emergency power, such as a generator. A diesel motor comes mounted on the same skid as the pump. The system uses a combustion engine to turn the impeller. Unlike the electrical motors, diesel-driven pumps require a lot more infrastructure and maintenance, such as on-site diesel storage fuel tanks, batteries for engine startup, combustion air for the engine, increased ventilation and an engine exhaust system in the room and a governor system. The governor system is important because the risk of the diesel engine producing too much power and spinning the impeller too fast.

The last type is steam turbine systems, as found in Chapter 13. These are very rarely used. This is because steam has to be generated by a separate unit (boiler, steam generator, etc.) and steam must either be available at all times or there is a delay while the steam is generated and the generators need to be provided with emergency fuel and power. The only places these pumps are seen are in older installations that are using steam for other process, such as power plants, factories and other industrial settings. Like the diesel engines, steam-driven pumps require governors to keep from overspinning the impeller.

What Is A Jockey Pump

Another critical part of the fire pump design is the pressure maintenance pump, commonly called the jockey or makeup pump. The pumps are used to maintain the pressure in a system without the main fire pump turning on, but they are not powerful enough to keep up with the demand of an active system. Sizing the pumps is tricky as NFPA 20 does not give a firm guidance. Section 4.26.1 calls out that a means of maintaining pressure in the system shall be provided by:

• A jockey pump.
• Water mist positive displacement units (mostly for high-pressure, low-flow applications such as water mist system).
• Other approved methods.

Most fire protection engineers use the 1% rule for finding the flow rate, taking 1% of the rated flow of the main fire pump for sizing. The pressure rule of thumb is to add 10 pounds per square inch to the pressure of the system. These rules do not fit all applications. Systems with higher leakage rates such as underground piping can be undersized and will not keep up with the leakage. To see the impact on leakage rates, consult NFPA 24: Standard for the Installation of Private Fire Service Mains and Their Appurtenances Chapter 10.

One way to tell if a pump is undersized is if the fire pump turns on when there is no use of the system. This means that the jockey pump is not keeping up with the leakage rates. Oversized jockey pumps will produce a water hammer effect. This is due to the pump quickly coming on and slamming off. The pump will also be short cycling, which can cause damage to the pump.

Two important terms to understand when designing fire pumps are cavitation and NPSH. Cavitation is when small vapor pockets are produced in the flow of liquid in the pump. When the pockets encounter the surface of the impeller or reach an area of high pressure in the pump, they implode causing damage to the surrounding components, most notably the impeller. This is caused by the liquid pressure falling below the vapor pressure of the fluid being pumped. This is seen in areas where the pump is located far above the water source, this is known as static loss. Recently there has been a push in some parts of the country to raise pumps above flood plain levels which increase the risk of cavitation. Another reason is if the supply pressure is too low. This can be due to too much friction losses in the piping, likely causes are supply piping too small, high friction loss fitting such as backflow preventors, valves and other fittings. Cavitation can be easy to find from problems with the pumps holding pressure to performance in the system changes, but one of the fastest ways to identify it is the sound the pump makes when running. For a normal pump you will hear the hum of the motor and the spin of the impeller. In a cavitating pump it will sound like there is gravel or marbles in the pump smacking around. Typically, this is a loud sound that can be heard when the pump is running, so ideally you would hear this during an annual test and not a real fire, so you had time to go and fix the problem.

Sizing a fire pump

A fire pump is designed to handle the most demanding fire sprinkler system. In a typical building like a high-rise office, there is the sprinkler demand on each floor, along with the standpipe. Other types of buildings may have a foam system for fuel oil storage or foam systems for helipads on the roof — or any number of different systems.

The first step in sizing a fire pump is to identify the most demanding system. In most commercial buildings, the standpipe is the most demanding system. The requirements for the design of a standpipe system are in NFPA 14: Standard for the Installation of Standpipe and Hose Systems. An example for determining the standpipe demand is below. This example used NFPA 14-2016 for all the section reference. Be sure to always verify the editions of the applicable code and reference standards that are required for project. This can be found in most U.S. building codes in the referenced standards chapter.

The building is a new high-rise office building, 280 feet in height to the roof, contains four staircases and is provided with a class I standpipe system, with one hose valve at each level in each staircase and no horizontal standpipes outside the staircases. The building is fully sprinklered and has a full emergency generator for life safety systems, including load for a fire pump.

In this case the most demanding system is going to be the standpipe system. This can be confirmed by running a hydraulic calculation for the most demanding sprinkler system and comparing it to the calculations of the standpipe system.

To find the flow rate of the fire pump we use NFPA 14-2016 Chapter 7.

• The first standpipe riser has a demand of 500 gallons per minute, each additional standpipe adds 250 gpm. The calculated demand would be 1,250 gpm (NFPA 14-2016 7.10.1.1.1 and 7.10.1.1.3).
• However, there is limit of 1,000 gpm for fully sprinklered buildings, so the demand drops to 1,000 gpm (NFPA 14 2016 7.10.1.1.5).

To find the pressure required it should be looked at as:

• Demand at the top + static losses+ pipe friction losses – source water pressure
• The demand at the top of the building is 100 psi for a Class I system (NFPA 14-2016-7.8.1).
• The static losses and pipe friction losses can be found using the height of the building and piping layout and a hydraulic calculation. For details on hydraulic calculations see NFPA 14-2016 Chapter 8.
  • Loss due to elevation would be 280 feet/2.31 (psi per foot) = 121 psi.
  • Pipe losses are another 20 psi, based on hydraulic calculations.
• The supply water pressure was found to be 30 psi at the required calculated flow.
• 100 psi + 121 psi + 20 psi – 30 psi = 211 psi is the pressure we need to add into the system with the pump.

The demand and pressure fall in line with the area for a centrifugal pump. In this case, because there are no details provided about the size of the room or other space requirements, we are free to select either a horizontal split-case or vertical turbine based off the pumping types and capacity ranges provided in the NFPA Fire Protection Handbook 20th Edition.

If it is determined that the sprinkler load is the most demanding, such as in a warehouse or a long flat building, then the pump is sized based off the hydraulic demand of the sprinkler system.

Fire pump components

Other components that need to be considered are the controllers for the pumps. They are what turn the pumps on and, in the case of the jockey pump, off again. This is done using small sensing lines. Each pump has its own line and it is tied between the controllers and the discharge or system side of the pump. When the pressure in the piping falls, it signals the controller to turn on the pumps. In the case of the jockey pump, the controller will it turn off when a set pressure is reached.

Test headers are used to test a fire pump by providing a route to flow water through the pump without sending it thought the rest of the system. Where a test header connection is used, it sits on the discharge side of the fire pump and is sized according to the fire pumps rated size. The table for centrifugal pump sizing is NFPA 20-2016 table 4.27.

Finally, check valves are needed on the discharge side of each pump to disallow water pressure or flow to force the pump to spin backward and cause damage to it.

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