Typical buyers of fire pumps are interested primarily in the hydraulic performance of the pump in question—specifically the gallons per minute (gpm) and the pressure boost (pounds per square inch [psi]) of the pump. If a pump can be found to deliver a specific gpm and psi, and it is listed by a trusted third-party agency, then the evaluation often ends there. Rarely is the type of fire pump considered. This article focuses on centrifugal fire pumps that carry either an Underwriters Laboratories (UL) or FM Global label, and are used specifically for stationary fire protection.
Image 1. Horizontal split case fire pump (Images courtesy of Steven Brown & Associates)Horizontal Split Case
Horizontal split case (HSC) pumps receive their name from the split design of the casing, where the cover can be lifted off the pump to expose the internal components. HSC pumps include two bearings, located on either side of the impeller, which are useful to withstand the vibration and thrust forces caused by water turbulence in the suction piping. The casings can be designed to handle higher working pressures and are often heavier. The durability of the HSC design allows the pump to be used for water flows in excess of 5,000 gpm.
An HSC pump is not always mounted horizontally. It is possible to have the same durability features with a vertical mount. The HSC pump is often connected to an appropriate driver by a coupling or drive shaft. When mounted horizontally, this can create a larger footprint. Because of a concern for floor space, HSC pumps are not typically chosen for flows below 1,000 gpm.
The efficiency of an HSC pump’s impeller is dependent on even water flow entering the eye (or inlet). There are two entry points of water into an HSC pump’s impeller, which is where the term “double suction” is used. If water enters the impeller unevenly, hydraulic imbalance can occur and cause stress to the pump shaft or bearings. The need for smooth, laminar flow in the suction piping of an HSC pump is why the National Fire Protection Association (NFPA) 20 has established strict rules regarding the length of straight piping required on the suction side of an HSC pump (see NFPA 20 2013 4.14.6.3.1, and 4.14.3.1). As a general rule, the larger the volume of water to be pumped, the more important it is to produce smooth laminar water flow into the pump casing.
The orientation of an HSC fire pump must be predetermined when the is manufactured and installed. The pump must be built for either right-hand (clockwise) or left-hand (counter-clockwise) operation from the vantage point of the driver. For diesel-driven fire pumps, the pumps can only be operated at a clockwise rotation. Failure to install properly can lead to a pump disassembly and reassembly to the correct rotation in the field (if driven by an electric motor) or rearranging piping (if diesel-engine driven).
The extra bearings, larger impellers and overall larger size of the horizontal split-case fire pump raise the cost. The initial investment can provide end users with a pump that will last longer and is easier to service.
End Suction Horizontal
End suction centrifugal pumps get their name from the pathway the water takes to enter the pump. Typically, the water enters one side of the impeller. On horizontal end suction pumps, this appears to enter “the end” of the pump. Unlike the HSC, the suction pipe and motor or engine are parallel to each other, eliminating the concern about pump rotation or orientation.
Since water enters one side of the impeller, the ability to have bearings on both sides of the impeller is lost. Bearing support is from the motor or the pump power frame, so they do not operate well on large water flow applications.
One benefit of an end suction pump can be a lower initial cost. If the pump can perform at a given rating and pressure, then the pump will work. If driven by a diesel engine, another advantage is the ability to situate the engine in parallel and near a wall (assuming proper air ventilation is considered).
The removal of the impeller requires removal of the motor to provide space. Often this is a complex procedure for those maintaining the pump.
End Suction Vertical Inline
Space savings and lower initial cost are two advantages of these designs. The process of water entering the impeller is identical to a horizontal end suction pump. However, the casing is designed so that the motor is mounted on top, and the pump flanges are made to coincide with the same elevation centerline. As far as centrifugal pumps go, this requires the smallest space in a pump room.
These pumps are also available to take advantage of smaller fire pump ratings: 50 gpm, 100 gpm, 150 gpm, etc., often with single-phase electric motors. Historically, HSC pumps have been manufactured at only 250 gpm and 500 gpm ratings. The vertical inline pump helps fill in the gaps of pump gpm ratings.
The mechanical design of a vertical inline pump (VIP) is often the least expensive. The pump impeller is attached to the motor shaft and is cantilevered in the pump casing. There is no hydraulic balance needed for the water entering the pump volute, but there is minimum bearing support for vibration. This results in a limited use in fire pumps above 750 gpm.
If debris clogs the pump impeller, the motor and impeller rotating assembly must be lifted up and out of the pump casing together, while still assembled. For large motors this can be difficult. If the motor size exceeds 50 hp, special equipment may be needed. Since these pumps are typically chosen for their small size, the pump room itself may be small as well. An allowance for space to accomplish future repairs should be made (see NFPA 20 2013 4.12.1.1.6, which cautions installers to allow for proper clearances).
Vertical Shaft Turbine
Vertical shaft turbine (commonly referred to as “vertical turbine”) pumps have impellers that are submerged in the water source. These pumps include a bowl assembly, which contains several impellers on a vertical shaft (where the discharge of one impeller directly feeds the suction of the next impeller, and so on), a column assembly built to a specified length, and a discharge head assembly, which holds the motor or right-angle gear drive. In a mechanical room, the discharge head assembly of a typical vertical turbine pump is the only visible part of the pump.
These pumps are helpful in areas where water is scarce, but also when installing an above-ground water storage tank is not desirable. An added advantage to a vertical turbine pump is that almost any length and discharge pressure can be ordered. Several impellers can be added for additional pressure. When properly installed, there is no longer a concern about air in the suction pipe or priming concerns. With each additional impeller, you are adding to the pump “churn” or shut-off point on its curve. So, additional unwanted pressure is often seen at lower operational flows.
These pumps can be challenging to service. The pump must be removed or disassembled, which often requires use of a roof hatch in a room so that a crane can lift it out for examination. Since these pumps are custom-built, the lead times can be long—from eight to 20 weeks depending on the manufacturer and models.
Special care must be made before operating a vertical turbine pump for the first time. The shaft and impellers must be properly adjusted to lift them off the bowl casings prior to operation.
Rotary Gear Pump
Rotary gear pumps are used for a wide variety of pump applications, from high viscosity or low NPSH requirements. Unlike the other pump types discussed, a rotary gear pump can be built to self-prime. Due to its small size, portable rotary gear pumps are often used in rural areas to pump water from ponds or streams.
The rotary gear pump operates through positive or fixed displacement of the liquid being pumped. This is accomplished through pump gears, which move a specific amount of liquid per gear revolution. There is no throttling this type of pump, so a relief valve is required.
In the fire protection field, rotary gear pumps are especially useful for high-pressure and high-hazard applications—specifically to handle foam concentrate. Some brands can operate dry since the rotating elements never touch due to timing gears. This is extremely important when operating with foam and petrochemical products.
While these pumps are smaller than centrifugal pumps, they can have more operating noise. Their hydraulic efficiencies are slightly less than other types of pumps, which is why they are often used with high-viscosity liquids.