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.
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 184.108.40.206.1, and 220.127.116.11). 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.