The advantages and disadvantages of the different pumps used for stationary fire protection.
Typical fire pump buyers are primarily concerned with the hydraulic performance of the pump in question, specifically the gallons per minute (GPM) and pressure rise (pounds per square inch [psi]) of the pump. If a pump can be found that delivers a specific GPM and psi and is listed by a trusted third-party agency, the evaluation often ends there. The type of fire pump is seldom considered. This article focuses on Underwriters Laboratories (UL) or FM Global centrifugal fire pumps that are specifically used for static fire protection.
Horizontal Split Case
Horizontal Split Case pumps get their name from the separate box design, so the cover can be lifted off the pump to expose internal components. Horizontal Split Case pumps have two chargers, located on each side of the impeller, which is helpful in resisting vibrations and thrust forces caused by water turbulence in the suction tubes. Housings can be designed to withstand high working pressures and are often heavier. The durability of the HSC design allows the pump to be used for more than 5,000 gallons per minute of water.
The HSC pump is not always installed horizontally. The same durability is possible with a vertical stand. The HSC pump is often connected to a suitable motor by means of coupling or driveshaft. When installed horizontally this can create more space due to floor space concerns, HSC pumps are generally not selected for currents below 1000 GPM.
The efficiency of the HSC pump impeller depends on the flow of water entering the eye (or inlet). There are two water inlet points on the HSC pump impeller, where the term “double suction” is used. If water enters the impeller unevenly, the hydraulic imbalance can occur and cause pressure on the pump shaft or bearings. The need for smooth laminar flow in the HSC pump suction tubes is the reason why the National Fire Protection Association (NFPA) 20 has established strict rules regarding the length of straight tubes required on the side HSC pump suction valve (refer to NFPA 20 2013 4.14.6.3.1 and 4.14.3.1). As a general rule of thumb, the larger the volume of water to be pumped, the more important it is to produce a uniform laminar flow of water in the pump casing.
The direction of the HSC fire pump must be determined in advance when it is manufactured and installed. The pump must be constructed to operate with the right hand (clockwise) or left hand (counter-clockwise) from the driver’s point of preference. In the case of diesel fire pumps, the pumps can only run clockwise. If not installed correctly, the pump can be disassembled and reassembled to correct for field rotation (if driven by an electric motor) or pipeline reorganization (if the diesel engine is running).
Additional bearings, larger impellers, and larger overall size of the horizontal fire suppression pump add to the cost. The initial investment can provide end-users with a pump that lasts longer and is easier to maintain.
End suction horizontal
Centrifugal pumps get their name from the path the water takes to enter the pump. Usually, the water enters the side of the impeller. On horizontal suction pumps, this appears to enter the “end” of the pump. Unlike the HSC, the suction tube and the motor or motor are parallel to each other, eliminating concern about pump rotation or direction.
Since water enters one side of the impeller, the ability to get bearings on both sides of the impeller is lost. Bearing bracket for motor or pump power frame, therefore does not perform well in high water flow applications.
One of the benefits of an end suction pump can be the lower initial cost. If the pump can run at a certain rating and pressure, the pump will run. If powered by a diesel engine, another advantage is the ability to place the engine in parallel and close to the wall (assuming adequate air ventilation is observed).
Removing the impeller requires removing the motor to save space. This is often a complicated procedure for those who maintain the pump.
End Suction Vertical Inline
Space-saving and low initial cost are two advantages of these designs. The process of entering the water into the impeller is identical to the horizontal suction pump. However, the casing is designed so that the motor rests on top, and the edges of the pump are made to conform to the same centerline of height.
These pumps are also useful to take benefit of smaller fire pump ratings: 50 GPM, 100 GPM, 150 GPM, etc., often with single-phase electric motors. Historically, HSC pumps were only manufactured at 250 GPM and 500 GPM. The vertical inline pump assists in filling the gaps of GPM pump ratings.
The mechanical design of a VIP inline pump is often the least expensive. The impeller of the pump is attached to the shaft of the motor and is fixed in the pump housing. No hydraulic balance is required for water to enter the scroll pump, but there is minimal support for bearing vibration. This results in limited use in fire pumps over 750 gallons per minute.
If debris clogs the impeller, the motor and rotating propeller should be lifted up and out of the pump housing together, with assembly continuing. For larger motors, this can be tricky. If the engine volume exceeds 50 hp, then special equipment may be required. Since these pumps are usually chosen for their compact size, the pump room itself may be small as well. Space should be set aside for future repairs (see NFPA 20 2013 4.12.1.1.6, which warns installers to allow proper clearance).
Vertical Shaft Turbine
Vertical turbines have impellers submerged in the water source. These pumps include a bowl assembly, which has several impellers on a vertical shaft (where a single impeller discharge feeds the suction of the next impeller, etc.), a shaft assembly built to a length, and a discharge head assembly, which holds the motor at right angles. In a mechanical chamber, the discharge head assembly of a typical vertical turbine pump is the only visible part of the pump.
These pumps are useful in areas where water is scarce, but also when installing a water storage tank above ground is not desirable. An additional advantage of the vertical turbine pump is that virtually any length and vacuum pressure can be requested. Several impellers can be added for additional pressure. When properly installed, there is no longer any concern about the air in the suction tube or preparation concerns. With each additional impeller, you add to the pump a “churn,” or shutdown point, on its curve. Therefore, additional undesired additional pressure is often seen in lower operating flows.
These pumps can be difficult to service. The pump must be removed or dismantled, which often requires the use of a sunroof in a room so that a crane can lift it for inspection. Since these pumps are custom-designed, delivery times can belong – from eight to 20 weeks depending on the manufacturer and model.
Great care must be taken before operating the vertical turbine pump for the first time. Shaft and impellers must be properly adjusted to lift-off bowl casings prior to operation.
Rotary Gear Pump
Rotary gear pumps are used in a wide variety of pump applications, ranging from high viscosity or low NPSH requirements. Unlike the other types of pumps discussed, a rotary pump can be built for self-operation. Due to their small size, pumps with portable rotary gears are often used in rural areas to pump water from ponds or streams.
The rotary gear pump works by positive or constant displacement of the liquid being pumped. This is achieved through the pump gears, which transmit a certain amount of liquid in each gear cycle. There is no throttling for this type of pump, so a relief valve is required.
In the fire protection field, rotary gear pumps are particularly useful for high-pressure and high-risk applications – particularly for handling foam concentrates. Some brands can run dry since the rotor elements never touch due to the timing gears. This is very important when working with foam and petrochemical products.
While these pumps are smaller than centrifugal pumps, they can have greater operating noise. Their hydraulic efficiency is slightly lower than other types of pumps, which is why they are often used with highly viscous liquids.