Date: August 6, 2013
THE ANATOMY OF A FIRE PUMP
As I considered a topic for this article, it occurred to me that there has been little discussion on the anatomy of a fire pump, and I believe it is important to spend more time discussing this with our firefighters. Understanding the anatomy of a fire pump can be a key to maintaining a long-lasting, strong-running pump for many years.
The “centrifugal pump”, a critical component of the fire truck, is probably one of the most misunderstood components. A fire pumps is made up of three major parts: the speed increaser/pump transmission, the impeller shaft assembly, and the pump body. All play very important roles in the flow of water through the pump and into the discharge hose for firefighting operations.
The Speed Increaser/Pump Transmission
A speed increaser/pump transmission is not always a necessary part of a fire pump. But, relative to the majority of firefighting conditions, it plays an important role, particularly for the midship-style fire pump, (which will be the focus for this article).
The speed increaser/pump transmission on a midship pump serves two purposes: (1) it transmits power (horsepower and torque) from the driveline of the apparatus to the pump or rear wheels, and (2) it increases the speed to the pump. The midship-style speed increaser/pump transmission is also referred to as a chain-/gear-driven split shaft pump transmission. It incorporates either a sprocket set with chain (drive sprocket, driven sprocket, and chain) or a gear set (drive gear, idler gear, and driven gear); a main through shaft that is split, referred to as the drive shaft (input); and coupling shaft (output).
This split shaft allows the engine to supply power either to the rear wheels for road position or to the pump for pump position. A sliding shift collar, which is part of this main split shaft, can either couple the two main shafts for road position or be slid across, thereby disengaging from road position and engaging the pump drive sprocket, or drive gear, for pump position.
In general, a fire pump requires an average of about 3,500 revolutions per minute (rpm) to achieve the necessary volume and pressure required for firefighting operations. Factors impacting this would be whether the pump is a single-stage or two-stage. The diesel engines on virtually all of the fire apparatus built today are governed somewhere between 1,800 and 2,100 rpm and, at typical pump operations, these engines are running between 1,500 and 1,700 rpm. Therefore, speed increaser/pump transmission ratios land on or near 2.25 to 1.
Impeller Shaft Assembly
The impeller shaft assembly, the heart of any fire pump, consists of three components: the impeller shaft seal, the impeller shaft, and the impeller. The impeller, the most important part of this assembly, will be the focus of this discussion.
An impeller comes in many sizes but incorporates key common characteristics – the eye, vanes, exitway, shrouds, hubs and wear rings. The eye, the point at which water is first introduced to the impeller, and referred to as the low-pressure side, can be either a single-eye impeller or a dual-eye impeller. Both designs are very effective and perform well. Operators will notice the biggest difference under drafting conditions. As the altitude of the pumping location increases, the dual-eye impeller can be upward of 50 percent more efficient during this operation-i.e., a 1,500-gpm single-eye impeller will perform up to about 4,300 feet of altitude; a dual-eye impeller will perform at up to 10,300 feet of altitude (both with two six-inch intake hoses no less than 20 feet long).
The vanes grab the water from the eye and direct it through the impeller to the exitway (high-pressure side), but not without the help of the shrouds-the walls on either side of the vanes. These two parts control and direct the water through the impeller, finishing at the exitway, where the water now becomes the high pressure (the operating pressure) needed for firefighting operations.
An inherent characteristic in every centrifugal fire pump is the high-pressure side (exitway) water wanting to return to the low-pressure side (eye), also referred to as recirculation. The hubs and wear rings work together to minimize this.
The hub is the rotating part directly surrounding the eye. The wear ring is the stationary part and seated into the pump body. With this design feature, there is no way to create a positive seal-nor do was want to-because some water flow between the hub and wear ring interface is important for cooling. However, it is important to minimize this so that the pump is as efficient as possible. Typical clearance is 0.005 inches. The more pressure lost through this interface, the less efficient the pump will be and the more horsepower and speed it will take to get proper pump performance. Because of the potential for wear in this area, departments need to perform yearly pump tests. Annual pump testing proves the necessary data to determine continued fire pump capability.
The Pump Body
The pump body is the part of the fire pump that controls and directs the flow of water from the inlet side through to the discharge. Within the pump body are three main parts: the intake section, the volute section, and the discharge section. The volute section directly surrounds the impeller and controls and directs the water as it leaves the exitway of the impeller. This section incorporates a stripping edge, which does exactly what its name implies: It strips the water off the exitway of the impeller, directing it through the discharge side of the pump. It is an important part that requires a specified clearance, depending on the performance of the pump. Generally pump designs incorporate one stripping edge. However, other high-flow pump designs will incorporate two, and these two stripping edges normally oppose each other at 180 degrees-i.e., one at the nine o’clock position and one at the three o’clock position, mainly for pump balance at the higher flows. Intake and discharge sections can be fabricated of either stainless steel weldments, typically handled by the truck manufacturer, or fully manifolded cast iron, designed and manufactured by the pump manufacturer.
Finally, an additional design characteristic of a centrifugal fire pump allows for the impeller to use incoming pressure, unlike the characteristics of a positive displacement pump (piston pup, rotary pump, and so on). Because of this characteristic, it is possible to design and manufacture multi-stage centrifugal fire pumps-but this is a discussion for another time.
STEVE TOREN, director, North American sales and marketing, has been with Waterous for 24 years. He has spent all of this time learning the pump trade in various positions at Waterous, including pump applications (two years), service and training specialist (six years) and sales/sales management (16 years).