Date: October 14, 2019
THE HEART OF THE FIRE PUMP: THE IMPELLER
The fire pump is sometimes referred to as the ‘heart’ of the fire truck. For without a pump, a fire truck is just a transport vehicle that can bring firefighters and equipment to the scene, but is incapable of extinguishing any fire. In order to get the wet stuff on the red stuff, a fire pump is needed.
If the heart of the fire truck is the fire pump, then the heart of the fire pump is the impeller. The impeller shaft assembly (or rotating element as some people call it), is what delivers the water to the nozzle at the appropriate pressure and flow for the successful extinguishment of the fire. Therefore, it is crucial that the pump’s heart (impeller) is designed for optimum efficiency and longevity.
When choosing the capacity of the pump, remember that size matters. The National Fire Protection Association (NFPA) – 1901 Standard for Automotive Fire Apparatus requires pumps to be able to flow their rated capacity at 150 psi, 70% of their rated capacity at 200 psi, and 50% of their rated capacity at 250 psi. These somewhat arbitrary performance points were selected to ensure that the pumping system is capable of various flow rates at various pressures that may be needed to extinguish fire. Of course all pumps sold in the United States (and other countries) will meet the requirements of NFPA 1901, but they may not be designed to efficiently operate at the performance points needed in real life situations – for example, smaller fires may only require 60 gpm at 125 psi. So the pump must be capable of flowing large volumes of water when extinguishing a lumberyard fire, and smaller amounts of water when extinguishing a dumpster fire. Generally speaking, the larger the capacity of the pump – the more inefficient it becomes at the lower flows. So bigger isn’t necessarily better. Choose the size of your heart wisely!
The use of multi-stage pumps (I.e. two-stage pumps) allows for both high and low flows since they give the operator the ability to select the appropriate firefighting mode needed at the time (Volume or Pressure). The advantages of two-stage pumps over single-stage pumps is beyond the scope of this article, but is one of the most important considerations when selecting a pump for your new fire truck.
Once the capacity of the pump and the number of stages are determined, the next factor to consider is longevity. As noted above, every fire pump sold that meets NFPA 1901 requirements will pass the required pump tests – when it is new. But as time goes by, that may change. Centrifugal pumps are not 100% efficient. Due to their design, a small amount of water is allowed to bypass from the discharge (exitway) of the impeller back to the intake (eye) of the impeller. This water – in effect – short-circuits the impeller, and causes the pump to lose efficiency because some of the water from pump discharge is travelling back to the intake side of the pump instead of flowing out the end of the hose – where we would like it to go.
The trick to building a better mousetrap in the pump industry is to reduce the amount of water that is bypassing the impeller. All pump manufactures accomplish this by fitting the impeller with a wear ring that can be machined to within a few thousandths of an inch from the impeller. Any water that bypasses the impeller is forced to flow between the wear ring and the impeller. The smaller the clearance is between the impeller and wear ring, the less water will be able to bypass the impeller and the more efficient the pump will be.
No matter how pure the water that is leaving the nozzle appears to be, it always has a certain amount of sand, sediment, minerals or other foreign particles in it. Over time, the sand and other particles in the water wear away at the different parts of the fire pump. The water that bypasses the impeller by flowing between the impeller and wear ring also has some of this sand in it. As the sand passes through this narrow gap, it wears both surfaces (it sort of sandblasts the surfaces). As the two surfaces wear, the clearance increases allowing more water to short circuit the impeller. What can sometimes happen is the sand in the water can wear away at these surfaces to the point where the gap becomes so large that the impeller is bypassing so much water from discharge back to intake, that the pump can no longer deliver its rated capacity. When that happens, it’s time for open heart surgery (an expensive pump overhaul).
How long it will take for that to happen is difficult to say. It depends primarily on the amount of sand and other minerals in the water that is being pumped, how often the pump is used, and what pressures it is operated at – all of which are out of our control. So from a design standpoint, we need to do whatever we can to minimize the amount wear in this area. We have two ways of accomplishing this.
First, we need to limit the amount of water (and sand) that flows between the impeller and wear ring. There are two ways to do this. The first is to leave only a very small gap between them. This requires precision machining and more importantly – precision support of the impeller shaft. As the impeller and shaft are spinning inside the pump, there are forces in play that are trying to push the impeller against the wear ring as it is rotating. Waterous uses heavy duty, deep-groove, anti-friction ball bearings (as opposed to sleeve bearings) which are capable of absorbing this radial thrust and prevent the impeller from contacting the wear ring (which would result in catastrophic failure) – even though the clearance between the two is about the thickness of a human hair. The impeller/wear ring interface design is also crucial in restricting the amount of water that flows between the two parts. Our exclusive labyrinth design forces the water (and sand) to change directions multiple times when flowing through the interface. Every time the water changes directions, it slows down. This not only results in increased efficiency when the pump is new, it also results in reduced wear over time because there are less abrasives flowing through the interface.
The second way of reducing the amount of wear that occurs in this crucial area is by using a material that is wear resistant. Most impellers and wear rings are made of bronze or aluminum because of their excellent corrosion resistant properties. Unfortunately, these materials are relatively soft and do not withstand the ‘sandblasting’ that occurs when pumping. About 75% of the wear that takes place between the impeller and wear ring occurs on the impeller. This is because the impeller is rotating and the wear ring is stationary. It is because of this wear problem that Waterous introduced a unique flame plating process to the fire service back in the 1950’s, and it is still a Waterous exclusive today. The flame plating process consists of adding tungsten carbide to the wear surfaces of the impeller. This unique process produces an extremely hard, well-bonded, wear-resistant coating which outwears hard chrome plating, and even tool steel (filing on a flame plated impeller will have no effect on the impeller and will ruin the file). Flame plating the impeller will eliminate 75% of the normal wear inside the fire pump, which means that the pump will last three times longer than comparable pumps without flame plated impellers. For most departments, this translates into a lifetime of service without the need for an overhaul.
When purchasing your next fire apparatus remember that all pumps are not created equal. With today’s shrinking budgets, it is more important than ever to consider the total cost of operating your equipment over its lifespan (20+ years?). Specifying the right pump today could save you the expense of a pump overhaul in the future. Consider the options available to you – take good care of your (pump’s) heart.