A pump is the workhouse of an apparatus. Within this complex mechanism are many critical parts that work in tandem to ensure consistency, accuracy and reliability. One of the most important mechanics within that pump is the impeller. The impeller shaft assembly drives the pump and is what delivers the water to the nozzle at the appropriate pressure and flow.
When specifying a pump, fire teams must look closely at impeller quality to ensure it fits their needs. Here you will find considerations when making a selection:
Size and stage
When choosing the correct capacity of a pump, the right size matters. Whether you follow NFPA, EN or CCC Standards, it is important to select a pumping system capable of various flow rates at various pressures that may be needed to extinguish fire.
Of course, all pumps are required to meet the required standards, but they may not be designed to efficiently operate at the performance points needed in real life situations. For example, smaller fires may require less flow. Therefore, 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. This means bigger isn’t necessarily better.
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. Choosing between a two-stage pumps and a single-stage pumps is one of the most important considerations when specifying.
A large part of ensuring pump efficiency is controlling the direction of water flow within the pump.
For instance, centrifugal pumps are not as efficient as many would hope. With their design, a small amount of water bypasses the discharge of the impeller back to the intake. This water short-circuits the impeller and causes the pump to lose efficiency.
The trick to building a better mousetrap in the pump industry is reducing the amount of water bypassing the impeller. All pump manufacturers accomplish this by fitting the impeller with a wear ring machined 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 – increasing pump efficiency.
The next factor to consider when assessing a pump is longevity.
No matter how pure the water appears leaving the nozzle, it inevitably will include sand, sediment, minerals, and other foreign particles within it. Over time, the sand and other particles chip away at the components within the pump.
The water that bypasses the impeller by flowing between the impeller and wear ring move through a very narrow gap, essentially sandblasting both surfaces. As the two surfaces wear over time, the clearance increases, allowing more water to short circuit the impeller. Over time a gap will develop, and the impeller will bypass water from discharge back to intake. When this happens, a pump will need extensive repairs.
How long it will take for that to happen is difficult to say. It depends on the number of minerals in the water, frequency of pump is use, operating pressure. From a design standpoint, it is important to develop an impeller that can withstand these environmental stresses. We have two ways of accomplishing this:
First, limit the amount of water that flows between the impeller and wear ring by leaving a tiny gap between them. Leaving this gap requires precision machining and more importantly, precision support of the impeller shaft. When impeller and shaft spin inside the pump, force tries to push the impeller against the wear ring.
Waterous uses heavy-duty, deep-groove, anti-friction ball bearings capable of absorbing radial thrust, preventing the impeller from contacting the wear ring. The clearance between the two, which is about the thickness of a human hair, prevents catastrophic failure. 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 to change directions multiple times when flowing through the interface.
With this design, the water slows down with each change of direction. The result is increased efficiency when the pump is new and reduced wear over time.
Using non-corrosive material is the second way of reducing the amount of wear. Bronze or aluminum is the metal of choice for most impellers and wear rings 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 percent of the wear that takes place between the impeller and wear ring occurs on the impeller. To combat this, Waterous introduced a unique flame plating process. Specific to Waterous impellers, the flame plating process consists of adding tungsten carbide to the wear surfaces of the impeller. This produces a tough, well-bonded, wear-resistant coating that outwears hard chrome plating and tool steel.
Flame plating the impeller will eliminate 75 percent of the normal wear inside the fire pump. This means pumps 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.
Making the right choice
When purchasing your next fire apparatus, remember that all pumps are not equal. With today’s shrinking budgets, it is more important than ever to assess the total cost of operating your equipment over its lifespan. Specifying the right pump today, that includes the best quality impeller, could save you the expense of a pump overhaul in the future.
For more information, go to www.waterousco.com