Single-Stage Vs Two-Stage Water Pumps

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Pump speeds and horsepower required to obtain various common fireground flows are shown for 1,500-gpm single-stage and two-stage pumps.

When drawing up purchase specifications for pumpers, a fire department must choose the type of pumping equipment to be used – a complex task.

Consideration must be given to the pump rated capacity, number of pump stages required, how the pump is to be driven, pump location, the need for pump-and-roll capability, special pump performance requirements, the need for special materials for pump construction (based on local water conditions), the type of pressure control system, the size and location of intake connections and discharge outlets – and the list goes on. These are all important considerations, and each one must be carefully studied to ensure that the apparatus will function as intended.

One of the most important considerations is the number of pump stages required.  Many people struggle with this decision. There seems to be a lack of understanding of the options available and the advantages of each. We will deal with the differences between single- and two-stage pumps. Three- and four-stage pumps are also readily available, but generally are only specified when very high discharge pressures are required.

Four main criteria should be used when deciding whether a single-stage or two-stage pump should be specified:
1. Performance
2. Pump life
3. Simplicity of Operation
4. Cost

Performance

It should be noted that the National Fire Protection Association 1901 Standard for Automotive Fire Apparatus makes no distinction between a single-stage and a two-stage pump. Both must pass the same performance tests listed in the standard. So in essence, either pump will do the same thing. The difference is that they are not designed to do the same thing.

One pump will operate more efficiently than the other at certain operating conditions. The reasons for this are beyond the scope of this discussion, but the actual differences are worth exploring.

Single-stage pumps are designed to flow large volumes of water.  If you have an apparatus that frequently flows at or near the rated capacity of the pump, then a single-stage pump will probably be the best choice. It will flow rated capacity more efficiently (with less engine speed) than a two-stage pump (even though both pumps may have the same 1,500-gpm rated capacity).

Rated Capacity

Single-stage pumps work best when pumping at or near their rated capacity, but most fire departments pumpers don’t operate at or near their rated capacity all the time. In fact, most commonly pumps are used at only a fraction of their rated capacity. Look at the apparatus in your fleet. When was the last time a pump flowed rated capacity at a fire call? For many departments (including the big ones) most fires are extinguished with one handline flowing about 100 gpm.

Two-stage series/parallel pumps are really ‘two pumps in one.” When operated in the volume (parallel) position, they mimic the design of the single-stage pump. In the pressure (series) position, they will flow up to 70 percent of their rated capacity more efficiently (with less engine speed) than a single-stage pump.

The chart on the facing page shows the pump speeds and horsepower required to obtain various common fireground flows using a 1,500-gpm pump.  The actual numbers may vary between pump manufacturers, but the comparisons will be similar.

Pump Life

So the two-stage pump is more efficient at most flow rates, but what does that really mean to me? The short answer is it means a lot. If you are using a single-stage pump to flow less than rated capacity, you will need to spin it faster than a two-stage pump. This results in additional fuel consumption for the engine. A typical diesel engine consumes about 20 gallons of fuel per hour when run at governed speed. This may not sound like much, but over the life of the apparatus – and with today’s fuel prices – the dollars can add up quickly.

Also, running the engine faster than necessary results in additional wear-and-tear on the engine, chassis transmission and driveline components. This is not a huge consideration for most departments since today’s engines, transmissions and drivelines used in fire apparatus are basically the same design that are used in over-the-road trucks, which run hundreds of thousands of miles before needing to be overhauled. But there is still no getting around the fact that using a two-stage pump will decrease the amount of wear-and-tear on these components.

By far the biggest advantage to using two-stage pumps is they will last longer. As stated above, if you are using a single-stage pump to flow less than rated capacity, you will need to spin it faster than a two-stage pump. This will cause the pump to wear out faster. For busy departments this could result in an unexpected premature budget-busting pump overhaul job.

More importantly, the faster you run a diesel engine, the more horsepower it will develop. The pump, however, will only require a fraction of the horsepower being sent to it from the engine. The excess horsepower will be converted into heat energy. This will cause the water in the fire pump to heat up. If the water gets too hot, a catastrophic pump failure is imminent.

Finally, while single-stage pumps are designed to flow large volumes of water, they are not designed to produce high pressures. In fact, to develop 125 gpm at 200 psi (a typical 300-foot 1-1/2-inch attack line), a single-stage pump will need to spin faster than it would to pump 1,500 gpm at 150 psi. At this speed, the impeller wants to flow in excess of 1,500 gpm. But the nozzle at the end of the hose is limiting the flow through the impeller to 125 gpm. This results in an extreme amount of turbulence inside the pump, as the water leaving the impeller has no place to go. This phenomenon results in what is known as “recirculation cavitation,” which can quickly destroy a centrifugal pump.

Using a two-stage pump to develop the same 125 gpm at 200 psi requires about 25 percent less speed, which greatly reduces the amount of recirculation cavitation inside the pump.

Simplicity of Operation

There is no question that a single-stage pump is simpler to operate than a two-stage pump because the operator of the latter must decide whether to place the pump in volume or pressure. No such decision is necessary with a single-stage pump.

If more flow or pressure is required when operating a single-stage pump, the operator only needs to increase the engine speed to attain it. However, that is the only thing the operator can do. With a two-stage pump the operator has the ability to transfer the pump from pressure to volume (or vice versa) to obtain the desired output without necessarily changing the engine speed.

In actuality, this decision needs to be made rarely because the two-stage pump can deliver up to 70 percent of its rated capacity in the pressure mode. This means that the operator can leave the pump in the pressure mode for the vast majority of working fires. On the rare occasion when more than 70 percent of rated capacity is required to extinguish the blaze, the operator will quickly observe that not enough water is being pumped at full engine throttle and realize the need to transfer the pump to the volume mode.

Cost

Two-stage pumps are slightly more expensive than single-stage pumps, but the cost differential is not a significant amount when compared to the overall cost of the complete apparatus over its useful life. And, as noted above, the additional initial cost will be offset by fuel savings over the life of the apparatus.

So there are a number of reasons why two-stage series/parallel pumps may be preferred over single-stage pumps:
• Pumper performance is better with a two-stage pump over almost the entire operational range, and particularly at capacities less than 70 percent of rated capacity and at pressures greater than 150 psi.
• Pump life is greater for two-stage units, particularly if pumpers are used often for low-capacity/high-pressure applications.
• The additional operator training required for pumpers having two-stage series/parallel pumps is minimal.
• The additional initial cost of a two-stage pump is insignificant and is offset by fuel savings over the life of the apparatus.

Steve Morelan, who has 32 years of fire service experience, is the assistant fire chief of the Little Canada (Minn.) Fire Department and global service manager at Waterous Company. He has instructed hundreds of pump operation, maintenance and overhaul classes worldwide. He is also a Minnesota state certified apparatus operator.

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