Chassis Components, Pumpers

Pump Theory and Engine Purchasing

Issue 9 and Volume 23.

Apparatus with fire pumps are not unique concepts in the fire service; rather, they are the cornerstone of every department.

Hours are spent on the bells and whistles of new apparatus, whether to purchase an aerial platform rather than a straight stick, and even if a rescue-pumper is the best option for a department. Yet, the biggest part of a pumper is often overlooked, or at least proper attention is not placed on the most important component of the engine—the fire pump.

Pump Theory

In pumper operator class, the hardest lecture to swallow is pump theory. True pump theory has complex formulas and equations to determine maximum capabilities of pumps under various conditions. Unless you are a math geek, most of the fire service is lost at the word formula. We take a complex component of the apparatus and bring everything down to the easiest of ways to interpret complex formulas and develop pump charts. These charts are the backbone to rapid response; they get the operator very close to the actual pressures needed. Soon the formulas are forgotten, and the operator is reliant on a chart and, as a result, the theory of the pump is gone too.

So, what this relates to is sending drivers or officers who are not up on pump capabilities to a prebuild for an engine. As a result, departments are forced to rely on the manufacturer’s build team for advice or recommendations. Thus, larger pumps are put into apparatus, and larger pumps need larger motors to drive them. All of this leads to a larger price tag.

The better choice is to look at the needs of the department and community. What fire loads are present in the district? What are the water demands to meet those needs? What water tower operations or large lines need to be supported? Do you have high-rise buildings? Do you operate a water tower with dual nozzles at the bucket? What flows are your hydrants capable of producing? Do you use a water shuttle operation? Answering these questions will help in the process, which is much like homework. My department operates a 2,250-gallon-per-minute (gpm) pump that is rated at 1,500 gpm. The capability of the pump is a combination of the pump and engine needed to drive the pump. It meets our needs to supply greater than 2,000 gpm from a hydrant to be able to use both nozzles on an aerial platform and also feeds the needs of the downtown high-rise district. The pump is Underwriters Laboratories-derated to ensure that the it will pass pump tests. If we rated our pumps at 2,250, then any decrease in engine or pump performance would yield an apparatus that failed a pump test—and that means being out of service.

None of these questions seem to have anything to do with pump theory. But, I would argue that they have everything to do with it. Do the ratings on your new pumper meet the needs of the community, or are you purchasing the engine that you had because it was adequate? What are the future developments in your area, and are you purchasing an apparatus that is good today but not 15 or 20 years from now when the apparatus is in reserve status?

Pressure or Volume

Generally, with fire pumps, there is a choice of pressure or volume. The lower the pressure on the pump, the more volume can be pushed. The higher the pressure on the pump, the less volume can be pushed. This seems to be a simple concept until an engineer is in a situation where more water is needed. The reaction almost all the time is to throttle up and give more pressure. But, with more pressure we are creating more problems than we are solving. When pressure is increased from a supply engine to the attack engine, the attack engine may not have the ability to provide the safe pressure on a hoseline. The best option for this situation is to have an additional supply line laid to provide additional volume at low pressure. If you don’t believe me about the higher volume at a lower pressure, look at the boilerplate on the pump. Ours states 1,500 gpm at 150 pounds per square inch (psi) and only 750 gpm at 250 psi. The reason for the decrease in pump volume discharge is that the pump is spinning harder to make pressure, and a decrease in volume is the result.

Large municipal departments generally don’t have to worry about water—hydrants usually are present to satisfy the needs at fires. Sharing resources with other departments may change that philosophy. Most large cities are located on a body of water or a waterway, which makes drafting a distinct possibility. Drafting is when pump theory really comes in handy. Even if there is no chance ever that your department will draft, the concepts are still relevant—especially to be able to share services with suburban departments that use portable tanks and drafting.

As water enters the pump, it is accelerated by the vanes of the pump. As the water accelerates, it creates a vacuum that pulls more water into its place (decreases pressure on the inlet side of the pump). This is a pressure differential created by the pump. Most importantly, the pump does not need to be completely full of water to create a pressure differential.

Pumps mounted on apparatus are almost exclusively centrifugal pumps, which are not self-priming. But by using the concept of the pressure differential in the pump, an operator can use the pump to act as a self-priming pump. If your department uses dry pumps in the winter, using tank to pump and tank fill while throttled up to 1,000 revolutions per minute will prime the pump in almost the same time as using the primer (with some time variations related to pump and primer manufacturer and piping). The faster the pump spins, the faster the priming effect.

When a pump is wet, there is a strong pressure differential on the intake (vacuum) side vs. the discharge (pressure) side of the pump. The centrifugal pump can pump air through a discharge and still maintain a prime while drafting. This allows an operator to use the method of circulating water from the tank to pump and tank fill to pull water from a portable tank or even a cistern at a level much lower than the inlet of the pump. The faster the pump spins, the more draft created on the intake side of the pump; and, as a result, air will be exhausted into the tank. This operation has worked with or without a master intake valve (MIV) present. In fact, better results have been achieved with aggressively opening the pump’s MIV.

So, what does all this mean while going over the specifications of your apparatus? Knowing pump theory will help you understand the capabilities of your apparatus. Using the apparatus to its fullest capability will give you a true understanding of whether it will meet the performance needs of your department. The spec committee should have an operator that knows the pump inside out, including theory. As the design of the apparatus changes, that individual is there to ensure that the performance requirements are being met or if that apparatus will meet the needs if used in a different manner.

Understand pump theory and practice concepts by applying theory to find new ways to accomplish a task. Take that information to a manufacturer, along with the operational plan of the department and the requirements (both present and projected) of the department’s fire district. Know your department’s needs and purchase accordingly. Train and share information on problem solving with pumps and pump theory.


DOUGLAS PIETZ is a lieutenant with the Milwaukee (WI) Fire Department. He is the vehicle operations training coordinator for the department.