Interior Pump Pressure Loss
DEPARTMENTS
Herman E. Collier’s Training Notebook
Fire departments using largediameter supply hose are often disappointed or puzzled when, after making the calculations on the blackboard, they’re unable to deliver the desired flow through a single outlet.
To understand this problem, a little must be known of the limitations that physics puts on the mathematical calculations. Any time a large amount of water is flowing through a single discharge opening, part of the total pressure built up can’t be discharged. When flow rates exceed 500 gpm through a single discharge, the design of the pump and piping prohibits some of the pressure from being released.
This is called “interior pump pressure loss.” Unless it’s understood and accounted for, the frustrations will continue.
An example of this phenomenon is an air compressor. When air is being released for filling car tires, the pressure in the tank drops to the point at which the air compressor turns on. The compressor can build pressure faster than it can be released through the single air hose. The pressure builds back up in the tank, and then the compressor shuts off. All this occurs while you’re still filling the tire.
The centrifugal pump does the same thing. The Class “A” pump is designed to deliver its rated capacity from a draft at 150 psi net pump pressure. However, to accomplish this, more than one discharge port needs to be used, preferably one from each side of the discharge manifold. This is because most centrifugal fire pumps are designed to deliver their rated capacity through more than one discharge opening.
You can observe this inability to move all the pressure from the pump by comparing the main pump pressure gauge with the port discharge gauge. When we begin moving water, the two gauges register the same pressure. As we increase the flow rate, both gauges will continue to register the same rate until the flow reaches about 500 gpm. At this point, the port discharge gauge will begin registering a lower pressure than the main pump pressure gauge.
In tests performed on pumps of various brands and capacities, this difference in pressure began showing up at about the same discharge. The pressure loss was from 5 psi at 500 gpm to more than 30 psi at 1,000 gpm.
For example, at 500 gpm, the main pump pressure gauge records 125 psi, and the port gauge records 120 psi. At 700 gpm, the main pressure gauge records 145 psi, and the port gauge records 125 psi—a 20-pound pressure loss. At 1,000 gpm, the pressure loss exceeds 30 psi.
The same loss occurs when you directly supply a piped deck gun instead of large-diameter hose, but you can’t observe it unless there’s a discharge port gauge. Only through the use of a pitot gauge, which measures velocity and converts it to a pressure reading, or use of an inline gauge at the tip can you measure the loss between the pump and the point of discharge.
When a 2-inch nozzle is supplied at 1,000 gpm through a piped deck gun on a pumper, for example, there’s a loss of about 60 psi between the main pump pressure and the pitot gauge at the deck gun. Of this, about half is normal loss in the deck gun caused by piping configurations; subtract that from the 60 pounds total loss, and you see the effect of interior pump pressure loss.
To realize 1,000 gpm on a prepiped deck gun, you’ll need to reach pressures up to 160 psi on the main pump pressure gauge when using a fog nozzle and 140 psi when using a 2inch open tip (remembering that fog nozzles require more pressure than open tips to reach the same flow). By using a mounted deluge set with two short lengths of 3-inch hose and placing one into each side of the pump discharge manifold, you can provide the same discharge at approximately 30 psi less pressure.
Pressure build-up inside the pump may be the answer to the puzzle of vanishing pressure at the discharge. If you think you’re not flowing the water you should be, or you’re having problems with excessive pressure, check the “interior pressure loss” in your pump and piping.
