# Thread: Pressure Correction

1. ## Pressure Correction

My department recently was conducting our annual pump tests on our apparatus and the subject of pressure correction came up. I have always heard that you subtract the FL from the PDP for pressure correction and the officer in charge of the test said that we added it to the PDP. Which is it and why?

Thank You Brothers and Sisters for your help.

2. [ 10-12-2001: Message edited by: Daron ]

3. I may be missing something, but here-goes:
By subtracting FL from the PDP, you get the pressure at the nozzle (NP). For example, if you want 130 psi at the nozzle, and your FL is 20, you should add the FL and the nozzle pressure (NP) to find the PDP.(130 + 20 = 150 psi)
On the other hand, if you are pumping at 150 psi, and you have FL of 20 psi, subtract the FL from the PDP to find the NP. (150 - 20 = 130 psi.
Whether you add or subtract FL depends on which variables you have

4. I am not sure what you are asking, for pump test you have to have PDP match what part of the test you are doing, 150psi, 200psi, or 250psi. The only correction is FL on the intake side through the hard suction and strainer. If this is what you are asking then you subtract it. Lets use the 150psi test, the reason for this is Total pump preasure must equal 150psi. If FL and Elevation lost equal 9psi, then the pump must produce 141psi = 150psi becuase it is over coming 9psi lost on the intake.

When I do my test i dont figure in the intake lost. If I am doing the 150psi test I set my PDP at 150psi. The truck will be able to do it, if I need that extra 10-12psi to make the test, then it is time to start looking for problems anyway. This test is higher than the standard, but you can go higher.

5. You do give the pump credit. It has already performed work by lifting the volume of water into the pump. If you would like the formulas to calculate this contact me.
camaintenance@hotmail.com And I will fax or send them via email to you.

6. A pump test differs from most standard fireground calculations in that it is trying to simulate the initial acceptance test conducted by Underwriters Labratories. You are trying to duplictate as accurately as possibe the conditions that were present when the initial test was made. All of the data from this initial test is located on the pump panel, on the PUMP TEST PLATE / Underwriteres Plate. So basically, you can compare your annual results to the first test ever conducted on your pump- and see if its performance has degraded any.

The original tests were conducted from a draft- with a lift of no more than 10 feet.

What you need to calculate is NET pump pressure- the total pressure that a fire pump develops. But since a centrifugal fire pump- operating from a draft - actually has negative pressure on the intake side- and positive pressure on the discharge side of the pump -- the NET pump pressure is actually an algebraic sum of these two pressures.

The best way to determine this is by converting the the intake pressure from inches of mercury (suction) into pounds per square inch (psi)of negative pressure.

1" of mercury equates to approx. 1/2 (.5)psi -- equivalent to about 1 foot of lift.

Here's an example-
The pump is operating from a draft, with a discharge of 150 psi. The compund gauge on the intake side of the pump shows 20" of mercury. Convert the 20" of mercury to a pressure. [This is work that the pump is doing in order to allow the pump to actually pump water, so as CAM said - you do give the pump credit] In the case of this scenario - that pressure is to 10 psi, you credit this to the pump --- Because of the 20 inches of mercury (suction) on the intake side of the pump.

150 psi discharge pressure + 10 psi of 'work' being done to 'lift' water to the pump -- the pump is "actually" pumping at 160 psi.

Remember, that by their operating principle - centrifugal fire pumps cannot 'suck' - or generate suction. When you draft, you need to pull a prime, or prime the pump.

'Pulling a prime' uses a smaller oil driven pump to suck air from the pump housing itself- and lower the atmospheric pressure inside the pump. Once that is accomplished - the atmospheric pressure acting on the surface of the body of water-- pushes the water up the hard suction.

You can also 'prime your pump' by backflowing water from the tank. Open your tank to pump valve- and let water flow into the pump- and down the hard suction to the surface of the water. Now there is no air in the pump -- but only water- and water is what a centrifugal pump can move.

[ 10-12-2001: Message edited by: FF McDonald ]

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