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Permalink Reply by Paul Spencer on October 12, 2011 at 12:18am

Hey, I hear your frustration.  You highlight a very interesting point.  In my mind "net pump pressure" describes the work energy the impeller is putting into the water.  Burn diesel, spin the impeller, the pressure rises.  There is certainly a difference between static and dynamic pressure rise given a fixed amount of throttle but understanding that is way above my pay grade.  None the less, static or dynamic the pressure rise in the pump is still refered to the same as NPP.  Subtract the intake psi from the discharge psi and you have NPP.  Don't know if that helps.

 

Where it comes into play in my world is in pump testing.  A fire pump is tested in its ability to create a pressure rise at a certain flow.  We think of 150, 200 & 250 psi as the pressures we test at.  It's really a NPP of 150, 200 and 250 we test at.  At draft the intake pressure is negative and we deduct that from our discharge pressure.  The actual discharge pressure for the 150 psi test might actually be 145 psi after a negative intake pressure is deducted from it.  The pump test is actually an impeller test.  If you use the wrong intake hose you can cause a significant negative pressure in the intake however you can still see a 150 psi NPP rise.

 

Hope I haven't clouded the issue.

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Permalink Reply by Paul Shapiro on October 12, 2011 at 7:45pm

Here it is plain and simple. Net pump pressure is the pressure created from the pump without the assistanace of an incoming pressure from a hydrant or other pressureized evolution . Net pressure operations are used in static water supply operations such as drafting and booster tank operations. The thing that is important to understand is that pumps are rated at net pressure at 150 psi. The higher the net pressure goes from 150 the lower the pump rating is. With this being said the higher a incoming pressure is (as long as the water is there) the lower the net pressure becomes. EXAMPLE a 150 psi discharge pressure with a 100 psi intake pressure makes the net pressure 50.150 minus 100 = 50 . So, this tells me that the pump can go up another 100 psi and still increase in flow( the magic number is 150 psi net.) This is how pumps are able to surpass there rated capacity. I achieved a 3125 gpm flow from a 1500 gpm pump using two very strang hydrants and 5" supply lines.

I hope this helps.

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Permalink Reply by Ed Hartin on October 13, 2011 at 9:07am

Net pump pressure is the pressure actually being developed by the pump. When pumping from a pressure source such as a hydrant with a centrifugal pump, you subtract the residual pressure from the discharge pressure. If the pump was not in gear, water would pass  through the pump and discharge would be similar to the residual pressure (less the friction loss resulting from the water passing through the pump). When the pump is in gear, additional energy is added and discharge is higher (this is the net pressure developed by the pump).

When drafting it is a bit different. In this case, you need to addthe height of lift (converted to psi) and friction loss in the suction hose to the discharge pressure. In this case a discharge pressure of 145 with a lift of 10' would result in a NPP of approximately 150 psi.

The concept of net pump pressure is useful in understanding why a centrifugal pump can pump much more than its rated capacity when working from a pressure source. The energy developed by the apparatus engine can be used to develop flow or pressure. If the net pump pressure is lower than 150 psi, more energy can be used to develop flow.

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