Packaged Booster Pump Selection and Design

[doopedeng]

Hi All, I'm trying to verify my opinion of domestic water pressure in a 93 ft high office building serving mainly just washrooms over the 8 floors of the building. The city water pressure at the suction side of the pressure booster varies between 50-60 psi. The required pressure at the 8th floor fixtures 93ft above the incoming city main is 30 psi. Elevation difference is 93ft/2.31 = 40 psi. Therefore, the pressure boost required is about 20 psi. After the system was installed, the pressure gauge on the discharge side of the constant volume booster pump (I know, should be variable volume) is reading 120 psi or higher. So, to my understanding the pump is boosting the pressure by 70 psi (50 to 120) and not 20 psi, yet the pump rep is saying this high pressure should be expected - that the weight of the water (40psi) should be added to the 50 psi from the city water, giving 90 psi, plus the 20 psi boost (more at low or no flow conditions). What am I missing here?

Additional info - no pressure tank installed on system; check valve installed at the booster pumps; and the building has virtually no load due to low occupancy right now.

Any help would be appreciated.
Thanks,
Dooped

 

[jonr12]

Look at it this way… If you have a minimum of 50 PSI coming in, and you lose 40 PSI before it gets to the top of the 93’ building, you should have 10 PSI on the top floor. If you want 30 PSI on the top floor, how much more pressure do you need to add? 30 – 10 = 20 PSI. So your booster pump needs to pick up the 50 or 60 coming in and increase it to only 70 PSI going up.

Another way to look at it…. If you have 120 PSI at the booster pump, and you only lose 40 PSI going up the 93’, you should now have 80 PSI on the top floor instead of the 30 that you want.

I would also use a pressure tank, especially for low occupancy.

Weight of the water has nothing to do with it. Sounds like you have a pump that is almost three times as large as you need. I will bet the rep is still claiming the Variable Speed Drive will save energy. Not gonna happen! What you really need is a new pump rep!