negative absolute pressure

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ndlaham
Posts:3
Joined:Tue Sep 06, 2011 9:29 pm
negative absolute pressure

Post by ndlaham » Tue Oct 11, 2011 3:33 pm

Hey all, I'm having trouble understanding something, hopefully someone can help.

What does it mean if I am calculating a negative absolute pressure in a flow circuit. The flow is pumped up about 40 feet from an atmospheric tank and then runs horizontally for around 400 feet in 1.5 inch pipe, and return back down to the tank.

I'm using a pump curve from a high pressure, low flow pump, operating at about 30 feet of head once the flow is steady state, and calculating around 20 GPM after all friction head losses. The pump has plenty of head pressure to get up the building. Maximum says 148 feet at 0 GPM. Here is the spec sheet, http://www.pexuniverse.com/docs/pdf/968 ... mq3-45.pdf

Anyway, I am seeing a negative absolute pressure (which is impossible) at the top of the circuit right before it returns down to the tank. If I use a lower, say 20 feet, I don't see this problem.

Why is this happening if the pump has enough head to get through the circuit and over the building? Shouldn't the gravity create suction and give me a constant flow with normal pressures. I think it makes sense to have negative gauge pressure at the top since the flow is being dropped down 40 feet, but why I am calculating impossible numbers?

Any ideas?

Thanks,
Nick

PipeFlowCalcs
Posts:57
Joined:Sat Sep 17, 2011 8:50 am

Re: negative absolute pressure

Post by PipeFlowCalcs » Sat Oct 15, 2011 10:52 am

Well you can not have negative absolute pressure, might be your gauge is not accurate for low pressure readings.

Your pump might be able to give 148 feet head, actual pump working pressure is just enough to over come all friction losses plus gravity head difference between to tanks level. If both tank are on the same height, than pump give head that is required only for friction losses.

As you have upward going pipe, pressure drops and it might go into vacuum very easy. With 40 feet pressure is zero. Head of 33.8 feet vacuum is absolute zero.
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ndlaham
Posts:3
Joined:Tue Sep 06, 2011 9:29 pm

Re: negative absolute pressure

Post by ndlaham » Fri Dec 14, 2012 4:27 pm

I actually am calculating these values to be negative. I am not reading any gauges on a real life system.

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