Pump Head - Flow Calculation

[jebatty]

This seems to be easy, but I must be tired. I have a flowmeter with a 2 psi pressure drop (4.6 ft head). If I have 8 gpm (1" pipe) with the flowmeter (about 11 feet of head), and now I remove the flowmeter (head decreases by 4.6 feet), what is the approximate flow? Circ is Grundfos UPS 15-58FRC operating on HI.

 

[jebatty]

If it helps, Med =7 gpm, and Lo = 5 gpm.

 

[Nofossil]

To a first approximation, you'd think about 13gpm.

However, I think for a given plumbing loop, the effective head increases with the flow rate, and it's nonlinear. I'd expect the actual flow to be a bit lower as a result.

I haven't been initiated into the inner mysteries (actually, it's been too many years since Reynold's equations) so I won't try to be more specific.

 

[jbrown9709]

This thread reminds me of something that prompted a light bulb moment for me a while ago. I am pumping through about 50' of 1" Pex, and don't want to add the head loss of a flowmeter to my piping. But, since I'm working with a boiler I built myself, I'd like to know what the flow rate is, so combined with temperature differential at the boiler, I can calc BTU output. My thought is this: If I put pressure gages immediately upstream and downstream of the pump (taco 007) for which pump curves are readily available, can I take the pressure differential across the pump and back-calculate the flow rate based on the pump curve?

Thanks,
Jeff

 

[jebatty]

Interesting thought, but my first reaction is "no" because water does not compress. I think you will have equal pressure throughout the system. While I think it is true that the circ pump creates a pressure differential which causes the water to circulate, I'm not sure that is measurable in any way that would provide the info you want.

I hope you get a knowledgeable "yes" answer. A back door approach is as good as a front door approach.

My thought with the flowmeter was to obtain a known flow rate along with the amount of head added by the flowmeter. From that I thought I could get a good approximation of the actual flow rate without the meter. An interesting observation is that as the water temp rises, the flow slightly declines. The flowmeter measures flow by the force of moving water against a weight sliding on a rod. My guess is that the density of the water is falling as temp rises, causing the weight not to lift as high and showing a slightly reduced flow rate. I'm thinking the flow rate has not actually changed. Water expansion from 0C to 100C is about 4%, so I'm also guessing that the flowmeter would show about a 4% decrease in flow over this same temp range.

 

[jebatty]

Back to my original question. At flow of 5 gpm, the flow chart shows head of about 5 feet. At 7 gpm, head is 9 feet, and at 8 gpm, head is 11 feet. It appears that for about every 2 feet in change of head, gpm changes 1 gpm for this circ and this flowchart.

Based on this I'm thinking that if I remove the flowmeter, head reduces about 4.6 feet, which will result in about a 2.3 gpm increase. Being a little conservative, and thinking that head increase with increase in flow is not linear, my estimate is that without the flowmeter, the circ on HI will have flow of about 7 gpm on LO, 9 gpm on MED and 10 gpm on HI.

What do you think?

 

[Nofossil]

Back to my original question. At flow of 5 gpm, the flow chart shows head of about 5 feet. At 7 gpm, head is 9 feet, and at 8 gpm, head is 11 feet. It appears that for about every 2 feet in change of head, gpm changes 1 gpm for this circ and this flowchart.

Based on this I'm thinking that if I remove the flowmeter, head reduces about 4.6 feet, which will result in about a 2.3 gpm increase. Being a little conservative, and thinking that head increase with increase in flow is not linear, my estimate is that without the flowmeter, the circ on HI will have flow of about 7 gpm on LO, 9 gpm on MED and 10 gpm on HI.

What do you think?

I think that seems pretty reasonable. I think Jeff is right, though. The fact that water isn't compressible doesn't prevent dynamic pressure differences. Under dynamic conditions you can clearly develop a pressure difference. I'd be interested to know if anyone has actually made that measurement across a circulator.