re: Pump Curves and DP measurement across the Pump

[TurbineBlade]

Hi all,

I have two pressure indicators; one in the suction and the other in the discharge sides of the pump. I would like to find out the flow rate of the system. The questions that I have are:

1. Does the DP measurement across the pump reflect accurately to the overall system pressure drop?

2. If I take the DP measurement, convert to head (ft) and read it from the pump curve to get the corresponding Q. Is this correct approach?

3. My system is heavy water, which is 10% heavier than light water, and operate at a higher temperature. I believe that the pump curve from manufacture is generated based on light water at STD condition. When I do the DP to head (ft) conversion based on heavy water density, do I have to further convert the head (ft) to light water density before reading the corresponding Q on the pump curve?

Thank you for your help

Turbineblade

 

[SeanB]

The difference between the discharge pressure and the suction pressure is the delta P across the pump (assuming in psi). You then have to convert this into ft of water by multiplying the delta P (in psi) by 2.31 and dividing by the specific gravity of your fluid. This is the only conversion. Find this value on your pump curve and you will find the corresponding flow rate.

 

[electricpete]

Some things to remember:
- If the pump is not operating at the same speed as the curve, there would be an error unless you correct for speed.
- Pump performance can change / degrade. For example increased recirculation due to wear ring wear. That means your pump might not meet its pump curve.
- I think pump curve would be based on pressure at height of centerline of pipes. If you gage is offset vertically, correct for the height of the fluid.

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[TurbineBlade]

HI,

Thanks for the responses.

Electricpete,

What instrumentation will show direct/indirect the pump speed?

Thanks again.

Turbineblade

 

[bk19702]

A few ways to determine driver speed:

- Tachometer (for a turbine drive)
- Tape the shaft (do this when the pump is down), then measure with a handheld tach while in service.
- Strobe light (dial in the flash rate to the corresponding RPM)

 

[TurbineBlade]

Hi,

If the pump suction pipe diamater is different than pump discharge pipe diameter, then I need to account for the difference in velocity head?

thanks

 

[BigInch]

The pump curve's differential head includes velocity heads. If its significant, you need to account for them, although usually it can be ignored. If you have a high velocity and low head pump, include it.

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