Pressure drop differences in piping

Question

Pressure drop differences in piping

Liad (Chemical)

(OP)

25 Feb 21 13:35

Hello All,
I got into a discussion with some other mechanical ENGs about pressure drop in pipe.
We are running some tests for a gear pump and want to simulate a flow rate with back pressure in the end of a long line that as a connection to a 10 bar steam line. I suggest to use a short line and a gate valve and to close it until getting the right back pressure at the discharge of the pump (about 15-20 bars), it means we need to close the valve almost completely.
The others said that it does not represent the real condition (with the steam line).

The question is, will there be a difference in the capabilities of the pump and the flow obtained by using a gate valve to create the back pressure or using for example a pressure tank (without any diameter decrease)?

Thanks a lot.
Liad

RVAmeche (Mechanical) · Answer 1 · 2021-02-25 13:37:39

A gear pump displaces the same amount of fluid as long as the RPM stays the same.

I think a test line with a manual valve (globe would be more precise) would be fine.

LittleInch (Petroleum) · Answer 2 · 2021-02-25 13:43:50

Well using a gate valve is a poor way to create pressure drop / back pressure. only in the last few percent open will you get a lot of pressure drop so controllability is limited.

A globe valve or even maybe a needle valve would be much better and more controllable.

But in the end pressure is pressure. How you generate it is up to you. In "real life" this is created by flow in your long pipe and then an end pressure.

In your test environment this is created by a pressure drop across some sort of restriction - in your case a partially open gate valve (bad choice as said).

But 15 bar is 15 bar. The fluid doesn't know how that was generated nor does the pump, neither should "the others".

Maybe ask them how they think pumps are tested? By installing them in the real pipeline / plant or on a test rig where the back pressure is generated by pressure drop across aa variable restriction (Control Valve). I'll give you a hint - it's the second one.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Latexman (Chemical) · Answer 3 · 2021-02-25 15:23:58

The steam pressure provides a constant backpressure. The "long line" provides a variable back pressure. At your design flow rate calculate the total backpressure. Then, to test the pump, put a pressure gauge followed by a control/needle valve on the discharge of the pump; no "long line" and no steam. Open the CV 100%, on the pump, and start closing the CV until the backpressure equals the total backpressure you calculated. Now, measure the flow rate and compare to what was expected. Finished.

Good Luck,
Latexman

Latexman (Chemical) · Answer 4 · 2021-02-25 15:48:25

Oh yeah, after a number of months or years of wear and tear, that gear pump (same RPM) won't flow as much as when tested. It's clearances will open up and there will be more slippage and internal recycle. Just so you know. Been there, done that.

Good Luck,
Latexman

Compositepro (Chemical) · Answer 5 · 2021-02-25 20:39:40

Have a safety relief valve on the pump discharge because the amount of valve movement needed to go from 10 bar back pressure to 200 bar may be small.

Come Join Us!

Posting Guidelines

Pressure drop differences in piping