Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Pressure drop differences in piping

Pressure drop differences in piping

Pressure drop differences in piping

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.

RE: Pressure drop differences in piping

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.

RE: Pressure drop differences in piping

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.

RE: Pressure drop differences in piping

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,

RE: Pressure drop differences in piping

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,

RE: Pressure drop differences in piping

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close