Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

0

Trouble With Control Valves: Flow Rate Declines After Opening


10 replies to this topic
Share this topic:
| More

#1 jstall

jstall

    Brand New Member

  • Members
  • 3 posts

Posted 01 December 2021 - 04:03 PM

We have a tank feeding a pump, which feeds 3 lines - one through a heat exchanger and back to the tank, and two through control (equal percentage globe) valves. When one of the control valves is opened, there's a corresponding increase in the flow rate, but it declines afterwards. This makes control of the system very difficult. I attached a screenshot of the problem as well as a basic P&ID. The pressure at the pump outlet seems to stay pretty steady at around 110 psi.

 

Does this look familiar to anyone?

 

Thanks for reading, and for any help you can provide

Attached Files


Edited by jstall, 01 December 2021 - 04:04 PM.


#2 latexman

latexman

    Gold Member

  • Admin
  • 1,672 posts

Posted 01 December 2021 - 05:20 PM

I assume the pump is a centrifugal, right? Can you turn gridlines on the major x and y values and repost? Thx!

#3 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 3,528 posts

Posted 01 December 2021 - 11:16 PM

You need to show more of the system. And a pump curve with some data. I don't understand the pressures that you display.

 

Bobby



#4 Pilesar

Pilesar

    Gold Member

  • Members
  • 1,347 posts

Posted 02 December 2021 - 09:09 AM

Could it be you are aspirating air? The flow pattern makes me think of a vacuum leak.



#5 breizh

breizh

    Gold Member

  • Admin
  • 6,292 posts

Posted 02 December 2021 - 09:45 AM

Hi,

How is managed the level in the tank feeding the pump? 

How do you mitigate the risk to generate a vortex ?

What about cavitation or vapor lock?

 

Breizh 



#6 jstall

jstall

    Brand New Member

  • Members
  • 3 posts

Posted 02 December 2021 - 10:20 AM

I assume the pump is a centrifugal, right? Can you turn gridlines on the major x and y values and repost? Thx!

 

Yes, it's a centrifugal pump. I've attached a copy of the graph with gridlines.

 

You need to show more of the system. And a pump curve with some data. I don't understand the pressures that you display.

 

Bobby

 

Unfortunately I don't have most information about the system handy, without visually inspecting or asking around for it. I'm only responsible for the instrumentation and controls (including these valves)

 

Could it be you are aspirating air? The flow pattern makes me think of a vacuum leak.

 

The vacuum is logged at several points in the system. It seems to remain steady, and this is a new system that was thorougly inspected for leaks a few weeks ago (but a leak might be possible).

 

Hi,

How is managed the level in the tank feeding the pump? 

How do you mitigate the risk to generate a vortex ?

What about cavitation or vapor lock?

 

Breizh 

 

The level is managed manually by operators. It's a large tank, the level remains steady.

I believe the tank has a feature at the bottom to prevent vortices, but I'm not aware of any other safeguards.

 

Thanks, all, for the responses. We're going to try shutting off the line through the cooler to see if that improves control.

Attached Files


Edited by jstall, 02 December 2021 - 10:23 AM.


#7 latexman

latexman

    Gold Member

  • Admin
  • 1,672 posts

Posted 02 December 2021 - 11:06 AM

Based on the last 2-3 terraces of flow on reflux and receiver, to my eye, they seem to asymptote to a steady-state value after 7-9 minutes.  Is that your opinion too?



#8 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 3,528 posts

Posted 02 December 2021 - 12:04 PM

Troubleshooting requires much more information than you have presented. So, if you need help, you need to gather information first.

 

 

Bobby


Edited by Bobby Strain, 02 December 2021 - 03:38 PM.


#9 jstall

jstall

    Brand New Member

  • Members
  • 3 posts

Posted 02 December 2021 - 01:38 PM

Based on the last 2-3 terraces of flow on reflux and receiver, to my eye, they seem to asymptote to a steady-state value after 7-9 minutes.  Is that your opinion too?

Sometimes it takes longer, like 20 minutes, but the flow does eventually reach some steady asymptote, yes.



#10 latexman

latexman

    Gold Member

  • Admin
  • 1,672 posts

Posted 02 December 2021 - 01:43 PM

And it always does it.  That suggests to me the issue is not the pumps or fluid flow, but in the instruments and/or controls.  That's my $0.02



#11 horatorres

horatorres

    Gold Member

  • Members
  • 88 posts

Posted 03 December 2021 - 10:38 PM

1. check the flow measurement and the electric signal l to the control valve positioner. I guess the valves are open manually due to the fast increase in signal. Means the operator saw a real low flow. Where he saw the disturbance

 

2. Check pressure downstream of the cooler, draw with the flow behavior.

 

3.  Record the amp of the pump, draw with the flow

 

I think that flow measurement is laying, it must be proved

 

Horacio






Similar Topics