INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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.

Jobs

Refrigeration Level Control Valve Tuning

Refrigeration Level Control Valve Tuning

(OP)

Hello All,

I am in the process of doing some PID tuning on the level controllers for our refrigeration system.

I am trying to tune the Level controllers for the Economizer and Shell/Tube Chillers. The level controllers are all upstream on the vessels. The Economizer feeds 3 Chillers that maintinan a propane level to cover the tubes. The Chillers than have a back pressure controller that discharge gas back to the compressors.


Surge Vessel > Level Control Valve > Economizer >
Level Control Valve > Chiller
Level Control Valve > Chiller
Level Control Valve > Chiller

With the current tuning parameters, the system goes into a perpetual swing when there is any kind of sudden flow change in the plant. (ie surge of gas due to a PIG or dehy beds switching. The surge of Gas flashes propane at the Chillers and the level controller swings open. This will than cause a constant swing of all the level controllers until their is operator intervention.

In this kind of system does the Economizer need to have the fastest tuning to accommodate the actions of the chillers?

Control Action is Reverse
P= Gain Multiplier
I= Integral Time in Minutes
D=Derivative Time in Minutes

RE: Refrigeration Level Control Valve Tuning

Sounds like the LCV for each chiller is pulling in too much propane when there is a gas surge in order to keep up with the required high rate of vaporisation. And hence the control loop interaction between these level controllers connected in parallel as they compete for the limited feed ex the economiser.

Try modifying each level loop to a cascade LIC - FIC loop with a max flow limit SP on the FIC. Also add a ramp up rate limit on the FIC setpoint reset signal from the LIC to smoothen out the demand rate rise.

But this change will mean that during a gas surge / overload, you may need to dump the excess gas that the chillers cannot handle ( as these will be locked in at max feedrate of propane ) which would otherwise cause a temp spike in the combined exit gas from these chillers.

RE: Refrigeration Level Control Valve Tuning

Is there a master temp controller ( sensing exit gas temp from the chillers) that resets the LC setpoint on each chiller ? Temp controllers are prone to cause fluctuations in the controlled variable, and this is due to the long sensing lag inherent in the RTD and thermowell.

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!


Resources


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