×
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!
  • Students Click Here

*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

Jobs

ON/OFF control

ON/OFF control

ON/OFF control

(OP)


How does a BMS or PLC ontroller control the sequencing ON and OFF of say 4 nr. pumps that are trying to maintain a set point pressure in a pipe of say 2 bar. The usage of water and a subsequent drop in pressure will call for more pumps to maintain the set point. How does it do that in control logic?  I know how it would work if it was a single pump on a variable speed drive but how does PI control work when you have say 4 stages of ON/OFF. It would also expalin how say 4 nr cooling towers in parallel with fans on them stage ON/OFF to maintain a set pont return temp??



 

RE: ON/OFF control

With a four stage controller

The Model UCS-421 is a solid-state multi-stage device
used for floating/tri-state control of VAV boxes, or multistage
control in HVAC systems from a single analog signal.
The UCS-421 can be used to obtain a digital output
from a voltage or current producing sensor. In its base
configuration, the UCS-421 has four relay outputs. Units
may be "daisy chained" to provide additional stages of
control. The UCS-421 is supplied with mounting track
for easy installation.

go here: http://www.hvacc.com/pdf/kele/UCS-421.pdf

RE: ON/OFF control

Remp,

First note: these pumps should have a DP control bypass valve (sort of a poor man's VFD) if you want some kind of automated pump staging.

With this type of setup and this type of variable flow control: at a standard operating point with 3 pumps maintaining a 20 psid set point with the bypass valve fully closed; the shift to the 4th pump should occur after the DP set point diminishes to say, 18 psid with the bypass fully closed after X amount of time.

The stage-back to 3 pumps should be the point that with 4 pumps running and the bypass open X percent (found out by testing) produces a DP of, for example, 16 psid.

With what you have:

Test with a fixed load to see that (for example) 3 pumps produce 20 psid, 2 pumps produce 15 psid, and 4 pumps produce 24 psid: Allow more slop. If the loop DP set point is 20 psid and you are at 13 psid for X amount of time, add a pump. If you are at 25 psid for X amount of time, drop a pump.

The cooling tower scenario is easier as fan operation doesn't have an immediate impact on CWS temp.

CB
 

RE: ON/OFF control

Assuming all 4 pumps are VSD controlled, all pumps would be controlled in parallel as per single pump control.  If the VSD speed drops to minimum speed, turn one of the pumps off, if the VSD speed increased to maximum speed, start another pump.

Not that operating 4 pumps in parallel would result in a very flat pumping curve making pressure control more difficult.

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