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Can someone explain to me how this lead / lag pump configuration works?

I have three pumps (P1, P2,P3). I make P1 as lead, P2 as lag and P3 as standby. Each pump can be started or stopped remotely in DCS.

If P1 (lead) trips, P2 (lag) and P3 (standby) can starts automatically on trip of a running pump. When this happens, which pump (P2 or P3) will be lead and which one will be lag?

Please advise how should the pump configuration be made for lead/lag/standby.

Hi peter,

Just done a lead/lag/standby compressor system on a refrigeration unit. Used the following:

P1lead,P2Lag,P3Standby
if P1 fails, P2 Leads, P3Lag once P1 is back online this becomes Standby

P2lead,P3Lag,P1Standby
if P2 Fails P3Leads P1Lags, Once P2 is back onlne this becomes Standby...

P3Lead,P2Lag,P1Standby
if P3 Fails P2Leads P1Lags, Once P3 is back onlne this becomes Standby...

If the middle Pump fails in any case the pump at standby becomes lag and the pump returning from fault becomes the standby.

Just a rolling sequnce really in all honesty.

I had a manual control which allowed for manual selection of lead/lag/standby, it also included a duration running switch which changed the sequence to suit the time in service.

Hope this helps,

Si..

I am trying to configure logic for lead/lag/standby control for 3 pumps (P1, P2, P3), and need to develop a logic diagram.

>P1lead,P2Lag,P3Standby
> if P1 fails, P2 Leads, P3Lags. Once P1 is back online this becomes Standby.
>
> P2lead,P3Lag,P1Standby
> if P2 Fails, P3Leads, P1Lags. Once P2 is back online this becomes Standby.
>
> P3Lead,P2Lag,P1Standby
> if P3 Fails, P2Leads, P1Lags. Once P3 is back onlne this becomes Standby...
>
> If the middle Pump fails in any case, the pump at standby becomes lag and the pump returning from fault becomes the standby. <

Just a like rolling sequence. A manual control is also available to allow for manual selection of lead/lag/standby.

Can some logic wizard help me with this configuration?

The logic sequence is as follows:
Create three variables:
1. Lead Pump
2. Lag 1 Pump
3. Lag 2 Pump
When the controller first boots, look at any of the above variables to make sure they are not sitting at a zero. If it is, assign a default vaue to them:
Lead Pump = 1
Lag 1 Pump = 2
Lag 2 Pump = 3

Next, create a latching variable and make it turn on and off on some variable. Runtime works great here, for example:

If a timer is > setpoint
Pumplatch = 1
else
Pumplatch = 0
endif

Next, you will zero the timer when the latch point value goes to zero. Make the following additional variable:

Last lead pump
Last lag1 pump
Last lag2 pump

Next, look at the boolean logic and perform a function as such when the latch value is true and the last lead pump is 1:

Lead pump = 2
Lag1 pump = 3
Lag2 pump = 1

Next, look at the boolean logic and perform a function as such when the latch value is true and the last lead pump is 2:

Lead pump = 3
Lag1 pump = 1
Lag2 pump = 2

Lastly, look at the boolean logic and perform a function as such when the latch value is true and the last lead pump is 3:
Lead pump = 1
Lag1 pump = 2
Lag2 pump = 3

To complete your logic sequence, do the following when the latch is false:

Last Lead Pump = Lead Pump
Last Lag1 Pump = Lag1 Pump
Last Lag2 Pump = Lag2 Pump

No matter what code you use this logic in, it will work and alternate your pumps. Your final statement will write the variables to outputs. If you have software overrides, you will need additional variables and more code.

PChan,

if you were able to find some info pls, share with me thanks.

The PLC code for an Allen Bradley AB5 can be found here:

http://www.plcs.net/downloads/index.php?PHPSESSID=aece13f 6837f9c564c72e0463342e41f&direction=0&order=&directory=Allen_ Bradley&PHPSESSID=aece13f6837f9c564c72e0463342e41f

Select "Allen_Bradley_PLC5_Templates"