How do you use Weekly Rotation Logic?
Lead and Lag are terms used to denote which pump, fan, chiller, or other piece of equipment gets used first and last respectively. In order to prevent excessive wear and tear on any one piece of equipment, when there is more than one piece available for the same job, Lead and Lag equipment need to be rotated on a regular basis. There are many different central plant configurations, and many different ways of setting up the rotations within them. Here are some ideas and examples that might help you with your own central plant equipment rotation.
Scenario 1
Suppose that you have 2 pumps that perform the same job and only one is needed at any time. But, the building manager wants to rotate them on a weekly basis. Here's one way to do it.
Step 1
Add a Software Binary Output (Event Sequence) and name it Rotation Schedule. Go to the Point Program screen for that point and put a Schedule on it. Make the Schedule come on at 10:00AM on Wednesday and go off 1 minute later.
If this point were commanded ON by Operator, it could cause problems, so you need to add a Logic Sequence to it. Make the Logic say:
If the priority of Rotation Schedule is Operator Command then auto Logic Test Rotation Schedule
This Logic should evaluate every 15 seconds and should be Priority Logic 3. This way, it will override the Operator Command and will never be ON by Operator Command for longer than 15 seconds.
Step 2
Add another Software Binary Output (Event Sequence) and name it Lead Pump. From the Point Program screen, click the field below Relay ON and change the units form Start/Stop to Pump 1 and Pump 2. Add a Logic Sequence that says:
If Rotation Schedule is ON and Lead Pump is Pump 1then Pump 2 Lead Pump wait 00:01:00 else if Rotation Schedule is ON and Lead Pump is Pump 2 then Pump Lead Pump wait 00:01:00
Make the Logic evaluate every 45 seconds. Since the Rotation Schedule is only ON for 60 seconds every week, and the Lead Pump Logic only evaluates every 45 seconds, there is no way it can change states twice in one week.
Step 3
Now all you have to do is add logic to Pump 1 and Pump 2 that starts them if they are the Lead Pump and there is a need for water to be pumped.
Scenario 2
Step 1
If you had more than 2 pumps, make the point in Step 2 a Software Analog Output. On the Point Program screen, make the range of the point 1 to 3, or however many pumps you want to rotate.
Step 2
Another way to make a weekly rotation is to add a Day of Week point, which is a Software System point. The point will have a value of 1 to 7 DOW, depending on what day it is, with Sunday being Day 1.
On the Lead Pump logic point, add logic similar to this:
If DAY OF THE WEEK is equal to 1 DOW and ST SECONDARY CHW PUMP LEAD is equal to 3 then command ST SECONDARY CHW PUMP LEAD to 1command ST SECONDARY CHW PUMP LAG to 2 else if DAY OF THE WEEK is equal to 1 DOW and ST SECONDARY CHW PUMP LEAD is equal to 1 then command ST SECONDARY CHW PUMP LEAD to 2 command ST SECONDARY CHW PUMP LAG to 3 else if DAY OF THE WEEK is equal to 1 DOW and ST SECONDARY CHW PUMP LEAD is equal to 2 then command ST SECONDARY CHW PUMP LEAD to 3 command ST SECONDARY CHW PUMP LAG to 1
If ST SECONDARY CHW PUMP LEAD is equal to 1 and ST SEC CHW PUMP 1 is Alarm then command ST SECONDARY CHW PUMP LEAD to 2 If ST SECONDARY CHW PUMP LEAD is equal to 2 and ST SEC CHW PUMP 2 is Alarm then command ST SECONDARY CHW PUMP LEAD to 3. If ST SECONDARY CHW PUMP LEAD is equal to 3 and ST SEC CHW PUMP 3 is Alarm then command ST SECONDARY CHW PUMP LEAD to 1
Important: Make the Logic Evaluate Every 24 hours. Otherwise, the Lead and Lag pumps will rotate all day long on Sunday.
In the logic shown above, the Secondary Chill Water Pump Lead and Lag Pumps are rotated between 3 pumps every Sunday. The second half of the logic is a "fail-safe" to make sure that if the lead pump is not working for some reason, the next pump in the rotation will become the lead pump. If the Lead pump is in Alarm, it means that it is commanded ON, but the Status shows that it is OFF. This "fail-safe" portion of the Logic is something that you might want to add to the logic in Scenario 1.
Scenario 3
Now let's look at an even more complicated scenario where you have 3 Chillers and you want to rotate weekly the order in which they come on as needed depending on the load.
Step 1
Add a Software Binary Output (Event Sequence) and name it Chiller Rotation Schedule, just like in Step 1 of Scenario 1.
Step 2
Add a Software Analog Output (Setpoint) called Chiller Rotation Switcher and make the range 1 to 7 on the Point Program screen. Add a logic sequence to it that says:
If CHILLER ROTATION SCHEDULE is ON then adjust CHILLER ROTATION SWITCHER by 1 wait 00:03:00
Since the Chiller Rotation Schedule is only on for 1 minute, and there is a wait of 3 minutes in the logic, it will only increment 1 number per week.
Step 3
Now go back to the Chiller Rotation Schedule point and add a Logic Sequence that says:
If CHILLER ROTATION SWITCHER is equal to 7 then command CHILLER ROTATION SWITCHER to 1
This takes care of rotating the switcher back to 1 when it gets to 7.
Step 4
Now add 6 Software Binary Input (Logic) points called:
CHILLER ROTATION 1,2,3 (1)
CHILLER ROTATION 2,3,1 (2) … through
CHILLER ROTATION 3,2,1 (6)
On CHILLER ROTATION 1,2,3 (1), add logic that says:
If CHILLER ROTATION SWITCHER is equal to 1
Then command CHILLER ROTATION 1,2,3 (1) ON
On CHILLER ROTATION 2,3,1 (2), add logic that says:
If CHILLER ROTATION SWITCHER is equal to 2
Then command CHILLER ROTATION 1,2,3 (2) ON
Add similar Logic to CHILLER ROTATION points 3 through 6 also.
Step 5
Now add points that determine whether or not a Lead and Lag Chiller is needed, then add logic to the chillers that check which Rotation point is in effect and turns on the Lead/Lag Chillers.
Here's an example of logic from a Chiller Start/Stop point.
If CHILLER ROTATION 1,2,3 (1) is ON
or CHILLER ROTATION 1,3,2 (4) is ON
and CHILLER 1 STATUS is OFF
and CHILLER LEAD PROGRAM is ON
then START CHILLER 1 CHW PUMP S/S
wait 00:00:03
START CHILLER 1 CW PUMP S/S
START CHILLER 1 START/STOP
command CHW SUPPLY SETPOINT (BUILDING) to 42 DEGF
command CHILLER 1 LOAD LIMIT to 60.0 %
wait 00:20:00
command CHILLER 1 LOAD LIMIT to 80.0 %
wait 00:15:00
command CHILLER 1 LOAD LIMIT to 100.0 %
(This takes care of starting Chiller 1, if it is the Lead Chiller)
If CHILLER ROTATION 3,1,2 (3) is ON
or CHILLER ROTATION 2,1,3 (5) is ON
and CHILLER 1 STATUS is OFF
and CHILLER LAG PROGRAM is ON
then command CHILLER 1 LOAD LIMIT to 60.0 %
command CHILLER 2 LOAD LIMIT to 60.0 %
command CHILLER 3 LOAD LIMIT to 60.0 %
command CHW SUPPLY SETPOINT (BUILDING) to 42 DEGF
wait 00:03:00
START CHILLER 1 CHW PUMP S/S
wait 00:00:03
START CHILLER 1 CW PUMP S/S
START CHILLER 1 START/STOP
wait 00:05:00
command CHILLER 1 LOAD LIMIT to 80.0 %
command CHILLER 2 LOAD LIMIT to 80.0 %
command CHILLER 3 LOAD LIMIT to 80.0 %
wait 00:10:00
command CHILLER 1 LOAD LIMIT to 100.0 %
command CHILLER 2 LOAD LIMIT to 100.0 %
command CHILLER 3 LOAD LIMIT to 100.0 %
(This takes care of starting Chiller 1 if it is the Second Chiller, among other things)
Well, it continues on from there, but hopefully you get the idea. With CBAS, you are free to make your Rotation Schedule as simple or as complicated as you need it to be. If you need examples of logic, just ask and Computrols Tech Support can get the examples you need. If it's never been done before, we can help you write it.
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