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Unread January 13th, 2020, 05:05 PM   #1
prpatel21
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PID question

Hello I am trying to control speed of the 5hp vfd pump, based on the differential pressure sensor. i am trying to write a rslogix 5000 pid loop so it regulates the pump speed based on differential pressure. regulate speed so Delta P should never be more than 22psi. sensor range is 0 to 100 psi.

Can someone explain me relation with DP and pump speed and how to set a pid loop parameters.
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Unread January 13th, 2020, 07:33 PM   #2
Peter Nachtwey
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The pressure is not linear. The pressure increases with the square of the flow.
The resistance to flow also plays a part. Look at the link below.
https://www.engineeringtoolbox.com/p...ves-d_635.html

If you are only trying to control at one pressure you can just assume the pressure will be linear enough at that pressure.
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Unread January 14th, 2020, 09:56 AM   #3
prpatel21
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Thanks, but i am just trying to understand the relation between differential pressure and pump speed and understand how they co relate. Once i understand i want to setup pid so pump speed is regulated based on change in differential pressure. DP might change because of multiple zones turning on and off.
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Unread January 14th, 2020, 10:15 AM   #4
parky
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Usually, a differential pressure sensor relies on the difference across some type of orifice or filter, etc. If you have a pump driving into what could be a dead-end if all valves are closed this is not a good idea. having a return with restricted flow is often used and PID control of the pump speed depending on pressure between the feed pipe and return pipe is quite easy, however, PID could probably not respond quickly enough with multiple valves opening & closing without overshoot/undershoot. too be honest I looked into this and it was easier to use a reverse acting hydraulic control valve this is self-regulating very much like a pressure control valve used in steam regulating systems
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Unread January 14th, 2020, 11:21 AM   #5
drbitboy
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Quote:
Originally Posted by prpatel21 View Post
Hello I am trying to control speed of the 5hp vfd pump...
Is the pump centrifugal, positive displacement, or summat else?

And what does the process, of which the pump is a part, look like?

What kind of response times would be satisfactory?

Where are you measuring the differential pressure e.g. across the pump, or across a restriction or orifice meter or long length of pipe?

Last edited by drbitboy; January 14th, 2020 at 11:26 AM.
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Unread January 14th, 2020, 11:22 AM   #6
Peter Nachtwey
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The pressure will increase with the flow squared if the the restriction to flow is constant.
https://en.wikipedia.org/wiki/Orifice_plate.

I usually control pressure by controlling the valve as parky suggested. Valves respond much faster than motors with a lot of inertia.
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Unread January 14th, 2020, 11:58 AM   #7
danw
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The measured DP might not be flow - could be some other pressure the OP's trying to maintain. 22psid is not a common maximum flow element pressure drop, although it could be . . .
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Unread January 14th, 2020, 02:28 PM   #8
prpatel21
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Basically DP is between supply and return on a hot water system. regulate pump to 22psi dp is probably overkill, but that set point is adjustable.

I just want to control the speed of the pump so dp never goes above not 22psi but lets say 10psi.

here is a sequence. When pump is commanded on, it starts at 100% speed because dp sensor is reading 0psi at that time, as system gets flow going dp reads a value, if dp reads above 10psi(user adjustable) pump slows down to a point where the dp shouldnot be more than 10psi and regulate pump speed based on the dp value.

just was trying to figure out pdi parameters for that.
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Unread January 14th, 2020, 02:50 PM   #9
celichi
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Quote:
Originally Posted by prpatel21 View Post
Thanks, but i am just trying to understand the relation between differential pressure and pump speed and understand how they co relate. Once i understand i want to setup pid so pump speed is regulated based on change in differential pressure. DP might change because of multiple zones turning on and off.
Go to the MFG and get the pump curve.
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Unread January 14th, 2020, 04:01 PM   #10
cjd1965
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on start up consider running PID in manual mode to prime the system. You may need to experiment such as 25% for 2 minutes or whatever suits your system, this will prevent your pump going 100% because the return sensor pressure is 0
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Unread January 14th, 2020, 04:05 PM   #11
celichi
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on start up consider running PID in manual mode to prime the system. You may need to experiment such as 25% for 2 minutes or whatever suits your system, this will prevent your pump going 100% because the return sensor pressure is 0
This is a good idea, also make sure you implement bump-less transfer.

PV move to PID SP and PID CV Tracks the pump speed.

I usually do this by controlling the pump manually within the PID loop.
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Unread January 14th, 2020, 04:30 PM   #12
drbitboy
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Quote:
Originally Posted by drbitboy View Post
Is the pump centrifugal, positive displacement, or summat else?

And what does the process, of which the pump is a part, look like?

What kind of response times would be satisfactory?

Where are you measuring the differential pressure e.g. across the pump, or across a restriction or orifice meter or long length of pipe?
Actually, all of that is probably irrelevant, although it might provide rough guidance. Peter Nachtwey's answer, that it is linear enough, is the best. I think the suggestion behind that is that you can set up your process to a stable state, with the pump running on manual at a speed that maintains the DP at, say, 18PSI. Take note of the speed setting. Then change the speed up manually and gradually until the DP is 22 PSI, and take note of that speed setting. Now drop the speed back to the setting that yielded 18PSI in a single step, and log how the DP responds. That is your characteristic response, and, though very rough, from there you should be to able apply any of several algorithms* to set some initial PID parameters.

Because without knowing the process, none of us can "explain [... the] relation [between] DP and pump speed."

Also, controlling pressure with pump speed is, as parky suggested, rare, even for a VFD.

* The best would be to send Peter or someone similar a purchase order.
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Unread January 14th, 2020, 04:43 PM   #13
celichi
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Quote:
Originally Posted by drbitboy View Post
Actually, all of that is probably irrelevant, although it might provide rough guidance. Peter Nachtwey's answer, that it is linear enough, is the best. I think the suggestion behind that is that you can set up your process to a stable state, with the pump running on manual at a speed that maintains the DP at, say, 18PSI. Take note of the speed setting. Then change the speed up manually and gradually until the DP is 22 PSI, and take note of that speed setting. Now drop the speed back to the setting that yielded 18PSI in a single step, and log how the DP responds. That is your characteristic response, and, though very rough, from there you should be to able apply any of several algorithms* to set some initial PID parameters.

Because without knowing the process, none of us can "explain [... the] relation [between] DP and pump speed."

Also, controlling pressure with pump speed is, as parky suggested, rare, even for a VFD.

* The best would be to send Peter or someone similar a purchase order.
Not looking to pick a fight, but from my experience in process control for water application, I have seen very few pumps that are linear, and the same can be said for pressure control valves. Would be nice if they were linear, then you would not need a PID, you could just use a look up table.

Hence the pump curve can give you a good idea for CV limits.

Using a VFD and speed control to control pressure is very common. Unless you want to waste energy and use a pump chase or pressure control valve.

Pump type plays a major factor in your control strategy (Cent. vs Multi-stage vs PD).

If you really want to figure this out, try to get a demo of Control Station's Loop Tuner, and 90% of your loops are done in 10 minutes.

Most of my loops are typically Flow-Press cascading or Press only.
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Unread January 14th, 2020, 07:28 PM   #14
SD_Scott
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Not sure what type of pump and motor combo you have, but there may be a published NPSH curve for that pump. It will show you flow and DP. If you know the suction and discharge pressures you can calculate DP. Then you can control on suction, discharge or DP(flow). This is common in the pipeline industry. It's called a low select. You select the PID based on which one has the lowest CV or error for example. If there is only one pump station, you probably don't need a select.

Forgot to mention, you can look up Pump Affinity Laws.

https://en.wikipedia.org/wiki/Affinity_laws

Last edited by SD_Scott; January 14th, 2020 at 07:50 PM.
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Unread January 14th, 2020, 09:37 PM   #15
Gene Bond
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The application sounds like a normal HVAC hot or chilled water loop pump. A simple PID setup for differential pressure is very common here, given that there are typically several zones which open or close. When all zones close, the pressure is satisfied easily and the pump shuts down.

Simple enough to manually tune. Most often, the default PID values in the drive's PID loop is very close and only needs small adjustments. Rolling your own in the PLC is overkill, IMO. But, I see it all the time, and am forced o adjust the accell/decel in the drive to tune the loop (integral) since the PLC dude is never around when you need him... 'The drive is hunting'... Yep, just like it's being told to. 'Well how are you going to fix it?' Hang on while I change the laws of physics!
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