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Old November 1st, 2018, 03:07 PM   #1
sparkie
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Pump House Controls - Line Pressure PI(D?)

So went to check out a pump house job the other day that feeds roughly 150 residences. There is a storage pit/tank that holds water from 4 separate well pumps with a 4-float system. There is also a pair of series pumps with 7.5 HP. They mount vertically on the pumps (but I have not gotten information off of the pumps themselves yet). These pumps are running via a control loop with a VFD and pressure sensor on the pump outlet line. I'm wanting to know two things:

In particular, what is the relationship between the motor/pump RPM and the line pressure? I'm wanting to look more at the components of this control loop before I decide if we should take the job, and I have never done an RPM/Line pressure control loop before.

On a non-technical note, how do you guys handle it when a customer is demanding something be done in a specific manner, even though you know it will lead to issues? An example would be that on the 4-float system in the tank, the bottom float (low-low), in my opinion, should be an emergency float that shuts down the system if the tank level gets too low and throws an alarm to prevent the system from running dry, however the customer is wanting the pump to just run at a very slow RPM if the water level drops below the last float.

I've dealt with well-storage-pump systems before, but I've never had a couple of the curveballs that this one was thrown.
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Old November 1st, 2018, 05:57 PM   #2
mdim
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Maybe the things from slide 17 can help you a bit:
http://www.swowea.org/plantops/1-Pump%20Curves.pdf

Other than this, what do you mean by slow rpm. If it moves water, you ca run dry and get damage, if you don't move, you feed energy (heat) and again can damage something.
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Old November 1st, 2018, 06:47 PM   #3
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I agree with you completely, you have high and low warning for process control actions i.e. the tank is low level so reduce pump speed, and then you have high-high and low-low which are process interlocks i.e. tank is too low, stop the pump.

In my opinion the low level should be used to reduce pumps speed, and the low-low should be used to shut the pump down. If the customer thinks they can do a better job, then let them do a better job!

If you really can't see an alternative, then atleast use a switch in the pipework before the pump to detect water. If this switch is de-energised, then interlock the pump.

Regarding the pressure/rpm the type of pump you use can have an influence on this, for example the outlet pressure from a centrifugal pump is influenced by the pressure at the inlet, so varying tank levels can affect this.
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Old November 1st, 2018, 07:00 PM   #4
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It is fairly common to have a constant pressure system for water supply to an area. It is all but mandatory to shut down a pump if a low level condition could result in cavitation.

In your case, the wells should be called to prevent that from happening, but in a case where the wells fail to keep up, you might be able to switch to a lower pressure setpoint which may result in reduced output of the pump station. I am not sure how much time that would buy you if any. But I agree that you shouldn't set up a situation where the pump can run dry.
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Old November 1st, 2018, 07:20 PM   #5
sparkie
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I should be a bit more specific. There is a holding tank with the 4 float switches. This is fed by 4 well pumps that are just ran from contactors. I appreciate that document mdim, I'll give it a read tonight. Perhaps I can dig up some documentation on the original system. I'm curious, though, as to why they would series the two pumps unless it was intended for them to run together when the site was constructed. How does that flow work if you have two pumps in series like that? You have to forgive me, there as I have not dealt a ton with fluid pressure systems in this sense. I'm also open to mathematical explanations . It would be fun to put some of this calculus and diff EQ I've spent the last year or so learning to use so I can get an idea of how this PID loop would function.

Also Okie, I saw that you use the P2K processors from AD as well. Have you installed any in caustic environments? I was thinking about using one here, but I'm also thinking about just going with an MLX 1400, as I know that they are tough old birds and can handle the environment just fine. The price would be roughly the same, the P2K is just more modern and has those features to boot.
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Old November 1st, 2018, 07:22 PM   #6
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I would agree with your thoughts on the LL level action to shut off the pump. But at the end of the day if it's their equipment and they understand the risks, then it's their decision and I would go with that.

I would expect the PID tuning itself to be fairly straightforward.
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Old November 1st, 2018, 07:55 PM   #7
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in most cases I would agree there is little value I running a pump at slow speed but there is 1 value and that is to keep the motor moving and keeping it heated to prevent condensation from building up in the motor. Most large water pumps have heaters mounted in the frame to keep condensation from forming, some place use a small DC current through 2 motor winding to heat them when not in use. You can buy them


as for what to with a client as you described well some jobs you just have to walk away from there just not worth the problems if you do decide to stick it out cover you but get everything in writing and signed of on by the client for when it hits the fan
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Old November 2nd, 2018, 12:28 AM   #8
Peter Laing
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Hi Sparkie,
Pressure is proportional to torque on a centrifugal pump. So if you configure your VFDs to torque mode you'll get the best performance. You might not even need to set up a PID loop. However, you will probably find that a PID pressure control loop will work fine with the VFD in normal speed mode. I've had good success controlling flow rate from centrifugal pumps using a simple PID loop driving a speed mode VFD.
Cheers,
Pete
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Old November 2nd, 2018, 03:42 AM   #9
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Quote:
Originally Posted by GaryS View Post
in most cases I would agree there is little value I running a pump at slow speed but there is 1 value and that is to keep the motor moving and keeping it heated to prevent condensation from building up in the motor. Most large water pumps have heaters mounted in the frame to keep condensation from forming, some place use a small DC current through 2 motor winding to heat them when not in use. You can buy them
I agree with that. I know even some of the VFDs have built in this function to provide some current to the windings for heating and drying. Never used it but I've seen it in manuals. Just not sure which exactly, if it is PF755 or Danfoss or something else.
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Old November 2nd, 2018, 09:07 AM   #10
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Sparkie I worked for a large water utility out west for 11 years as an industrial electrician/programmer. Even the electricians had to have distribution certifications through the state. When there are more than about 30 or so (I can't remember exactly) users (homes, businesses, etc.) water distribution systems fall under state regs and laws, based on the safe drinking water act. My system was in a large city, and almost all the distribution was "paralleled" so to speak, so if one area had a pumping station down or lost power, then water pressure came from another part of the system to keep pressure up.
Your customer probably wants the pump to run at low speed with a very low level to prevent back flow issues, which are very bad as far as health and safety concerns. At certain pressure levels, the water utility is required to warn residents and issue a "boil water" directive, which is huge as far as PR, and a real no-no.

You may wish to research this a little more to see if this is a job you want to take on. Just my experience and hope this helps.
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Old November 2nd, 2018, 09:24 AM   #11
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Quote:
Originally Posted by saultgeorge View Post
Your customer probably wants the pump to run at low speed with a very low level to prevent back flow issues, which are very bad as far as health and safety concerns. At certain pressure levels, the water utility is required to warn residents and issue a "boil water" directive, which is huge as far as PR, and a real no-no.
Good thinking.... how about adding another float then, one for slow speed then the all stop float
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Old November 2nd, 2018, 09:59 AM   #12
Tom Jenkins
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Quote:
Originally Posted by sparkie View Post

In particular, what is the relationship between the motor/pump RPM and the line pressure? I'm wanting to look more at the components of this control loop before I decide if we should take the job, and I have never done an RPM/Line pressure control loop before.
Pump flow varies directly proportional to speed. Pump discharge head varies with the square of the speed. The relationships are known as the affinity laws.

https://www.engineeringtoolbox.com/a...aws-d_408.html

https://www.linkedin.com/pulse/apply...y-tom-jenkins/

This relationship doesn't apply to a current operating point - the whole pump curve must be redrawn and then the system curve is drawn over the new pump curve. The intersection point identifies the new operating flow and head.

Your system, unlike the one illustrated in the engineering tool box site, has a high proportion of static (constant) head to friction head (parabolic). That means that at a fairly high speed your pump won't make enough pressure to move any water. This is known as dead heading. You need to keep your pump above the dead head speed or you will build up heat, ruin the seals, and possibly destroy the pump.

And the low-low float switch is there to stop the pump.

Last edited by Tom Jenkins; November 2nd, 2018 at 10:01 AM.
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Old November 2nd, 2018, 01:27 PM   #13
GaryS
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Without the benefit of seeing the full system I would offer this.
Why not use a level transducer with a high and low float switch as an override.
With the transducer you can them just set the levels from an HMI
You should also set up a lead lag pump configuration along with an equal run time setup to increase reliability.
You could also eliminate the PLC altogether by using the Yaskawa Q-Pump VFD’s they take care of the controls without a PLC.
There are many options to chose from.
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Old November 7th, 2018, 11:48 PM   #14
sparkie
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Thanks for the info so far guys. They just blew another pipe up, so I'm going to go through what they have at the moment and make it, well, not blow up lines. I think the PID with the VFD and the pressure transducer needs checked out. I won't be able to start on a new system for about a month, though. In the mean time, are there any not-so-obvious specs to look for while I'm checking out the piping and such?

I'm going to get all of the pump info I can. They said they had some documentation on the original system, so I'm hoping I'll get an idea of system head from that. I also need to get tank volume and level above pump inlets and the information from their well pump motors.

I also sat down and spoke with the customer. Now that he has a plan to take back to the owners to get this issue resolved he is in better spirits and I bought myself some time to dig into this. Also, no one that services this equipment has any kind of license. My state is generally pretty willy-nilly about regulation.
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Old November 8th, 2018, 05:42 PM   #15
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What pipe if blowing? Like, between the pump(s) and tank or after the tank?
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