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Old July 23rd, 2020, 06:03 AM   #16
drbitboy
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Quote:
Originally Posted by rzuhair View Post
The pump manufacturer is my customer. I am building a PLC Controlled VFD package for their Pump with sensors and Tank Level automation. The customer is adamant on keeping the pump within the curves.

The pump will always somewhere on its curves, so I find this statement very odd.



Quote:
But, if the Pressure and Flow are correct at a certain point, should I go for a lower Pump Speed if the Pump can still maintain that Pressure and Flow?

@rzuhair there have been three us now, me, then Peter, then Ron, saying the same things: if the flow is set by the process*, then throttling flow to set a discharge pressure to operate pump as max efficiency loses money; power is flow times pressure rise; etc.


* If the pump flow into or out of the tank is the control element to maintain tank level, then there is a corresponding flow out of or into the tank, which sets what the pump needs to match to maintain steady-state level. If that corresponding flow it outside the customer's target range on the pump curves, then do they expect the control system maintain target flow and let the tank either overflow or drain until empty?



Thermodynamics do not lie, and something is amiss in this thread, which is not at all uncommon.


You say the customer adamantly wants this, and also that the customer is a pump manufacturer. That seems odd, that a pump manufacturer does not understand how pump curves and system curves work. Is this customer the pump manufacturer engineering staff, or sales staff, or someone else?


#1 Why would there be a control valve in a system with a VFD pump? Certainly there needs to be a block valve for maintenance, but it would be left wide open during operations.


Wait a minute, you have mentioned controlling flow, and controlling pressure, but not tank level. what's up with that?



Is the pump feeding the tank, and the pressure is a proxy for tank level? It doesn't seem that would be the case as you say the pressure measurement is between the pump discharge and the control valve.


What don't we know?


From what we do know, it seems you may have two choices: convince the customer they are asking for the wrong thing; or, give them what they want, get paid, and move on.
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Old July 23rd, 2020, 10:06 AM   #17
Tom Jenkins
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Quote:
Originally Posted by Tom Jenkins View Post
This is obviously a centrifugal pump.

3) Varying pump speed creates flow by reducing the flow itself.
Someone rightly pointed out that the statement I made was clear as mud.

Here is a better explanation:

Centrifugal pumps have a volume associated with the impeller, dictated by diameter and vane height and geometry. When speed is reduced the impeller turns fewer revolutions per unit time. That results in producing flow, but less flow. What I probably should have just said is that flow is proportional to speed.

But there's more, as they say in the infomercials. The pump head also reduces, proportional to the square of the speed. That shifts the intesection point of the pump and system curves further, resulting in even less flow.

Hopefully this will help: https://www.linkedin.com/pulse/apply...y-tom-jenkins/
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Old July 23rd, 2020, 12:01 PM   #18
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Sorry Peter, I take that back, I was thinking of temperature not flow regarding two controllers, it's me in a mucking fuddle lol
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Old July 23rd, 2020, 02:25 PM   #19
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After following this thread since it started, a few thoughts have come together in my mind. World domination has not occurred, just a bit of clarity.

The control system is being designed for the pump manufacturer. Their purpose is not to maximize system efficiency but to demonstrate the pump performance to the satisfaction of the owner's representative. The discharge valve is used to force the pump to various operating points so that actual curves can be drawn for that specific pump. In municipal work, it's commonly part of the specification for larger pumps and requires the manufacturer to have a licensed PE oversee the testing.

Mike
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Old July 23rd, 2020, 02:52 PM   #20
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Quote:
Originally Posted by Mike Lamond View Post
After following this thread since it started, a few thoughts have come together in my mind. World domination has not occurred, just a bit of clarity.

The control system is being designed for the pump manufacturer. Their purpose is not to maximize system efficiency but to demonstrate the pump performance to the satisfaction of the owner's representative. The discharge valve is used to force the pump to various operating points so that actual curves can be drawn for that specific pump. In municipal work, it's commonly part of the specification for larger pumps and requires the manufacturer to have a licensed PE oversee the testing.

Mike
I had asked that question earlier: http://www.plctalk.net/qanda/showpos...04&postcount=9 but the OP stated that this is for a tank filling application. But I had the same thought as you. If they are qualifying the pumps then yes, they would have to load them up and verify the curves. We have built pump test stands and this is what they do.
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Old July 23rd, 2020, 03:48 PM   #21
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Quote:
Originally Posted by ndzied1 View Post
We have built pump test stands and this is what they do.



If it's a test stand then why does it have "Tank Level automation?" Also, the OP said "I do agree on achieving Total System Efficiency to be the goal."


Neither of those is consistent with a test stand; I'm not saying it's not a test stand, but neither is there is any explicit evidence yet that it is, while there is some evidence, if correctly understood, that it is not.



A test stand would take water from the tank and put it back, so there would be no need to control tank level, other than a visual check to ensure there was some level in the tank before starting the pump.


Like I said, we don't have enough information about what the actual goal is here, and there is a likely disconnect between our assumptions and reality.
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Old July 23rd, 2020, 04:21 PM   #22
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Correct me if I am wrong. No fancy math just common sense.
1)pump curves have nothing to do with efficiency, they simply show at a certain rpm what the flow would be
2)everyone knows or should know (vfd and infinity's law by now) the slower you can run the motor the less energy used
3) the process dictates what the flow requirement is so run the pump at the required flow for the process, without any flow control let speed be the flow control, if flow demands change then adjust vfd speed to maintain the flow, that's as good as it gets
4) to summarize the slower you can run the motor the less energy used, but the required flow for the process will dictate this
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Old July 23rd, 2020, 04:38 PM   #23
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[QUOTE=AkaHammer;857043]Correct me if I am wrong. No fancy math just common sense. .../QUOTE]


No correction needed.


@rzuhair: That's now at least four people now saying the same thing. Plus the possibility that this is a test stand and level control has naught to do with it.



Please explain in detail what the heck is going on with what your pump manufacturer/customer is trying to accomplish. A P&ID plus some explanation would not be a bad start.
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Old July 23rd, 2020, 05:23 PM   #24
Tom Jenkins
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Quote:
Originally Posted by AkaHammer View Post
Correct me if I am wrong. No fancy math just common sense.
1)pump curves have nothing to do with efficiency, they simply show at a certain rpm what the flow would be
2)everyone knows or should know (vfd and infinity's law by now) the slower you can run the motor the less energy used
3) the process dictates what the flow requirement is so run the pump at the required flow for the process, without any flow control let speed be the flow control, if flow demands change then adjust vfd speed to maintain the flow, that's as good as it gets
4) to summarize the slower you can run the motor the less energy used, but the required flow for the process will dictate this
Kinda sorta.

1) Most manufacturer's pump curves also show efficiency values superimposed on the flow/head curves. https://blog.craneengineering.net/ho...gal-pump-curve

3) the process dictates both flow and head. You can't separate the two. The process flow vs. head requirement is the system curve. It is usually a combination of stactic head and friction head. A pump curve without the system curve doesn't tell you what the actual flow and power are. You need the intesection point between the pump curve and the system curve to identify the operating point.
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Old July 23rd, 2020, 10:18 PM   #25
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What is the OP's or his customer's definition of efficiency?
There is efficiency of the pump's motor
The efficiency of the pump.
Then there is the problem of maximizing the area in the rectangle defined by the operating point's pressure and flow relative to the input power.
That requires a little simple calculus.
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Old July 28th, 2020, 02:41 PM   #26
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Originally Posted by drbitboy View Post
Please explain in detail what the heck is going on with what your pump manufacturer/customer is trying to accomplish. A P&ID plus some explanation would not be a bad start.
Sorry for the delayed response as I was in the field. So here's the complete application. This is a Horizontal Pumping System used for Surface Pumping in Oil and Gas mostly for Salt water Disposal. The tanks that are in the middle of nowhere collect water from Drilling sites in the vicinity and that water needs to be disposed off. So this Pump manufacturer provides the end user the H-Pumps with a VFD package. The VFD package is there to ensure that the end user doesn't abuse the operational limits of the H-Pump.

These Pumps are either leased or bought and when they come back for service/repair/rebuild, the Pump manufacturer provides their end-user a witness test that this VFD package when used with the Pump operates just like they mentioned while selling.

Last edited by rzuhair; July 28th, 2020 at 03:04 PM.
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Old July 28th, 2020, 03:04 PM   #27
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Quote:
Originally Posted by rzuhair View Post
...the Pump manufacturer provides their end-user a witness test that this VFD package when used with the Pump operates just like they mentioned while selling.

So the PLC app you are writing is to run the test, and reproduce the pump curve, or at least one point on the pump curve? So basically a way for the end-user to kick the tires?

Last edited by drbitboy; July 28th, 2020 at 03:09 PM.
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Old July 28th, 2020, 03:28 PM   #28
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So the PLC app you are writing is to run the test, and reproduce the pump curve, or at least one point on the pump curve? So basically a way for the end-user to kick the tires?
Yes. Basically, it's a Pump protection system. The end-users like to Pump beyond the Pump's capabilities as they buy a undersized Pump and make it work overtime(instead of buying a higher rated pump for a different site's specs). This results in part failure and a lot of finger pointing between them and the manufacturer.
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Old July 28th, 2020, 04:36 PM   #29
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Before you reinvent the wheel I suggest you do some research: https://empoweringpumps.com/hydrauli...dynamic-pumps/
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Old July 28th, 2020, 06:23 PM   #30
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Quote:
Originally Posted by rzuhair View Post
Yes. Basically, it's a Pump protection system. The end-users like to Pump beyond the Pump's capabilities as they buy a undersized Pump and make it work overtime(instead of buying a higher rated pump for a different site's specs). This results in part failure and a lot of finger pointing between them and the manufacturer.

So the supplier says "it was working to spec when we leased it to you, you broke it you pay to fix it."


Is that it?
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