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Why VFD pumps produce flow more than rated flow at low speed
2

Why VFD pumps produce flow more than rated flow at low speed

Why VFD pumps produce flow more than rated flow at low speed

(OP)
I newly joined here and hope to get help. We have three identical centrifugal pumps with VFD parallelly installed in a system. The pumps are designed and tested at 560 M^3/hr @ 100% speed (1750 rpm); But these pumps could get 676 - 699 m^3/r at only 58.9-86.7% speed. What did it happen? Does the system curve too flat ( I don't think the throttle valves are installed on discharge piping). What should I do for that? Thanks a lot!

RE: Why VFD pumps produce flow more than rated flow at low speed

What controls the discharge head on the pumps and what is the discharge head at 100% and what is the discharge head at the reduced speed?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

Building on what Artisi said, determine the actual head that the pumps need to develop based on field measurements and compare that to the design point of the pumps.

At this point with no data I realize it's speculation but I wouldn't be surprised if you find out that the "design point" the pumps were designed to operate at (x m3/hr @ y m head) is quite a bit different than the conditions they are actually operating under.

RE: Why VFD pumps produce flow more than rated flow at low speed

What was the suction head at test conditions?
What is the suction head at your installation?

Mike Halloran
Pembroke Pines, FL, USA

RE: Why VFD pumps produce flow more than rated flow at low speed

At low flow the system piping (without additional controls) provides far less resistance, roughly proportional to (q/Q)1/2 thereby allowing speed and discharge flow to increase, otherwise known as "run away" flow.

Get those throttle valves installed and working and you should return to normal.

RE: Why VFD pumps produce flow more than rated flow at low speed

The pump at 100% speed produces 575m3/hr at a differential head = H1
At 60-90% of full speed, it produces 680-700m3/hr at a differential head = H2

This can happen only if H2 << H1. If this should be corrected, then find out why imposed backpressure / discharge pressure is so low at low speed.

RE: Why VFD pumps produce flow more than rated flow at low speed

Just to summarise, the flow will be a result of the head imposed by the system (head,friction etc). The design and factory testing of xx@yy is rarely ever met as an installed condition, but that is another discussion.
If the site requirements are being met in terms of flow, the head and speed are irrelevant. If site head is lower than design, flow will be higher. It is not unknown for a pump spec.to be wrong or to have a wrongly specified / selected pump.
The site conditions need to be analysed and the pump selection to be re-evaluated so an engineering decision can be made to address the problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

It is apparent that someone over estimated the system discharge head.

The pumps specification sheet is accurate. The pumps are operating at a different point on the pump curve because the system head curve has less headloss than expected.

RE: Why VFD pumps produce flow more than rated flow at low speed

Oldox you don't mention head or pressure in your initial post which is key. However the data you give is very odd. If an increase in speed from 58% to 86% makes only 30 m3 difference in 600odd then this is a very steep system curve and would not seem to suit a vfd.

What is the system this pump is feeding?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Why VFD pumps produce flow more than rated flow at low speed

Oldox hasn't mentioned anything following the initial post, so either the problem is solved / gone away or wasn't really a problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
I appreciate so much here for your kind response and help! Sorry I couldn't provide some pump ops data last weekend as they are at my office. Yes, I used to witness once locally that the local suction pressure gauge showed 80 psi and discharge pressure gauge showed 250 psi. DCS recorded flow was 690 m^3/r and speed was 87% speed. The pump head is calculated as 128 ft which pumping fluid is oil emulsion. But I haven't made converted curve yet to look if it is still on the pump curve. I have some following methods I am thinking, one is just to make a converting curve to look it running point; or operate the pump at 100% speed to look how much flow it will get; or last one to make a real system curve then konw the BEP. Do I need to do all of them or just one is enough to verify the system curve is too flat? Thanks again!

RE: Why VFD pumps produce flow more than rated flow at low speed

With one speed, flow & head point you can guess. With 2 a better guess. With 4 points, 2 at different speeds and flows, should narrow it down to a pretty good idea of what's going on there.

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
I will try. But the system curve is most suspicous which I worry about the pump could not be run at 100% speed as too much flow than rated. Thanks BigInch!

RE: Why VFD pumps produce flow more than rated flow at low speed

Um, 170 psi differential head comes to 392 feet of water.
What's the s.g. of that emulsion?

Mike Halloran
Pembroke Pines, FL, USA

RE: Why VFD pumps produce flow more than rated flow at low speed

You wouldn't necessarily have to run any of those points at 100%. Try 85% and 60% speeds for example.

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Sorry, the head should be 128 meter (422 ft). S.G is 0.93

RE: Why VFD pumps produce flow more than rated flow at low speed

In some pumping systems where the pump could go down to end of curve as the imposed backpressure drops (due to some failure or change in operating conditions), is to ensure the motor HP is good for end of curve operation also - in this case, that would be EOC at 100% speed.

RE: Why VFD pumps produce flow more than rated flow at low speed

It is apparent that the system design and the actual operating conditions do not agree with each other - WHY? that is what needs to be established.
The very first step is to review design conditions compared to site conditions for variance.
Check the pump selection against the design conditions. What was specified to the pump supplier as the pump duty requirements, is this the same as the design requirements?
Did the pump supplier supply the pumps to the specified design duty.

At this stage anything else is just spinning wheels - you need to establish what was wanted against what is installed. Once that is known, any necessary changes can be reviewed to establish what should be done to correct the problem, that's if any problem actually exists or changes need to be made.
Is the pump running over capacity at a lower speed a problem?

Note that your OP asked why flow was higher and speed lower - well the answer lies in the above - once that is answered you can decide what to do next.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Thanks Artisi and georgeverghese! The pumps have had some vibration but I don't exactly know what were the running condition during that happened but I will talk to the vibration technicians. The document of original design seems not easily to be got it as it is a old plant but I will try. But the reality might have changed referring original requirment. By the way, the problem for this issue has being asked by Ops, do you think the good explanation is to creat the real system curve as I asked before although it might not be easy?

RE: Why VFD pumps produce flow more than rated flow at low speed

I just re-read the OP and it says you have three VFD drives in parallel?

If you have common inlet header and outlet header then you might just be getting one pump running at a different place than the supposedly identical parallel pump even if running at the same speed. If you have vibration that often indicates running at end of curve - too much flow and not enough back pressure from your system

Artisi has it correct (as usual) - first establish data from the pump side and the system side. Obtain as many flow / pressure points as you can and understand how the system control works and then compare.

If you can draw out a schematic of your system that would help us and you understand what is possibly happening here

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Why VFD pumps produce flow more than rated flow at low speed

So, it's an old plant - has the overcapacity always been evident / problem?
Has anything within the circuit been replaced, removed, or otherwise altered.
Is there any flow regulating system / equipment in the circuit?

My signature at the end of the message holds true for everything in life and pump systems smile2.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

Agree, if the pump Q-h curve is flat as you say, then there would be significant difference in flow between these pumps if there is even a small variation in imposed differential head. You may need to manually chock back the pump that is running EOC for the time being - ideally, there should be an FIC loop on pump discharge on each of the pumps (FIC modulating VFD). If there is a master PIC on common pump discharge, you could run these FICs' in cascade mode with setpoint reset from this PIC. Include hi limit in FC setpoint for each pump to stop the running pump from going EOC (at full speed) when any one pump trips.

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Thanks to everyone again! We don't have any control valve installed on pumps discharge piping which is different from the pump with constant speed system. If I try to avoid creating (calculating)the system curve which seems a little complicated and I have never done it before, and also if Ops could accept reduced flows, I am thinking to manually chock back pressure, as georgeverghese mentioned above, at BEP during pumps running at 100%. Then redseal those chock valves. Pumps design required system (curve or resistance) will established, pumps could continue run with VFD, but along with the system curve. Do you think it is a simple and straigt way to solve this problem?

RE: Why VFD pumps produce flow more than rated flow at low speed

A system curve is quite easy providing you can vary and accurately measure pressure and flow at the common discharge point. Two points migh get you somewhere, but three, four or more over your normal operating range wold be better. Then you need to overlay with your pump curves to see how flat or steep they are and where they intersect

Only then can you see where your issue lies.

We can't see your system or know where the pressure gauges / transmitters and flow figures are being measured, hence it is impossible to say much more.

I don't actually know what your problem is - is it the vibration / running at end of curve or what? Throttling and running at 100% just wastes energy, but yes it could work to get you back onto a better place and depending on how far out you are, might actually use less power due to better efficiency.

Do the work, take the readings, create the system curve and then look at the pump curves. Then the solution will become apparent.

BTW how many pumps are running at once? Does each pump have its own flow meter? A decent system sketch / PFD would help A LOT.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Thanks LittleInch, it looks you encourage me to creat the system curve and I will try . It might be more apparent to over reveiw whole system. To trigger this issue is that we have had mechanical seal and bearing failures and we figure out vibraiton is one of the factors, and then we reviewed the operation data and found this concern. We run two of three pumps most of time and they have their idividual flow meters and suction/discharge local gauges as well.

RE: Why VFD pumps produce flow more than rated flow at low speed

Do you have the pump curves for the different speed ranges, or even just one speed

Can you post them?

It sounds like you have a mismatch between pump and system and the only long term solution is to fix that issue.

Read and understand the posts by artisi and look up parallel pumping on this forum - there is lots of information there.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Why VFD pumps produce flow more than rated flow at low speed

Interesting curve - pretty flat.

Any idea why the design is 560 @ 143m , but performance test is 556 @ 150m?. Probably "within spec" but clearly you're starting with a pump delivering more than the design.

Given your pump curve stops at 619 if you're operating at nearly 700 you might run into some issues, but it is strange that the efficiency curve doesn't flatten off.

If you provide that serial no to the pump vendor you might be able to get some better information on these units.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Yes, I have sent email to pump OEM ysterday. Thanks!

RE: Why VFD pumps produce flow more than rated flow at low speed

Previous response was based on a pump with a fairly responsive Q-h curve, which isnt the case now
Send us a process description with process controls brief on what is happening downstream and we'll see if we can save these pumps - we'll have to pull some other tricks out of the bag for this one.

RE: Why VFD pumps produce flow more than rated flow at low speed

Time to employ a qualified pump engineer to review the operation and sort out all the conflicting information - such as earlier info about 80 psi inlet pressure etc.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Thanks! I will review all the system when I create the system curve, and also will discuss with ops about the downstream real requirement, such as flow and pressure.

RE: Why VFD pumps produce flow more than rated flow at low speed

Initially all that is required is an accurate flow and an accurate total head at the current operating condition and an understanding of what the initial operation philosophy and what is is currenrly, and why are the units speed controlled - was this to balance various flow conditions by a combination of 1,2,3 pumps in operation?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why VFD pumps produce flow more than rated flow at low speed

(OP)
Well, I willl discuss with ops. The system which is located at a steam plantis that the pumps pump oil emulsion from a tower where the emulsion comes from oil well to a process plant which is located around 20 km away. The system seems a bit complicated. Thanks!

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