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