VFD vs trim impeller
VFD vs trim impeller
(OP)
Hello All:
I have a pump which is way oversized. The pump best efficiency point is 450 gpm at 150 feet of head
My process requirement is only about 220 gpm at 80 feet
The motor horsepower is 20 hP
I want to slow down the pump to reduce flow and head
I have two options a) Trim the impeller b) Install a VFD
Installing a VFD is quite easy, while installing a trim impeller is costly since it involves downtime and also draining the system to isolate the pump
I'm leaning towards installing a VFD unless they are cons to it
What are the things I should be mindful when using a VFD or a trim impeller
Thanks for your help in advance
Araza
I have a pump which is way oversized. The pump best efficiency point is 450 gpm at 150 feet of head
My process requirement is only about 220 gpm at 80 feet
The motor horsepower is 20 hP
I want to slow down the pump to reduce flow and head
I have two options a) Trim the impeller b) Install a VFD
Installing a VFD is quite easy, while installing a trim impeller is costly since it involves downtime and also draining the system to isolate the pump
I'm leaning towards installing a VFD unless they are cons to it
What are the things I should be mindful when using a VFD or a trim impeller
Thanks for your help in advance
Araza
Asif Raza





RE: VFD vs trim impeller
VFD's have shorter lives than contactors. You will probably have to replace it if this is an application with more than, say, a 10 year life.
VFD's require more maintenance, even if some is just basic stuff like changing of filter media.
If you're trying to save money then you need to weigh up the costs above against the saving in energy. I would also compare them to replacing the pump itself with a smaller one.
RE: VFD vs trim impeller
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: VFD vs trim impeller
[Edit]To get 80ft you will need to reduce speed to 70% of current speed (affinity rules) but flow will only reduce to 70% from current value so you will still be off peak efficiency as you will need to throttle the pump. VFDs use power themselves. Check whether the motor can a) is suitable for VFD, b) whether cooling fan can work at reduced spewed effectively.
Best bet remains a properly sized pump and motor.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: VFD vs trim impeller
Not BY 70%, it's TO 70%.
If your existing operating point is, say, at 1780 rpm, you can do the new one at roughly 1250 rpm. Or trim the impeller.
My question would be, is this a permanent change, or do you foresee the need to change conditions later? If change is possible, a VFD would make that easier.
RE: VFD vs trim impeller
I used to count sand. Now I don't count at all.
RE: VFD vs trim impeller
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: VFD vs trim impeller
Yes, this would be permanent change. I do not foresee any more changes in the future with respect to flow or head
yes, I have folks who can take care of the VFD's with regards to maintenance
I've calculated a speed of 2500 rpm to go down up to 80 feet of head. The motor is running at 3550 rpm
So all in all I'm hearing that VFD is the way to go to deal with higher head and flow
Kind regards,
Araza
Asif Raza
RE: VFD vs trim impeller
RE: VFD vs trim impeller
Very old motors do fine with VFD's due to the extra mass of the equipment, but it might be wise to change it out anyway for the better efficiency.
Motors built just before VFD's became prevalent will burn up their bearings in 3 years or less unless they are retrofitted with proper equipment such as grounding rings.
Also check the power supply can handle a VFD, Certain harmonics from the rectifier are back fed into the grid and can make things weird, especially with self excited backup generators
Been there done that...
Hydrae
RE: VFD vs trim impeller
RE: VFD vs trim impeller
Hydraulic power required for 220gpm@80ft is approx. 4.5hp for which you will be utilising a 20hp 2pole motor speed reduced.
My suggestion is you run a calculation on your idea in terms of power costs including all introduced inefficiencies of the pump hydraulics, motor and VFD compared to a correctly sized pump over a years 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: VFD vs trim impeller
" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden
RE: VFD vs trim impeller
Johnny Pellin
RE: VFD vs trim impeller
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: VFD vs trim impeller
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: VFD vs trim impeller
If this was 10-15% change then either trim (permanent) or VFD (flexible) would make sense.
But both will cost significant efficiency in the long term, I'll wager that the eff alone will pay for the new equipment.
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P.E. Metallurgy, Plymouth Tube
RE: VFD vs trim impeller
I took the OP at their word that the only options are impeller trim or VFD. I can trim the impeller in a day in my shop at a cost of about US$500. Installing a VFD would take 6 months depending on project resources and likely cost $10,000. For such a small pump, the energy savings between these options would only be few hundred dollars. Given the tight budgets for project work these days, I could not justify the VFD economically.
I really should have insisted on seeing a curve before making a recommendation. But, we usually don’t get that luxury. Trimming the impeller to minimum would certainly improve the situation. The rest could be dropped across a control valve. Even if the pump was still at 75 percent of BEP, it would probably run well there.
Johnny Pellin
RE: VFD vs trim impeller
Secondly, you can always have buffer without, again, having to change impeller.
20HP VFD won't cost may not be significant.