VFD vs trim impeller 1

[asifraza0]

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

Asif Raza

 

[ScottyUK]

Do you have the technical skills to configure and maintain the VFD, or will you need to hire them in?
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.

 

[Artisi]

ScottyUK is on the money, simply replace the unit with one suited to the duty, problems solved, also a cheaper solution in ongoing maintenance and overall running costs.

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.)

 

[LittleInch]

That's a big ask of either a VFD or trimmed impellor to reduce flow and head by 50%.

[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.

 

[TenPenny]

LittleInch, check your math or your grammar: "To get 80ft you will need to reduce speed by 70% (affinity rules)"

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.

 

[SandCounter]

Keep in mind you start to lose a lot of efficiency when trimming an impeller more than 15%. Also, if you are using affinity laws to determine speed or impeller size, you need to also account for lift. From your two points, I'm guessing you have between 45 and 60 ft of static head. A VFD will allow you to find the sweet spot.

I used to count sand. Now I don't count at all.

 

[LittleInch]

Ten penny. You're correct. Post edited.

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

 

[asifraza0]

Hello:

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

 

[georgeverghese]

Beware of shaft seal cooling issues that may occur at lower speeds on a VFD that operates at less than approx 50% of design speed. Check with your mechanical engineers / pump vendor. This may also be the case if the actual operating speed (as a result of variable feed flows) in plant operations is varied to match the line hydraulics / process requirements.

 

[hydrae]

Check that the motor is VFD compatible:
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

 

[lilliput1]

With VFD also call for shaft grounding ring on the replacement inverter duty motor or else pitting of the bearings will occur. Also use smaller size motor and corresponding sized VFD otherwize you may have to throttle valves to create more head so the VFD would operate at more than the 20% minimum recommended duty on VFD.

 

[Artisi]

VFD, the panacea for all problems (so many seem to believe) not withstanding the addition problems you are likely to introduce.
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.)

 

[jraef]

As to the efficiencies involved, also understand that a VFD itself is only 97% efficient at BEST, at 70% speed it may drop closer to 93% eff. So yes, the affinity law helps you reduce energy, but the efficiency losses, when the speed reduction is permanent, will add up. As a general rule, VFDs are a great way to reduce flow in many VARIABLE flow applications because they save significant energy compared to throttling valves. But for PERMANENT flow changes, they introduce undesired effects. Even though I am in the VFD business, I generally recommend impeller trimming for permanent fixed flow changes.


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[JJPellin]

In our facility, trimming the impeller would be much less expensive and produce fewer risks and problems in the longterm. VFD's are very good in some applications. But, in the situation described above, I would trim the impeller.

Johnny Pellin

 

[LittleInch]

JJ - Could you trim it enough to halve the head ? Still seems a big ask to me and I think you'll have other issues coming into play when flowing that low through an impellor and casing deigned for twice the flow.

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

 

[Artisi]

Without a pump curve, an impeller trim is not a reliable recommendation.

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.)

 

[EdStainless]

Others have said it but I'll repeat it, buy the correct size pump and motor.
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.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[JJPellin]

I agree, my recommendation is not optimal. But a VFD is not optimal either. Neither option can drop both the head and flow by 50 percent. Since almost all of my pumps have a control valve controlling flow to a set point, there will be waste and inefficiency.

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

 

[quark]

I would prefer VFD for two reasons. Starting from (450,150) you can't exactly achieve (220,80). Since head is critical, your new flow rate will be 73%, as already indicated. If 80 feet is system resistance (calculated) then it can be a higher value than what exact system resistance is. If you trim your impeller and your system resistance is lower than 80 ft, you will get more flow and consume more power. With VFD, you can do trial and error to achieve optimum running condition.

Secondly, you can always have buffer without, again, having to change impeller.

20HP VFD won't cost may not be significant.