Impeller Trimming

[impeller1]

Good day,

We're considering trimming the impellers on our 6 vertical turbine Pumps. Pumps are consuming too much power and we'd like to reduce it by trimming them. I would like to know, is there any particular method / procedure to trimming impellers? The impellers are semi open impellers. Do you simply skim off 1/4 or 3/4 of an inch as necessary off the impeller O.D.?

Thanks

 

[1gibson]

"Too much" meaning more than design, or per design but you want to reduce power further? Any chance of impellers rubbing, check the lift setting. Rub will show up at shutoff or lower flows because of increased thrust.

Before you make the decision, increase the lift 10-20% and check. Not sure of pump size, but this shouldn't be more than 0.010" or so. This reduces efficiency, so reduces head but increases power.

But if the impellers are rubbing the liners at current lift, you can eliminate the rub and your power reduction will be greater than the increase due to lost efficiency...

If they are multistage, you can reduce labor by trimming only the top stage, do not go below manufacturer's minimum diameter.

But the procedure is trim, underfile, balance, reinstall.

 

[NGLENGR]

You should go ahead and contact the manufacturer and ask for the existing configuration's thrust calculations. I've found that having this information can be good for solving all kinds of problems. They should be able to provide it.

Changing lift is not something I've done much, but it can help.

Another thing to remember is excessive trimming of the impeller will degrade the efficiency of the pump and can be counter-productive. If there is a good pump curve available, try and plot the head at each proposed diameter and compare with existing process data. Understand what your process is doing to require "too much" power.

 

[impeller1]

Ok let me explain the full story..sorry, I should have done this from the start..the decision was made based on Engineering review that we should fit larger impellers on our Pumps. (6 single stage vertical turbine pumps rated at 8287 usgpm @ 191 feet of head)The larger impellers were supposed to give us 10,000usgpm @ 191 feet of head at around 595 BHP. We supplied the manufacturer with the pump's wet end (column pipes, suction bowl and strainer all assembled together). Manufacturer claimed that due to poor condition of bowl, pump was requiring more than 600 BHP to produce the flow we wanted. (Motors are rated at 600HP). We were able to get 9,730 usgpm @ 599 BHP. So, we settled for that, however, when we installed the pumps, we found that we had to close down on the discharge valve far too much to keep the power within 600hp and this is turn was causing too much recirculation and damaging the pump's bearings. Now, we are forced, in an attempt to save the motors and bearings, to trim the impellers. The manufacturer recommended a trim from 16.614 inches down to 16.417 inches to safely operate within the 600 horsepower limit. So this is why I would like to know what are the procedures in performing this trim and also, an associate told me that instead of trimming the impellers' O.D., you can actually leave it and trim the vanes of the impeller. I haven't spoken to manufacturer about that yet but I'm not really certain if that's adviseable as won't this affect the pump's flow passages? And won't you also have to trim the corresponding vanes on the diffuser as well?...Ok well that's the full story.

Thanks Again!!

 

[1gibson]

Trimming the vane only (and leaving the shroud) can be done on enclosed impellers, as an attempt to avoid excessive discharge recirculation by allowing the shroud to remain and help guide flow into the case diffusers.

For this situation, 0.2" trim on semi open, trying to leave the back shroud would only complicate the process. Never heard of doing this on a semi-open.


Service is water? Can you coat the hydraulic passages on the bowl to improve surface condition (and improve efficiency)?

 

[NGLENGR]

I would agree about the coatings. There's several companies (Belzona comes to mind) that offer coatings that are claimed to improve efficiency between 2-6% at BEP conditions.

I'd be curious to see how exactly the system works, because if you're turning down these pumps enough to wreck bearings, the larger impellers are well off what you were expecting.

Do what 1gibson said, and keep the shrouds as flow guides. You'll still get some recirculation at the tip depending on the system demand, but it should keep things relatively stable.

 

[JJPellin]

All of my large vertical turbine pumps have impellers with angled OD's. I would cut the amount recommended by the manufacturer. I would cut it at the same angle as the current OD. I would not leave the back shroud as I have never seen this done on this type of impeller. I would rebalance the impellers after they are cut.

We coat the bowls and columns on all of our big vertical turbine water pumps. I believe it not only improves the efficiency, but it increases the life of the parts by delaying or eliminating the corrosion.


Johnny Pellin

 

[1gibson]

Most likely will require a dual plane balance, see original balance record from OEM to confirm.

 

[dabluffrat]

Agree with all previous comments about checking the lift and coating. If the lift setting on a semi open impeller is too low you could have a rub, too high and you're losing efficiency. Semi open impellers are trimmed on an angle. I'd recommend sending in one complete bowl assembly and asking the OEM to trim and test one before trimming all. No, you don't trim the bowl diffuser vanes.

Do you have an accurate system curve, how do you know you need 191 ft of pump head?

Did you know that 76.4% of all statistics are made up...