Sizing for Smaller Pump Impeller
Sizing for Smaller Pump Impeller
(OP)
We have a centrifugal pump that is pumping too fast for our process. The medium is water and currently it is pumping at 635 GPM. We need it to pump 425 GPM of water to our reactor. Our maintenance staff wants to trim the impeller instead of just throttling back a valve in the discharge line (or a flow orifice). I discussed with our pump vendor a procedure to get the correct pump head vs. flow rate so they can give us a new impeller size. My thoughts were:
- Read the pressure gauge right after the pump at the current flow rate of 635 GPM
- Throttle back the ball valve after the pump AND after the pressure gauge until the flow rate to our reactor is 425 GPM (by local flow meter).
- Whatever that pressure gauge reads at the 425 GPM rate is the new head that we need for impeller sizing (at full open ball valve after pump)
The vendor is saying actually we need a gauge after the throttled valve to get the head we need to size a new impeller.
From your experiences, which one of us is correct? Can you please explain why if my procedure is incorrect?
Thanks in advance.
- Read the pressure gauge right after the pump at the current flow rate of 635 GPM
- Throttle back the ball valve after the pump AND after the pressure gauge until the flow rate to our reactor is 425 GPM (by local flow meter).
- Whatever that pressure gauge reads at the 425 GPM rate is the new head that we need for impeller sizing (at full open ball valve after pump)
The vendor is saying actually we need a gauge after the throttled valve to get the head we need to size a new impeller.
From your experiences, which one of us is correct? Can you please explain why if my procedure is incorrect?
Thanks in advance.





RE: Sizing for Smaller Pump Impeller
Regards
StoneCold
RE: Sizing for Smaller Pump Impeller
If maintaining the same pump differential and delivery pressures is not important (and I assume it isn't, based on what I read between the lines regarding the application), then an impeller trim by itself can get you where you need to be. Otherwise other adjustments in the overall system might be required anyway.
Regards,
SNORGY.
RE: Sizing for Smaller Pump Impeller
Thanks!
RE: Sizing for Smaller Pump Impeller
Upstream of the throttle is "pump rating".
The pump rating has to meet or exceeds system.
If not you don't get 400+ gpm flow.
RE: Sizing for Smaller Pump Impeller
If your system is very stable with no major variations in pressure drop, trimming the impeller is the easiest, quickest and cheapest way to get what you want. But, if the system may be changing over time, I agree that a VFD would give you more flexibility. Based on what you have provided, I would agree that trimming the impeller sounds like a good option. If you have a very tight flow requirement (ie 435 gpm is OK, 450 is too much, 410 is not enough), then you may want to leave a little extra meat on that impeller at the first pass. You can always go back and trim more if you still get too much flow. In other words, if the calculation, based on the affinity laws, shows that a final impeller diameter of 8.450" is correct, I might trim it to 8.500" and test again. It is hard to put that metal back onto the impeller if you take off too much.
Johnny Pellin
RE: Sizing for Smaller Pump Impeller
Capital expenditure justification can be made on the energy savings alone.
Additionally, with a VFD there is futute flexibility built-in if process conditions change in the future.
Go back to the pump vendor and see if he will give you a set of "VFD pump curves" with an operating range for your particular pump.
Not all pumps can (safely) operate in the range that a VFD can put out.
My thoughts only
-MJC
RE: Sizing for Smaller Pump Impeller
I did look to the affinity laws and saw that flow and impeller diameter is a 1:1 ratio. So if I want to cut flow from 635 GPM to 450 GPM, I would need to cut the diameter of the impeller 70.9% (450/635*100). We currently have a 7.0" impeller in this pump, so the affinity laws are saying we need a (.709*7") = 5" impeller.
Am I doing the affinity law incorrect or does that look right to you guys?
Thanks again.
RE: Sizing for Smaller Pump Impeller
You are still looking for the intersection of the system curve with the pump curve. The affinity laws can approximate the new pump curve. But, you still have to account for the system curve. I use a simple spreadsheet program to generate a new curve for a change in speed or impeller diameter. I have attached it. You can generate new curves for different impeller diameters until you find one that crosses at your new operating point.
Johnny Pellin
RE: Sizing for Smaller Pump Impeller
There will be a minimum diameter to which you can trim - so be careful.
RE: Sizing for Smaller Pump Impeller
Nice spreadsheet. Is it public domain? Can I request using a copy?
Regards,
SNORGY.
RE: Sizing for Smaller Pump Impeller
Johnny Pellin