Changes in Pump Performance with Adjustments in Pump Lift.

[sandriver]

I have a one stage mixed flow deep well pump which uses a closed impeller. In our last performance test, we were 0.10 of a pound low from our acceptable limit. I instructed the that the pump lift be adjusted from 5/8" to 3/16". Past performance tests have shown that an icrease of 0.75 lb has occured when this is performed. This adjustment did increase the discharge pressure by 0.50 lbs. My question is are there any papers, articles, etc. that show that this is the case (or not)? What I have read indicates that there is no effect if the impeller is a closed impeller. I believe that what they mean is that there is no negligible affect, however for a 16,000 gpm pump operating at a discharge head of 60 feet, an incrase of 0.50 lb in discharge head in this instance is significant and affects operability. I made sure that the test method was correct, and that the precision test gauges were in cal.

 

[checman]

sandriver
With enclosed impellers you do not have an end clearance per say. You have wearing ring clearance instead. It appears that your wear rings are worn to a point they should be replaced. By lowering the impeller you are most likely increasing the insertion of wearing rings and that gives an addition pressure drop which decreases the slippage past the rings. You may also be inserting areas of the wear rings that do not have the same amount of wear because they where not in close proximity to its mating ring which also decreases the amount of slippage. You must be careful because sudden changes in discharge pressure or other service conditions may cause the impellers to contact the bowls causing a major problem and or a more expensive repair. The design clearances are there for a reason.

Regards checman

 

[JJPellin]

There is one other possibility. We have pumps that sound very similar. Ours are single stage, vertical turbine water pumps pumping about 15,000 gpm at 50 psi. Ours also have radial wear rings. But in addition to the radial wear rings, our pumps have a lateral wear ring that sets off the end of the impeller ring. This lateral ring is rubber coated aluminum and the proper lift setting for this pump is only 0.030". Our mechanics did not realize that the pump had the lateral ring and were setting the lift at 1/4" for many years. Dropping it down to the proper lift setting significantly increased the performance. Otherwise, the previous reply was absolutely right. You are just inserting more of your wear rings and using some of the less worn area to reduce wear ring losses.

 

[sandriver]

I accounted for thermal growth, thrust, and shaft stretch in my calculations, increasing the insertion of the wear ring increased the annulus depth, and should lead to less internal recirculation. My question is; is there a correlation between the wear ring area depth and the change in efficiency (i.e. discharge head)? Past performance testing using different float adjustments has yielded slight increases in performance. The pump is new and all wear ring clearances were in spec.