Centrifugal screw pump impeller clearances

[oldfitter]

Hi Guys,

We have a couple of horizontally mounted Hidrostal mixer pumps on a thickened sludge storage tank. The motor, coupling assembly with the impeller were recently withdrawn from the volute and sent away for repair. ( At present, all i know is that they had been probelmatic for some time, eventually failed and were sent away for repair.)
Once the assemblies were returned to site, they were fitted back into the volutes by out craft team and put back into operation.
Ops have reported that they are both vibrating and one is especially noisy, i confirmed this when i went to site.
On site i tried to check the flow with an ultrasonic strap on flow meter ( which i have used with a good success rate in the past ) For some reason, although the signal strength was ok it did not register any flow. I looked over the top of the open tank at the mixing pattern, there was very little in the way of movement. In the past ops say that when the mixers have been working, there is a good swirling pattern in the sludge. From this i believe there is little or no flow being produced by the pumps.
The inspection cover was removed from the pumps to check for blockages. The pump volutes seemed clear and when the suction and disharge valves were opened flow from the inspection cover seemed free and uninterrupted indicating no up or downstream blockages.

After talking to the fitter who reinstalled the pumps, it seems he is unsure of the procedure for setting the impeller / adjustable suction liner clearances. On installation, he said that on tightening the mounting nuts up, the impeller jammed. Instead of backing off the suction liner, he installed another 3mm thick horseshoe washer on each stud to stop the impeller from fouling.

Conceivably, there could be almost 3mm of clearance between the impeller and liner. Could this clearance be such that the capacity of the pump is greatly reduced and if so, how much % wise per mm of clearance.

There are issues around the control of the mixers that lead to excessive starts / stops which could have led to the past failures and i am addressing those.

When the pumps are running, i'm wondering if the wear is causing the pumps to run too far left on the curve leading to loss of flow and vibration and noise.

For info, panel ammeters show the noisy pump pulls 13 amps and the other pump pulls 24 amps. FLC for both pumps is 33.6 amps. The pumps are 22kw. The operating band of the pumps is between 40lps @ 17m and 170 lps @ 5m. The pumps were originally specd to give 105 lps @11m.

Thanks in advance for any comments

 

[Artisi]

Check rotation.

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 was my first thought as well. Incredibly common to not check rotation.

I hesitate to ask this, but was the pump and motor aligned properly before they fitted the coupling as the last thing to be done, i.e.after bolting it down and attaching the pipework / dining these mods. The fact that the impeller jammed tells me something isn't right with the whole assembly.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[oldfitter]

Sorry Guys, forgot to tell you that we did check the rotation and it is ok. The reason the impeller jammed could also be that maybe the the mixers were fitted in the wrong volutes. I doubt whether they were marked up on removal from site.
Am i way off the mark thinking the clearances can have such a dramatic effect on the capacity. I've read that 0.05mm of excess clearance can reduce capacity be 1%. Hence 2mm extra clearance would reduce the capacity by 40%. This seems a lot to me. Would this apply to cntrifugal type screw impellers.

 

[LittleInch]

I think it would. I had to look up their website to see what these were exactly, but it looks to me like the clearance of the screw part against the liner is critical. A 3mm gap all the way round will add up to a large "hole" pouring fluid back to the suction side. Also looks like getting the shaft and liner centralised is also pretty crucial, but would be difficult to get as exact as it was before you took it out.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

Not an expert by any means on hydrostal but wouldn't think 3mm clearance would have a major impact on performance.
How was performance prior to rebuild compared to now?
Keep looking

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.)

 

[MikeHalloran]

The impeller looks remarkably similar to the units sold for pumping paper stock, but I don't remember them having an adjustable conical suction liner, the presence of which strongly suggests that clearance is a huge issue.

The jam on initial assembly suggests that the impeller, apparently supported entirely by a C-face motor or a bearing assembly similarly piloted to the volute, was not concentric to the suction liner, so the clearance to the suction liner is at least 3mm (axially) on the tight side, and higher by some unknown amount on the loose side. ... i.e., way too much.

IOW, most of the flow is happening within the pump. It needs to be disassembled far enough to inspect (and probably more thoroughly clean) all of the faying surfaces involved in establishing a relation between the impeller and the volute and the suction liner.

Your fitter will probably need to be issued a putty knife, a razor blade scraper, and a dial indicator kit and given good lighting and plenty of time to do the re-re-installation properly. Ask him/her to use clay or Plastigage to check the clearance between the impeller and the suction liner.




Mike Halloran
Pembroke Pines, FL, USA