Sites: GlobalSpec.com | GlobalSpec Electronics | CR4 | Electronics360
Login | Register
The Engineer's Place for News and Discussion®

Previous in Forum: Pumping Problem   Next in Forum: FORMULA TO CALCULATE M/C TIME
Close

Comments Format:






Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







13 comments
Power-User

Join Date: Jul 2008
Location: India
Posts: 457

Centrifugal Pump Impeller - Casing Clearance

12/09/2008 12:38 AM

how can i understand the clearance between impeller and casing. both side of impeller.

__________________
thoughts becomes things.
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Anonymous Poster
#1

Re: centrifugal pump impeller and casing clearance

12/09/2008 12:54 AM

http://www.centrifugalpumpguide.com/centrifugal-pump-impeller.html

(Welcome back Sandeep - I haven't seen any questions from you in awhile)

Register to Reply Score 1 for Good Answer
Power-User

Join Date: Jul 2008
Location: India
Posts: 457
#3
In reply to #1

Re: centrifugal pump impeller and casing clearance

12/09/2008 11:22 AM

Thank you very much that you are missing me.

I left the company. waiting for new challanges.

so no work , no observation, no questions.

feeling sick without work.

Frankly speeking somebody ask me this question in interview and i dont have that answer.

still i dont understand how can i measure that 0.003" gap?

how much it will affect the performance of pump.

__________________
thoughts becomes things.
Register to Reply
Anonymous Poster
#4
In reply to #3

Re: centrifugal pump impeller and casing clearance

12/09/2008 10:45 PM

I am sorry to hear that - hopefully you get back to work soon.

The front clearance between the impeller and the casing is set to achieve an acceptable pump efficiency. If this clearance is allowed to grow, either through corrosion or erosion, the pump will lose efficiency. More power will be required to achieve the same flow rate from the pump. This is caused by increased recirculation from high pressure zones to lower pressure zones across the front of the casing. While this recirculation may not be great, the flow disturbance also contributes to a loss of efficiency. The back clearance must also be controlled to allow the pump-out vanes to do their job, which is to reduce the pressure at the back of the impeller. If this clearance is allowed to grow outside of tolerance, an increase in axial thrust can lead to early thrust bearing failure.

Register to Reply
Anonymous Poster
#2

Re: Centrifugal Pump Impeller - Casing Clearance

12/09/2008 10:21 AM

how can i understand the clearance between impeller and casing. both side of impeller.

What I have come to conclude that you are asking the magnitude of the clearance to be maintained between the impeller neck ring and the casing wear ring. It is by thumb rule should be maintained .003" per inch of the neck ring diameter.Clrearance should be restored after it reaches the value double of the original.

Register to Reply
Associate

Join Date: Apr 2006
Posts: 41
Good Answers: 3
#5

Re: Centrifugal Pump Impeller - Casing Clearance

12/10/2008 12:42 AM

Rear clearance can be measured with feeler gauges, after impeller installation and before the volute is installed. Front clearence can be measured with a special clay used by engine builders to measure piston-to-head clearance. The down-side is you have to apply the clay to the impeller, assemble the pump, dis-assemble the pump and measure.

The quick & dirty method is to install the volute w 0 clearance, and back off the shaft & impeller just a hair. This method ignors rear clearance.

__________________
Finish your beer! There are sober kids in Africa!
Register to Reply
Associate

Join Date: May 2008
Location: Seoul, Korea
Posts: 34
Good Answers: 1
#6

Re: Centrifugal Pump Impeller - Casing Clearance

12/10/2008 4:03 AM

Pump is the mechanical device that changes speed energy to pressure energy.

Impeller expose the speed energy to the fluid inside while casing changes its energy to pressure term.

So gap (clearance) will leak back the discharged fluid to suction side again which will lead to low efficiency and flow loss.

Impeller is rotating component and casing is static one. So, Gap and/or clearance is a necessary evil.

Reducing clearance will improve internal recirculation susceptibility and improve efficienty also. On the other hand, Exteremely narrow clearance will occur excessive eddy current between impeller and casing and it will cause erosion. (This is the reason why we have to have replaceable impeller and casing wear ring. If it is eroded, you can change and it is new pump.)

Due to the configuration of almost impellers, it has two clearance. One is suction side and the other is discharge side. You can understand the same manner with one difference. Suction side wear ring is exposed to suction pressure and discharge side one is exposed to discharge pressure.

You can measure this clearance very easily. I forgot its name. There are many metal plates with proven thickness. If you insert it one by one, you will know the gap's size.

In practical, there is no apparent (or noticeable) efficiency drop when the wear ring is eroded not much. Such efficiency drop frequently misunderstood as efficiency drop by other reason since it is quite small. It is recommended to dismantle the pump while plant turn around.

One more tip. Since wear rings are always under eddy (strong or weak), one side (either casing side or impeller side) wear ring must have higher hardness than another one. This will ensure that only weaker side will be eroded and you can save spare part money.

Register to Reply
Power-User

Join Date: Jul 2008
Location: India
Posts: 457
#7
In reply to #6

Re: Centrifugal Pump Impeller - Casing Clearance

12/10/2008 7:48 AM

please send me the pictures if you can to measure this clearances.

is it similar for all type of pumps or its varies as per the pump.

is it mentioned in pump manual.

__________________
thoughts becomes things.
Register to Reply
Power-User

Join Date: Feb 2008
Location: Northeast Ohio, USA
Posts: 263
Good Answers: 9
#8

Re: Centrifugal Pump Impeller - Casing Clearance

12/10/2008 8:57 AM

Sandeep, It appears that you have been given ample information about the reason for the close tolerance between the impeller and the suction side of the casing as well as the backside of the impeller to the casing. However, you sound like you still are apprehensive regarding the measuring of those tolerances. As someone suggested, you can accomplish this measurement with "feeler gages". These are small strips of metal with the actual manufactured thickness imprinted on them. You insert these metal strips, using alternately increasing thisckness into the space between the volute and the impeller. When you are unable to make the strip enter the space, then that strip is too thick. Go to the next smaller strip and re-insert it into the space. That will be the thickest strip you can insert. The fit will be snug but not requiring you to force it into the space. Read the thickness stamped on that strip and it will be the width of the space. If that measured width is more than you are allowed, take the strip with the allowable number on it, insert it into the space and adjust the space to the thickness of that strip. The adjustment will then have made. Some pumps have jack bolts to move the shaft assembly with the impeller attached for adjustment. Some require the installation of shims to accompish the adjustment. Consult the pump operation and maintenance manual for the correct way to accomplish that task.

The backside of the impeller is not as important to the efficient operation of the pump as the suction side. That is due to the fact that the backside is, in actuallity, another "mini-pump". The backside has what are called "pump-out vanes" which actually act as a small impeller to continuously pump the medium away from the seal area to 1) - reduce the pressure in that area and 2) - remove debris away from the seal so that it doesn't cause harm to the seal or shaft. Therefore, the small amount of leakage that may be experienced in that region is minimal.

Good luck in your search for a new position. Hope it works out well for you.

__________________
All that is necessary for the triumph of evil is for good men to do nothing. - Edmund Burke
Register to Reply
Power-User

Join Date: Oct 2008
Location: Meherrin Virginia
Posts: 306
Good Answers: 6
#9

Re: Centrifugal Pump Impeller - Casing Clearance

12/10/2008 9:49 AM

You could be asking one of two questions, the purpose of the wearing rings is to provide a seal between the suction and discharge areas of the pump. If you have reason to measure these it is usually accomplished during disassembly of the pump and is done best with inside and outside mikes.

If you are looking for casting clearances between the casing and the impeller, now you have to get inventive. In the turbine business we use anything pliable that will hold it's shape. This is not a usual check on pump internals because the clearances are liberal and fairly easy to predict.

__________________
If you fail to follow through, you will fail.
Register to Reply
Associate
Hobbies - CNC - New Member

Join Date: Dec 2008
Location: Winnipeg, Manitoba, Canada
Posts: 38
Good Answers: 2
#10
In reply to #9

Re: Centrifugal Pump Impeller - Casing Clearance

12/10/2008 2:16 PM

"If you have reason to measure these it is usually accomplished during disassembly of the pump and is done best with inside and outside mikes."

If your pump has radial wear rings this is true. I think most of the other comments assumed axial wear rings, which can be adjusted for clearance, and can usually be checked through a handhole without dissassembly.

As far as clearance on the outside diameter of the impellor, in a volute casing it is desirable to keep the clearance between the impellor plates and the cutwater to a minimum. Impellor vanes are sometimes machined to a smaller diameter to change the curve.

Best regards

Rick

__________________
too soon old, too late smart
Register to Reply
Power-User

Join Date: May 2007
Posts: 272
Good Answers: 5
#11

Re: Centrifugal Pump Impeller - Casing Clearance

12/11/2008 2:23 AM

have a look at this site, hope this can help u

http://www.lightmypump.com/

S

corrosion prevention

Register to Reply
Anonymous Poster
#12

Re: Centrifugal Pump Impeller - Casing Clearance

12/24/2008 5:13 AM

my dear

i think the clearance between the impller and the casing for centrifugle pumps is 0.015":0.030"

thank you

Register to Reply Off Topic (Score 5)
Power-User

Join Date: Jul 2008
Location: India
Posts: 457
#13
In reply to #12

Re: Centrifugal Pump Impeller - Casing Clearance

12/24/2008 11:32 AM

Is it same for expeller type impeller and wear ring type impeller.

__________________
thoughts becomes things.
Register to Reply Off Topic (Score 5)
Register to Reply 13 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (4); otha (1); RickZillman (1); rockbit (1); sandeep lokhande (3); strider6 (1); The Commoner (1); yeonhoyu (1)

Previous in Forum: Pumping Problem   Next in Forum: FORMULA TO CALCULATE M/C TIME
You might be interested in: Centrifugal Pumps, Wellhead Equipment, Industrial Fans and Industrial Blowers