Clearance between impeller and casing body 1

[safiamoiz]

In mixed flow type impeller pump, what should be the clearance between the impeller and casing. I am not asking the clearance between impeller wear ring and case wear ring. Rather when the impeller is going to rotate in side the casing body, what should be this radial clearance. My pump is a vertical turbine pump and the impeller is worn out at its outer periphery. At this outer edge the the angle is 10 degree. Any advice or literature in this regard please.

Moiz

 

[JJPellin]

You should provide a cross-section of the pump to clarify. Pumps in my plant of this type do not typically have any close clearances between the OD of the impeller and the inside of the casing. I have attached a sketch that I made when I was analyzing performance of a large vertical turbine pump. There are no critical radial clearances other than the wear ring.

Johnny Pellin

 

[DubMac]

safiamoiz,

I think the term you are referring to is "Tip Clearance". JJPellin is right that it is not much of a consideration in a true vertical turbine pump. But you say yours is a mixed-flow??

In mixed-flow and axial pumps the discharge periphery of the impeller blade will run against a replaceable liner that is bolted into the bowl and is essentially a replaceable "wear plate". This is the clearance I think you are talking about and it is very much a proprietary value among manufacturers and varies quite a bit according to your service. Call the manufacturer with the serial# of the pump and ask for the tip clearance for that pump; they may hand it out or not.

I used to be very involved with large circ water pumps like this in power plants. Pretty much on the large machines the tip clearance was the call of an expert pump repair mechanic rather than any data from Engineering. If there was excessive bumping, liner and impeller tip damage, then we opened up a bit. If you could build no head or power was exceeding motor size, then we closed it up.

If impeller/propeller is in decent shape, this clearance can many times be adjusted by shimming the hub against a shaft shoulder. I've known expert mechanics that heat up the blades and essentially bend the angle up or down as required; those guys were good...

If there is quite a bit of impeller damage and needs repair, you can build-up weld the impeller/propeller tips to add diameter and then machine, or replace liner with a larger one (usually very expensive if a large pump).

May all be way beyond the scope of what you were looking for; your question reminded me of a fascinating time in my career. Watching true artistry and creativity from guys with gloves on.

 

[CaracasEC]

I agree with the above infos. We have a lot of vertical turbine pumps and during inspection/overhauling, the clearance of the impeller and casing wear ring are being mandatory as per OEM

 

[Artisi]

As already indicated a sketch or drawing of the pump impeller / bowl assembly needs to be provided - as there are a number of configurations for mixed flow pumps and without this info we cannot answer your question correctly.

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.)

 

[safiamoiz]

I am attaching the drawing for clear understanding. The red arrow points at the gap between IMPELLER & SHROUD. I want to estimate this clearance. The pump is 30 years old and we are unable to trace the original manufacturer. The original manufacturer has closed its business.
We estimate the original gap as 0.125 inch. This has increased to 0.500 inch now. How this gap is going to effect the pump performance. I understand that the head & flow would decrease. Does it is going to create any unbalanced forces on impeller, in radial direction especially. Our shaft bearings are wearing out. I would like to know if this excessive clearance has any indirect effect on pump shaft or on bearings.

Moiz

 

[Pumpsonly]

So you have an open impeller mixed flow pump.But it looks more like a axial flow pump.
The clearance between the impeller blade and the shroud can be adjusted using the 3 piece rigid coupling between pump and motor shaft.
The clearance will affect the output / efficiency of the pump and is specified by the manufacturer taking into consideration of the down thrust and the shaft stretch an is usually state in the name plate.

This is the most important step for a start up engineer to perform.

If the shroud is worn out an uneven, vibration will be your problem.

 

[1gibson]

How do you estimate 0.125" original clearance?

The clearance should be zero with the pump uncoupled, impeller resting on liner. Impeller should be machined to the exact same profile as the liner. Then you set the lift (maybe the lift is 0.125") but the clearance will be a smaller number due to the angle of the vanes/liner.

If the liner was completely vertical, lifting 0.125" would not change the clearance at all. The liner is not vertical (you said 10* angle) so calculate the clearance with trig. It will be pretty small.

Basically as soon as the impeller contacts the liner and some material rubs off, it is all downhill until you repair and balance the impeller. Once that impeller wears, your balance is ruined and that is the reason bearings will go. Especially with an overhung impeller like this (no bottom bearing.) 0.500" (is that on the radius!?) seems an extremely large number.


P.S. that is a pretty good drawing for a 30 yr old pump with manufacturer out of business.

 

[DubMac]

You have a pretty good sized pump there safiamoiz; from the flow and speed, I am guessing 54"-60" ?? 1/8" tip clearance sounds very reasonable for that size machine. In terms of performance, a UNIFORM tip clearance is more important than worrying about whether the clearance is 1/8" or 5/32". Your worn impeller is out of balance and will cause more problems than losing a point or two of efficiency.

If you are going to regain the 1/8" clearance you will have to buildup-weld the blade tips and then machine them back to ORIGINAL DIAMETER. Because of the shroud angle (10 degrees), every "x" reduction in blade diameter must be offset with approx. 5 times "x" movement in vertical direction. This will screw up your bearing settings if you reduce blade diameter very much. especially with your enclosed lineshaft. And the tail bearing cap will most likely be the limiting factor in the amount of overall vertical travel.

 

[clay87]

for what it's worth, our large vertical circulator (~250kGPM) is lifted 0.340" with an actual blade/shroud clearance around 0.050".