High Suction Pressure 2

[RS82]

Could somebody please explain to me the reason behind removing the back wear rings and balance holes on a high suction, end suction pump? I know it is to balance thrust force, but how does this happen in theory? By having higher pressure between impeller and throat bush are we not having a higher thrust force going towards the impeller?

Also, if the application is very light, SG <0.4, we want the stuffing box pressure to be close to discharge..should we have a close clearance throat bushing or should we open it up and have an orifice on the plan 13 line to back up enough pressure?

 

[bimr]

The thrust bearing for single stage overhung pumps is subject to axial loading caused by exposure of one end of the shaft to suction pressure and forces on the impeller due to the differential between suction pressure and discharge pressure. When single stage overhung pumps are used for suction pressures in excess of 250 psig (17 MPa), it is common practice to “unbalance” the impeller wear rings. This is done by making the diameter of the back wear rings smaller than that of the suction side wear rings, or by eliminating the back wear rings completely. This produces an unbalanced axial thrust load in the opposite direction to that created by high suction pressure and, thus, decreases the net axial load on the thrust bearing.

SUBJECT: Controlling the pressure in the stuffing box area

http://www.mcnallyinstitute.com/04-html/4-10.html

 

[JJPellin]

It is desirable to have the shaft in tension, rather than compression. The high suction pressure acting on the end of the shaft would tend to put it into compression. Removing the back wear rings lets discharge pressure on the backside counter this thrust and put the shaft back into tension.

In the example you describe, I would leave a standard throat bushing clearance of about 0.035" on the diameter and orifice the plan 13 flush line to keep the seal chamber pressure high.


Johnny Pellin

 

[RS82]

Why would it be better to have the shaft in tension as opposed to compression? Whats the benefit?

Also, by having a higher pressur between the impeller and throat bush, are we not have a net thrust towards the impeller? How does this really balance?

Sometimes I hear they plug only one or two of the balance holes why would they do that?

Thanks for your help folks!

 

[BigInch]

Tension won't buckle the shaft. A smaller shaft diameter can be used.

What would you be doing, if you knew that you could not fail?

 

[JJPellin]

The last example that I worked this issue had front and back wear rings of the same diameter. So, the pressure on each side of the impeller was approximately the same. These forces cancelled each other out. There was one force left that was not balance. The suction pressure acting on the end of the shaft where the impeller mounts was not countered with an opposing force on the other end. For my pump, the suction pressure was almost 400 psi and the shaft area was almost 5 square inches. This gave a thrust load of almost 2000 lbs. with the shaft in compression. We removed the back wear rings and plugged the balance holes. The additional force of the discharge pressure acting on the back of the impeller produced a force toward the impeller eye of about 2,500 lbs. Thus, the shaft is running in tension with a net thrust load of about 500 lbs. That is much better than a shaft running in compression with a net thrust load of 2000 lbs.

Some applications restrict the balance holes but do not eliminate them. The only time I have done this is for services in vacuum where I want to maintain positive pressure in the seal chamber. This is trickier. You have to try and balance the leakage across the back wear ring with the pressure drop across the restricted balance holes to try and hold some intermediate pressure between suction and discharge. The balance holes are not likely to change over time. But the wear ring clearance will. So, the flow through the balance holes will change as the pump wears and the seal chamber pressure will also change.


Johnny Pellin

 

[bimr]

See page 9-134:

http://85.185.231.196/mekanik/CD_3/pump handbook/petrolmum industry.pdf