Two stage pump and L3/D4

[jet1749]

I have a Hayward Tyler 2 stage end suction overhung centrifugal pump suffering from mechanical seal and thrust bearing problems. The outboard end of the shaft is supported in a steady bush inside the pump case (similar to a wear ring)operating speed is 2950rpm. Can the stiffness of the shaft of this style of pump be reviewed by the use of L3/D4, or would it be considered a "between bearings" pump. If it is suitable for review by L3/D4, where would the "L" be calculated from? from centre of NDE bearing to mid-point between impellers, or to mid-point of outboard impeller? If it is considered a "between bearings" pump is there a comparable calculation for shaft stifness?

 

[JJPellin]

A two stage overhung pump is a very poor configuration. The addition of a tail bushing at the far end of the shaft can improve the situation marginally. But ultimately, it is still overhung and will tend to have more problems with seal and bearing reliability. We have perhasp 4 of this style pump left in our plant (out of 1400 centrifugal pumps). One of them was successfully retrofitted with a product lubricated tail bushing and the reliability of that pump was much improved. But the tail bushing is still not a true bearing and should not be considered as one when evaluating shaft stiffness. I would consider the full length of the shaft from the radial bearing to the the mid point of the farthest impeller.

The only true solution for a pump of this type is to replace it with a proper between bearings pump.

 

[Artisi]

One thought and it could be an option - is the fit a cutless-rubber bearing at the outboard end of the shaft. This would in effect become a "true" support bearing for the overhung shaft.
It probably needs modification to the existing design and is also dependent on the product being pumped.

I have seen this used in small and very large axial / mixed flow vertical line-shaft driven pumps to support the shaft on the inlet end of the impeller/s (turbine).

Although I would agree with JJPellin - change the unit to a better design.

Naresuan University
Phitsanulok
Thailand

 

[Artisi]

Just to add a bit to the previous post, the modification would be a support bearing spider in the pump inlet or prior to the inlet flange.

Naresuan University
Phitsanulok
Thailand

 

[jet1749]

Thanks for the replies. Just to add a little more information, the pump is a 1960's vintage pump on hot hydrocarbon service. Approximate dimension between NDE bearing and mid-point of outboard impeller is 20" the shaft diameter under the seal sleeve is approximately 2" giving L3/D4 of 500 which is way over the recommended maximum limit. At present the system runs on one pump and problems are reduced, however, periodically duty and standby pumps are required to run in parallel to maximise flowrates due to feedstock changes. Usually the problems occur when the system goes back to single pump operation (or should do) we usually find both pumps operating with the discharge gagged in by a control valve and both pumps running near shutoff.