API Plan 23 1

[rota1]

Hi guys! Please clarify?
Here are two contradicting views on the clearances for the throat bushing in a API plan 23 environment overhung pump.
One view is the throat bushing must be installed at the bottom of the seal cavity to help isolate the seal chamber from the pump flow, close clearances are not required.Diametral clearances of 0.025 in to 0.030 in are adequate.
The other view is
A close clearance throat bushing is to isolate the stuffing box from the pumping area,it should be as tight as possible depending on the material. If the stuffing box is not isolated from the pumpage the desired cooling would not be achieved,the pressures would equalize, it would however restrict turbulence in the chamber which would have some effect on the movement of product from the stuffing box but not as effective as the tight throat bush. Also excessive throat bush clearance would create too large a duty on the heat exchanger for effective cooling (stuffing box volume plus behind impeller).
However the pump manufacturer does not recommend using a back wearing on this pump so as to reduce load on the thrust bearing, the impeller is a single suction type with no balance holes.

 

[JJPellin]

No matter what the impeller configuration, I would suggest a throat bushing as tight as possible to reduce mixing with the hot product in the pump. As you noted, this reduces the load on the heat exchanger. Some seal manufacturer's build in a tight clearance bushing of sorts right into the cartridge seal. This is nice since it will be renewed at every seal change even if the mechanic does not think to measure the throat bushing. Even if the seal has this built-in tight clearnace bushing, I will still use a close clearance bushing at the bottom of the box to further restrict the mixing of hot and cooled products. Our standard is for a 2 to 3 inch shaft to have 0.014" clearance on the diameter, renew at 0.020".

 

[Enkidu]

There does seem to be a trend towards tighter throat bushing clearances. I'm really not convinced there is a significant reduction in heat load on the cooler, since all we are trying to do is limit the intermixing of the hot and cooled liquid. Since there is no pressure difference on either side of the throat bushing, there really is no flow.
However, after spewing all that, several seal manufacturer's seem to be recommending floating throat bushings on Plan 23's. On a 3" shaft, Clearances would be in the .004" to .006" range. I have yet to hear any positive or negative feedback, or any reduction in cooling water requirments.