Bolton Triple Marine engine – boiler feed pumps

[mike robinson 2Participant @mikerobinson2]

Ref: Model Engineer August 15th 1986 page 204 (drawing) and 205 (build text)

Before I blindly follow the drawings, I have a question about chambering of the boiler feed water pump of which there are 2 on this model.  The drawing shows a 7/32” diameter reamed cylinder which is relieved to ¼” diameter. The remaining bearing length is .812”.  The build text says such chambering can be omitted because of the potential air locks and the author, Mr Bertinat, then says he has reduced chambering to a minimum. I assume chambering, with some air trapped, provides some damping from hydraulic shock in the boiler feed system? What would be wrong in reducing to the counterbore depth to where I have put the red line? Or even a little higher at 11/16’’ deep? That would give 1 1/8’’ of bearing surface or 38% more than as designed. I appreciate your guidance

Thanks, Mike

 

[Charles LamontParticipant @charleslamont71117]

Can’t see any point in it being any higher than the top edge of the 1/8 port hole. You don’t want any air in a boiler feed pump because it will expand on the suction stroke and compress while trying to deliver. With enough air it can stop the pump feeding at all. If an air cushion is needed, it should be a chamber downstream of the pump delivery valve. This would be at a (more-or-less) constant pressure and does not affect the pump.

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