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.

[mike robinson 2Participant @mikerobinson2]

Charles, thanks for the reply, that all makes sense to me. I will look at the valve block drawings but it should be possible to squeeze in a small chamber as an “add-on” if that becomes neccessary. There are more than enough parts to make as it is….

 

[bernard towersParticipant @bernardtowers37738]

Can somebody please explain the reasoning behind the enlarged chamber as for the life of me can see no reason for it at all. The several piston water pumps that I have built in the past all have parallel  bores.

[Charles LamontParticipant @charleslamont71117]

Its the way great grandad did plunger pumps, and he didn’t know any better.

In full size it would make reboring easier (and more frequently necessary)?

[bernard towersParticipant @bernardtowers37738]

So what you are saying is the drawing is uneccesarily complicated?

[mike robinson 2Participant @mikerobinson2]

It does seem strange to include a feature that could unduce airlocks then say the counter bore is kept to a minimum then give the builder the option not to include the feature. I can see no logic for it.

[Charles LamontParticipant @charleslamont71117]

I am not sure what you mean.

[JasonBModerator @jasonb]

Seems common on model marine engines, Stuart triple has a larger chamber than the ram as does the 1923 ME compound. Also the Stuart steam pump has a larger dia chamber.

[mike robinson 2Participant @mikerobinson2]

Charles, I was refering to Mr Bertinat’s text in my original post quote “some builders may prefer to omit the chambering of the bore as it provides a potential source of air locks – I have reduced it to the minimum with this in mind”

Jason, interesting to know it seems common on marine engines but is there an engineering / hyraulic reason for such chambering only on marine engines Vs other stationary engines (my Stuart no.9 water pump doesn’t have it).

I will be machining that feature at the weekend and based on Charles’s advice will stop the 1/4″ diameter just above the 1/8″ feed hole.

Thanks for all the helpful comments

Mike

[mike robinson 2Participant @mikerobinson2]

Should have tried this before, Copilot 365 AI replied with 3 resason, these are 2 of them

The larger diameter hole at the bottom of the cylinder in a vertical water pump, often referred to as a chambering operation, serves several important purposes:

Pressure Relief: It helps to relieve pressure that may build up within the cylinder, ensuring smooth operation and preventing potential damage to the pump components.

Seal Lubrication: The larger hole can facilitate better lubrication of the seals by allowing a small amount of fluid to pass through, which helps in maintaining the integrity and longevity of the seals.

[Charles LamontParticipant @charleslamont71117]

Posts crossed I think. I was asking what Bernard meant.

It would be interesting to know where the bottom end of the plunger comes to when it is at the bottom of its stroke.

Copilot has found some explanantions but I find the first does not make much sense, and the second is unconvincing.

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