Design Error on Leak compound Edwards Air Pump?

[malcolmd]

I am building one of the AA Leak compounds (the smaller one with designs provided by Camden Books), this post might only make sense for someone familiar with this design.

I think there is a problem with the air pump design/drawings...

This looks like it's not a problem with the drawings (there are a fair number of these, which I have been tracking http://sy-befur.co.uk/leak-hints-and-tips/), so much as an oversight in the design...

The outlet flap valve assembly is not fixed/located by anything vertically within the outlet chamber... neither is the lower valve plate sealed against the lip it rests on above the cylinder (this is - part 3 on P20 of "the book" - (the part with the holes))....so (as I noticed when I completed a trial assembly) the whole valve (both plates and the rubber washer) move up and down with the piston rod, and even if the vacuum held it down, I feel sure any hard won pressure difference would be lost as the air leaked back into the pump round and under the valve plate.

My current plan is to make a nylon "block" which is a light push fit in the outlet chamber, has "ports" to let the air-water out to the delivery port, and is slightly longer than the available space, so that when the top cover is fitted it will press the lower valve plate down onto a gasket I am fitting under the valve plate....

does anyone have any better ideas/opinions? am I missing something?

[steamboatjack]

Malcolm,

Yes you are correct, as usual Mr leaks drawings leave out details.
First the hole in the plate can be a clearance hole not reamed as per the drawing.
I would say your idea is sound, this will prevent the plate lifting and also reduce the volume above the valve a bit. Make sure the insert cannot rotate & block the outlet port.
Incidentally this is a genuine Edwards type pump, these were designed for surface condensers and are therefore the lowest part of the system so the condensate runs into the bottom, probably not so efficient when using a keel condenser? Other users report them as noisy in this context.

Regards
Jack

[malcolmd]

Thanks Jack, a little confirmation is worth a lot...

I am planning to cut a large grove around the insert to ensure if it rotates the port will not be blocked...

On the noise front others have suggested that running the pumps on the engine from the cross head at up to ~500rpm is responsible for the noise, so I am gearing them down 3:1 (see this note on the topic from the blog http://sy-befur.co.uk/2015/01/19/retirement-beckons/). Hopefully this will help to silence them.

On the condenser front, I am torn between a keel condenser (I have one ready to fit) and building a surface condenser, for the reasons you state - some have suggested that some "foot valves" near the condenser will overcome the problem, but I am not sure the pump will be able to prime itself without being "flooded".... thoughts?

[Dhutch]

Struggling to fully visualize what is being talked about, is the one of the 'double acting edwards pumps' or 'modified edwards pump' I have seen on other leak engines including our own? Otherwise called a Frogg pump I think, effectively the top half of an edwards pump, at both ends. Or is the design for a true edwards pump?

I currently have a thread on the non-engines section (didnt know if this counted as an engine or not...) on the same topic if you want to look.

Daniel

[johngriffiths]

An Edwards air pump is single acting and the coned end of the piston descends into the coned bottom cover displacing the water at high velocity around the piston through ports cast in the cylinder wall. The water, having insufficient time to run back, is moved away to the valve plate which is below the top cover.

Unless an Edwards pump is flooded you will not get a vacuum. To give mine something to pump at all time I have a pipe with regulating valve leading from hotwell to condenser. Also, Arthur Leak's design of delivery valve sometimes failed to seat if the pump lifted much water so I fitted a light stainless spring. Another problem encountered was dishing of the valve which occurred twice so I fitted a 1/8" oil resisting rubber sealing disc beneath the valve plate. There was a problem with air pump "hammer" overcome by fitting an air vent on the top cover of the pump, the vent pipe leading to the hotwell.

With the surface condenser and the Edwards pump mine can obtain quite high vacuum, 24"-25", at the expense of cold feed water so a bypass on the cooling water to the condenser feeds the cooling plate which forms the ash pan floor.

On his Leak Geof Newton used a keel condenser with an Edwards pump but I'm unsure how it performed overall although he never seemed to have a problem on Windermere.

Not relevant to this thread but starting/simpling valves are called starting valves on marine engines.

John G

[malcolmd]

JohnG, thanks for the reply... there are some points here which prompt of further questions/thoughts:

1 - I like the hotwell-to-condenser feed idea, although I am leaning more towards a surface condenser solution which I am guessing will obviate the problem (if the pump is the lowest point) However, I am going to post some questions on my condenser plans in a minute.
2 - The "dished valve" is an interesting point... I just tried out the pump in the kitchen sink and initially thought it had seized after one upstroke, but of course what had happened was that the flap valve was closed and the "piston ring" I had fitted was working, so it was attempting to pull a vacuum above the piston and this is clearly going to put some stress on the valve plate. I did not full understand how your

fitted a 1/8" oil resisting rubber sealing disc beneath the valve plate.

would help overcome this - can you explain?
3 - I'll keep a watchful ear for any signs of hammer...
4 - at least cold feed water will give the feed water heaters in the Yarrow something to do although we might have a "lukewarm well"

Mal

[johngriffiths]

Didn't intend to confuse, Mal; take out 'plate'. The rubber disc is between valve and valve plate. Rubber was also used in full size practice as were thin brass discs.

On full size marine plant and the small machines we use temperature of feed water had/has a considerable effect on steam raising.

Water seals are quite effective on the pistons of Edwards pumps and are, of course, a form of labyrinth seal.

John G

[malcolmd]

OK John, yes I understand - currently I have fitted a hi-tech rubber seal cut from a motorcycle inner tube - hopefully it will survive a while....

As to the seal on the piston, unfortunately the air pump was the only component that the people I purchased the casting set (2nd hand from) had done any work on - and the cylinder is about 20-thou oversize, so the piston was a really bad fit - so I decided that I would fit an o-ring seal as a last resort, as the size of the o-ring is quite large (3/16th inch) and playing with it I think there is a chance that it will not get chopped up in the ports... if it does I will need to make a new piston, but scrapping a 1/2kilo of phosphor bronze piston is a significant waste...

Also, if you are interested, I have published some pictures of the nylon block (and the o-ring) on Befur's blog as part of the "issues with the drawings" section....

thanks again for the comments....

p.s. please also look at my questions on surface condensers, just published - any answers or thoughts gratefully accepted

[steamboatjack]

Malcolm,
If you are running & air and feed pumps at reduced speed, you may find that the air pump if not both are of insufficient capacity??

Engine power: Camden states the engine to develop 11.2 IHP at 120PSI and 450RPM I DISPUTE THIS.
My calculation (as published in this forum some time back)
gives 6.1 IHP at these parameters.
You would need a more realistic pressure of 175PSI & 550 RPM
to develop 11 IHP.

Regards
Jack

[malcolmd]

Yes, I agree on both counts: I made a spreadsheet to compute the required capacity, and increased the ram's to 0.5inch which should be enough, I'm hoping the air pump will manage, as it really does not need to shift much water and I will try to keep air leaks to a minimum.... time will tell...

and yes (based on your and other input), I am intending to run on a boiler pressed to 225 or 250psi (a watertube Yarrow) but steam chest pressure will be a bit lower, and allow the engine to run at up to 550rpm...

Mal