Mamod safety valve

Gentlemen,

I was given A Mamod TE1A Traction engine for my Birthday when I was fourteen and forty two years later I want to refurbish it, I would like to make a new safety valve, I am aware I could buy a new one but I have the means and ability to make and test one, the operating pressure is 10 - 12 psi, my only problem is how do I calculate the spring required and again I could measure my old one but then I learn nothing.

Martin P

Edited By martin perman on 25/06/2020 14:50:36

Edited By martin perman on 25/06/2020 14:52:02

I had one of their stationary engines bought at a jumble sale back in the sixties. The screw in safety valve/ water filling plug had a very poor seal. I cut a short section out of a bike pump connector to replace it. I guessed the length, not having any means to test it and was lucky not to have blown myself up.

Force is pressure times area. If you had 10psi and an area of 0.1 square inches you would need a spring applying 1lb of force. This 1lb would be from an initial compression so you need to know the spring rate, the relaxed length and the assembled length. The area is probably the inside area of the rubber o ring's hole. I think the likely area is a lot less than 0.1 so a spring applying a lot less than 1lb will be required.

Martin C

As Martin says but remember a safety valve has hysteresis. The force on the spring to get it to open initially is as stated but as it opens the spring compresses (closing force increases) and the area exposed to pressure increases (so opening force increases) so unless you choose a spring with the correct 'rate' the boiler pressure drops below the lifting pressure before the valve closes - wastes precious steam. To determine the rate you also need to estimate the lift and calculate the force from the steam pressure on the increased area - this isn't so easy as the steam is expanding at this point too where it is throttled by the valve so its a dynamic state rather than a steady state. Not so easy to calculate and the reason why on many miniatures the valve lifts at say 100psi but does not reseat itself until the pressure has dropped to say 80psi or less. One reason why you see people clouting model valves with shovels lol. 5 psi hystereresis is probably pretty good for model valves in general use although pop type valves can be tuned better.

Paul.

Tubal Cain's (T D Walshaw) Book on springs in the Workshop Series is an invaluable source of information, worth buying if you want to know how to make springs.

Andrew.

If the old safety valve still exists, then refurbishing it would be a better way to go than redesigning it.

Mamod safety valve is quite weak so fit a soft spring and try it if it to high when you trial it it's no big deal the pressure if to high will push the piston off it's seat

H

Thank you all,

I will take on your thoughts, I dont want to redesign the valve just make a new one, the spring being the main issue.

Martin P

Hi Martin, what thread sizes did Mamod use for the boiler fittings? (Whistle, valve, drain). Dad has a couple or three engines with the various bits missing.

Steve

Steve,

Good afternoon, the safety valve and whistle are 1/4" BSF and the water drain plug is 3/16" BSF.

Thank you,

Martin P

I don't what you reckon your time is worth, but new Mamod safety valves are only about a tenner from Forest Classics and no doubt others as well.

This talk of safety valves brings to mind a friend who used to work in Stuart Turner Models when they were still in Henley. One day a school teacher came in with one of Stuarts oscillating engines with a boiler. He had some issue with the engine, but the chap in Stuarts was horrified to see that the teacher had removed the spring and ball from the safety valve 'because it kept sizzling" and replaced it with a screw and nut. The teacher(?) got a good telling off before he left. I hope he learned something!

Maurice

An excellent series on Spring Design was in MEW 234-237 by Linton Wedlock. It included useful charts and good advice.

Neil

There was a series in EIM about safety valves and springs, and many years ago one by Bill Hall in ME. Basically you have to ensure that not only will the spring keep the valve shut until it reaches operating pressure, but it will then open wide enough to release all the steam the boiler can make without allowing the pressure to rise too much. Probably not a big problem with a Mamod boiler.

Nick,

Its not about how much my time is worth its the fact that I'm capable of making one, I dont make engineering models I make parts whilst restoring old stationary engines to get them running again and making the safety valve is easy enough but I have to get the spring correct otherwise it will leak or damage the boiler hence my question.

Neil,

I dont have any access to ME magazines.

Martin P

Edited By martin perman on 26/06/2020 18:14:24

Maurice's story is reminiscent of the first boiler explosion in North America.

The fireman on the "Best Friend of Charleston" objected to the noise of the safety valve, so he tied it down.

He paid for his error with his life.

Post WW2, a large B & O articulated loco had a boiler explosion. The huge boiler ripped off the chassis and turned end over end, tearing up the track at the same time. A frightening picture!

Steam at 50 psi is at about 170'C, from memory, and so contains a LOT of heat energy, which it gives up when it condenses. Which is why it will strip skin from people nearby.

Any pressure vessel should be treated with respect., even if the pressure is only a few psi.

Remember the work extracted from atmospheric pressure in Newcomen engines.

Howard

Don't bother calculating it Martin.

Instead make the valve adjustable my having a threaded stem (towards the end) with two nuts. This is how some other manufacturers made theirs, Fleischmann comes to mind I think.

Find yourself a nice stainless spring out of a pen and give it a go. If its a bit soft you may have to look around a bit but it is likely you will find it works fine.

There are far too many parameters to design a Mamod SV without encountering tolerance problems; with a little adjustability you can be spot on.

Roger,

Thanks for that, never even crossed my mind, the simple things are always the best.

Martin P

You are more than welcome, not a new idea though.

Here we go:

You can do a lot worse than copy Kevin:

https://www.ita-shop.co.uk/ourshop/prod_5496551-Mamod-14-BSF.html

https://www.ita-shop.co.uk/ourshop/cat_1283563-SAFETY-VALVES-cc-pg_2.html