mamod safety valve quotation

The 20-25 psi valve is available in a 1/4 x 26 tpi thread for standard loco boiler, usually fitted in place of whistle when used with Water Top-up Valve.

If you require an alternate blow off pressure please add your request to the comments at the checkout and the valve will be set before dispatch. Currently the allowable range for custom setting is 20 to 60 psi.

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.

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.

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.

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

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.

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.

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!

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.

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.

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.

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.

Mamod 1/4 " Long Safety valve,  this is the one that is used on Mamod Mobiles such as the traction engine and roller,   it is also used on their stationary engines produced after 1967.

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