View Full Version : steam release valve

08-10-2005, 09:01 AM
im making a steam boiler(donkey engine)that will hold approx 1/8th litre of water, i have a piece of seamless stainless steel tube 60mm diam with 5mm thick wall for the boiler, what i need to make is a suitable pressure release valve, would any one have any info on one please?


08-10-2005, 09:07 AM
That's something you want to get correct, as a blown-up boiler would ruin your whole day.

Since you're in England....
Model Engineer magazine has had extensive discussions of boilermaking over the years. TEE Publishing sells a wide variety of modeling books, some of which must cover boilermaking, safety, etc.

08-10-2005, 09:14 AM
According to "So You Want To Build A Live Steam Locomotive" by Joe Nelson...

Safety Valve,
Required Minimum area = 0.2074 * (W / P)
W = Weight of water evaporated per hour
P = Gauge Pressure of the Boiler

For 2 safety valves,
Minimum inside diameter =
sqrt((Area + 2) / .7854)

Make sure you build your Boiler with a safety factor of 6.
Boiler Shell Working pressure
(1/6th Tensile Strength of metal * thickness of plate) / 1/2 diameter of the boiler barrel

08-10-2005, 10:06 AM
60mm?? thats a small boiler. a pressure relief valve is a simple concept, stainless ball bearing held against a seat until the the pressure is great enough to lift the spring (stainless) and hence limit the pressue the boiler can get to. the compression of the spring needs to be adjustable and they generally screw into a bushing that is part of the boiler. you can make them or buy them

check out

lots of UK links there that will sell you what you need, or there have been numerous diy examples in model engineer over the years.

if you are new to this, as SGW said, its potentially dangerous and not to be trifled with - like making a stick of dynamite in the shop, there are some good books on the subjetlike Martin Evans (ME author)and/or the one BillH mention as some good stuff.

consider going with copper for a boilder this size. stainless is the most difficult material to make a boiler out and i believe requires welding by a certified welder (to meet boilder codes).

[This message has been edited by Mcgyver (edited 08-10-2005).]

08-11-2005, 02:31 AM
I don't remember exactly, but somewhere I read about making small scale boilers and pressure relief valves. One thing that struck me was the suggestion to make darn sure you control where that steam is vented to. Pretty easy to get burned during a sudden or unexpected release of pressure.

I seem to recall also reading that the vent opening should not be too small. It's possible for pressure to keep building while the vent is open if there's any restriction to the flow rate. Superheated water can also exit this valve, so if the opening is too small, the pressure may not be released as quickly as it might need to be. I'm going from a poor memory of this here, but there was some discussion about using a hinged cap that just swings down onto the end of a hole- with some suitable gasket. Since it doesn't slide or turn in a bore, there's little chance that it could jam.

08-11-2005, 06:35 AM
the flip side to being too small is that if its too large you potentialy induce the explosion you are trying to avoid. Bill gave the formula

Ian B
08-11-2005, 07:54 AM

5mm wall thickness of stainless on a 60mm tube is *awfully* thick - that's halfway between schedule 40 and 80.

That's about the same wall thickness that we'd use to contain 1,000psi of gas pressure offshore! As long as the end caps are similarly rated and properly welded on, you'd have a (small) pressure vessel that would have a working pressure in the order of 2,000psi. Not bad for a boiler for a donkey engine...

What pressure are you aiming for? Around 60psi? From making water rockets, I know that the plastic 1.5 liter fizzy drinks bottles can take 125psi. Puts things in perspective.

As Mcgyver says, consider copper - much better heat transfer, easier to braze etc, and about 2mm wall thickness should be enough.


08-11-2005, 06:25 PM
yeh 60mm x 5mm is a bit heavy but "hey" i got it for nowt!, its an end off a gas trap so one side has been code welded, and i am a welder anyway, so ill pop another end on and "ive a boiler", i will not be showing it at any time, so a certificate is not needed, ill only fire it up once then it will be a show piece on my mantle piece, i did have one when i was a kid, a mammod and i thought now i have the machinery and the time ill have a bash at making one! actually i need a rest from making tools! its all i have done for some months now and i thought a different project was in order, i like the idea of a fully working stainless steel orniment, actually i might set the boiler to explode and give it to that fella that sold me the dodgy 3 jaw chuck! ha!


08-11-2005, 06:39 PM
One of Tubal Cain's 2 books on making steam engines has plans for a pressure relief valve.

Just be careful.