Im making a steam boiler for my steam engine and need to know how to make a safety valve for it.
A spring loaded ball value would work but you don't need to make it that complicated, at least on a stationary engine. All you need is a short vertical tube from the boiler, and a metal cap with a blind hole drilled in it so it fits loosely over the pipe and seals it with its weight. You can set the pressure from the area of the hole in the pipe and the weight of the cap. That's the way the valves on pressure cookers work. They usually have 2 or 3 caps with different weights for different maximum pressures. Put a rubber plug in the bottom of th hole in the cap, so it seals against the top of the pipe (which obviously needs to be cut square and polished smooth). The cap needs to be a LOOSE fit over the end of the pipe so it can't jam shut through thermal expansion etc, but if the center of gravity of the cap is below the top of the pipe it will sit in a stable position and seal properly. As the other answer said, you have to be careful with safety valves, because the boiling point of water is higher under pressure, so when the valve opens there can be a rush of steam generated. The other issue is that when the valve opens you obviously get steam released without any warning, so you may need a steam deflector (e.g a larger diameter tube surrounding the complete safety valve) so the steam can't blow out sideways and be a safety hazard to people.
You could even take a blanking fitting (as I have) and drill a small hole in the end. Then arrange it so the end of a tapered metal rod pokes fit's into/through that hole. Use a blanking fitting that's small enough, and it'll fit in one side of a thread reducer. The other side or the bore can be mated with a fitting that connects into your steam lines. The reducing fitting will still have a external thread exposed, and if it's big enough you can use that to connect up some pipe work around the metal rod, to hold a T (allowing the steam to ducted off) and after that hold guide bushes for the metal rod.
There are several aspects to safety valves. The first is determining the amount of steam you need to relieve. Second is the correct pressure setting. Third is to have a device that is reliable. Failure to do any of these things correctly can lead to serious injury and death. With this idea in mind, I can only tell you that you need to consult an experienced engineer who can work with you to get this correct. You do not want to rely on answers you will get here. I am amazed that people still provide suggestions to the question. Don't you realize that you might be abetting someone who because of their ignorance could be building something equivalent to a bomb.
There are certainly greater efficient thoughts. An aerosol can may well be greater advantageous than sturdy adequate. you're actually not likely to construct that lots tension to exceed what the can is rated for.
I'd go for Rob T.'s answer, but use a silicone rubber, not standard rubber. It is the simples and most reliable form of pressure relief, which is why it is used on pressure cookers. If you use a ball type, ensure it is stainless steel. A silicone rubber seat will eliminate the need to grind the seat to the ball, as the ball will also be damaged in that process. The spring will also need to be stainless steel.
