Hey all, Long time lurker, first time poster. I am in the process of building a monotube flash steam boiler, hopefully to drive a small kart-type of car. (I am aiming for 1hp). I am in the process of pressure testing the various pieces and it is going fine, but I am having real trouble trying to figure out what kind of safety valve I need on the steam side of the coil. Basically, what I have is a vertical coil of copper tubing, with a jet of natural gas blowing up it. Water goes in the bottom as water, goes out the top as steam. I am shooting for 100 psi working pressure, and I'd obviously like to get a safety release valve or 2. I figured that I would get a 125 psi and a 150 psi. However, the steam safety valves I can find out there for that pressure steam are mostly huge and expensive. Is that the only option? There are lots of those 1/4 inch type spring valves for air compressors out there, and of course water heater valves at low pressure, but the steam ones are few and far between. What are people out there using for steam safety valves? Can I just use the air compressor type ones? Some people also suggest that I don't even need one since I just have a copper coil, and maybe that's true (I did build it this way so it would be safer than a boiler with standing water in it) but I'd rather be safe than sorry. I'd really appreciate any advice!