Re: NNG: Safety Valve Question; Why are there two different types?

Until the 2017 ASME Code is published in July what we know as steam locomotive safety valves are not allowed in new construction. Post July they are legal. How National Board will treat these is yet to be seen. How jurisdictions will treat them is yet to be seen. FRA has always allowed their use. Other jurisdictions often allow one to be used if it is set to pop below working pressure with one or two new "code" valves set at MAWP (maximum allowable working pressure) and higher.

Bottom line is that steam locomotive safety valves are designed for high cyclic service. They act as a safety valve as well as a pressure regulating valve to some degree. Since the demise of widespread steam, the design was dropped in favor of valves that are technically only required to work once and be discarded, since more and more the tendency is to operate boilers well below their design pressure which is MAWP for steam locomotives. Typically, the new valves operate quite well at least once but because they are not as robustly built or held to as tight a tolerance as steam loco valves, they tend to not be reliable over a great many cycles.

Strasburg does make these valves, and because of the small quantities, they are very expensive. We only started making them in order to ensure that we had good, reliable valves for our own use. They have proven to be more popular then we thought simply because they work very well.