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

I have worked on a couple of locomotives that had ASME approved safety valves. As Linn said they worked a few times, lifting at the desired pressure and having about 15 lbs. of blow down. From there they went down hill. The lifting pressure dropped steadily, and blow down became an intolerable 20-25 lbs.

In most cases, if the valve lifted and knocked the steam back 20 lbs., first of all with a the engine working hard with a big fire in her, the safety valve was roaring away for several minutes before it seats again. Losing 20 pounds of pressure made a serious dent in the pulling power of the locomotive, that you had to recover in order to get the power needed to pull the hill. In addition you blow lots of valuable water out the safety valve. I read once where an open safety valve blows 20 gallons of water a minute out the valve. I instructed my fireman "do not under penalty of death pop this engine off". Of course he was instructed that a good fireman kept the stack pitch black and the safeties lifting, and he told me so. I told him to keep the engine at least 10 lbs. below the pop. If it got any higher, I would start dropping the Johnson Bar down until the steam started back, then I would hook it back up again. He gave me a black look. I told him this was MY engine for the day and he would fire it as I instructed (there I go with the "my engine" thing again). He bitched to someone upstairs, and I never ran an engine at that place again.

BTW this was at a well known former logging line tourist railroad...