by Ron Lucier, ASNT NDT Level III
They are several types of steam valves:
- Isolation Valves
- Control Valves
- Non-Return Valves
- Safety Relief Valves
How you inspect them is different, but first, a little bit about steam. When you boil water at atmosphere it will reach 212˚F (100˚C). Put it in a pressure vessel and continue heating, adding another 960 or so BTU’s and it becomes steam, but still at 212˚F (100˚C). Pretty useless, though. Keep heating it and the pressure goes up. That heat is called enthalpy. That’s the energy steam turbine blades extract via pressure drop to make power and it’s an important concept in steam valve leakage diagnosis.
Isolation Valves
Steam is hot and many valves and lines are insulated. This is a problem for inspection. I used to have holes in insulation cut anywhere from 10 to 16 “pipe diameters” away from the valve, on the bottom of the pipe and use high temp black paint. I would also cut a hole in a convenient location upstream of the valve and within a day of startup establish a baseline.
