Trouble shooting a failed diaphragm pressure or temperature control valve can make your task of repairing the valve much simpler and less expensive. The primary manufacturers of this type of valve include companies such as Fisher, Spence, Spirax Sarco, and Watson McDaniel to name a few companies. Valves of this type can be readily identified by a large diaphragm case and external copper tubing connecting a pilot to the main valve.

When trouble shooting a failed valve, the key is to isolate if failure is in the main valve or the pilot. On all valves of this type, steam is first fed to the pilot. When the pilot opens, it feeds steam to the main valve diaphragm case which then opens the main valve.

To determine which is failed, close off the main steam supply to the entire valve and bleed off the pressure. Remove both pieces of copper tubing. One of the copper lines will run to the diaphragm case, the other will be a pressure relief line which allows the pressure on the diaphragm case to be relieved when the pilot closes.

On all brands of valve you will find two small bleed orifices located either in the copper tubing or in the tubing fittings. Make sure they are open and not plugged with trash. They will be about 1/16" in diameter so a paper clip is a good cleaning tool. If either orifice is plugged, the valve will fail open.
Next turn steam pressure back on the valve. You should see no steam from any of the open ports. If steam flows from the pilot port, the pilot is failed. If steam flows from the main valve port, then the main valve is failed.

If no steam flows from the pilot port, turn the adjustment screw to the on position and see if the pilot opens and closes properly. Usually this process will allow you to see which of the valve components has failed.

The pilot on all of these styles of valves tends to fail first so repairing or replacing the pilot is typical. If the main valve fails to open, failure is probably due to a ruptured diaphragm.
Also make sure you have a drip trap installed ahead of this type of valve to reduce failures. Have questions? Give us a call. We can provide parts for all major brands as well as repair of complete valves.