"I've got a customer with a safety valve that weeps. Is there any common cause for this?"

Answer:

There are several causes. The most common is the set (or popping) pressure of the valve is too close to the operating pressure of the boiler or pressure vessel. A safety valve exposed to pressure at or near its set pressure will tend to weep or " simmer".

Check the operating manual for the recommended differential between the safety valve set pressure and system operating pressure. If the manual is not available, a good rule-of-thumb is to set pressure at 1) 10% of operating pressure; or 2) Five PSI; (which ever is greater), above the systems operating pressure. Remember, the greater the difference between set pressure and operating pressure - the better your valve will operate.

Here is a quick reference to the types of pressure associated with safety valves.

Operating Pressure:

The actual pressure at which a vessel is maintained in normal operation.

Allowable Working Pressure:

Maximum design pressure of a new vessel in accordance with applicable codes.

Set Pressure:

The pressure at which the valve opens.