Steam Safety Valve With High Set Pressure and Excessive Blowdown
A steam boiler has a MAWP of 250 psig and a required relief capacity of 12,000 lb/hr. The installed safety valve is a direct-spring valve with a set-pressure nameplate of 250 psig and a rated capacity of 13,500 lb/hr at 250 psig. During an inservice test, the valve begins to open at 258 psig, reaches full lift at 262 psig, and reseats at 238 psig. The boiler operating pressure is 225 psig. The valve has no capacity certification tag, and the repair shop that last serviced it is not VR-stamped. The inspector must decide whether the valve is acceptable, what testing and repair requirements apply, and whether the boiler can remain in service.
1. Evaluate set pressure and reseat pressure against Code expectations. Is the valve acceptable?
Read the worked answer
The valve is not acceptable as tested. The set pressure is 258 psig against a 250-psig nameplate, an 8-psi or 3.2 percent deviation; for a Section I steam safety valve over 100 psig the set-pressure tolerance is typically plus or minus 2 percent, so this is outside tolerance. It reaches full lift at 262 psig and reseats at 238 psig; the 20-psi blowdown is about 8 percent of set pressure, which is excessive for a steam safety valve and indicates the valve is not resetting properly. The boiler operating pressure is 225 psig, so the valve may simmer or leak before set pressure, which can damage the seat. I would not accept the valve for continued service. The boiler should not be left in service unless a properly set and certified valve is installed or the existing valve is repaired and retested.
2. What does the missing certification tag and non-VR shop imply for capacity and repair?
Read the worked answer
The missing capacity certification tag is serious because the inspector cannot verify that the valve has the required 12,000 lb/hr capacity at 250 psig. A nameplate rating of 13,500 lb/hr is not enough if the capacity was not certified or if the valve has been modified. The non-VR repair shop means the last repair was not performed by an organization holding the NBIC VR stamp, so the repair is not acceptable for a Code relief device. The valve must be repaired or rebuilt by a VR-stamp holder, with certified capacity and set pressure, and a new capacity certification tag as applicable. I would treat the valve as unverified until the VR shop provides documentation.
3. What actions and records are required before return to service?
Read the worked answer
Remove the valve from service, replace it with a spare that is set at 250 psig and has a certified capacity of at least 12,000 lb/hr, or send it to a VR-stamp shop for repair and testing. After repair, require a bench test for set pressure, reseat, and capacity certification; install a new tag; and record the test. The boiler may remain in service only with an acceptable valve. The inspector should complete an inspection report noting the deficiency, notify the owner and jurisdiction, and verify the replacement or repair before signing off. Records should include the valve data, test results, VR stamp, and capacity certification.