- Published on Wednesday, 22 April 2009 22:02
- Written by Jim Knox
Our company routinely receives inquiries from end users about their safety-relief valves.
Here are some of the most frequently asked questions...
Q: What is the proper way to install a safety or safety-relief valve?
A: Safety and safety-relief valves should be installed vertically with the drain holes open or piped to a convenient location. All piping must be fully supported.
Q: How often should I test/ inspect my valve?
A: Maintenance should be performed on a regular basis. An initial inspection interval of no longer than 12 months is recommended. The user must establish an appropriate inspection interval depending on the service conditions, the condition of the valve and the level of performance desired.
The ASME Boiler and Pressure Vessel Code does not require nor address testing installed valves. The only thing the code states are design and installation requirements, such as some valves must have a lifting lever. For instance for Section VIII:
“Each pressure relief valve on air, water over 140° F, or steam service shall have a substantial lifting device which when activated will release the seating force on the disk when the pressure relief valve is subjected to a pressure of at least 75% of the set pressure of the valve.”
Q: What mounting orientation should be used to install a safety valve?
A: Installing a safety valve in any position other than with the spindle vertical and upright may adversely affect performance and lifetime.
Q: Why is there a hole in the valve body?
A: This drain hole is required on some models by the ASME Boiler and Pressure Vessel Code. It is intended to prevent any condensate from accumulating in the body that may freeze or corrode internal valve parts and prevent the valve from opening. The drain hole should be piped away to safely dispose of any discharge or condensate.
Q: Which end should be connected for vacuum valves?
A: This is often a confusing topic. The correct installation often looks backwards from what appears to be correct. A paper instruction tag illustrating the proper connection is attached to each valve. Vacuum valves should have the NPT threads that are cast integral to the body attached to the vacuum source. See the assembly drawing for additional clarification.
Q: What set pressure should the valve be set to open?
A: Typically, the valve should be nameplate set to open at the MAWP (Maximum Allowable Working Pressure) of the vessel the valve is intended to protect. There is a tolerance to actual set pressure, which means a valve set at 100 psig nameplate may open slightly above or below 100 psig. Consult the current ASME Boiler and Pressure Vessel Code for tolerance classes and special situations when the set pressure may be different than the MAWP.
Q: Why is my valve leaking?
A: It is normal for spring-operated safety valves to exhibit leakage or simmer/warn, as the system operating pressure approaches the nameplate set pressure, typically in the 80%-90% range of nameplate set pressure. The ASME Boiler and Pressure Vessel Code does not require a specific seat tightness requirement. A certain level of leakage is allowed per manufacturers’ literature and API-527 Seat Tightness Performance Standards, both of which can be found in the Technical Reference Catalog and in the Data Supplement, summarized as follows:
- Factory Standard Seat Tightness Performance: No visible (no audible for air service) leakage for 15 seconds (30 seconds for liquid or Section IV steam service) at 20% below nameplate set, or 5 psig below nameplate set, whichever is greater. EXCEPTION: Section IV steam service is checked at 12 psig.
- API-527 Standard Seat Tightness Performance: A Functional Test Report (FTR) is automatically provided for valves ordered to API-527. See API 527 for complete details.
At very low set pressures, the ratio of the downward spring force as compared to the upward pressure force is very small. In these cases it may be impossible to achieve seat tightness.
Use soft seats for superior seat tightness, assuming the application falls within the soft seat temperature limitations. Although soft seats will typically provide a higher degree of seat tightness than metal seats, Factory Standard does not ensure bubble-tight seats, regardless of seat material.