Safety Valve Basics FAQ
Everything you need to know about safety valves, relief valves, and pressure relief devices. Understand the types, terminology, and applications.
What is a safety valve?
A safety valve is a pressure relief device that automatically opens at a preset pressure to protect equipment from overpressure. Safety valves are designed for compressible media like steam, air, and gas. They open rapidly (pop action) and are governed by ASME Section VIII or Section I codes.
What is a relief valve?
A relief valve is a pressure relief device designed for liquid service. Unlike safety valves that pop open, relief valves open proportionally as pressure increases above the set point. They are commonly used in water heaters, hydraulic systems, and liquid process lines.
What is the difference between a safety valve and a relief valve?
Safety valves are for compressible media (steam, air, gas) and open with a rapid pop action. Relief valves are for incompressible media (water, oil, liquids) and open proportionally. A safety relief valve can handle both. The key difference is the opening characteristic and the media they are designed for.
What is an ASME-certified valve?
An ASME-certified valve has been designed, tested, and stamped per ASME Boiler and Pressure Vessel Code. The UV stamp indicates certification for unfired pressure vessels (Section VIII), while the V stamp is for power boilers (Section I). ASME certification ensures the valve meets strict safety and performance requirements.
What does set pressure mean?
Set pressure is the predetermined pressure at which a safety or relief valve begins to open. It must be set at or below the Maximum Allowable Working Pressure (MAWP) of the equipment being protected. Set pressure is factory-calibrated and stamped on the valve nameplate.
What is accumulation on a safety valve?
Accumulation is the pressure increase above set pressure during valve discharge, typically expressed as a percentage. ASME Section VIII allows 10% accumulation for single valve installations and 16% for multiple valve installations.
What is blowdown on a safety valve?
Blowdown is the difference between the set pressure (opening) and the reseating pressure (closing), expressed as a percentage. A valve set at 100 PSI with 7% blowdown reseats at 93 PSI. Proper blowdown prevents valve chatter and ensures clean reseating.
Can I use the same valve for steam and air?
Not always. While some valves are rated for both, the capacity requirements differ significantly. Steam uses BTU/hr calculations while air uses SCFM. Always check the ASME nameplate for certified media. Using a valve on the wrong media voids its certification.
What valve do I need for a water heater?
Water heaters require a temperature and pressure (T&P) relief valve, typically set at 150 PSI and 210 degrees F. For commercial water heaters, you need an ASME-rated relief valve sized to match the BTU input. Aquatrol 69 Series valves are commonly used.
What type of valve do I need for an air compressor?
Air compressors require ASME Section VIII safety valves with a UV code stamp. Set pressure should match your compressor maximum working pressure. The valve must handle the compressor full output in SCFM. Aquatrol 130 Series and 88/89 Series are popular choices.
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