When a relief valve call comes in, the first thing I usually hear is a set pressure and a pipe size. That's a good start, but it isn't enough to put the right valve on your system. A relief valve is the last line of defense against overpressure. Getting it right the first time is worth a few extra minutes of homework.
Here are the seven details I ask for before anyone orders one.
1. Set Pressure
This is the pressure where the valve starts to open. It should be based on what the system and its lowest-rated component can safely handle, not on what the old valve happened to be stamped. Leave room between your normal operating pressure and set pressure. A valve that runs too close to set will weep, which we covered in Why Is Your Relief Valve Leaking?
2. Media and Its State
Tell us exactly what's in the line and whether it's a gas, a liquid, steam or a two-phase mix. Gas and liquid valves behave differently when they open, and the trim and sizing method change with the service. "Water" and "air" are fine answers. "Process fluid" is not.
3. Required Relieving Capacity
The valve has to pass enough flow to keep pressure from climbing, whatever the worst-case scenario is: a blocked outlet, a failed regulator, a fire, or thermal expansion in a blocked-in liquid line. This number drives the orifice size. If you don't know it, your process engineer should. Mercer's general PRV sizing sheet is a handy way to gather the data in one place.
4. Operating and Relieving Temperature
Temperature affects the spring, the seat and the body material. Give us the normal temperature and the temperature the media could reach during a relief event. They aren't always the same.
5. Connections and Sizes
Note the inlet and outlet type and size: NPT, flanged, tube fitting or something else. The outlet is often larger than the inlet. Keep the inlet line short and unrestricted, because a long or undersized inlet can make the valve chatter and chew up its seat.
6. Body and Seat Materials
Match the body and seat to the media and temperature. Soft seats seal tightly but have temperature and chemical limits. Metal seats handle more heat but may not seal as tight. Stainless bodies are common for corrosive or clean services. If you're unsure, tell us the service and we'll help you sort it out.
7. Discharge Conditions and Code Requirements
Where does the valve vent? Straight to atmosphere, to a safe location through a discharge line, or into a closed header with back pressure? Back pressure changes how a valve performs, so we need to know. Also confirm whether the vessel or system requires an ASME code-stamped valve. Your site's engineering standards will say.
For small instrument lines and analyzer systems, a compact proportional relief valve is often the better fit. You'll find options in our relief valves collection.
Get Help Matching the Valve
A-Best Industrial Inc. Fittings, Valves, and Tubing is an authorized distributor of Mercer Valve pressure relief valves. Bring us these seven details and we'll help you match the valve, set pressure and materials to your service. Learn more on our Mercer pressure relief valves page.
We're Here to Help
Call A-Best Industrial Inc. Fittings, Valves, and Tubing at 310-833-3353 or email sales@abesttube.com. We're open Monday–Friday, 8:00 AM–5:00 PM Pacific, with after-hours emergency service.