A relief valve that drips, hisses or "simmers" at normal operating pressure is one of the most common calls we get. Most techs' first move is to crank the adjustment screw a few turns and walk away. Please don't. That valve is the last thing standing between your system and an overpressure event, and a weeping valve is telling you something worth listening to.
Here's how I work through it after years of chasing these down in plants and on skids.
1. Check How Close You're Running to Set Pressure
This is the cause I see most often. A spring-loaded relief valve doesn't snap from fully closed to fully open. As system pressure climbs toward set pressure, the force holding the seat closed gets smaller, and the valve can start to leak a little before it ever reaches set.
If your normal operating pressure sits right up against the set pressure, some weeping is normal behavior, not a defect. A common rule of thumb is to keep operating pressure at least 10% below set pressure, but follow the manufacturer's guidance for your specific valve. If the margin is too thin, the fix is a system or setpoint review, not a tighter spring.
2. Look for Pressure Spikes and Pulsation
Your gauge may read a steady number while the line is seeing short spikes from pumps, compressors or fast-closing valves. Those spikes can crack the valve open again and again. Watch the gauge closely during start-up and cycling, and consider a snubber on the gauge so you can actually read it. If pulsation is the problem, address it at the source.
3. Suspect Debris on the Seat
Once a valve has relieved, it can carry a bit of scale, thread sealant, tape or weld slag onto the seat. Even a small particle will hold it off seat and cause a steady drip. This is why I always tell folks to blow lines clean before installing a relief valve, and to keep tape and paste off the first threads where they can shred into the flow path.
4. Inspect for Seat Damage from Chatter
A relief valve that's oversized for the flow, or that sits on a long or restricted inlet line, can chatter: open, slam shut, open again. That hammering damages the seat and spring. Once the seat is marked up, it won't reseat cleanly, and cleaning won't fix it. The valve needs repair or replacement, and the sizing or inlet piping needs a second look so it doesn't happen again.
5. Confirm Materials and Installation
Check that the seat material and body material fit your media and temperature. A soft seat that's swelling or hardening in service will leak. Also look at the piping: a heavy discharge line hanging off the outlet can put enough strain on the body to distort the seat. Support the discharge piping on its own.
What Not to Do
- Don't raise the setpoint above what the system and its weakest component are rated for.
- Don't plug, cap or restrict the outlet to "stop the drip."
- Don't adjust or repair a relief valve unless you're qualified and your site procedures allow it.
If you're not sure which of these you're dealing with, that's what we're here for. A-Best Industrial Inc. Fittings, Valves, and Tubing is an authorized distributor of Mercer Valve pressure relief valves, and we're glad to help you match the right valve, set pressure and seat material to your service. You can learn more on our Mercer pressure relief valves page or browse our relief valves.
Need a Hand?
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