A safety relief valve that weeps, simmers or won’t reseat is one of the most common calls we get. There are plenty of possible causes — debris on the seat, corrosion, a valve that has relieved too many times, the wrong valve for the service. But in our experience two causes account for most of it, and both are fixable without replacing anything.
1. The valve isn’t installed vertically
A spring-loaded safety relief valve is designed to sit upright, with the spindle vertical. Mount it at an angle or horizontally and the spring and disc no longer load evenly against the seat. The result is a valve that leaks below set pressure, reseats poorly, or wears out early.
As the industry guidance puts it: installing a safety valve in any position other than with the spindle vertical and upright may adversely affect performance and lifetime.
This one is worth checking first because it costs nothing to look. Walk out to the valve and see how it’s actually mounted — not how the drawing says it should be.
2. Operating pressure is too close to set pressure
This is the cause people miss.
A spring-operated safety valve doesn’t snap from perfectly sealed to fully open. As system pressure climbs toward the set pressure, the force holding the disc on the seat falls away, and the valve begins to simmer — a small, audible escape before it actually lifts. That typically starts somewhere in the range of 80–90% of nameplate set pressure.
So if your system normally runs at 95 PSI and the valve is set at 100 PSI, the valve isn’t faulty. It’s doing what a spring valve does.
The rule of thumb: keep at least a 10% gap between normal operating pressure and set pressure. A 20% gap gives noticeably better seat tightness and longer valve life.
If you can’t open that gap by lowering operating pressure, the answer is usually a valve with a higher set pressure — assuming the vessel’s MAWP allows it — not a replacement of the same valve, which will do exactly the same thing.
What to check, in order
- Is the valve mounted vertically, spindle up?
- What is normal operating pressure, and what is the set pressure on the nameplate? Is the gap at least 10%?
- Has the valve relieved recently, or been manually lifted? Debris on the seat after a lift is common.
- How old is it, and when was it last tested?
If the first two check out and it still leaks, it’s a valve problem rather than a system problem, and it needs testing or replacement.
Send us the part number from the nameplate and we’ll work out what you have and what the options are. Requests received before 3 PM ET are typically quoted the same business day.
For information only. This article is general technical guidance, not a specification or a recommendation for any particular installation. Valve selection, sizing, set pressure and inspection intervals depend on the equipment’s design data and the code it was built to. Always confirm with a qualified engineer and your jurisdiction’s boiler or pressure vessel inspector before specifying, changing or relying on a relief device.