Pressure relief valves fail for a handful of core reasons: corrosion and mineral buildup on the seat, worn or broken springs, improper sizing, excessive back pressure in the discharge piping, and simple lack of maintenance. Some of these cause the valve to leak constantly, others prevent it from opening when it should, and a few can cause it to open too early or too late. Any of these failures can create a serious safety hazard, since the valve is the last line of defense against a catastrophic overpressure event.
Seat Damage and Internal Leakage
The most common failure you’ll actually notice is a valve that won’t stop leaking. The sealing surface where the disc meets the seat is precision-machined, and even tiny imperfections can let pressure escape. Corrosion, mineral scale from hard water, or particles carried in the fluid can all pit or scratch that surface over time. Once the seal is compromised, the valve weeps or drips continuously, which wastes energy in industrial systems and, in a home water heater, sends water down the discharge pipe when it shouldn’t.
Leaking can also stem from manufacturing quality. Poor lapping (the polishing process that makes the disc and seat mate perfectly) leaves microscopic gaps from the start. If you notice a valve leaking shortly after installation, this is one likely explanation.
Spring Fatigue and Breakage
The spring is what holds the valve closed until system pressure reaches the set point, then allows it to reseat once pressure drops. Springs weaken over time, especially in high-temperature environments where the metal gradually loses its tension. A weakened spring lets the valve crack open below the intended set pressure, releasing fluid when the system is actually operating normally. A broken spring means the valve has essentially no holding force at all, and the system can’t build or maintain pressure.
Temperature plays a direct role here. When a valve operates at elevated temperatures, the metal bonnet that houses the spring expands, and the spring itself relaxes. Manufacturers account for this by calibrating the valve slightly higher during room-temperature testing, typically within a few percentage points of the desired set pressure. A valve rated for 400°F service, for example, might be set about 1.3% higher on the test bench so it opens at the correct pressure once it’s hot. If this temperature correction wasn’t applied during installation, or if the valve ends up in a service temperature it wasn’t designed for, the set point drifts.
Incorrect Sizing
A valve that’s too small for the system it protects is one of the more dangerous failure modes because it looks like it’s working until an actual emergency. An undersized valve simply can’t relieve enough flow during an overpressure event, allowing pressure to climb past the equipment’s safe limit. According to engineering guidance from the American Institute of Chemical Engineers, mixing different industry rating standards (ASME and API values) during the sizing calculation can overestimate a valve’s capacity by 13% or more, resulting in a valve that’s too small without anyone realizing it.
The problem gets worse when a valve designed for gas or vapor service is used in a liquid scenario without adjusting its rated capacity. In that case, the valve’s actual liquid-handling ability can be overestimated by 66% or more. That’s not a subtle error. It means the valve might handle only a third of the flow it needs to during a liquid overpressure event.
Oversized valves cause different problems. They tend to open briefly, dump pressure fast, then slam shut, only to reopen moments later. This rapid cycling is called chatter, and it destroys the valve from the inside.
Chatter and Rapid Cycling
Chatter happens when the valve opens but the system can’t sustain enough flow to keep it fully open. The valve pops open, pressure drops instantly, the spring slams it shut, pressure builds again, and the cycle repeats at high speed. Research on valve degradation in nuclear power plants found that this kind of oscillation can reach frequencies of 100 to 120 cycles per second. At that rate, the disc hammers against the seat hundreds of times in seconds.
The damage accumulates quickly. The contact surfaces between the seat ring and the disc erode, and the repeated impacts can create tiny crevices in the metal. If corrosive fluid enters those crevices, it accelerates the damage through localized corrosion. After disassembly, valves that have experienced significant chatter typically show visible degradation on both the seat ring and the disc. Once those surfaces are damaged, the valve can no longer seal properly, leading to constant leaking even when chatter stops.
Chatter is often caused by oversized valves, but excessive back pressure in the discharge piping can trigger it too. If the piping downstream of the valve creates resistance greater than about 10% of the valve’s set pressure, the valve can’t achieve full lift. It opens partway, loses the pressure differential it needs to stay open, closes, and the cycle begins.
Back Pressure in Discharge Piping
Back pressure is one of the less obvious causes of failure because it has nothing to do with the valve itself. It comes from the piping connected to the valve’s outlet. Long pipe runs, small diameters, elbows, or a shared discharge header where multiple valves vent into the same line can all create enough resistance to interfere with valve operation.
Experimental data shows that valve lift stays constant until back pressure reaches roughly 10% of the set pressure. Beyond that threshold, every additional increase in back pressure causes a proportional drop in how far the valve can open. A valve that can’t fully lift can’t relieve its rated flow, which means an oversized discharge pipe might be the difference between a valve that protects the system and one that doesn’t.
Some valves are designed with internal features (balanced bellows or piston designs) that compensate for back pressure. If a standard valve is installed where a balanced design was specified, it will underperform in exactly the conditions where it matters most.
Assembly and Alignment Errors
Pressure relief valves are assembled with tight tolerances, and small manual errors during manufacturing or rebuilding can cause significant problems. Misalignment of internal components, particularly the spindle and bushings inside the nozzle, is a known contributor to leakage. Using the wrong wire size during assembly is one specific cause: it throws off the fit between mating parts and creates a path for pressure to escape.
The overlap between the disc and seat also matters. If the mating parts overlap too much, the valve needs more pressure than intended to open, delaying its response during an overpressure event. If they overlap too little, the valve opens prematurely at pressures below its set point. Either condition means the valve isn’t protecting the system at the pressure it was designed for.
Warning Signs of a Failing Valve
Three indicators cover most failure scenarios. First, your system can’t reach its normal operating pressure. This points to a valve that’s stuck open, has a broken spring, or has a plugged balance hole that prevents it from closing properly. Second, the system exceeds its maximum rated pressure, meaning the valve isn’t opening when it should. Third, you notice visible leaking or hear chattering from the valve during normal operation.
Leaking is the trickiest to catch because it can be gradual. A small drip from the discharge pipe might not affect system performance enough to trigger alarms, but it signals seat damage that will only get worse. In industrial settings, a leaking valve also means lost product, wasted energy, and potential environmental issues depending on what’s being vented.
Chattering is harder to miss. It produces an audible rapid clicking or buzzing and, if left unchecked, will destroy the valve’s internal surfaces within a relatively short period. If you hear it, the valve needs to be taken out of service and inspected before the seat damage makes it unable to seal at all.