experienceLESER Safety Valve knowledge - Malfunction chattering

No operator wants a chattering safety valve in their plant, but what is it and how does it happen?

According to ASME PTC 25, 2018 Chapter 2.7, chattering is defined as abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disk contacts the seat.

 

It is therefore a rapid opening and closing of the safety valve in which the disc and seat strike each other. As a result, not only are the sealing surfaces damaged, but there is also an impermissibly high-pressure increase. The permissible pressure increase of 10% above the set pressure can quickly be exceeded and, in the worst case, this can become a hazard to people, the environment and industry.

In the lift-pressure diagram, the function curve then looks like this as an example:

 Bild1        Chattering
 Lift-pressure diagram  Chattering

 

But how can it happen that a safety valve starts to chatter? Essentially, two aspects can play a role here:

  • Inlet pressure loss
  • Back pressure

Effects of increased inlet pressure loss at the inlet of the safety valve

Due to line length, pipe bends and other installations in the supply line to the safety valve, friction losses occur during the opening of the safety valve and thus a pressure drop from the system to be protected to the inlet of the safety valve. Inlet pressure losses >3% are not permissible according to the internationally applicable regulations to avoid the risk of an unstable function.

A too high-pressure loss can lead to the valve closing again due to the spring force acting from above and the lower pressure in the safety valve inlet. However, the pressure in the system to be protected has not yet dropped back to the normal operating pressure, which causes the pressure to rise again via the inlet of the valve - the valve opens again. Due to the renewed downstream flow, pressure loss occurs again and the valve opens and closes with high frequency, this is referred to as chattering.

Bild2
Built-up Back Pressure

Effects of back pressure

A similar effect can occur on the outlet side of a safety valve. When the safety valve opens, a so-called inherent back pressure occurs due to line length, diameter and components such as bends or silencers. If this exceeds the permissible 15% of the set pressure for a conventional safety valve or 35% or 50% of the set pressure* for a spring-loaded safety valve with bellows, the valve may also close again. This happens because the back pressure on the disc together with the spring force then becomes so high that the safety valve closes again. The pressure in the inlet continues to rise again or even increases further because the actual overpressure scenario has not yet been solved. The back pressure at the outlet decreases with a reduced lift while the safety valve is closed, which allows the safety valve to open again. This also causes the valve to open and close rapidly, it starts to chatter.

*) 35% for all valves except API Type 526 / 50% for API Type 526

The following measures can be taken to stop chattering or fluttering due to excessive inlet pressure losses or build-up back pressures:

  • Reduction of the capacity of the safety valve (lift limitation, smaller safety valve)
  • Optimization of the pipeline (reduce length and components, increase diameter)

Depending on the plant situation, other effects such as pressure surges and resonance effects can cause a safety valve to start chattering. However, these are currently still the subject of research.

All these effects must be sufficiently considered during planning and design to ensure safe operation and to avoid an impermissibly overpressure in the system.

Fun Fact:

The demonstration plant at LESER Kontor in Hamburg got its name from chattering safety valves and was given the name "Chatroom".