Common reasons for Bellows failure 2

[Nasim12345]

Dears

Can you pls share me the common reasons for the Bellows failure in PSVs other than listed below.

1. If the PZV has been exposed to excessive backpressure(constant/variable) then the bellows will damage.
2. If the PZV starts to chatter (rapidly opening and closing), as well as flow instability, could inevitably cause valve damage such as premature fatigue failure of the bellows.
3. If the operating pressure is too close to the set pressure, then pzv spindle is moving (maybe just simmering rather than popping open), causing enough movement to fatigue the bellows.
4. If there is any corrosion, bellows may damaged.


Thanks
Nasim

 

[The Obturator]

5. Badly designed bellows / Bellows manufacturing faults / Non OEM bellows replacements.
6. PRV seeing service other than what is specified in data sheet.
7. Solidification of media.
8. Impact from loose objects in outlet stream (welding rod bits etc)

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[don1980]

Engineers intuitively understand that all metals will eventually experience fatigue failure when they're exposed to compression forces. What's not understood (and not intuitive) is that the number of bellows compression cycles is not equal to the number of PSV open-close cycles. Almost all PSVs experience flutter instability during the opening phase and during the closing phase. And in many cases there are periods of flutter instability during the time the PSV is open. All of this flutter instability is generally unnoticed by the observer. It can only be seen by using specialized (high speed and highly sensitive) instrumentation on a test stand.

When the PSV goes though a single opening/closing cycle, the bellows may experience hundreds of compression cycles. So the key to preventing bellows failure is to prevent the PSV from opening in the first place. And when bellows failures do occur, one shouldn't jump to the conclusion that the bellows has some defect. The use of Inconel as the bellows material will help minimize failures. Inconel resists fatigue failure better than other alloys, which is why Inconel is one of the standard materials offered by manuafcturers.

Bellows PSVs should be checked for leakage after each opening. It's an easy test - use a Drager Tube device to "sniff" the bonnet vent. If the bellows is cracked, gas from the PSV outlet piping system can/will freely flow through that crack, and out to the atmosphere through the bonnet vent.

 

[Nasim12345]

Thanks for your feedback. Existing PSV bellows material is SS316l & PSV manufacturer already recommended to replace with Inconel Bellows, which is advice by don1980.

Regards
Nasim