Alternatives and plugging of bonnets of bellow balanced relief devices

[sheiko]

The bonnet of bellow balanced Relief Devices shall not be plugged, because in the situation of a leaking bellow, you will get an unwanted situation that the opening pressure increases with the backpressure. Disadvantage is the situation of a leaking bellow is that you will have the product/gas escaping from the unplugged bonnet. Solution can be to plug the bonnet with a manometer (incl. frequent checks) or a pressure transmitter for detecting a failing bellow. So the required repair can be taken without a relief of chemical content.
I know that there where manufacturer’s (e.g. Alstom) in the past that had developed “Bellows Leak Detector valves”. That valve allows the atmospheric pressure to enter into the bonnet cavity, thus guarantying the true balance of the valve and preventing pressure built-up inside the bonnet due to atmospheric air thermal expansion. In case of bellow failure, the backpressure builds up into the bonnet and pushes a piston out. That piston shuts off the hole in the wall of the bonnet, preventing the medium from leaking into the atmosphere, AND shows a visual indication outside the valve casing indicating the failure of the bellow, which can be checked frequent.

My questions are:
1.: Does anyone have experience with these kind of “Detector valves”?
2.: What are alternatives?

"We don't believe things because they are true, things are true because we believe them."

 

[rutherford703]

Alternative: Pipig the bellow vent to a safe location.

 

[Chance17]

I have no experience with the Detector valve.
My experience is like rutherford703 - vent to A SAFE location.

One additional comment
For services below ambient conditions, the bellows and chamber can freeze with ice.
My experience is to purge the bellows with nitrogen

 

[don1980]

Sheiko - Some time ago I came across this Alstom leak detector device and thought it was a clever idea. My only concern was whether the piston will get stuck and not pop out to reveal a bellows failure. I spoke with someone in the engineering department of a major global valve manufacturer to ask if they were familiar with this, and whether they offered a similar device. The answer was no to both questions.

If this turns out to be a reliable device, then I think it will be copied by other manufacturers. Meanwhile, I have no plans to try it.

In recent years I've been sensitized to this issue. As others have mentioned, the current best practice is to pipe away the bonnet vent if the leak risks are intolerable. In the past, I don't think many people gave this risk a passing thought, but today I see a number of companies assessing the hazard of vent-hole leak, and installing a drain/vent pipes when the risks are high. Also, there are some chemicals that pose accute inhalation or skin exposure hazards. In those services, I generally recommend avoiding the use of a balanced PSV.