Questioning Balanced Bellows common wisdom 1

[robsalv]

Apologies for the cross post, but the discussion has run out in the other forum.

I have a query about balanced bellows PRV's. We all know that a balanced bellows stops superimposed and developed back pressures having an additive effect on the set pressure of a spring op PRV. This is due to the bellows area and the bellows being referenced to atmospheric pressure.

My questions are: What happens when the bellows is cracked?? Is the PRV still getting some back pressure compensation benefit if the crack is small?

I've always assumed that any failure takes the bellows out of the picture and the back pressure additive effects are now in play... but thinking about it further, since the bonnet is vented, therefore relieving superimposed back pressure, I'm wondering whether my long held assumption has been too hasty.


Most bellows fail via a fatigue crack in a convolution, which is usually a small crack.

In the case of a PRV connected to a closed discharge header with some nominal back pressure - is the effect on set pressure effectively zero because of the pressure drop across the cracked bellows (going from header pressure down to atmospheric pressure).

The pressure drop and vented bonnet means that the back pressure really wont have an additive effect on set pressure. Does anyone violently disagree?

Similarly during a relieving event, the amount of pressure that could build up in the bonnet would be related to the back pressure caused by the pressure drop through the bonnet vent AND the cracked bellows. The set pressure is likely to be impacted, but not by the full measure of the developed discharge pressure.


Am I on the right thinking path here?

Happy to take discussion on this topic!

Cheers

Rob

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[robsalv]

Wow, the silence is deafening...

- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[JimCasey]

Depends on whether the bonnet vent is plugged.
If it is, then the crack in the bellows allows pressure o build in the bonnet. If not, then your crack has to approach the size of the bonnet vent before pressure starts to build in the bonnet. Of course if the bonnet vent is open, whatever is in the system will be venting out atmosphere, and that is seldom a good thing.

 

[robsalv]

Thanks for your post Jim. I agree.

The more I look at it, the more I'm convinced that while the bonnet vent can vent what product is in the bonnet (and yes, this is seldom acceptable), the bellows will continue to provide back pressure compensation to some degree - as near as complete for a small fatigue crack and lesser the larger the crack is.

The prime action of the bellows is to provide a bellows area equivalent to or greater than the nozzle/disc area which then shields the disc from the back pressure. The inside of the bellows is kept at atmospheric pressure to provide the full back pressure compensation. So any cracking would start to remove effective area from the picture and possibly allow some back pressure in the bonnet.







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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."