Challenging the common Bellows wisdom.

[robsalv]

A balanced bellows stops superimposed and developed back pressures having an additive effect on set pressure by virtue of a vented bonnet and a sound bellows.

What happens when the bellows is cracked??

I've always assumed that this takes the bellows out of the picture and the additive effects are now in play... but thinking about it further, since the bonnet is vented, 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.

Now say that the BB PRV is discharging into a flare header with some nominal header pressure, there'd be a pressure drop across this crack, i.e., superimposed header pressure one side, atmospheric the other since the bonnet is vented.

The pressure drop and vented bonnet means that the set pressure isn't really going to be impacted is it?

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 PRV is still likely to chatter, but the full developed discharge pressure wont be fully additive.


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..."

 

[edanyel]

Mine is far away from being an answer, it's just a thought.

A flare is an open vent isn't it? at it's end point your waste is burning to the atmosphere.
But anyway you always have your header pressure present.
What I mean is that a new little open vent (the hole in the bonnet)won't make a difference in the superimposed BP

Having that in mind, I think you are adding all the BP without drop at the back of your seat when your bellows breaks.

Maybe if the crack in the bellows is very, very small then there could be less addition, but think that the cause of the breaking is usually fatigue because of chattering, so it's just a question of short time to have your bellows totally broken and we get back to my first paragraph.
As I told you, I'm just thinking loudly.

Bst rgds
Daniel

 

[robsalv]

Thanks edanyel, your points are fair, but I'm not wanting to skew this discussion away from the OP.

A bellows does a few things, but predominantly eliminates back pressure issues.

My question is, how is this function impacted if the bellows is "holed"? Or alternatively, how will a conventional PRV operate with a failed bellows?





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

 

[MikeClay]

Since the backside of the bellows is at atmospheric pressure, a small leak should have little effect on the backpressure compensation. If there is a major leak or bellows failure backpressure compensation may be effected if the leak rate causes backpressure on the PSV telltale vent.

--Mike--

 

[edanyel]

My answer is then:

My question is, how is this function impacted if the bellows is "holed"?

Your balanced PRV is not balanced any more.

Or alternatively, how will a conventional PRV operate with a failed bellows?

It'll pop at higher pressure, how much higher depends on the BP, which now has a vertical component.

Be aware of one more thing, once the BP goes through your hole to the inner side of the bellows, builts up itself because the only way to get to the bonnet is the very little space around the guide rod. It´s not a freeway to reach the open bonnet vent.

Rgds
Daniel

 

[robsalv]

Hmmmm. Mike and Daniel, your responses appear to be at odds.

Daniel, before pondering this issue, I was thinking along the same lines as you. But now I'm leaning more towards Mike's point of view.

I'm leaning towards a bellows needing to fail in a big way for it to lose all back pressure compensation benefits.







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

 

[MikeClay]

In otherwords, how big is big.

 

[robsalv]

Mike, you are a philosopher indeed. LOL

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

 

[lizking]

Remember the bellows is protecting against built-up backpressure as well. If that valve opens, how will the additional backpressure affect the capacity? I would say, after a "short" time period, you're in "conventional" mode. I know, I know.... how short is short?