[Balancing piston design in JOS-E Emerson] 2

[gguliye]

Dear All,
The Crosby has incorporated the balancing piston to its valves and it functions as the secondary bellow. When the bellows ruptures, the unbalanced area of disk holder will expose to the back pressure and this back pressure in turn will affect the set pressure of the PRVs. So, the Crosby has installed the balancing piston to take the force coming from back pressure to itself (the direction of the force will be upward), so the set pressure will not be affected(the force applied on unbalanced area of disk holder will be equal to the force applied to the piston in the upward direction). My question is as following:
can we use the balancing piston alone without any bellows to avoid from the effects of the back pressure completely?

 

[The Obturator]

Theoretically Yes. BUT you have to realise that the Balanced Piston "BP" design component only comes into play should the bellows unit fail by rupture or other. Then the set point is maintained by the BP. However, and its a BIG However...any back pressure/fluid will be constantly exiting via the open bonnet vent. You cannot close this off since the balancing effect requires atmospheric pressure. Should you have a toxic fluid or liquid, you will see a constant bleed of the fluid from the outlet piping coming out of the bonnet vent.

The name given to the Crosby BP design is "Supplementary Back Pressure Balancing Piston". It is supplementary to the balancing bellows and so designed for those applications where the user cannot have a set point variance. The BP if you like, is intended as a temporary fix to bellows failure. Hence a conventional BP is not offered.

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

 

[gguliye]

Dear Avalveman,
Suppose the bellow has been ruptured by some means. the back pressure will go up between the guide and holder and will pass through the cylinders of BP. Later, on its way up to the bonnet vent hole, the back pressure may affect the set pressure by being applied to the top of the disk holder (point where the spindle is attached to the holder). So the set pressure will vary by some amount. Am I right? Is that what u mean by temporary fix?

 

[The Obturator]

Not quite right. There is a Bonnet Vent that will not "hold" the back pressure to make set ppressure higher. The temporary fix means that the set pressure will not be affected if the bellows ruptures. At which point the bonnet vent will leak the back pressure fluid until the bellows will be replaced. Some users will have a device of sorts to indicate bellows failure.

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

 

[gguliye]

Dear Valve man,
In order for the fluid to move from Bonnet to the atmosphere through hole we have to have pressure difference. If the outside is atmospheric pressure we have to have greater pressure than that in the bonnet. That's why BP way of working doesn't make any sense to me I am sorry for that

 

[The Obturator]

The BACK PRESSURE FLUID is being vented out the bonnet hole/vent while the main valve is closed.

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

 

[The Obturator]

Re my last note.....In case of bellows failure/rupture

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

 

[gguliye]

Valve man
Thank u Very much for ur explanations !!

 

[don1980]

Adding to what Valveman has already explained, constant leakage to atm will occur if/when the bellows has failed and the PSV outlet is connected to a flare header or some other closed system. Whatever system that is, it will have a higher pressure than atm, thus there's a driving force for leakage (backflow) from the outlet side through the failed bellows to the atm. The whole purpose of this particular design is to ensure that the set pressure remains relatively constant, despite the failure of the bellows. You may ask, why doesn't the balance piston prevent this leakage. The answer is that this is not a sealed (leak-tight) balance piston. The balance piston is closely fitted to the valve stem but there's no o-ring that seals off leakage like there is in small balanced piston PSVs (PSVs with just a balance piston and no bellows).

That's how this valve is intended to work. You may also ask, when would you want to use such a valve? It's for applications in which one prioritizes the importance of holding the set pressure constant, despite the failure of the bellows. Obviously, this means that one is willing to tolerate the continuous leakage (through the cracked bellows) from the outlet system to the atm. I think there are few applications in which this is a tolerable circumstance. So, understand that trade-off and recognize that this design isn't for broad application. It's seldom tolerable to allow a constant leakage to the atm. Also, recognize that it makes no sense to use this valve in an application where the discharge is routed directly to the atm.

 

[gguliye]

Don 1980
Thank u Very much highly appreciate your help