[ MAXIMUM BACK PRESSURE PERCENTAGES ON GAS/VAPOUR APPLICATIONS ] 4

[gguliye]

Dear Team members,
why balanced valves becomes unstable when the constant back pressure is greater than 50 % of set pressure? does it bend the bellow?
regards,

 

[RVAmeche]

The bellows isolates most of the disk assembly but there's still some surface area exposed to the backpressure (see Fig 4 in API 520). A very high backpressure will still produce a closing force, resulting in reduction of lift/reduced capacity. I don't think it's physically deforming the bellow, but I may be wrong.

 

[gguliye]

Dear Sir,
Thank u for ur answer
We have two areas on the spindle namely balanced and unbalanced. right?
The unbalanced area is covered with the bellow, and in the balanced area the pressure acting on both top and the bottom are equal so they cancel each other due to the same area.
Please let's wait to see what others think

 

[RVAmeche]

 

[gguliye]

I see different disk holder designs per manufacturer

 

[The Obturator]

gg To answer your initial question. 50% maximum SUPERIMPOSED VARIABLE BACK PRESSURE in a Spring Operated Pressure Relief Valve is the maximum published in API-520. It comes from a time when this was the general concensus between major PRV manufacturers at the time. A conservative capacity correction factor was established to put everybody on the same page. This 50% has been taken as the limit by many people ever since.

The facts to understand here are as follows;-
i. Some manufacturers can exceed 50%. Some can't go as high, some can't use bellows at all because of valve size/pressure. Some have unbalanced bellows purely to protect internals.
ii. The 50 % correction curves are from earlier extrapolations of earlier tests at lower back pressures.
iii. Some manufacturers higher pressure PRV designs have exposed guiding in a bellows design but are still balanced etc.
iv. Some manufacturers have conducted new back pressure tests that have not entirely produced expected results. You won't hear about this.
v. If not sized correctly, Variable back pressure adds to the probablity of unstable operation during opening.
vi. Everyone is awaiting rules for back pressure testing and correction flow factors by manufacturers type to be finalised in an unpublished ISO-4126 issue (maybe -8). Don't expect to hear on this for some time, as manufacturers in USA, Italy, Germany cannot agree the testing parameters. Some manufacturers are already selling their "tested solution", since everywhere you will find "Consult the Manufacturers for High Back Pressure"

For the Bellows to balance, its area must be equal or just over, the nozzle bore area. Ie, the nozzle bore is being shielded from back pressure above the disc. Hence only atmospheric pressure is present above the disc allowed by the bonnet vent.

You'll note maximum back pressures listed in API-526. Not everybody can meet these. Many can go over. Again, API-526 was a general concensus at the time of what could be offered. The back pressure limits here are not of the outlet flange (actually the body shell), but the bellows maximum pressure at 100'F

Hope that explains.

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

 

[gguliye]

Thank u very much abturator