Balanced Bellow Pressure limitations 1

[Sawsan311]

Dear All,

it is conventionally known that as part of flare system design, the total backpressure limitation based on the type of PSV (superimposed + built-up backpressure) is a key criteria for the flare system network hydraulics. For example, as per API 520 P I balanced bellow can accommodate up to a backpressure of 50%. However, the limitation of the bellow pressure ratings as included in the API 526 tables specify lower pressure limits than the backpressure acknowledged limits and even the PSV set pressure.

My question is:

a) what is the rationale behind the bellow pressure limit ? is it due to its manufacturing tolerances as per ASME B16.34 for flanges.
b) Is it for this reason some designs introduce balanced piston as an auxiliary backup for the case if bellow fails
c) In absence of the auxiliary piston, do you agree that the hydraulic calculations for the flare system shall accordingly take into account the bellow limits as a crucial check such that the values in API 526 tables are not exceeded??

Thanks

Regards,

 

[The Obturator]

a) The back pressure limits in API 526 are based on design limitations from a group of manufacturers whom agreed those limits when API was being set up. The outlet pressure limitation is basically the limit the body shell can take and also the limitation of spring designs for larger orifice sizes - this why larger PRV's cannot have high set pressures.. The larger the orifice the more limiting the spring design availability. Some manufacturers can exceed the back pressure limits, and some can't even meet those in API-526. This is due to the limitation of high pressure bellows design availability. Again some can exceed some not. For pilot operated PRV's also in API-526, you effectively have full inlet and outlet ratings since there us no (large) bonnet spring or heavy walled bonnet or any bellows, to limit these ratings. The pilot controller does not change in size for any orifice.

b) Balanced Piston is purely a supplementary device to counteract back pressure effects of set pressure in case of bellows failure. There will be leakage of the back pressure fluid out of the bonnet vent in case of bellows failure. Balanced piston is used in critical applications where set pressure must be maintained.

c) The bellows limitations in API-526, or what the manufacturer can offer, should always be taken into account for any application.

Back pressures are generally limited to 50% like you stated (from API-520) This will apply to spring operated PRV's only - See also my earlier comments within thread1203-459294 regarding back pressure application limitations.

See also my FAQ on API-526 nuances faq1203-2099



*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sawsan311]

Thanks Obdurator for your great response.

With regards to the statement included in the first post: ''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 consensus 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''

I believe for a greenfield design and at early stage where vendor's feedback on the bellow limitations is not available, Engineering process hydraulics will have to respect the backpressure limits of the balanced bellow PSVs being maximum total backpressure of 50% of the PSV set pressure while taking into account figure 30 of API 520 P1 which accounts for the backpressure correction factors in case backpressure approaches 30%-40% depending on the overpressure of the scenario, see API 521 section 5.5.2 (backpressure) . Similarly, one should not overlook the manufacturing tolerances of the bellows which are somehow of lower limits than the PSV outlet flange limits because they need to be flexible enough to allow proper lift at the desired set point. Refer to API 520 P1 section 4.2.1.3.3.

Accordingly, I don't tend to accept that vendors or designers specially at early design stage where flare hydraulic piping modelling is done- don't meet the acknowledged bellow limitations :''You'll note maximum back pressures listed in API-526. Not everybody can meet these. Many can go over. '' For achieving a safe design, at least API 526 bellow limits need to be considered as a starting basis.

Regards,

 

[The Obturator]

I agree, For new construction, system engineers/designers should be aware of API-526 limits (for both spring and pilot operated PRV's), since as I have advised "some vendors can exceed". This basically means that PRV designs capable of exceeding the API-526 limits, are based on a case by case basis, and more often than not, result in a custom engineered PRV design. Such a special being a one-off, will tend to be expensive and on a long lead time.



*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***