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Bellows Test Specification in relation to system Back Pressure

Bellows Test Specification in relation to system Back Pressure

(OP)
Hello all,

What is regarded as a correct test pressure for bellows on PSV's.

I have seen various valve service companies specify that they test bellows to 1BARG and 2BARG (depending on the company), these pressures are stipulated regardless of the set point of the valve and( as far as im aware) does not take into account ant back pressure .

Am i right to assume that PSV's should have have the bellow's tested to the maximum theoretical back pressure of the system? Is there any testing standard that covers this?

Are bellows (generally speaking) designed to hold a certain percentage of pressure of the set point of the valve ?

Any information on this subject would be much appreciated.

Thanks in advance

Carlos

RE: Bellows Test Specification in relation to system Back Pressure

Carlos,

Generally, the SRV maker will design a bellows and get a specialist subcontractor to manufacture the bellows capsule, sometimes also attaching a free issued part to allow a full assembly to be made. The bellows manufacturer will also carry out the necessary test as agreed with the designer (the SRV maker). As a minimum, the maximum outlet pressures stated in API-526 (for bellows valves) will be the base. However, due to some other constraints, not every SRV manufacturer meets even this criteria, on the other hand, some exceed it (with special designs for a particular application). The bellows is delivered to the SRV maker with the appropriate certification. No other bellows test as such is carried out as normal.

What the SRV maker does do is to carry out bellows integrity tests. That is to say, to test if the bellows is leak free in an assembled SRV.

A nominal pressure of approx 1 bar / 14.5 PSIG is applied through the SRV outlet. Should the bellows be damaged in any way, it will leak. Leakage is detected via the open bonnet vent found on all balanced spring SRV's. Detection using soapy water. The pressure is purposely low as you could get the valve trying to open since during this test since there is zero inlet (set) pressure. If the bellows is damaged, just a low pressure will detect it. Any leakage is subject rejection.

There is another test usually performed by the SRV maker, and that is the shell pneumatic test. Here the entire SRV internal envelope is pressurised to test the SRV joint integrity (nozzle to body location, body to bonnet etc). In the case of a bellows SRV you again pressurise via the outlet at typically 100 PSIG but you MUST equalise the pressure in the bonnet chamber since it is sealed off from the body cavity by the bellows. Hence, pressure is also applied at the same time through the bonnet vent location (normally 1/2" NPT). A differential no more than 5 PSIG should be seen between the 2 pressurised chambers. During this pressure hold the joints of the SRV are sprayed with soapy water to detect any leakage. In the case of a conventional valve, pressure is applied through the valve outlet only.

Your last question regarding % back pressure Vs set pressure warrants several chapters. This has many many variables in order to provide an answer and certainly will change from manufacturer to manufacturer.

Hope this helps. Any extraordinary tests are usually discussed and agreed with the SRV manufacturer.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

RE: Bellows Test Specification in relation to system Back Pressure


avalveman,

"There is another test usually performed by the SRV maker, and that is the shell pneumatic test. Here the entire SRV internal envelope is pressurised to test the SRV joint integrity (nozzle to body location, body to bonnet etc). In the case of a bellows SRV you again pressurise via the outlet at typically 100 PSIG but you MUST equalise the pressure in the bonnet chamber since it is sealed off from the body cavity by the bellows."

Why must the pressure be equalized in the bonnet chamber while we know in SRV operation the bellows outside will sense the back pressure and the pressure in bonnet chamber is atmospheric?

RE: Bellows Test Specification in relation to system Back Pressure

The bonnet chamber is pressurised in this case to allow the shell pneumatic test. You need pressure in the bonnet to test the cap and bonnet to body joints. Equal pressure in the body and bonnet is used to prevent unecessary bellows damage. You need to do it this way to obtain pressure in the body.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

RE: Bellows Test Specification in relation to system Back Pressure

STOP PRESS ! Ignore my earlier first comment, para 3, about the valve opening during test. It would not happen. Apologies.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

RE: Bellows Test Specification in relation to system Back Pressure

(OP)
Thankyou all for very useful input.

If an integrity test is only carried out at 1barg for the bellows and the SRV is used on a system which generates a back pressure (for arguments sake) of 4 barg, then how would one know that the bellows are fit for purpose?

RE: Bellows Test Specification in relation to system Back Pressure

Carlos, per my first reply, normally the bellows vendor will carry out testing on the SRV makers instructions.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

RE: Bellows Test Specification in relation to system Back Pressure

Carlos,
As per Saudi Aramco Engineering Procedure(SAEP-319), Para 6.2.4.5 States that " A test shall be conducted by pressurizing the external side of the bellows to 26 psig (138 kPa) or 25% of CDTP, Cold Differential Test Pressure (i.e., the pressure at which the RV is adjusted to open on the test stand) whichever is smaller. Over pressure can damage the bellows".

Every manufacturer in KSA is following the above rule.

Thanks
Nasim

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