shell test pressure

[arash807]

Dear members

According to the most testing standards like API598-API6D and etc...
shell test pressure is 1.5max allowable pressure @38c.
my question is that.why shell test pressure is greater than MAWP which is specified in ASME B16.34
in other word,increasing the pressure more than MAWP dose not damage the valve body?

regards
Arash

 

[reverman]

It is a safety factor.

Almost every single industry does something similar to this. Your Cell phone manufacture probably warns you never leave your phone in a location over 50°C. But in reality they probably test it at 60-70°C.

It's just bad practice to design something that may fail right at the margin. Variances in casting quality, machining, etc. can all cause problems. But you test well above the design pressure then you can have a reasonable degree of confidence that the valve will preform correctly at design pressure.

That said you should never use the valve in service over the rated pressures. While it is tested at higher pressures these are all short duration tests. Prolonged service above the pressure ratings can lead to failures which is potentially dangerous

 

[gerhardl]

In addition to revermans posting:

A valve has always a double function:

1. Part of the pipeline, where the piping itself are available to withstand shocks and pressure peaks that may unintentionally occur, and be above nominal pressure.

2. An opening and closing device, able to close against a given pressure (MAWP).

This, in addition to additional cost and complexity of construction, is the reason that worldwide different classification societies, all describes a higher body shell test (with open valve) than the test requiered for closed valve fluid pressure against the closing element of the valve.

 

[valvit]

API 598, API6D/ISO 14313, ASME B16.34 all state the MINIMUM shell test pressure shall be 1.5X the MAOP. The manufacturer is free to design and test the valve in excess of this number, often at the requirement of the end-user.