Allowable stress for B 31.1.

Dear members,
When I compare the allowable stress for 31.1 and 31.3, I found that 31.1 has lower allowable stress than 31.3.
Can any one tell me the reason why 31.1 has lower allowable stress than 31.3?

Thank you in advance.

Dear member,
Kindly share your knowledge.

Regrads,

Dear Mr.Peter,
Please keep in mind, ASME B31.1 is more stringent than B31.3.
Becoz of power plant application. (there will be pipelines with
high pressure and temperature) when compared to B31.3 (oil and gas piping).

Allowable stress value ---20000 psi (B31.3)
-------do-----------------15700 psi (B31.1)

Its all for more safer operation for the pipe design pressure and temperature.

B31.3 piping (refineries, petro-chemical plants) can have just as high pressure / temperatures as B31.1 piping.

My understanding that the reason for the lower allowables in B31.1 is to provide higher margin of safety / higher reliability to B31.1 piping, due to the increased consequences of failure.

If some B31.3 piping fails, typically the impact is to the immediate plant and to the Owner of that plant.

If some B31.1 (eg. power plant) piping fails, the impact will be the plant and the plant's Owner, but also may be to the general public and other industries that rely on the power from the power plant.

Thank you very much Mr satpiping and Mr Dave Clark.

Have a nice day.

Thanks & regards,