OPERATING AND DESIGN PRESSURE/TEMPRATURE

[sadra]

hellow,
in our project in blowdown line the operating temprature
is more than design temprature.
some body how is it possible.

 

[MortenA]

Poor design?

Best regards

Morten

 

[sadra]

Dear MortenA
we cannot sayoor design I think it is related to firecase
relief study because maximum operating temp. will be happend
in this case.but actually I dont have any information about that.
Regards
sadra

 

[tad123]

Normally, the PSV and its piping system are not designed for fire case but at the maximum operating Pressure and Temperature plus margin. When the system exposes to fire, the temperature in the pipe may exceed the specified design temperature. However, this can not be considered as normal operating temperature but upset conditions. The piping is derated at high temperature. However, ANSI B31.8 (if I recall it right) allows the piping to expose to an upset pressure of 20% or even 33% of its design pressure for a certain times and hours in a year. If the pressure associated with the upset temperature falls below the allowance above, the piping design is still OK.
Cheers,
TAD123

 

[MortenA]

I agree with tad - you should then print Operating case: Fire relief

Remember that if you are in europe theres a new pressure vesselv directive that overrules the old api 520 "fire case" 21% - now it alway (no exceptions) max 10%

Best regards

Morten

 

[TD2K]

B31.3 allows short term excursions to 120% and 133%. B31.8 is limited to 10% I believe, definitely not 20% or 33%.

 

[gunnarhole]

Sadra,

Do you think it may be possible that the design case temperature for your blow-down system is a minimum temperature chosen to account for the JT effect of the relieving gas? It might have an effect on the materials and welding procedures selected for this service.

Regards,

Gunnar