28 replies to this topic

[jprocess]

Dear Phil,
Thanks a lot for your valuable comments and clarification.
The MP steam header is designed for 12 barg.

Per ASME B31.3, you may exceed design pressure based on frequency of events and duration but must never exceed test pressure. ASME B31.3 does not specifically talk about scenarios so I would fall on both ASME Section VIII, Div 1 and API Std 521. In this system it appears you may be able to exceed the test pressure of the MP steam pipe if there were a tube rupture so in this system I would say you need a PSV on the MP steam line to limit this pressure excursion.

Sorry, but I misunderstood your mean! You supplied additional information to support this idea that tube side can exeed its design pressure(depend on frequency and duration) and consequently need not to be provided with a psv but after that you proposed to use a psv to protect the tube side against tube rupture scenario!
As a rough estimation the tube side test pressure can be 150% of its design pressure(12 barg) which equals a value of 18 barg.The shell side operating pressure is 20 barg.If tube rupture occures the tube side will expose to a pressure higher than its test pressure and so should be provided with a protective relieving device.It's OK?

[pleckner]

I think you mis-worded your question when you tried to paraphrase what I wrote, "You supplied additional information to support this idea that tube side can exeed its design pressure(depend on frequency and duration) and consequently need not to be provided with a psv...."

In this new system we are discussing, both the design and test pressures of the piping can be exceeded if there were a tube rupture. Also in this system, you do have a credible tube rupture scenario and that will result in viloating ASME B31.3 so you need to protect against this.

[jprocess]

And if the tube rupture be a credible scenario but the resultant pressure do not exceed the test pressure we do not need to provide a relieving device?!
In this case there will be another problem that steam distribution network will be infected by hydrocarbons! Right?

[pleckner]

Maybe to the first question and yes to the second.

Unless you are requied by some Code to provide over pressure protection whether there is a credible relieving scenario or not (ASME Section VIII, Div. 1 for example), you do not need to provide that relieving device.

ASME B31.3 says you need over pressure protection if you can exceed test pressure but you still may need over pressure protection if you just exceed design conditions. This is dependent on severity and frequency over the life of the system. I'm including some wording from B31.3 below to give you an idea:

(1) Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than (a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr, or (b ) 20% for no more than 50 hr at any one time and no more than 500 hr/yr The effects of such variations shall be determined by the designer to be safe over the service life of the piping system by methods acceptable to the owner. (See Appendix V.)