ASME Section 1 Overpressure 1

[RugRat2]

Hey Guys,

I am currently evaulating a boiler relief valve at my plant for a future PHA. This is my first time dealing with a boiler PSV vs Section VIII Pressure vessel.

I know the boilers are allowed a 6% overpressure based on ASME, Section 1, PG 67. This is where is starts getting fuzy. Is it correct to only use a 3% overpressure when actually sizing the valve?

In pressure vessels we usually use the overpressure that is allowed (i.e., fire 21%, others 10%). The reason I noticed the 3% was due to the capacity tables in the relief valve catalogs.

I just dont understand why we use 3% overpressure.

Thanks for the help.

 

[insult2injury]

I believe that 3% is required per the code.

I2I

 

[EGT01]

RugRat2,

I'm not an expert in boiler relief system design but the discussion in this other thread seems to make sense...
thread408-62290

 

[RugRat2]

It appears the allowable overpressure is 6%. But there is also a requirement for a maximum lift of the relief valve after 3%. I will check with my reference material once I get to the office.

Thanks for the link . I thought I did a good search before posting .

 

[insult2injury]

I don't have my Section I here but I believe that 3% is required by code for sizing the relief valve and that the 6% is the as tested maximum overpressure of the relief valve or combination of relief valves.

I2I

 

[EGT01]

As always, we need to be careful with the proper use of terms regarding overpressure and accumulation.

For the case involving 2 relief valves with the first set at MAWP and the second set at MAWP+3%, the second PSV is limited to 3% overpressure in order to not exceed the allowable 6% accumulation. Obviously, the first PSV will experience 6% overpressure and it would seem reasonable to be able to take credit for its capacity at 6% overpressure.

What is not completely clear to me is for the case of a single PSV set at MAWP. It would seem that for a single PSV set a MAWP, you should be able to size it for 6% overpressure since 6% accumulation is allowed. Not having experience with these systems, that is the question I would like to clarify.

For what it's worth, here's an excerpt from a 1998 edition of ASME Section I...
PG–67.2 The safety valve or safety relief valve capacity for each boiler (except as noted in PG–67.4) shall be such that the safety valve or valves will discharge all the steam that can be generated by the boiler without allowing the pressure to rise more than 6% above the highest pressure at which any valve is set and in no case to more than 6% above the maximum allowable working pressure.

Just looking at the last part...
"without allowing the pressure to rise more than 6% above the highest pressure at which any valve is set and in no case to more than 6% above the maximum allowable working pressure."
it would seem...
"a single PSV set at MAWP is okay using 6% overpressure."

Now, on another note and hoping not to confuse the issue, the above also seems to imply that you should not design your system such that any PSV experiences more than 6% overpressure.

I would appreciate additional comments.

 

[insult2injury]

ASME Section I SRVs are nameplate rated at 3% accumulation or 2 psi (whichever is greater). This is why all the manufacturers' products are rated at the 3%, which should be used for the relieving capacity calculations, not the 6%.

I2I