Section VIII Safety Valve Blowdown 3

[Sharik]

In ASME Section I there are requirements for PSV blowdowns (it must close no lower than 96% of the set pressure and the minimum blowdown must be not less than 2% of the set pressure, PG-72.1).

Are there such requirements for ASME Section VIII PSV's? What are the minimum and maximum PSV blowdowns for PSV's protecting pressure vessels?

 

[metengr]

Review ASME B&PV Code, Section VIII, Div 1, Part UG-125. Continue to read up until Part UG-129. Pressure relieving devices for unfired steam boilers must follow the rules of Section I. For all other unfired pressure vessels, the blowdown capacity of the pressure relieving device is determined based on preventing the pressure to rise by x% above the maximum allowable working pressure.

 

[Sharik]

I'm not looking for blowdown capacities. I know the ASME Section VIII capacities must be such as to prevent the pressure from rising more than 110% of the MAWP (for single PSV installations) or 116% of the MAWP for muliple PSV installations.

I'm looking for blowdown pressures. In ASME Section I, a PSV with a set pressure of 100 psig must close between 96 and 98 psig. It can't remain open down to 90 psig for example.

For a PSV with a set pressure of 100 psig protecting an ASME Section VIII pressure vessel (not a steam generator), what would be the required closing pressure or pressure range?

 

[Hipressure]

Have a look at the NBIC Appendix F Part IV This may help

 

[EGT01]

I'm curious to know what the NBIC Appendix F Part IV has to say but unfortunately I don't have access to it. If it is not too much trouble, is it possible for someone to summarize and post?

Just as additional info, here's some references to what you will find in ASME Sec VIII.

From the Anderson Greenwood website...

http://www.andersongreenwood.com/literature.asp

Anderson Greenwood
Technical Seminar Manual

http://www.tycoflowcontrol-na.com/ld/Tech Sem Manual.pdf

Includes a one page, side by side comparison of Sec VIII and Sec I valve requirements.

Crosby
Pressure Relief Valve Engineering Handbook

http://www.tycoflowcontrol-na.com/ld/CROMC-0296-US.pdf

Includes excerpts from ASME Sec VIII. In particular, look at UG-131(c)(3)(a), UG-136(c)(3)(b) and Appendix M.

 

[metengr]

2004 Edition of NBIC Appendix F
"Pressure Differential Between Safety or Safety relief Valve Setting and Boiler or Pressure Vessel Operating Pressure"


F-5000 Pressure Vessels
Only general guidelines are provided. After a self-actuated release of pressure, the valve must be capable of re-closing above normal operating pressure. Example given;
if a valve is set at 100 psi with a 7% blowdown, it will close at 93 psi. The operating pressure must be below 93 psi to prevent leakage or flow in service.....


For set pressure up to 70 psi, the recommended pressure differential is 5 ksi

For set pressures between 70 and 1000 psi, the recommended pressure differential is 10% of set pressure.

For set pressures above 1000 psi, the recommended pressure differential is 7% of set pressure...

 

[Hipressure]

Due to the variety of service conditions and various designs of pressure relief valves, only general guidelines can be given regarding differentials between the set pressure of the valve and the operating pressure of the vessel.
After a self-acuated release of pressure, the valve must be capable of reclosing above the normal operating pressure. For example: Valve set @100# with 7%blowdown, it will close at 93#, The operating pressure must be maintained below 93#in order to prevent leakage or flow from a partially open valve. Taken from NBIC 2001 Edd.App F Part IV

 

[EGT01]

metengr, hipressure

Thanks for the replies. Just wanted to make sure it wasn't saying something different than what the other references indicate.

 

[pve5]

metengr/hipressure

The set pressure tolerances for pressure relief valves per UG-126 of ASME Section VIII Div.1 are , plus or minus, shall not exceed 2psi for pressure up to 70 psi and 3% for pressure above 70 psi.

 

[Sharik]

PVE5,

You are describing the set pressure tolerances allowed by ASME Section VIII. I was looking for the blowdown (valve closing) tolerances.