ASME I vs. VIII Safety Reliefs 1

[Welder4956]

What is the difference between a Safety Relief Valve manufactured to ASME I vs. ASME VIII as far as deign and function? This would be for steam service at pressure below 250 psi.

 

[davefitz]

a good summary is in the book by Marty Bernstein and Loyd Yoder "Power Boilers" 1998 chapter 12.

Variations between sect I and VIII include the types of relief devices allowed, the max allowable overpressure, and setpoints.

 

[JimCasey]

This would be a good question for the safety valve forum. A section 1 valve is certified for a fired pressure vessel. The pop and blowdown are 3% of set pressure. There are TWO rings to adjust to control the pop and the blowdown, and a secton 1 valve usually has an exposed spring to keep it cool. A lifting lever is mandatory. SOme section 1 valves have dual outlets to reduce reaction loads on the piping.

Section VIII valves are for unfired pressure vessels. The spring is usually enclosed, but open springs and/or alloy are optional. Rated capacity is at 10% overpressure,16% permissible with multiple valves and 21% permissible for Fire rating. Blowdown is customarily also 10% but I was told recently that the code does not stipulate this and I have not checked to verify. There is only one adjustment ring. A lifting lever is only mandated for air, steam, or water over 140F. Section VIII valves are available with optional bellows-balanced discs so they can be piped to a recovery system with variable backpressure. Section VIII valves are also available with pilots instead of spring-loading, where the process pressure is used to hold the valve closed until the pilot trips. Pilot valves are much more compact and seal tighter as the pressure increases---usually have soft seats. But with the small flow passages to and in the pilot they are not tolerant of contaminants in the process

 

[Ashereng]

Nice summary Jim.

 

[JAlton]

The difference between the two designs has to do with the pressure at which the valve reaches full lift and therefore maximum capacity. ASME Sec. I - Power Boiler Safety Valves are designed to reach full lift by 103% of Set Pressure (the Two Ring Design mentioned earlier helps achieve this) the Open Yoke (exposed spring) design limits them to vapor service. ASME Sec. VIII - Pressure Vessel Safety-Relief Valves are not as highly engineered and may not reach full lift until 110% of Set Pressure. The enclosed spring design makes the SRV capable of both vapor and liquid service. Hence the name Safety-Relief Valve. Currently ASME Sub-Committee Safety Valve Requirements is considering two items related to this topic. One item will make it permissible for the first time to install Pilot Operated Relief Valves on Boilers and the other will lower the minimum set pressure from 15 psi to 3 psi. Last year, the ASME Sec. I Addendum removed the requirement for production testing of Blowdown of Safety Valves on Boilers. ASME Sec. VIII has no Blowdown requirements for production testing.

J. Alton Cox

http://www.delucatest.com