Over-Pressurization Devices (Relief Valves, Safety Relief Valves)

There are four types of over pressurization devices:

• Rupture Disks

• Relief Valves

• Safety Relief Valves

• Safety Valves

What should you expect on an over-pressurization device?Every device must have a name tag. The name tag must have one of the following ASME symbols ‘UV’ [spring loaded over pressurization device] ‘UD’ [rupture disk]. The name tag will have the set pressure and capacity. The set pressure should never be greater than the pressure vessels MAWP [maximum allowable working pressure].Here are some special rules to the set pressure-

1. Relief Valves, Safety Relief Valves, and Safety Valves should be set at or below pressure vessel ASME nameplate MAWP.

2. Rupture Disks should be set not higher than the PV [pressure vessel] nameplate. Under special circumstances the rupture disk can be set up to 110% of the MAWP. In addition to this special condition, whenever there is a possibility of internal fire in the pressure vessel the rupture disk can be set not higher than 160%, but a pressure vessel engineer will need to be consulted.

Remember There should never be a shut off valve between the PV and over pressurization device.

How should the over-pressurization device be installed?

1. Always in an Upright installation.

2. Installed the over pressurization device a few pipe diameters away from the PV, but consult the code and manufacture for maximum distance.

3. Make sure the exhaust discharges safely away, so no one could be injured.

Relief Valves -Relief valves open at a set pressure and re-close at the same pressure. These devices are suitable for liquid service.

Safety Relief Valves -These valves differ from safety valves in that they are meant to handle fluid streams that have liquids and vapor. These valves pop open at a set pressure and reset [blow down] at a lower pressure [very much like a safety valve].

Safety Valves -Safety valves are strictly for vapor or gas service. The vapor or gas should be relatively clean to ensure continued and successful operation. A typical vapor is steam, an example for gas would be compressed air. These are not meant for liquids. These valves pop open at a set pressure and reset at a lower pressure called blow down.

Rupture Disks -Rupture disks are probably the most versatile over- pressurization device. These can only be used once. They are the only device that can be used in conjunction with other over-pressurization devices.