Types of Pressure Relief Devices
Rupture Disc Device (CG-1)
It is a pressure operated non-reclosing device designed to
function by the bursting of a pressure containing disc at the test
pressure of the cylinder. Once the disc is ruptured, there is no
way to prevent the complete release of the contents.
Note – For compressed gas UN cylinder, test pressure of the
cylinder is 1.5 times the working pressure of cylinder. For
liquefiable gas UN cylinder, test pressure & the corresponding
filling ratio is given in P200 of ADR. Test pressure of a DOT
cylinder is 5/3 times the working/service pressure.
Fusible Plug Device (CG-2/CG-3)
It is a thermally operated non-reclosing device designed to function
by the yielding of a fusible metal at a set temperature. These devices
do not protect from overpressurization at temperatures below their
melting point but are designed to protect the cylinder from
overpressurization caused by exposure to excessive heat. In the
event a cylinder is exposed to fire or excess heat, the fusible plug is
designed to melt & release the cylinder contents preventing product
within the cylinder from creating excessively high pressures, caused
by high external temperatures, & rupturing the cylinder. These
devices are limited to use on cylinders of 500 psig service pressure or
less due to risk of extrusion of the alloy.
a) CG-2 plug uses fusible alloy that yields at a temperature
between 157°F to 170°F (69.4°C to 76.7°C)
Nominal temperature 165 °F (74 °C)
b) CG-3 plug uses fusible alloy that yields at a temperature
between 208°F to 224°F (97.8°C to 106.7°C)
Nominal temperature 212 °F (100 °C)
Combination Rupture Disc/Fusible Alloy (CG-4/CG-5)
It is a combination of pressure & temperature operated nonreclosing
device consisting of a rupture disc backed by fusible alloy
on the atmospheric side of the disc. In case of fire or high
temperature, the fusible metal yields & cylinder overpressure
caused by the heated gas is relieved by the bursting of the rupture
disc. Both the pressure & temperature requirements of the device
must be satisfied for the device to actuate. This device will not
protect a cylinder from overpressurization if the fusible alloy is not
heated to its yield temperature. The fusible metal prevents
premature rupture disc failure from momentary overpressurization
& also protects the disc from external corrosion
a) CG-4 plug uses fusible alloy that yields at a temperature
between 157°F to 170°F (69.4°C to 76.7°C)
Nominal temperature 165 °F (74 °C)
b) CG-5 plug uses fusible alloy that yields at a temperature
between 208°F to 224°F (97.8°C to 106.7°C)
Nominal temperature 212 °F (100 °C)
Pressure Relief Valve (PRV, CG-7)
It is a spring-loaded device designed to relieve excessive pressure &
reclose & reseat to prevent further flow of gas or fluid from the
container after resealing pressure is achieved. When the cylinder
pressure exceeds the pressure setting of the spring in the relief valve,
the valve opens to discharge the cylinder contents. Once the cylinder
pressure decreases to the PRV’s pressure setting, it reseats above the
pressure in a normally charged cylinder at 130°F (54.4 °C) after
venting sufficient gas to control the internal cylinder pressure. The
pressure setting of the pressure relief valve is between 75% & 100%
of the minimum test pressure of the cylinder.
This device does not protect against rupture of the container when
the application of heat weakens the container to the point where its
rupture pressure is less than the operating pressure of the device.
These devices are limited to use on cylinders with charging pressures
no greater than 500 psig.