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Relief Valve Orifice Sizes

What is a Relief Valve?
Relief Valves are mechanical devices designed to operate if an over-pressure situation occurs - they are used to safeguard the plant. Generally, they are the last line of defence; following on from ESD and operator intervention.
 
So what is a Relief Valve orifice?
The American Petroleum Institute has developed a series of inlet size, orifice, outlet size combinations for various pressure classes of flanged relief valves. These combinations have been widely adopted by engineers throughout the oil and gas and allied industries.
Central to these combinations are a series of fourteen standard orifice sizes each denoted by a letter ranging from D through to T. Each letter refers to a specific effective orifice area.
 
What do the orifices denote?
The valve sizing engineer (usually a process or instrument engineer) determines the controlling relieving rate from all possible scenarios, then the required relief valve orifice size is determined using the appropriate equation given in API. Knowing the required relief valve orifice size, an actual orifice size equal to or greater than the calculated orifice size is chosen from a the standard range. The maximum flow through this actual orifice will be the valve's capacity.
 
What are the standard orifice sizes?
The full list of letters and corresponding effective area is shown below:
 
Relief Valve Orifice Sizes
LetterBore Dimensions
in2cm2
D0.1100.71
E0.1961.26
F0.3071.98
G0.5033.24
H0.7855.06
J1.2878.30
K1.83811.85
L2.85318.40
M3.60023.23
N4.34028.00
P6.38041.16
Q11.05071.29
R16.000103.22
T26.000167.74

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Further Reading

Relief Valves
 
Pressure Relief Valves
Typical P&ID Relief Valve Symbol

C&I Resources

 
ATEX
Cable Gland Selector
CE Marking
Flange Dimensions
Ingress Protection
Pipe Schedules
Relief Valve Orifice Size
Temperature Class
Temperature Conversion
Thermocouple Types
Valve Leakage
Vortex Meters
Wires and Cables
 

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