Compressor and Downstream Equipment - PR valves are required for any

Compressor and Downstream Equipment - PR valves are required for any

compressor where the maximum pressure which can be generated during surge

or restricted discharge conditions exceeds the design pressure of the discharge

piping, downstream equipment or compressor casing.

For centrifugal compressors, it is usually economical to set the design

pressure lower than the maximum possible pressure that the compressor can

develop, and to provide appropriate PR valve protection on the discharge. In

some cases (e.g., where the flow through a PR valve would be the largest

release, or determining the size of a closed disposal system), it may be

advantageous to set the design pressure of the compressor casing and downstream

equipment equal to the maximum pressure that can be generated at the surge

point, assuming the most severe combination of speed, molecular weight, suction

pressure and temperature conditions that can occur as the result of a single

contingency. However, this is generally not desirable.

For positive displacement compressors, discharge PR valves are nearly

always required. Reliance on stalling of a reciprocating compressor is generally

not economically attractive, since driver stalling pressures are usually quite high

in comparison to operating pressure.

Low-pressure stage casings and interstage circuits on both centrifugal and

positive displacement multi-stage compressors are not normally designed for full

discharge pressure and must also be provided with overpressure protection.

Pressure relief valves for centrifugal compressors should be set higher than

the normal operating pressure by 170 kPa or 10% of set pressure, whichever is

greater. However, in the case of reciprocating compressors, a greater differential

than 10% may be desirable because of pressure surges. Interstage PR valves

should be set at least as high as the compressor settling-out pressure to avoid

valve lifting during compressor shutdowns.

PR valve capacity should be equal to the compressor capacity at the

emergency conditions. For centrifugal compressors, the combination of PR valve

set point and relieving capacity should be such as to avoid surge conditions over

the anticipated combinations of emergency conditions and operating variables.

Compressor PR valves should discharge to an appropriate atmospheric or closed

system, and not to the suction of the machine.Reference: PRESSURE SAFETY DESIGN PRACTICES FOR REFINERY AND CHEMICAL

OPERATIONS by Nicholas P. Cheremisinoff