The terms safety valve, relief valve, and safety relief valve
sometimes used interchangeably, and although the devices generally
provide a similar function (safety), they have important differences
in their modes of operation and application in HVAC systems
Safety valves open rapidly (pop-action). They are used for gases
and vapors (e.g., compressed air and steam).
Relief valves open or close gradually in proportion to excessive
pressure. They are used for liquids (e.g., unheated water).
Safety relief valves perform a dual function: they open rapidly
(pop-action) for gases and vapors and gradually for liquids. Typical
HVAC application is for heating water.
Temperature-actuated pressure relief valves (or temperature
and pressure safety relief valves) are activated by excessive
temperature or pressure. They are commonly used for potable hot
Application of these safety devices must comply with building
codes and the ASME Boiler and Pressure Vessel Code. For the remainder
of this discussion, the term “safety valve” is used
generically to include any or all of the four types described.
Safety valve construction, capacities, limitations, operation,
repair are covered by the ASME Boiler and Pressure Vessel Code.
For pressures above 15 psig, refer to Section I. Section IV covers
steam boilers for pressures less than 15 psig. Unfired pressure
vessels (such as heat exchange process equipment or pressure-reducing
valves) are covered by Section VIII.
The capacity of a safety valve is affected by the equipment on
which it is installed and the applicable code. Valves are chosen
based on accumulation, which is the pressure increase above the
maximum allowable working pressure of the vessel during valve
discharge. Section I valves are based on 3% accumulation. Accumulation
may be as high as 33.3% for Section IV valves and 10%
for Section VIII. To properly size a safety valve, the required
capacity and set pressure must be known. On a pressure-reducing
station, the safety valve must have sufficient capacity to prevent
an unsafe pressure rise if the reducing valve fails in the open position.
The safety valve set pressure should be high enough to allow
valve to remain closed during normal operation, yet allow it to
and reseat tightly when cycling. A minimum differential of 5
10% of inlet pressure (whichever is greater) is recommended.
When installing a safety valve, consider the following:
• Install the valve vertically with the drain holes open
or piped to drain
. • The seat can be distorted if the valve is overtight
or the weight of the discharge piping is carried by the valve
body. A drip-pan elbow on the discharge of the safety valve will
prevent the weight of the discharge piping from resting on the
• Use a moderate amount of pipe thread lubricant (first
2 to 3 threads) on male threads only.
• Install clean flange connections with new gaskets, properly
aligned and parallel, and bolted with even torque to prevent distortion.
• Wire cable or chain pulls attached to the test levers
should allow for a vertical pull and their weight should not be
carried by the valve. Testing of safety valves varies between
facilities depending on
operating conditions. Under normal conditions, safety valves with
working pressure under 400 psig should be tested manually once
month and pressure-tested once each year. For higher pressures,
test frequency should be based on operating experience.
When steam safety valves require repair, adjustment, or set pressure
change, the manufacturer or approved stations holding the
ASME V, UV, and/or VR stamps must perform the work. Only the
manufacturer is allowed to repair Section IV valves.