Page 172 - Valve Selection Handbook

Pressure Relief Valves 159

The second type of pressure relief valve is the pilot-operated pressure
relief valve that consists of a main valve and a pilot. The main valve is
the actual pressure-relieving device while the pilot positions the disc of
the main valve disc in response to the system pressure.
Traditionally in the U.S., direct-loaded pressure relief valves mainly
intended for boiler and steam applications are referred to as safety valves,
those mainly for liquid applications as relief valves, and those intended
for both compressible and incompressible fluids as safety relief valves.
This terminology is also applied in this book. None of these terms, how-
ever, fully describes the design or function of direct-loaded pressure
relief valves, and the traditional terminology has been replaced in ASME
Code and API RP 520 by the more generic term pressure relief valve.
Pilot-operated pressure relief valves in which the pilot is a self-actuat-
ed device are referred to in the ASME Code as pilot-operated pressure
relief valves and those in which the pilot is an externally powered device
as power-operated pressure relief valves.
The terminology used in the emerging European standards on pressure
1
relief valves differs to some extent from traditional U.S. terminology. In
these standards, the term safety valve is a generic term for self-actuated
reclosing pressure relief devices and covers direct-loaded and pilot-oper-
ated safety valves. Power-operated pressure relief valves are referred to
as controlled safety pressure relief systems.
Pressure relief valves are covered in this book under the following
headings:

• Direct-loaded pressure relief valves with subheadings safety valves,
safety relief valves, liquid relief valves, vacuum relief valves, and
direct-loaded pressure relief valves with auxiliary actuator.
• Piloted-pressure relief valves, consisting of a main valve in combina-
tion with a pilot that positions the disc of the main valve in response to
the system pressure.

The construction, application, and sizing of pressure relief valves is
subject to constraints by codes, or to agreement by statutory authorities
that must be followed by the valve user.

1
The emerging European standards for safety devices are labeled EN 1268, Parts 1 to 7.
At the time of writing this book, the standards were still in draft form.