Page 15 - Valve Selection Handbook

2 Valve Selection Handbook

The discussion on valve seals begins with the definition of fluid tight-
ness, followed by a description of the sealing mechanism and the design
of seat seals, gasketed seals, and stem seals. The subject of flow through
valves covers pressure loss, cavitation, waterhammer, and attenuation of
valve noise.
Manual Valves

Manual valves are divided into four groups according to the way the
closure member moves onto the seat. Each valve group consists of a
number of distinct types of valves that, in turn, are made in numerous
variations.
The way the closure member moves onto the seat gives a particular
group or type of valve a typical flow-control characteristic. This flow-
control characteristic has been used to establish a preliminary chart for
the selection of valves. The final valve selection may be made from the
description of the various types of valves and their variations that follow
that chart.
Note: For literature on control valves, refer to footnote on page 4 of
this book.

Check Valves

The many types of check valves are also divided into four groups
according to the way the closure member moves onto the seat.
The basic duty of these valves is to prevent back flow. However, the
valves should also close fast enough to prevent the formation of a signifi-
cant reverse-flow velocity, which on sudden shut-off, may introduce an
undesirably high surge pressure and/or cause heavy slamming of the clo-
sure member against the seat. In addition, the closure member should
remain stable in the open valve position.
Chapter 4, on check valves, describes the design and operating charac-
teristics of these valves and discusses the criteria upon which check
valves should be selected.

Pressure Relief Valves

Pressure relief valves are divided into two major groups: direct-acting
pressure relief valves that are actuated directly by the pressure of the sys-
tem fluid, and pilot-operated pressure relief valves in which a pilot con-