Page 61 - Valve Selection Handbook

48 Valve Selection Handbook

motion. Valves in which the closure member moves squarely on and off
the seat may trap solids and are therefore suitable only for essentially
clean fluids unless the seating material can embed trapped solids.

Valve End Connections

Valves may be provided with any type of end connection used to con-
nect piping. The most important of these for valves are threaded, flanged,
and welding end connections.

Threaded end connections. These are made, as a rule, with taper or par-
allel female threads, which screw over tapered male pipe threads.
Because a joint made up in this way contains large leakage passages, a
sealant or filler is used to close the leakage passages. If the construction
material of the valve body is weldable, screwed joints may also be seal
welded. If the mating parts of the joint are made of different but weldable
materials with widely differing coefficients of expansion, and if the oper-
ating temperature cycles within wide limits, seal welding the screwed
joint may be necessary.
Valves with threaded ends are primarily used in sizes up to DN 50
(NFS 2). As the size of the valve increases, installing and sealing the
joint become rapidly more difficult. Threaded end valves are available,
though, in sizes up to DN 150 (NFS 6).
To facilitate the erection and removal of threaded end valves, cou-
plings are used at appropriate points in the piping system. Couplings up
to DN 50 (NFS 1) consist of unions in which a parallel thread nut draws
two coupling halves together. Larger couplings are flanged.
Codes may restrict the use of threaded end valves, depending on
application.

Flanged end connections. These permit valves to be easily installed and
removed from the pipeline. However, flanged valves are bulkier than
threaded end valves and correspondingly dearer. Because flanged joints
are tightened by a number of bolts, which individually require less tight-
ening torque than a corresponding screwed joint, they can be adapted for
all sizes and pressures. At temperatures above 350°C (660°F), however,
creep relaxation of the bolts, gasket, and flanges can, in time, noticeably
lower the bolt load. Highly stressed flanged joints can develop leakage
problems at these temperatures.