Page 74 - Valve Selection Handbook

Manual Valves 61

Bonnet Joints

Bonnets may be joined to the valve body by screwing, flanging, weld-
ing, or by means of a pressure-seal mechanism; or the bonnet may be an
integral part of the valve body.
The screwed-in bonnet found in the valve shown in Figure 3-4 is one
of the simplest and least expensive constructions. However, the bonnet
gasket must accommodate itself to rotating faces, and frequent unscrew-
ing of the bonnet may damage the joint faces. Also, the torque required
to tighten the bonnet joint becomes very large for the larger valves. For
this reason, the use of screwed-in bonnets is normally restricted to valve
sizes not greater than ND 80 (NFS 3).
If the bonnet is made of a weldable material, the screwed-in bonnet
may be seal welded, as in the valves shown in Figure 3-6 and Figure 3-7,
or the bonnet connection may be made entirely by welding. These con-
structions are not only economical but also most reliable irrespective of
size, operating pressure, and temperature. On the debit side, access to the
valve internals can be gained only by removing the weld. For this reason,
welded bonnets are normally used only where the valve can be expected
to remain maintenance-free for long periods, where the valve is a throw-
away valve, or where the sealing reliability of the bonnet joint outweighs
the difficulty of gaining access to the valve internals.
The bonnet may also be held to the valve body by a separate screwed
union ring, as in the valves shown in Figure 3-2 and Figure 3-3. This
construction has the advantage of preventing any motion between the
joint faces as the joint is being tightened. Repeatedly unscrewing the
bonnet, therefore, cannot readily harm the joint faces. As with the
screwed-in bonnet, the use of bonnets with a screwed union ring is
restricted to valve sizes normally not greater than DN 80 (NFS 3).
Flanged bonnet joints such as those found in the valves shown in Fig-
ure 3-8 and Figure 3-9 have the advantage over screwed joints in that the
tightenening effort can be spread over a number of bolts. Flanged joints
may therefore be designed for any valve size and operating pressure.
However, as the valve size and operating pressure increase, the flanged
joint becomes increasingly heavy and bulky. Also, at temperatures above
350°C (660°F), creep relaxation can, in time, noticeably lower the bolt
load. If the application is critical, the flanged joint may be seal welded.
The pressure-seal bonnet found in the valve shown in Figure 3-10
overcomes this weight disadvantage by letting the fluid pressure tighten