Page 73 - Valve Selection Handbook

60 Valve Selection Handbook

Connection of Disc to Stem

The stem of a globe valve may be designed to rotate while raising or
lowering the disc, or be prevented from rotating while carrying out this
task. These modes of stem operation have a bearing on the design of the
disc-to-stem connection.
Most globe valves incorporate a rotating stem because of simplicity of
design. If the disc is an integral component of the stem in this case, as it
frequently is in small needle valves such as those shown in Figure 0-3,
the seatings will mate while the disc rotates, possibly resulting in severe
wear of the seatings. Therefore, the main field of application of such
valves is for regulating duty with infrequent shut-off duty. For all other
duties involving rotating stems, the disc is designed to swivel freely on
the stem. However, swivel discs should have minimum free axial play on
the stem to prevent the possibility of rapid reciprocating movements of
the disc on the stem in the near closed valve position. Also, if the disc is
guided by the stem, there should be little lateral play between stem and
disc to prevent the disc from landing on the seat in a cocked position.
In the case of nonrotating stems, as in the valves shown in Figure 3-6,
Figure 3-7, Figure 3-10, and Figure 3-11, the disc may be either an inte-
gral part of the stem (see Figure 3-11) or a separate component from the
stem (see Figure 3-6, Figure 3-7, and Figure 3-10). Nonrotating stems are
required in valves with diaphragm or bellows valve stem seal, as in Fig-
ure 3-6 and Figure 3-7. They are also used in high pressure valves such
as those shown in Figure 3-10 and Figure 3-11 to facilitate the incorpora-
tion of power operators.

Inside and Outside Stem Screw

The screw for raising or lowering the stem may be located inside the
valve body, as in the valves shown in Figure 3-2 through Figure 3-4, or
outside the valve body, as in the valves shown in Figure 3-5 through Fig-
ure 3-14.
The inside screw permits an economical bonnet construction, but it has
the disadvantage that it cannot be serviced from the outside. This construc-
tion is therefore best suited for fluids that have good lubricity. For the
majority of minor duties, however, the inside screw gives good service.
The outside screw can be serviced from the outside and is therefore
preferred for severe duties.