Page 77 - Valve Selection Handbook

64 Valve Selection Handbook

Direction of Flow Through Globe Valves

The question of direction of flow through globe valves has two
answers.
If the possibility exists that flow from above the disc can remove
either the disc from the stem or a component from the disc, flow directed
from below the disc is mandatory. In this case, hand-operated globe
valves with rotating stem and metal seatings can be closed fluid-tight
without undue effort, only if the fluid load on the underside of the disc
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does not exceed about 40-60 kN (9,000-13,000 lb). With a non-rotat-
ing stem and roller-bearing supported stem nut, as in the valves shown in
Figure 3-10 and Figure 3-11, hand operated globe valves with metal seat-
ings may be closed fluid-tight against a fluid load of about 70-100 kN
(16,000-22,000 lb), depending on the leakage criterion and the construc-
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tion of the valve. One particular advantage of flow directed from below
the disc is that the stuffing box of the closed valve is relieved from the
upstream pressure. On the debit side, if the valve has been closed against
a hot fluid such as steam, thermal contraction of the stem after the valve
has been closed can be just enough to induce seat leakage.
If flow is directed from above the disc, the closing force from the fluid
acting on top of the disc supplements the closing force from the stem.
Thus, this direction of flow increases greatly the sealing reliability of the
valve. In this case, hand-operated globe valves with a rotating stem may
be opened without excessive effort, only if the fluid load acting on top of
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the disc does not exceed about 40-60 kN (9,000-13,000 lb). If the stem
is of the nonrotating type with a roller-bearing supported stem nut, the
globe valve may be opened by hand against a fluid load of about 70-100
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kN (16,000-22,000 lb). If the fluid load on top of the disc is higher, a
bypass valve may have to be provided that permits the downstream sys-
tem to be pressurized before the globe valve is opened.

Standards Pertaining to Globe Valves

Appendix C provides a list of U.S. and British standards pertaining to
globe valves.