Page 64 - Valve Selection Handbook

Manual Valves 51

hand by the operating pressure and temperature in conjunction with the
applicable valve standard, and on the other hand by the properties of the
fluid, such as its corrosive and erosive properties. The selection of the
seating material is discussed in Chapter 2. The best approach is to consult
the valve manufacturer's catalog, which usually states the standard con-
struction material for the valve for given operating conditions. However,
it is the purchaser's responsibility to ensure that the construction material
of the valve is compatible with the fluid to be handled by the valve.
If the cost of the valve initially selected is too high for the purpose of
the valve, the suitability of other types of valves must be investigated.
Sometimes a compromise has to be made.
This selection procedure requires knowledge of the available valve
types and their variations, and possibly knowledge of the available valve
standards. This information is given in the following sections.

GLOBE VALVES

Globe valves are closing-down valves in which the closure member is
moved squarely on and off the seat. It is customary to refer to the closure
member as a disc, irrespective of its shape.
By this mode of disc travel, the seat opening varies in direct propor-
tion to the travel of the disc. This proportional relationship between valve
opening and disc travel is ideally suited for duties involving regulation of
flow rate. In addition, the seating load of globe valves can be positively
controlled by a screwed stem, and the disc moves with little or no friction
onto the seat, depending on the design of seat and disc. The sealing
capacity of these valves is therefore potentially high. On the debit side,
the seatings may trap solids, which travel in the flowing fluid.
Globe valves may, of course, be used also for on-off duty, provided the
flow resistance from the tortuous flow passage of these valves can be
accepted. Some globe valves are also designed for low flow resistance
for use in on-off duty. Also, if the valve has to be opened and closed fre-
quently, globe valves are ideally suited because of the short travel of the
disc between the open and closed positions, and the inherent robustness
of the seatings to the opening and closing movements.
Globe valves may therefore be used for most duties encountered in
fluid-handling systems. This wide range of duties has led to the develop-
ment of numerous variations of globe valves designed to meet a particu-