Page 83 - Valve Selection Handbook

70 Valve Selection Handbook

material. For this reason, parallel gate valves are normally used for
infrequent valve operation only.
• Loosely guided discs and loose disc components will tend to rattle vio-
lently when shearing high density and high velocity flow.
• Flow control from a circular disc travelling across a circular flow pas-
sage becomes satisfactory only between the 50% closed and the fully
closed positions. For this reason, parallel gate valves are normally used
for on-off duty only, though some types of parallel gate valves have
also been adapted for flow control, for example, by V-porting the seat.

The parallel gate valves shown in Figure 3-25 through Figure 3-28 are
referred to as conventional parallel gate valves, and those of Figure 3-29
through Figure 3-32 are referred to as conduit gate valves. The latter are
full-bore valves, which differ from the former in that the disc seals the
valve body cavity against the ingress of solids in both the open and
closed valve positions. Such valves may therefore be used in pipelines
that have to be scraped.

Conventional Parallel Gate Valves

The valves shown in Figure 3-25 through Figure 3-28 are representa-
tive of the common varieties of conventional parallel gate valves.
One of the best known is the valve shown in Figure 3-25, commonly
referred to as a parallel slide gate valve. The closure member consists of
two discs with springs in between. The duties of these springs are to keep
the upstream and downstream seatings in sliding contact and to improve
the seating load at low fluid pressures. The discs are carried in a belt eye
in a manner that prevents their unrestrained spreading as they move into
the fully open valve position.
The flow passage of this particular parallel slide gate valve is venturi
shaped. The gap between the seats of the fully open valve is bridged by
an eyelet to ensure a smooth flow through the valve. The advantages
offered by this construction include not only economy of construction
but also a reduced operating effort and lower maintenance cost. The only
disadvantage is a slight increase in pressure loss across the valve.
The seating stress reaches its maximum value when the valve is nearly
closed, at which position the pressure drop across the valve is near maxi-
mum; but the seating area in mutual contact is only a portion of the total
seating area. As the disc travels between the three-quarter closed to the
nearly closed valve position, the flowing fluid tends to tilt the disc into