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 Section 3.
VALVES AND FITTINGS
1 VALVES. Valves are used in heat conveying systems and in compressed air
distribution lines for the following purposes:
q To stop the flow of the medium
q To control the flow of the medium
q To
prevent
flow
reversal
q To
prevent
excess
pressure
1 . 1 Gate Valves. A gate valve basically consists of a body and a disk that
s l i d e s across the flow path and shuts it off. Gate valves are used in lines
w h e r e unrestricted flow is important, since with the valve fully open, a
s t r a i g h t w a y flow area is provided through the valve. They should not be used
f o r flow regulation or throttling service; when so used, the gate may vibrate
and chatter, with subsequent damage to the seating surfaces. T h r o t t l i n g a l s o
c a u s e s erosion of the lower edge of the seat rings. G a t e valves are properly
used as stop valves. T h e y are built in the following four different types,
a c c o r d i n g to the variations in their stem or gate construction.
(a) Outside Screw Rising Stem and Yoke. I n t h i s t y p e o f v a l v e ( r e f e r t o
f i g u r e 7-3) the stem rises while the handwheel remains in the same level. The
p o s i t i o n of the disk is indicated by the position of the stem. The exposed
threads of the stem should be protected from damage and corrosion.
( b ) Nonrising Stem. Here the stem rotates in the bonnet and is threaded
t o the disk. Wear on the packing is reduced but the stem is exposed to the
f l u i d passing through the valve.
( c ) Wedge Disk. T h e gate of this type of valve is a solid wedge that
slides between tapered seats.
Double Disk. The gate here consists o f two separate disks with a
(d)
spreader
mechanism between them. While the val v e is being closed, the disk
spreader
contacts the bottom of the valve body a n d forces the disks apart
against
parallel-faced seating rings.
1 . 2 Globe Valves. T h e globe valve is so calle d because of its globular
o u t l i n e . A s illustrated in figure 7-4, there is a directional change as flow
t r a v e l s through a globe valve, which results in greater frictional
G l o b e valves are used for throttling purposes and flow
r e g u l a t i o n . They may have plug or disk type valves and seats. The disks can
be of metal or composition. M e t a l disks are appropriate for throttling
s e r v i c e ; composition disks, however, are not recommended for this service
because of the severe cutting action which may damage the disk face. Globe
v a l v e s placed in a horizontal steam line should be installed with the stem in
a h o r i z o n t a l p o s i t i o n , t o prevent collection of condensate in the valve and
l i n e . F l o w direction through the valve depends on the service intended. In
f e e d lines, the valve is installed so that-the pressure acts under the disk; a
l o o s e disk in that case will not act as a check to prevent flow.
On open-end
and drain lines, however, some engineers recommend that globe val ves be
7-11
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