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 installed with the pressure on top of the disk. Drain valves with pressure
u n d e r the disk may vibrate open if not tightly closed. I n high temperature
s t e a m applications, v a l v e s should be installed so that the pressure is above
t h e disk. T h i s will eliminate leakage of the steam.
1 . 3 Angle Valves. I n this type of valve the fluid leaves at an angle of 90
f r o m the direction of the entering fluid. Construction and operating
c h a r a c t e r i s t i c s are similar to those of globe valves. Angle valves are mainly
u s e d for convenience in the installation arrangements, as when a 90 turn in
t h e f l u i d d i r e c t i o n i s r e q u i r e d . Their use eliminates an elbow and reduces
t h e number of joints.
1 . 4 Check Valves. Check valves are used to prevent flow reversal of the
f l u i d i n a l i n e . A Swing-check type of valve is illustrated in figure 7-5.
T h e flow of liquid through the swing-check valve is almost straight in its
d i r e c t i o n , being comparable to that of the gate valve. In the lift-check
v a l v e , there is a directional change as flow travels through the valve,
I n operation, b o t h types of valves
s i m i l a r to the flow through globe valves.
h a v e the fluid entering below the seat which lifts the disk and permits the
f l o w . W h e n flow ceases, the disk's weight causes it to close against the
seat. A n y flow reversal shuts the valve tightly. Operation is automatic.
1 . 5 R e l i e f V a l v e s . R e l i e f valves are designed to open automatically to
p r e v e n t pressure in the lines or in pressure vessels from increasing beyond
s a f e t y o p e r a t i n g l i m i t s . T h e y can be used with liquids or gases, including
steam. T h e main difference between a safety valve, used to protect boilers
from overpressures, a n d a relief valve is that the latter requires about 20
percent overpressure to open wide, while the safety valve opens wide at a set
pressure.
I n operation, the relief valve starts to close as the pressure
d r o p s , and shuts off at approximately the set pressure. The discharge pipe
f r o m a relief valve should be positioned to avoid injury to personnel when
d i s c h a r g i n g . A l s o , the installation should be designed to prevent the
d i s c h a r g e end from getting plugged with ice during freezing weather. Relief
valves should not be oversized. O v e r s i z i n g will prevent the valve from
o p e n i n g sufficiently, causing hunting and wire-drawing, with consequent
service trouble and heavy replacement costs.
P e r f o r m the following procedure.
2 OPERATION OF VALVES.
( a ) Close valves tightly, by hand only. Do not employ a wrench or
persuader.
Some large sized valves have impact wheels to help in closing and
opening.
( b ) Open and close valves slowly. This helps prevent water hammer in
water systems. In steam systems, i t permits slow warming of the lines. In
compressed air systems, e s p e c i a l l y those of high-pressure design, this is
required to reduce hazards and prevent surges.
( c ) Before operating large gate valves, use small bypass valves, when
p o s s i b l e , to equalize pressure on both sides of the gate.
( d ) Often a closed valve leaks, due to foreign matter trapped between
t h e seat and disk. This is sometimes eliminated by opening and closing the
v a l v e several times to flush out the deposits.
7-14
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