Quantcast Section 6. Pressure Reducing Stations

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PRESSURE REDUCING STATIONS
Section 6.
1 PRESSURE REDUCTION.  Steam is generally distributed at pressures
substantially higher than required for consumer uses.  Pressure reducing
stations are employed to lower high-pressure steam down to the working
pressure of the system.  Low-pressure steam can then be safely used for
heating purposes or for process work.
2 REDUCING STATION COMPONENTS.  A pressure reducing station may consist of a
p r e s s u r e reducing valve, pressure regulating valve, relief valve, strainer,
a u x i l i a r y valves, a n d pressure gauges (figure 3-16).
2.1 Pressure Reducing Valve.  This is the primary valve of the pressure
r e d u c i n g station.  Its main purpose is to reduce the incoming steam pressure
to a safe level below the rupture point of the equipment served. These valves
c a n be either self-operated or pilot-operated; single-seated or double-seated;
s p r i n g - l o a d e d or weight-lever operated.
2 . 1 . 1 Pilot-Operated Valves.  These valves may be operated by either internal
o r external pilot valves.  The unit consists of a small pilot valve and a
p i s t o n or diaphragm actuated main valve. In operation, the controlled
pressure governs the movement of the small pilot valve, which develops a
variable loading pressure that depends upon the controlled pressure.  The
loading pressure then controls the movement of the main valve. The control
fluid used by the pilot valve may be compressed air, steam, or it may be the
c o n t r o l l e d f l u i d i t s e l f .  F i g u r e 3-17 illustrates a pressure reducing valve
w i t h an external pilot valve.  The controlled fluid enters the chamber below
the pilot valve diaphragm through the control pipe and develops an upthrust
which is balanced by the adjusting spring.  This determines the position of
t h e pilot valve.  The fluid enters the pilot valve through pipe A. The amount
o f fluid passing through is determined by the position of the pilot valve.
T h e bleed port restricts the fluid flow from the pilot which builds pressure
under the main valve diaphragm and opens the main valve against the pressure
of the main spring.  I n operation, delivery pressure feeds back through the
c o n t r o l pipe to the pilot diaphragm.  As this pressure approaches a balance
w i t h the thrust of the adjusting spring, the pilot throttles the loading
pressure.  In turn, the main valve takes a position established by the loading
pressure where just enough fluid flows to maintain the set delivery pressure.
The set delivery pressure can be changed as required by changing the
compression of the adjusting spring.
2 . 1 . 2 Valve Seating.  Single-seated valves are used for deadend service,
where pressure must be maintained during no-flow periods.  This type of valve
can be closed tightly.  Balanced seating valves require smaller forces to
operate since the pressure acts on both sides of the disk.  They do not close
t i g h t l y .  Some valves are V-ported to provide throttling in the early stages
of opening and closing.
2.1.3 Valve Loading.  Self-operated pressure control valves may be either
spring-loaded or weight-loaded.  The controlled pressure can be adjusted by
changing the spring compression or by changing the lever position of the
weight.  In spring-loaded valves, the setting can be locked, which prevents
o p e r a t i n g trouble.  The setting of weight-loaded valves is more easily
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