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 5.1 Pneumatic. F i g u r e 6-9 illustrates a schematic diagram of a pneumatic
( a i r o p e r a t e d ) s i g n a l t r a n s m i t t e r . T h e transmitter is used to generate an
o u t p u t signal air pressure proportional to the reading of the measuring
T h e output signal, t h e r e f o r e , is related to pressure,
instrument.
t e m p e r a t u r e , or flow as measured by the measuring element installed in the
l i n e . When used in control systems, means are provided for changing the
s e t t i n g and response sensitivity of the equipment.
5 . 2 S o l i d S t a t e . P n e u m a t i c transmitters are being replaced wherever possible
b y s o l i d s t a t e t r a n s m i t t e r s , a s shown in figures 6-10 and 6-11. Solid state
t r a n s m i t t e r s are much more reliable, have no moving parts, are easily changed,
a r e more sensitive, and require less space.
6 AUTOMATIC CONTROLS. Automatic controls, when compared with manual control,
present a number of advantages, including:
q Elimination of human errors.
q Fastest and most accurate response to changing conditions.
q Predetermined pattern of operation.
q Release of operating personnel from routine assignments.
Operators
can be engaged in more productive duties.
T h e types of automatic controls generally used in a distribution system are
d e s c r i b e d in the following paragraphs.
6 . 1 Pressure Controls. Pressure controls are used for two main purposes:
f i r s t , to maintain a substantially constant pressure in one part of a system
r e g a r d l e s s of pressure variations, within the range limits, of the other part
o f the system; and second, to maintain a definite pressure differential
b e t w e e n two points and control the fluid flow. In either case, the fluid
p r e s s u r e upstream of the control is always higher than the control pressure.
P r e s s u r e controls consist of three elements:
( a ) A pressure sensing device
( b ) A regulating valve that controls the passage of the fluid
(c) A connecting element between the pressure sensing device and the
valve
T h e pressure control may be of the self-operated type, consisting of a device
w h i c h operates a regulating valve directly; or it may be of the pilot-operated
t y p e , operating the regulating valve through a pilot or relay valve; or the
v a l v e may be operated following the signal air pressure from a pressure
transmitter.
F i g u r e 6-12 illustrates a typical self-operated, spring-loaded
p r e s s u r e regulator valve. I n this type valve, the controlled pressure is
applied to the top chamber over the diaphragm and the diaphragm movement is
d i r e c t l y transmitted to the control valve through the valve stem.
6 . 1 . 1 T y p i c a l O p e r a t i n g P r i n c ip l e . I n operation, a n increase in the fluid
c o n t r o l l e d pressure pushes the d i a p h r a g m against the resistance of the spring;
6-13
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