Principle of Operation
The key to the
operation of a Type 99 regulator is
the yoked double-diaphragm pilot. Fast response and accuracy are made possible
by the amplifying effect of the pressure-balanced pilot and by the two-path
control system. The function of the pilot is to sense change in the controlled
pressure and amplify it into a larger change in the loading pressure. Any changes in outlet pressure act quickly on
both the actuator diaphragm and the loading pilot, thus providing the precise
pressure control that is a characteristic of a two-path system.
A typical pilot has an
approximate gain of 20, which means the outlet pressure needs to droop only
1/20 as much as a direct-operated regulator in order to obtain the same
pressure differences across the main diaphragm. Advantages of a pilot-operated
regulator are high accuracy and high capacity.
Upstream or inlet
pressure is utilized as the operating medium, which is reduced through pilot
operation to load the main diaphragm chamber. Tubing
connects the inlet pressure to the pilot through a filter assembly.
Downstream or outlet pressure registers underneath the main diaphragm through the downstream
control line.
In operation, assume
the outlet pressure is less than the setting of the pilot control spring. The
top side
of the pilot diaphragm
assembly will have a lower pressure than the setting of the spring. Spring
forces the diaphragm head assembly upward, opening the relay or inlet orifice.
Additional loading pressure is supplied to the pilot body and to the top side
of the main diaphragm.
This creates a higher
pressure on the top side of the main diaphragm than on the bottom side, forcing
the diaphragm downward. This motion is transmitted through a lever, which pulls
the valve disk open, allowing more gas to flow through the valve.
When the gas demand in
the downstream system has been satisfied, the outlet pressure increases. The
increased pressure is transmitted through the downstream control line and acts
on top of the pilot diaphragm head assembly. This pressure exceeds the pilot
spring setting and forces the head assembly down, closing orifice. The loading
pressure acting on main diaphragm bleeds to the downstream system
through a small slot
between the pilot bleed valve and bleed orifice.
Normally, excess
loading pressure slowly escapes downstream around the bleed valve (Figure 3)
or through the relief
valve body (Figure 4). Since loading pressure needs to exceed outlet pressure
only moderately to stroke the main valve fully open, a continued increase in
loading pressure differential extends the main diaphragm and the pusher post
assembly far enough to separate the bleed valve and
the bleed orifice. This
action permits quick dumping of excess loading pressure into the downstream
system.
With a decrease in
loading pressure on top of the main diaphragm, the main spring exerts an upward
force on the diaphragm rod connected to the main diaphragm, pulling it in an
upwards direction. This moves the main valve towards its seat, decreasing the flow
to the downstream system.
The pilot valve
diaphragm acts as a sealing member for the loading chamber and as a balancing
member to the upper pilot diaphragm. These two diaphragms are connected by a
yoke so any pressure change in the pilot chamber has little effect on the
position of the pilot valve. Therefore, the active diaphragm in the pilot is the upper pilot diaphragm and
the pressure on the top side of this diaphragm opposes the force of the pilot
control spring.
