Emerson Process Management 99 - Manual (Page 3)

Type 99
3
Type 99
A6469
January 2008
Type 99
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
ATMOSPHERIC PRESSURE
Principle of Operation
The key to the operation of a Type 99 regulator is the
yoked double-diaphragm pilot (letter keys in this section
refer to both Figures 2 and 3 unless otherwise noted).
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 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 self-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 (E) through the downstream control line.
Figure 2. Schematic of Type 99 Regulator with Type 61L (Low Pressure) Pilot
In operation, assume the outlet pressure is less
than the setting of pilot control spring (A). The top
side of pilot diaphragm assembly (F) will have a
lower pressure than the setting of spring (A). Spring
(A) forces the diaphragm head assembly upward,
opening the relay or inlet orifice (C). Additional loading
pressure is supplied to the pilot body and to the top
side of main diaphragm (E).
This creates a higher pressure on the top side of the
main diaphragm (E) 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 (F). This pressure
exceeds the pilot spring setting and forces the head
assembly down, closing orifice (C). The loading
pressure acting on the main diaphragm (E) bleeds to
the downstream system through a small slot between
the pilot bleed valve (D) and the bleed orifice (H).
Normally, excess loading pressure slowly escapes
downstream around bleed valve (D) (Figure 3) or
A6814
OUTLET PIPE
H
C
D
F
K
B
E
a g
InLET
InLET PRESSURE
OUTLET PRESSURE
LOaDIng PRESSURE
aTMOSPHERIC PRESSURE
Type 99
A6469
January 2008
Type 99
INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
ATMOSPHERIC PRESSURE