Data Sheet
Bulletin 71.2:92B
6
Figure 3. Type 92B Operational Schematic
Refer to Figure 3. Compression of the pilot spring
pushes diaphragm down and holds pilot valve plug
open. Outlet pressure is changed by varying the
amount of pilot spring compression.
When steam enters the inlet of the valve, it also enters
the pilot supply line and flows through the open pilot
valve to the top of the main diaphragm. The force
created by this steam pressure on the diaphragm
overcomes the force of the main valve spring opening
the valve plug and allowing steam to flow downstream.
Downstream pressure registers under the main
diaphragm through the control line and tends to
balance the diaphragm. Steam from the downstream
system also registers under the pilot diaphragm
through line. Pressure forces the diaphragm upward,
permitting the pilot valve plug to move toward the
closed position. Flow of steam to the top of the main
diaphragm is thereby reduced and the pressure on
main diaphragm drops due to the bleed through the
orice. The main valve moves toward the closed
position, allowing only enough steam flow to satisfy
downstream requirements.
When steam demand increases, the downstream
pressure decreases below the setting of the pilot
spring. The pilot opens to increase the pressure on the
main diaphragm. The main valve opens to increase
the flow downstream. Conversely, if the steam demand
decreases, the downstream pressure increases and
the pilot reacts to decrease the pressure on top of
the main diaphragm. The main valve throttles toward
the closed position and the steam flow decreases.
Thus, through the combination of pilot and main valve
operation, control of the downstream steam pressure
is maintained.
An internal check valve is included in all Type 92B
pilots to limit differential pressure on the main valve
diaphragm. In the event of a large decrease in
downstream pressure, the check valve opens to
relieve diaphragm loading pressure to the downstream
system. The check valve cartridge assembly has a
factory setting to limit differential pressure across the
diaphragm to approximately 40 psid / 2.8 bar d. If
diaphragm differential pressure reaches approximately
40 psid / 2.8 bar d, the check valve opens to relieve
diaphragm loading pressure into the downstream
CONTROL
LINE
MAIN VALVE
DIAPHRAGM
VALV E
PLUG
MAIN VALVE SPRING
ORIFICE
PILOT CONTROL LINE
PILOT SUPPLY LINE
PILOT
VALV E
PLUG
PILOT
DIAPHRAGM
PILOT SPRING
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
LOADING PRESSURE
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