Principle Of Operation; Type 92B Operational Schematic - Emerson Fisher 92B Instruction Manual

CONTROL LINE
ORIFICE
MAIN VALVE
DIAPHRAGM
VALVE PLUG
E0672_1
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
LOADING PRESSURE

Principle of Operation

Refer to the operational schematic in Figure 2.
Compression of the pilot spring pushes the diaphragm
down and holds the 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
orifice. 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
PILOT CONTROL LINE
MAIN VALVE SPRING
Figure 2. Type 92B Operational Schematic
PILOT SPRING
PILOT SUPPLY LINE
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.
A 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 exceeds 40 psid / 2.8 bar d,
the check valve opens to relieve diaphragm loading
pressure into the downstream system, thereby
preventing a high differential across the diaphragm
which might otherwise cause diaphragm damage. The
check valve closes and normal operation resumes
when the differential pressure across the diaphragm is
reduced to the proper level.
Type 92B
PILOT
DIAPHRAGM
PILOT VALVE
PLUG
3