Data Sheet
Bulletin 71.2:92B
7
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.
Refer to Figure 4. Once placed in operation, the upstream
Type 92B pilot (B) senses the intermediate pressure
between both valves and the Type 6492HM (A) pilot
senses pressure downstream of the second valve. As
demand for flow increases, intermediate pressure will
fall causing the Type 92B pilot to open. As the Type 92B
pilot opens, loading pressure to the main valve increases,
opening the main valve.
The Type 6492HM (A) safety override pilot remains
open because its setpoint is above the setpoint of
the downstream valve. In the unlikely event that the
downstream valve fails open, downstream pressure
will rise above the downstream valve’s setpoint. This
pressure is sensed by the Type 6492HM (A) safety
override pilot. As downstream pressure increases
the Type 6492HM (A) safety override pilot closes,
reducing loading pressure to the upstream main valve,
which positions the main valve to maintain desired
downstream override pressure.
In the event that the upstream valve fails, the
downstream valve will prevent downstream pressure
from rising above safe operating levels.
It is recommended to install some type of warning
system, such as a sentinel relief valve, to warn the
operator that a valve has failed in the system. This
will prevent prolonged operation with one valve, which
could cause valve trim wear and noise associated with
operation at high differential pressures.
E0794
Figure 4. Safety Override System Schematic