Principle Of Operation; Installation; Safety Override Pilot Principle Of Operation - Fisher 92S Instruction Manual

Type 92S

Principle of Operation

Pilot supply pressure is piped from the main valve inlet
(Figure 2) to the pilot inlet connection. Downstream
pressure registers on the main valve pistons through the
downstream control line and then on the pilot diaphragm.
When increased downstream demand lowers the
downstream pressure to a valve below the setting of
the pilot control spring, this spring forces the pilot valve
plug open to increase the loading pressure on the
main valve pistons. At the same time, the increased
demand lowers the downstream pressure on the
main valve piston(s). This opens the main valve plug,
increasing flow to the downstream system to satisfy
the increased demand and to restore downstream
pressure to the setting of the pilot control spring.
Decreased downstream demand increases the
downstream pressure registered on the pilot
diaphragm. The increased pressure overcomes the
force of the pilot control spring and allows the pilot
valve plug spring to close the pilot valve plug. As the
pilot valve plug closes, excess loading pressure bleeds
to the downstream system through the pilot bleed
restriction. At the same time, decreased downstream
demand increases the downstream pressure
registered on the main valve piston(s). This allows
the main valve spring to close the main valve plug,
reducing flow to the downstream system in response
to the decreased demand.
With a pressure-loaded or on-off pilot, the operation is
the same as for a standard pilot except that the pilot
control spring force on the pilot valve plug is aided
by pneumatic pressure from the loading device or
solenoid valve.

Safety Override Pilot Principle of Operation

Once placed in operation, the upstream Type 6492H
or 6492HT pilot senses the intermediate pressure
between both valves, and the Type 6492HM
or 6492HTM pilot senses downstream pressure of
the second valve. As demand for flow increases,
intermediate pressure will fall causing the Type 6492H
or 6492HT pilot to open. As the Type 6492H pilot
valve opens, loading pressure to the main valve
increases, opening the main valve.
The Type 6492HM or 6492HTM 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
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pressure is sensed by the Type 6492HM or 6492HTM
safety override pilot. As downstream pressure
increases the safety override pilot closes, reducing
loading pressure to the main valve, which positions
the main valve to maintain downstream pressure as
specified per ASME Boiler and Pressure Vessel Code,
Section VIII.
In the event that the upstream valve fails, the
downstream regulator 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.
When operating in most steam systems, valve
setpoints should be in strict accordance to ASME
Boiler and Pressure Vessel Code, Section VIII. The
Type 6492HM or 6492HTM safety override pilot should
be set at 10 psig / 0.69 bar or 10% above maximum
downstream operating pressure of the second valve,
whichever pressure is greater. For example, most
HVAC systems operate at 15 psig / 1.0 bar, so the
safety override pilot should be set no higher than
25 psig / 1.7 bar.

installation

!
Personal injury, equipment damage,
or leakage due to escaping steam or
bursting of pressure-containing parts
may result if this regulator is over
pressured or is installed where service
conditions could exceed the limits given
in the Specification section and on
the appropriate nameplates, or where
conditions exceed any ratings of the
adjacent piping or piping connections.
To avoid such injury or damage, provide
pressure-relieving or pressure-limiting
devices to prevent service conditions
from exceeding those limits.
Additionally, physical damage to the
regulator could result in personal injury
and property damage due to escaping
steam. To avoid such injury and damage,
install the regulator in a safe location.
WARNiNg