Instruction Manual

Types 1808 and 1808A
3
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As long as the inlet pressure is below set pressure,
the pilot control spring keeps the valve plug closed.
Inlet pressure passes through the pilot restriction and
registers as loading pressure on top of the diaphragm.
Force from the main spring, in addition to inlet pressure
bleeding through the pilot restriction, provide downward
loading pressure to keep the main valve closed.
When the inlet pressure rises above the set pressure,
the pressure on the pilot diaphragm overcomes the
pilot control spring and opens the pilot valve plug. The
pilot then exhausts the loading pressure from the top
of the main valve diaphragm. The pilot continuously
exhausts gas when the inlet pressure is above the set
pressure. The inlet pressure unbalance overcomes
the main spring force and opens the main valve.
As the inlet pressure drops, the pilot control spring begins
to close the pilot valve plug and the exhaust slows. This
causes the inlet pressure to build in the main valve
diaphragm casing, allowing the control spring to close
the main valve. Once the main valve is closed, the pilot
valve plug closes and the exhaust stops.

As long as inlet pressure remains below setpoint, the
pilot spring keeps the valve plug closed. Inlet pressure
passes through the upper port around the upper portion
of the valve plug then through the hollow passage in that
valve plug. Force from the main spring, along with inlet
pressure bleeding through the pilot, provide downward
loading pressure to keep the main valve closed.
When inlet pressure rises above the set pressure,
pressure on the pilot diaphragm overcomes the control
spring to close the upper port and stroke the valve
plug to open the lower port. The pilot then exhausts
loading pressure from the top of the main valve
diaphragm. The pilot exhausts only while repositioning
the main valve. The inlet pressure unbalance
overcomes the spring force and opens the main valve.
As the inlet pressure drops, the pilot control spring
begins to close the pilot valve plug and the exhaust
slows. This causes the inlet pressure to build in the
main valve diaphragm casing, allowing the control spring
to close the main valve. Once the main valve is closed,
the pilot valve plug closes and the exhaust stops.
Figure 2. Type 1808 with 6358 Series Operational Schematics
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E0075
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