X. Operating Principles (Cont.)
B. 2900 Series 39MV07 (Modulating) Pilot – Operational Description (Cont.)
MV07 Valve Modulating (Partial Relieving Position)
As inlet pressure increases, the pilot piston strokes and seals off the main valve inlet pressure from the
dome pressure. The pilot simultaneously opens the vent seal to relieve the dome pressure to the bottom of
the modulator piston. The modulator piston has a differential area with the smaller area being on top of the
modulator piston. The top of this piston always sees the main valve inlet pressure. When the dome pressure
is applied to the bottom of the modulator piston, there is a net upward force. This is due to both pressures
being equal (at this point), and the lower area being larger than the upper area. The modulator relieves the
pressure from the dome to the atmosphere until force from the inlet pressure on top of the modulator piston is
sufficient to move it to the closed
position. A certain amount of
pressure remains in the dome.
This pressure is controlled by the
differential area in the modulator.
Since the dome pressure has not
been dropped to atmospheric
pressure, the main valve only
partially opens at the set point.
The modulator piston will remain
closed until the main valve
disc is forced into higher lift by
increasing inlet pressure. As this
occurs, the modulator piston may
relieve further pressure from the
dome as necessary to achieve
the required main disc lift within
10% overpressure.
MV07 Fully Open
(Full Relieving Position)
As the inlet pressure increases
further, the net upward force on
the main valve disc increases,
allowing the main valve to relieve
more pressure. The disc obtains
full lift (full capacity) within
10% of set pressure. When
the discharging valve reduces
the inlet pressure to the preset
blowdown pressure of the pilot,
the pilot piston closes the vent
seal. Simultaneously, the inlet
seal is reopened in the pilot. The
main valve inlet pressure is again
allowed to enter the dome above
the main valve piston. As the
dome pressure equalizes with
the inlet pressure, the downward
force created by the differential
areas of the piston and disc
closes the main valve.
Shown
out of
Position
To
Dome
To
Dome
Figure 15: MV07 Valve Modulating (Partial Relieving Position)
Consolidated 2900 Series Generation II POSRV Instruction Manual
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