X. Operating Principles (Contd.)
B. 2900 Series 39MV07 (Modulating) Pilot – Operational Description (Contd.)
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 11: MV07 Valve Modulating (Partial Relieving Position)
24 | BHGE
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