Manual 45002 1907 Large Liquid Fuel Valve
Woodward 3
With ΔP held constant, accurate fuel metering is accomplished by controlling the
metering port opening.
Under operating conditions, fuel at inlet pressure (P1) flows to the metering
sleeve, one side of a bellows, to the bypass valve, and to an orifice. Metered fuel
at pressure (P2) is directed to the turbine and the opposite side of the bellows.
The bellows takes a position at which the sum of pressure P1, and the force of
spring S1acting on one side of the bellows, is equal to the sum of pressure P2
and the force of spring S2acting on the opposite side. When the balance of
forces has been established, the difference between the spring forces (S2–S1) is
equal to the difference between the pressures (P1–P2or ΔP). By varying the
force of spring S1, the ΔP can be adjusted to suit the requirements of a particular
application. The position of the bellows determines the position of the bleed-valve
check-ball, which then regulates the rate of fuel flow through the orifice. Pressure
Pr varies with the rate of fuel flow through the bleed valve, high flows resulting in
relatively lower pressures and low flows resulting in relatively higher pressures.
Pressure Pr plus the force of spring S3close the bypass valve piston. These are
opposed by pressure P1, which opens the bypass valve piston. The bypass valve
piston then takes a position at which pressure P1is equal to the sum of pressure
Pr and the force of spring S3. By varying the amount of fuel bypassed, pressure
P1is maintained at a constant differential above pressure P2, regardless of
variations in pressure P2or flow.
Opening the metering port to increase fuel flow to the turbine results in an
increase in pressure P2. This unbalances the forces across the bellows,
increases the force on spring S1and allows the bleed-valve check-ball to partially
close, reducing the rate of fuel flow through the bleed valve. With reduced fuel
flow, pressure Pr increases and results in an unbalance of the forces across the
bypass-valve piston. The piston moves to decrease the bypass flow and direct a
greater amount of fuel to the metering port. With more fuel being directed to the
metering port, pressure P1increases until the balance of forces across the
bypass valve piston and the bellows is established and further movement of the
piston or bellows is stopped.
Closing the metering port to decrease fuel flow to the turbine results in a
decrease in pressure P2. The resulting unbalance of forces across the bellows
forces the check ball further off its seat, and increases the rate of fuel flow
through the orifice. With the resulting decrease in pressure Pr, the unbalance in
forces across the bypass-valve piston causes the piston to move further open
and bypass a greater amount of fuel with less fuel directed to the metering port.
Pressure P1then decreases until the balance of forces across the piston and
bellows is established and further movement of the piston and bellows is
stopped.
The purpose of the orifice and relief valve (Figure 2-2) is to eliminate bypass
valve damping when a sudden increase in bypass flow is needed. The damping
is needed for ΔP stability in normal operation, and when change in inlet or
metered outlet flows are relatively small. Damping is provided by a restriction
between the bleed valve and bypass valve piston. When large decreases in
metered outlet flow or large increases in inlet flow occur rapidly, it is essential
that the bypass valve open immediately to prevent a transient rise in valve inlet
pressure. This transient rise would raise the pressure differential above the rating
of the ΔP sensing bellows. The damping restriction is provided by the orifice in
the relief valve plunger. If the pressure drop across this restriction exceeds 345
kPa (50 psi) when the bypass valve is moving in the open direction, the relief
valve opens to bypass flow around the damping restrictor. This allows the bypass
valve to open rapidly.
