Hamilton Sundstrand Company Gas Fuel Metering Valve HFG2.0 - Manual (Page 39)

Orifice Assembly
The orifice assembly contains a set of poppets and orifice plates. Fuel
flow is metered between the INLET port chamber and OUTLET port
chamber in proportion to the poppet position and resultant flow area.
The poppet assembly is connected to the extension rod of the actuator.
As the actuator retracts, the poppet assembly retracts to increase the flow
area between the poppets and the two orifices. As the actuator extends,
the poppet assembly extends to reduce the fuel exposure from the INLET
port chambers to the two orifices. When the actuator is fully extended, the
poppets seat in the orifices to close the valve. See Figure 2-3.
Soft Seats
The orifices contain pressure-energized soft seats and metal-to-metal
hard seats, which provide a leak-tight seal when the poppets are in a
closed position. See Figure 2-3.
Return Spring
A pre-loaded, fail-safe spring is located between the valve housing and
poppet assembly. The spring load increases as the valve opens. If a
power failure occurs, the spring causes the poppets to return to the soft
seats, thereby closing the valve. See Figure 2-3.
CH. 2: UNDERSTANDING THE HFG2.0 33