PT6 start, minimum pressurizing and shutdown valve

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snoopyWWIace
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PT6 start, minimum pressurizing and shutdown valve

Post by snoopyWWIace »

Question about P&W PT6 fuel pressure, but first a little context. I'm trying to simulate some PT6 engine data for a virtual reality project. I'm confused by the fuel pressure in different parts of the fuel system when the engine is starting up.

As I understand it (and correct me if I'm wrong) the FCU has a minimum pressurizing and shutdown valve that ensures the fuel pressure exiting the FCU is at least 100 psi (or maybe psia). HOWEVER, the primary manifold clearly fires before the secondary manifold. This is puzzling because the flow divider allows fuel to the secondary manifold if fuel pressure is greater than 17 psi. Well, the fuel pressure coming out of the FCU should be grater than 100 psi, so why doesn't the secondary manifold fire up immediately instead of after the first manifold?
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‘Bob’
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Re: PT6 start, minimum pressurizing and shutdown valve

Post by ‘Bob’ »

You are confusing two things.

The minimum pressurization and dump valve opens at 100 PSID. This makes sure that any fuel going to the nozzles is at the correct pressure. Remember that this is differential pressure between Po and P2 fuel pressures, not absolute.

The flow divider determines whether the fuel goes to the primaries or primaries and secondaries. Again, you have to pay attention to the type of pressure.

Primary valve opens at 9-13 PSI. No differential other than atmospheric pressure (maybe a tiny bit of compression) because there is nothing beyond except the nozzles. This means that if your minimum pressurization and dump valve fails, you can have fuel dribbling out of there at those low pressures. That pressure is only so the valve will stay closed when not in operation, and allow fuel to be purged at lower pressures on shut down.

Secondary opens at 17-22 PSID. This is the differential between P2 fuel pressure and the P3 air that is in the combustion chamber from the compressor. While the primaries open right away as soon as you move the condition lever on a normal start, the secondaries don’t open until about 35% Ng. This makes the start a lot cooler.

Common trouble points are the valve sticking in the cold.. leading to hung starts, and secondaries coming on too soon, leading to if not a hot start, a very very warm start.
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