Variable volume bearing compartment

1. A bearing compartment assembly comprising:

a bearing compartment comprising:

a bearing;

a plenum surrounding the bearing, the plenum having a plenum volume;

a buffer system that supplies pressurized air to an exterior of the bearing compartment to provide a buffer pressure that discourages leakage of oil from the bearing compartment when the buffer pressure exceeds a bearing compartment pressure within the bearing compartment; and

a reactive volume vessel that is in fluid communication with the plenum and with the buffer system, the reactive volume vessel comprising a housing with a cavity and a displaceable member that is movable within the cavity between a standard volume position and a maximum additional volume position in response to a pressure differential between the buffer pressure and the bearing compartment pressure to define a variable additional volume that is in fluid communication with the plenum volume, wherein, during a transition period when buffer pressure is decreasing, the displaceable member moves so that the reactive volume vessel adds the variable additional volume to the plenum volume to decrease bearing compartment pressure at a rate that is similar to or faster than a rate of decrease of the buffer pressure and thereby maintains the pressure differential positive during the transition period.

2. The bearing compartment assembly of claim 1 , wherein a first side of the displaceable member is fluidly exposed to the plenum and a second side of the displaceable member is fluidly exposed to the buffer system.

3. The bearing compartment assembly of claim 1 , further comprising:

an elastic member that biases the displaceable member toward the maximum additional volume position.

4. The bearing compartment assembly of claim 1 , wherein the displaceable member is a piston.

5. The bearing compartment assembly of claim 4 , further comprising:

a seal positioned between the piston and a housing of the reactive volume vessel.

6. The bearing compartment assembly of claim 4 , further comprising:

a spring that biases the piston toward the maximum additional volume position.

7. The bearing compartment assembly of claim 1 , further comprising:

a breather line in fluid communication with the plenum at a first end and to atmosphere at a second end.

8. The bearing compartment assembly of claim 7 , further comprising:

a check valve positioned between the first end and the second end of the breather line, the check valve configured to allow flow from the plenum to atmosphere.

9. A gas turbine engine comprising:

the bearing compartment assembly of claim 1 ;

a stationary component;

a rotating component; and

wherein the bearing is positioned between the stationary component and the rotating component; and

wherein the reactive volume vessel is positioned exterior to the bearing compartment in the gas turbine engine so that oil deposited in the reactive volume vessel from an air/oil mixture inside the bearing compartment will drain out of the reactive volume vessel to the bearing compartment during and after shutdown of the gas turbine engine.

10. A reactive volume vessel comprising:

a housing with a cavity comprising:

a first port configured to be in fluid communication with a bearing compartment of a gas turbine engine; and

a second port configured to be in fluid communication with a buffer system of the gas turbine engine;

a displaceable member positioned in the cavity, the displaceable member being positionable, based upon a pressure differential between a bearing compartment pressure at the first port and a buffer pressure at the second port, between a standard volume position and a maximum additional volume position to define a variable additional volume that augments a volume of the bearing compartment; wherein during a transition period when the buffer pressure is decreasing, the displaceable member moves so that the reactive volume vessel adds the variable additional volume to the volume of the bearing compartment to decrease bearing compartment pressure at a rate that is similar to or faster than a rate of decrease of the buffer pressure and thereby maintains the pressure differential positive during the transition period; and

an elastic member that biases the displaceable member in the additional volume position.

11. The reactive volume vessel of claim 10 , wherein the displaceable member is a piston that is fluidly exposed to the bearing compartment on a first side and is fluidly exposed to the buffer system on a second side, wherein a first surface area on the first side is equal to a second surface area on the second side.

12. The reactive volume vessel of claim 10 , wherein the elastic member is a spring.