Pneumatic trip valve partial stroking arrangement

An actuator assembly for a pneumatic trip system, the actuator assembly comprising:

• a cylinder;
• a piston configured to be connected to a trip valve, the piston being movably disposed within the cylinder between a first position corresponding to an open position of the trip valve and a second position corresponding to a closed position of the trip valve, and the piston dividing the cylinder into a first chamber and a second chamber;
• a biasing element disposed in the second chamber of the cylinder, the biasing element engaging the piston to bias piston toward the second position;
• a pneumatic circuit in communication with the second chamber of the cylinder;
• and
• a pneumatic mechanism configured to pressurize the first chamber of the cylinder,
• wherein the cylinder includes a first port for placing the first chamber in communication with an exterior of the cylinder such that the first chamber of the cylinder can be pressurized to cause the piston to remain in the first position,
• wherein the pneumatic mechanism is in communication with the first chamber of the cylinder via the first port and is configured to pressurize the first chamber to hold the piston in the first position and to release pressure from the first chamber of the cylinder to allow the piston to move to the second position under the bias of the biasing element,
• wherein the cylinder includes a second port for placing the second chamber in communication with the pneumatic circuit, and
• wherein the pneumatic circuit is configured to pressurize the second chamber of the cylinder to actuate the piston to move to an intermediate position between the first position and the second position corresponding to an exercised position of the trip valve while the first chamber is pressurized by the pneumatic mechanism.

A method for exercising a trip valve of a pneumatic trip system for a turbine, wherein the trip valve comprises a valve member operatively associated with a turbine flow path and a valve stem connected to the valve member, and wherein the valve member is configured to be actuated to engage the turbine flow path to close the flow path and prevent fluid flow through the turbine flow path, the method comprising:

• providing an actuator assembly operatively connected to the valve stem, the actuator assembly comprising:
• a cylinder;
• a piston movably disposed within the cylinder and dividing the cylinder into a first chamber and a second chamber;
• a biasing element disposed in the second chamber of the cylinder, the biasing element engaging the piston;
• a pneumatic circuit in communication with the second chamber of the cylinder;
• and
• a pneumatic mechanism configured to pressurize the first chamber of the cylinder,
• wherein the piston is operatively connected to the valve stem to move the valve stem and the valve member between an open position allowing fluid flow through the turbine flow path and a closed position preventing fluid flow through the turbine flow path,
• wherein the biasing element engages the piston to bias the valve stem and the valve member toward the closed position,
• wherein the cylinder includes a first port for placing the first chamber in communication with an exterior of the cylinder such that the first chamber of the cylinder can be pressurized to cause the piston to hold the valve stem and the valve member in the open position,
• wherein the pneumatic mechanism is in communication with the first chamber of the cylinder via the first port, and
• wherein the cylinder includes a second port for placing the second chamber in communication with the pneumatic circuit;
• pressurizing the first chamber of the cylinder with the pneumatic mechanism to cause the piston to hold the valve stem and the valve member in the open position;
• releasing the pressure from the first chamber of the cylinder with the pneumatic mechanism to allow the piston to move the valve stem and the valve member to the closed position under the bias of the biasing element;
• activating the pneumatic circuit to pressurize the second chamber of the cylinder to actuate the piston to move the valve stem and the valve member to an exercised position between the open position and the closed position while the first chamber is pressurized by the pneumatic mechanism;
• activating the pneumatic circuit to stop pressurizing the second chamber when the valve stem and the valve member reach the exercised position and retain pressure in the second chamber to hold the valve stem and the valve member in the exercised position;
• and
• deactivating the pneumatic circuit to release the pressure from the second chamber and allow the valve stem and the valve member to return to the open position,
• wherein the valve member allows fluid flow through the turbine flow path in the exercised position.