Quick closing non-return valves (QCNRVs), also known as steam extraction check valves, protect steam turbines from reverse flow in steam extraction lines. But have you ever wondered why these valves use different shaft configurations?
The answer lies in the way the shaft controls disc movement and interacts with the pneumatic actuator during an emergency trip. During normal operation, a QCNRV behaves as a conventional swing check valve. Steam flows freely through the extraction line with minimal pressure loss. If a turbine trip occurs, the pneumatic actuator forces the valve to close rapidly before reverse steam flow can damage the turbine.
The shaft may appear to be a simple mechanical component, but its configuration has a direct impact on friction, wear and closing performance.
Single shaft configuration
In a single shaft design, keys connect the shaft directly to the disc hinge. Both components rotate together throughout the valve stroke. Because of this direct connection, friction generated by the stem packing also acts on the disc. The flowing steam must overcome this additional resistance during normal operation. One important advantage of this arrangement is its simplicity. Operators can also determine the disc position directly from the external shaft throughout the complete angular stroke. The result is a robust and proven design with a direct mechanical connection between the actuator shaft and the swinging disc.
Double shaft configuration
A double shaft design separates the movement of the shaft from the movement of the disc. Instead of connecting both elements directly, the disc-hinge assembly swings independently during normal operation. A clutch transfers motion through a lost-motion mechanism only when the actuator needs to intervene. This configuration allows the disc to swing freely without transmitting packing friction from the shaft. It also reduces mechanical wear during normal service. When a turbine trip occurs, the clutch engages instantly. The pneumatic actuator immediately forces the valve to close without relying on the inertia of a conventional shaft. The result combines unrestricted disc movement during normal operation with the fast, positive closure required to protect the steam turbine.
Why Babcock offers both configurations
The difference between both designs goes beyond the mechanical arrangement. A single shaft configuration prioritises simplicity and provides direct indication of the disc position through the shaft. A double shaft configuration prioritises the free movement of the disc. By mechanically separating the disc from the shaft during normal operation, it minimises friction while keeping the actuator ready to engage instantly during an emergency trip.
Both configurations protect the turbine effectively. The difference lies in the way each design achieves that objective and the mechanical characteristics it prioritises.