Why are globe valves typically preferred over gate valves for throttling applications in power plant systems?

Globe valves are preferred over gate valves for throttling applications in power plant systems because their design provides better flow control and resistance to erosion when partially open. Throttling refers to restricting flow to control the flow rate or pressure in a system. A globe valve's disc moves perpendicular to the flow path, creating a tortuous path for the fluid. This allows for precise flow regulation by adjusting the distance between the disc and the seat. When a globe valve is partially open, the fluid velocity increases significantly as it passes through the narrow gap, and the resulting pressure drop is controlled. In contrast, a gate valve's disc moves perpendicular to the flow, either fully opening or fully closing the flow path. Gate valves are designed to be fully open or fully closed, offering minimal resistance to flow in the open position. When a gate valve is partially open for throttling, the high-velocity flow can cause significant erosion of the disc and seat due to cavitation and impingement. This erosion can damage the sealing surfaces, leading to leaks and premature valve failure. Furthermore, the vibration induced by the turbulent flow in a partially open gate valve can damage the valve stem and other components. Therefore, while gate valves are suitable for on-off service where minimal pressure drop is required, globe valves are better suited for throttling applications due to their superior flow control and erosion resistance.