Which action should never be performed on gate valves?

Throttling a gate valve is generally not recommended because gate valves are designed to operate either fully open or fully closed. They are not intended to regulate flow or to throttle, as doing so can cause issues such as vibration, wear, and potential damage to the valve over time. When a gate valve is partially opened to throttle the flow, it can lead to turbulence in the flow inside the pipe, which may increase wear on the seat and should be avoided to maintain the integrity and longevity of the valve.

In contrast, the other actions listed—closing completely, opening slowly, and regular lubrication—are all appropriate practices for the maintenance and operation of gate valves. Closing the valve completely ensures a tight seal and prevents backflow, while opening slowly helps to manage pressure changes and avoid water hammer, which can be damaging to the system. Lubricating the operating mechanism of the valve is also important for preventing wear and ensuring smooth operation.