Which type of valve gives good throttling action, takes less actuator force, but does not seal well?

The choice of a double seated valve is appropriate because these valves are designed to provide effective flow control and throttling action by allowing fluid to pass through between two separate seats. This configuration enables good control over the flow rate with less actuator force required compared to other types of valves, making them advantageous in applications where precise regulation of flow is necessary.

However, the nature of their design means that double seated valves do not seal as effectively as single seated valves; they rely on two seats and two plugs to create the seal, which can lead to potential leakage when the valve is closed. This is due to the complexity of their seating mechanism and the fact that the two plugs must align and seal simultaneously, which can introduce some inefficiency in sealing capabilities.

In contrast, valves such as direct acting and reverse acting typically prioritize good sealing over throttling ability, while single seated valves often excel in sealing but may require greater actuator force for throttling control. Therefore, double seated valves strike a balance between needing less force for actuation and providing reasonable throttling, but they compromise on sealing effectiveness.