What type of valve gives good throttling action but does not seal well?

A double seated valve is designed with two seats and two plugs, which allows for improved control over flow regulation, hence providing effective throttling action. The design facilitates balanced forces across the valve, making it less susceptible to wear and providing smoother operation during throttling. However, this construction generally leads to a compromise in sealing capability, especially under certain conditions—such as when there is a significant differential pressure or when the valve is partially open.

In practical applications, while double seated valves excel in applications where flow modulation is essential, their ability to create a tight seal when fully closed isn't as effective compared to some other valve types, such as globe or single seated valves. Therefore, while they offer good throttling capabilities, they might allow some leakage, impacting their sealing performance under certain operating conditions. This combination of features is essential for industries that require precise flow control without necessarily needing a perfect seal when the valve is in a closed position.