Which valve type provides minimal pressure drop when fully open and is best for isolation?

• A. Gate valve provides minimal pressure drop when fully open and is best for isolation.
• B. Globe valve provides throttling control but has higher pressure drop.
• C. Ball valve has the most resistance to flow when fully open.
• D. Needle valve is used for coarse throttling in steam lines.

Answer: (A)

The key idea is that the valve that gives the least resistance when fully open is the one with a straight, unobstructed flow path. A gate valve achieves this by moving a gate completely out of the flow, leaving a bore that matches the pipe diameter. When fully open, the path through the valve is essentially the same as through the pipe, so friction and turbulence are minimized and the pressure drop is very small. At the same time, gate valves are excellent for isolation because they can shut off flow tightly when closed.

In contrast, a globe valve routes flow through a more restricted path to provide throttling, so even when open it creates more flow resistance and a higher pressure drop. The needle valve is built for fine throttling, not a wide open path, and is not used for coarse isolation. The ball valve can offer a low pressure drop when fully open, but the situation described emphasizes minimal drop and reliable isolation, which aligns with a gate valve.