Which valve would cause the least head loss in a pipeline when fully open?

The gate valve is designed specifically to allow for minimal resistance when fully open, which is why it causes the least head loss in a pipeline. Unlike other valve types, the gate valve provides a straight-through flow path. This design minimizes turbulence and friction that typically arise with other valve configurations.

When a gate valve is fully opened, it presents a smooth bore that allows water to flow unimpeded, making it an excellent choice for applications that require maximum flow efficiency. Its mechanism, using a sliding gate, effectively eliminates obstructions in the flow path, further reducing head loss.

In contrast, globe valves and throttle valves are designed for regulating flow rather than for minimal loss. Globe valves, while useful for throttling and controlling flow rates, introduce more turbulence due to their restrictive shape when open, resulting in increased head loss compared to gate valves. Throttle valves, designed to adjust flow levels, typically have varying degrees of opening that create back-pressure and associated head loss as well.

Thus, in applications where minimal head loss is desired, the gate valve stands out as the most effective choice due to its design and functionality when fully open.