Which component can yield K-values ranging roughly 2-10 depending on opening?

• A. 90° elbow
• B. Gate valve throttling
• C. Sudden enlargement
• D. Tee fitting

Answer: (B)

A flow restriction that is adjustable and used to regulate flow can produce a wide change in loss coefficient as the opening changes. The loss coefficient, K, connects the head loss to the velocity head through the component (h_f = K v^2/(2g)). When a valve is throttled, the flow path tightens, velocity through the throat rises and turbulence increases, so the energy loss grows rapidly as the opening closes. A gate valve used for throttling fits this behavior best, with K ranging roughly from about 2 up to 10 depending on how far it is opened. The other items have more fixed, geometry-driven losses: a 90° elbow tends to have a relatively small, stable K; a sudden enlargement yields a K that depends on area ratio but is typically not large for common pipe sizes; a tee has losses distributed between through flow and branch flow with values that don’t vary widely with opening.