What are typical K-values for a 90-degree elbow, a throttling globe valve, and a sudden enlargement?

• A. 90° elbow ~0.9–1.5; throttling globe valve ~2–10; sudden enlargement K = (1 − A1/A2)^2.
• B. 90° elbow ~0.1–0.3; throttling globe valve ~0.5; sudden enlargement K ~0.1
• C. 90° elbow ~5–7; throttling globe valve ~12–20; sudden enlargement K ~5
• D. 90° elbow ~0.01–0.05; throttling globe valve ~0.2; sudden enlargement K ~0.2

Answer: (A)

Loss coefficients quantify how much head is lost as fluid passes through fittings, with head loss given by h_f = K (V^2)/(2g), where V is the velocity in the relevant section. For common fittings, typical ranges are:

• A 90-degree elbow usually causes a modest energy loss since the flow is redirected rather than severely restricted. The usual K range is about 0.9 to 1.5, depending on how sharp the bend is and the elbow’s radius.

• A throttling globe valve can produce a much larger loss, especially when the valve is throttled, because the flow is forced through a constricted path. Typical K values here span roughly 2 to 10, varying with how much the valve is throttled and the valve design.

• A sudden enlargement has a specific relationship: the loss coefficient is K = (1 − A1/A2)^2, where A1 and A2 are the cross-sectional areas before and after the enlargement. This means K is generally less than 1 for common enlargement ratios and depends on how big the area jump is. For example, if the downstream area is four times the upstream area, K ≈ (1 − 0.25)^2 = 0.56.

So the option listing elbow about 0.9–1.5, throttling globe valve about 2–10, and sudden enlargement with the given K expression matches typical practice. The other options propose ranges that are unrealistically low for elbows and globe valves or give incorrect K values for sudden enlargement.