When up-hole velocities decrease, which could be a probable cause?

• A. Reduced air pressure
• B. Blockage in the air stream
• C. Increased hole diameter
• D. Shorter drill string

Answer: (C)

The main idea here is how the velocity of air moving up the borehole depends on the cross-sectional area it has to travel through. With the same volume of air being pushed upward, increasing the hole diameter spreads that air over a larger area, so its speed must drop. This is the straightforward result of v = Q/A: if Q stays the same and A increases, v decreases. In air rotary drilling, a slower up-hole velocity reduces the air’s ability to lift cuttings, which can lead to slower cleaning and potential problems down the hole. Among the options, increasing the hole diameter directly explains a decrease in up-hole velocity, because it expands the flow path and reduces speed. A blockage would reduce flow more abruptly and create pressure changes rather than a smooth, diameter-driven drop in velocity. A shorter drill string or reduced air pressure affects the system but doesn’t inherently cause the straightforward, area-based drop in up-hole velocity.