Define suction dynamic head and explain why velocity in the suction line is important for pump suction performance?

• A. Suction dynamic head is the difference between atmospheric head and vapor head; velocity has no role.
• B. Suction dynamic head equals the static suction head only.
• C. Suction dynamic head is the vapor head present in suction.
• D. Suction dynamic head is the sum of static suction head and velocity head; high velocity in suction increases losses and can reduce NPSHa.

Answer: (D)

The idea being tested is that suction dynamic head combines the energy due to height with the energy due to motion. Suction dynamic head is the sum of the static suction head (the vertical lift to the pump) and the velocity head (V^2/2g) of the fluid in the suction line. When the velocity in the suction line is high, the velocity head term is large, so more head is needed at the suction to pull the fluid into the pump. In addition, higher velocity increases friction losses in the suction line, which lowers the actual pressure at the pump suction and reduces the Net Positive Suction Head available (NPSHa). If NPSHa becomes too small relative to the pump’s required suction energy (NPSHr), cavitation can occur. This is why the correct description is that suction dynamic head is the sum of static suction head and velocity head, and why high velocity in suction can increase losses and reduce NPSHa.