While pumping ballast the Chief Mate reports there is a decrease in vacuum on the pump suction when drawing from a partially full saddle tank. Which of the following could be a possible cause?
• Behavior of vacuum gauges on the suction side of a centrifugal pump • How air leaks vs flow restrictions (closed valves, clogged strainers) affect suction vacuum • Effect of drawing from a partially full saddle tank (possibility of air being introduced)
• Ask yourself: when the pump has to work harder to pull liquid (more restriction), does the suction vacuum reading usually go up or down? • Which options describe a restriction to flow, and which describe a condition that could let air into the suction line? • Why might the fact that the tank is partially full and is a saddle tank make it easier for air to get into the suction side?
• Identify which choices (B, C, D) would tend to increase suction vacuum because the pump is pulling against a restriction. • Identify which choice could introduce air into the suction side and therefore reduce the vacuum seen at the gauge. • Confirm in your mind that a drop in vacuum usually means the pump is not "pulling" as hard on solid liquid, often because air is entering or the effective suction head has changed.
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