While drafting and flowing water, a pump operator sees a reading of -75 kPa on the compound gauge. What does this indicate?

While drafting and flowing water, a pump operator sees a reading of -75 kPa on the compound gauge. What does this indicate?

• A. The maximum vacuum capability is achieved.
• B. The pump is delivering full discharge pressure.
• C. There is a cavitation condition.
• D. There is a leak in the intake.

Answer: (A)

On the suction side, negative readings on a compound gauge indicate vacuum. A reading of -75 kPa means the pump is pulling 75 kilopascals of vacuum below atmospheric pressure. At sea level, that puts the suction absolute pressure around 26 kPa, which is near the practical limit of what the pump can develop while drafting. This shows the pump has reached its maximum suction capability under the current conditions. It isn’t an indication of full discharge pressure, which would show on the discharge side as a positive reading. Cavitation would occur if the absolute pressure dropped below the water’s vapor pressure (typically a much lower value for room-temperature water), which isn’t the case here since the absolute pressure is still well above that threshold. A leak in the intake could cause a drop or fluctuations in suction, but a steady high vacuum reading primarily signals the pump is operating near its suction limit rather than identifying a leak.

On the suction side, negative readings on a compound gauge indicate vacuum. A reading of -75 kPa means the pump is pulling 75 kilopascals of vacuum below atmospheric pressure. At sea level, that puts the suction absolute pressure around 26 kPa, which is near the practical limit of what the pump can develop while drafting. This shows the pump has reached its maximum suction capability under the current conditions. It isn’t an indication of full discharge pressure, which would show on the discharge side as a positive reading. Cavitation would occur if the absolute pressure dropped below the water’s vapor pressure (typically a much lower value for room-temperature water), which isn’t the case here since the absolute pressure is still well above that threshold. A leak in the intake could cause a drop or fluctuations in suction, but a steady high vacuum reading primarily signals the pump is operating near its suction limit rather than identifying a leak.