Millwright Certification Pumps, Seals, and Packing 2

Question 1

What is cavitation in a centrifugal pump, and what causes it?

Accepted Answer

Formation and collapse of vapor bubbles when suction pressure drops below the liquid's vapor pressure, caused by insufficient NPSH available

Answer

Cavitation occurs when the pressure at the pump suction drops below the liquid's vapor pressure, causing bubbles to form. These bubbles collapse violently when they reach higher-pressure zones inside the pump, causing noise, vibration, and erosion.

Question 2

What is the primary advantage of a mechanical seal over packing in pump shaft sealing?

Accepted Answer

Mechanical seals provide near-zero leakage, lower friction, and longer service life than packing

Answer

Mechanical seals use precision-lapped sealing faces that provide near-zero visible leakage, significantly less shaft friction and power consumption, and longer service intervals compared to traditional compression packing.

Question 3

When repacking a pump stuffing box, why should the packing rings be installed with staggered joints?

Accepted Answer

Staggering the cuts prevents a continuous leak path through aligned joints, improving sealing effectiveness

Answer

If all packing ring joints are aligned, they create a direct leak path for the pumped fluid. Staggering joints (typically 90 degrees apart) forces any leakage to traverse the full ring before reaching the next joint, greatly reducing total leakage.

Question 4

What does the pump curve tell you about a centrifugal pump's performance?

Accepted Answer

The relationship between flow rate, head (pressure), efficiency, power consumption, and NPSHr at various operating points

Answer

A pump performance curve plots head (pressure) versus flow rate, showing how the pump performs across its operating range. It also includes efficiency, power required, and NPSHr curves at various flow rates.

Question 5

What is the function of a wear ring in a centrifugal pump?

Accepted Answer

To maintain a close clearance between the impeller and casing to minimize internal recirculation and maintain efficiency

Answer

Wear rings are replaceable clearance components installed on the impeller or casing (or both) that maintain a tight clearance to minimize internal fluid recirculation from high-pressure discharge back to low-pressure suction.

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Millwright Certification Pumps, Seals, and Packing 2