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

Excessive discharge pressure

Answer

Oversized impeller

Answer

Too much lubrication

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 are cheaper to install

Answer

Mechanical seals are easier to adjust

Answer

Packing always performs better than mechanical seals

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

To make the stuffing box look neat

Answer

To allow faster packing removal

Answer

Ring alignment does not affect performance

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

Only the maximum speed of the pump

Answer

Just the price and model number

Answer

Only the bearing type used

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

To protect the shaft from corrosion

Answer

To prevent the pump from running dry

Answer

To secure the impeller to the shaft

Question 6

Why must a centrifugal pump never be operated against a closed discharge valve for extended periods?

Accepted Answer

Energy added to the fluid converts entirely to heat, rapidly raising fluid temperature and potentially causing vaporization, seal damage, and casing failure

Answer

It will cause the motor to reverse

Answer

The pump will run faster than rated speed

Answer

It will reverse the flow direction