CPM Power Plant Machinery 2

Question 1

A boiler feed pump shows a sudden rise in noise and a drop in discharge pressure while suction pressure is low. What is the most likely cause?

Accepted Answer

Cavitation

Answer

Low suction pressure below NPSH required causes vapor bubbles that collapse noisily and reduce head.

Question 2

What is the main purpose of the balancing drum or balance disc on a multistage boiler feed pump?

Accepted Answer

Offset axial hydraulic thrust

Answer

Balance devices counter the axial thrust produced by the pressure difference across the impellers.

Question 3

On a steam turbine, which device trips the unit if shaft speed exceeds about 110% of rated speed?

Accepted Answer

Overspeed trip

Answer

The overspeed trip closes the stop valves when speed reaches its setpoint, typically around 110%.

Question 4

Why is a steam turbine placed on turning gear after shutdown?

Accepted Answer

To prevent rotor bowing during cooldown

Answer

Slow rotation lets the rotor cool evenly and prevents thermal sag or bowing.

Question 5

In a surface condenser, what does a gradual rise in backpressure with normal cooling water flow most often indicate?

Accepted Answer

Air in-leakage or tube fouling

Answer

Non-condensable gases and fouled tubes reduce heat transfer, raising condenser pressure.

CPM - Certified Plant Mechanic Practice Test