Free Millwright Pumps, Seals, and Packing Questions and Answers — Questions and Answers
Question 1: What is the primary cause of cavitation in a centrifugal pump?
- Excessive discharge pressure (dead-heading)
- Suction pressure dropping below the liquid's vapor pressure (Correct answer)
- Incorrect direction of motor rotation
- Misalignment between the pump and motor
Correct answer: Suction pressure dropping below the liquid's vapor pressure
Cavitation occurs when the pressure of the liquid at the pump's suction drops below its vapor pressure. This causes the liquid to flash into vapor bubbles, which then collapse violently as they move into higher-pressure areas of the pump, causing noise, vibration, and significant damage to the impeller.
Question 2: In a typical mechanical seal, what are the two primary, lapped sealing surfaces called?
- The gland and the shaft sleeve
- The O-ring and the gasket
- The stationary and rotating faces (Correct answer)
- The impeller and the casing wear ring
Correct answer: The stationary and rotating faces
A mechanical seal functions by using two ultra-flat, lapped faces that run against each other. One face is fixed to the stationary pump housing, and the other rotates with the shaft, creating a seal with a microscopic film of fluid between them.
Question 3: When installing new packing rings into a stuffing box, how should the joints of consecutive rings be oriented?
- All joints should be aligned at the top (12 o'clock)
- Staggered by 90 or 180 degrees (Correct answer)
- Randomly placed with no specific orientation
- Aligned with the lantern ring port
Correct answer: Staggered by 90 or 180 degrees
The joints or skives of each packing ring should be staggered, typically by 90 or 180 degrees, relative to the previous ring. Aligning the joints would create a direct path for fluid to leak along the shaft, defeating the purpose of the packing.
Question 4: What is the term for the condition where the pump and motor shafts are parallel but their centerlines are offset from each other?
- Angular misalignment
- Soft foot
- Axial float
- Parallel misalignment (Correct answer)
Correct answer: Parallel misalignment
This condition is known as parallel misalignment. The shafts are parallel to each other in both the vertical and horizontal planes, but their centerlines are not coaxial. The other primary type of misalignment is angular, where the shafts are at an angle to each other.
Question 5: What is the function of a lantern ring (or seal cage) in a pump's stuffing box?
- To prevent the packing from extruding
- To act as a primary bearing surface for the shaft
- To distribute sealing or flushing liquid to the packing (Correct answer)
- To scrape contaminants off the shaft before they reach the packing
Correct answer: To distribute sealing or flushing liquid to the packing
A lantern ring is a perforated ring placed between packing rings. Its purpose is to create a space for a liquid (usually from an external source or the pump discharge) to be introduced into the stuffing box to cool and lubricate the packing, or to flush abrasives away.
Question 6: A pump that uses intermeshing gears, lobes, or screws to move a constant volume of fluid with each rotation is best classified as what type of pump?
- Centrifugal pump
- Positive displacement pump (Correct answer)
- Eductor-jet pump
- Dynamic pump
Correct answer: Positive displacement pump
This describes a rotary positive displacement pump. These pumps work by trapping a fixed amount of fluid and forcing (displacing) it into the discharge pipe. Unlike centrifugal pumps, their flow rate is directly proportional to their speed and is not highly dependent on pressure.
Free Millwright Pumps, Seals, and Packing Questions and Answers
What is the primary cause of cavitation in a centrifugal pump?