Millwright Certification Pumps, Seals, and Packing 2 — Questions and Answers

Question 1: What is cavitation in a centrifugal pump, and what causes it?

Excessive discharge pressure
Formation and collapse of vapor bubbles when suction pressure drops below the liquid's vapor pressure, caused by insufficient NPSH available (Correct answer)
Oversized impeller
Too much lubrication

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

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.

Cavitation requires that NPSHa (Net Positive Suction Head available) falls below NPSHr (required by the pump design). Causes include: suction line too long or too small, suction lift too high, clogged suction strainer, fluid temperature too high, operating far right on the pump curve, or low suction tank level. Prevention: ensure NPSHa exceeds NPSHr by a minimum margin (typically 1-1.5 meters), reduce suction friction losses, or use a larger pump with lower NPSHr.

Question 2: What is the primary advantage of a mechanical seal over packing in pump shaft sealing?

Mechanical seals are cheaper to install
Mechanical seals provide near-zero leakage, lower friction, and longer service life than packing (Correct answer)
Mechanical seals are easier to adjust
Packing always performs better than mechanical seals

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

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.

Mechanical seals consist of two precision-lapped flat surfaces (one rotating with the shaft, one stationary in the housing) that create a seal with minimal contact and near-zero leakage. Advantages over packing: virtually no visible leakage, 80-90% less shaft friction, no shaft sleeve wear, longer service life (1-5 years vs. months for packing), and cleaner operation. Disadvantages: higher initial cost, requires more precise installation, sensitive to dry running, and more complex to replace in the field.

Question 3: When repacking a pump stuffing box, why should the packing rings be installed with staggered joints?

To make the stuffing box look neat
Staggering the cuts prevents a continuous leak path through aligned joints, improving sealing effectiveness (Correct answer)
To allow faster packing removal
Ring alignment does not affect performance

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

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.

Compression packing is installed as individual rings with butt or diagonal cut joints. Proper installation: remove all old packing and clean the stuffing box and shaft/sleeve. Cut new rings using a mandrel. Install rings one at a time, seating each firmly. Stagger each ring's joint 90 degrees from the previous ring. Ensure the lantern ring aligns with the flush/lubrication port. Install the gland and tighten only hand-tight initially. Start the pump and gradually tighten the gland until leakage is reduced to the desired rate (typically 40-60 drops per minute for water, as packing must leak to lubricate and cool).

Question 4: What does the pump curve tell you about a centrifugal pump's performance?

Only the maximum speed of the pump
The relationship between flow rate, head (pressure), efficiency, power consumption, and NPSHr at various operating points (Correct answer)
Just the price and model number
Only the bearing type used

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

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.

The pump curve plots: Head vs. Flow (the primary curve), Efficiency vs. Flow (peaks at the Best Efficiency Point or BEP), Power vs. Flow (shaft power required), and NPSHr vs. Flow (increases with flow). The system curve (head losses from friction and static head) overlays the pump curve: their intersection is the operating point. Operating far from BEP causes recirculation (low flow), cavitation (high flow), increased vibration, bearing loading, and seal problems. Best practice is to operate within 80-110% of BEP flow.

Question 5: What is the function of a wear ring in a centrifugal pump?

To protect the shaft from corrosion
To maintain a close clearance between the impeller and casing to minimize internal recirculation and maintain efficiency (Correct answer)
To prevent the pump from running dry
To secure the impeller to the shaft

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

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.

In a centrifugal pump, there is a pressure difference between the discharge zone (high pressure) and suction zone (low pressure) of the impeller. Without close clearance, fluid continuously recirculates internally, reducing efficiency and capacity. Wear rings are sacrificial clearance components installed at the impeller eye. Industry guidelines suggest replacing wear rings when clearance exceeds 2 times the original design clearance. Typical new clearances are 0.25-0.50 mm diameter depending on pump size.

Question 6: Why must a centrifugal pump never be operated against a closed discharge valve for extended periods?

It will cause the motor to reverse
Energy added to the fluid converts entirely to heat, rapidly raising fluid temperature and potentially causing vaporization, seal damage, and casing failure (Correct answer)
The pump will run faster than rated speed
It will reverse the flow direction

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

With the discharge valve closed, all energy from the motor converts to heat within the trapped fluid. The temperature rises rapidly, and in extreme cases the fluid can vaporize, destroying seals and potentially causing casing failure from thermal expansion.

A centrifugal pump operating against a closed discharge valve (deadheaded condition) converts all input power to heat within the small volume of trapped fluid. Temperature rise can be dramatic: several degrees per minute for larger pumps. Consequences include: rapid fluid temperature rise causing seal face damage, fluid vaporization, thermal expansion potentially causing casing cracking or gasket blowout, and impeller damage from recirculation. Pumps should have minimum flow protection: a recirculation line back to the suction source, pressure relief valve, or temperature switch.

Millwright Certification Pumps, Seals, and Packing 2

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