Millwright Certification Pumps, Seals, and Packing 1 — Questions and Answers
Question 1: What is the purpose of a lantern ring (seal cage) in a stuffing box?
- To act as the primary sealing element replacing packing rings
- To allow flush liquid to be introduced between packing rings for cooling and lubrication (Correct answer)
- To lock the gland follower in place under high pressure
- To center the shaft within the stuffing box bore
Correct answer: To allow flush liquid to be introduced between packing rings for cooling and lubrication
A lantern ring is a hollow spacer installed mid-way in the packing stack that aligns with a flush port, allowing clean liquid (or the pumped fluid) to be injected between packing rings. This provides lubrication, cooling, and in some applications prevents outside contaminants from entering.
Question 2: Which type of pump is classified as a positive-displacement pump?
- Centrifugal pump
- Axial-flow pump
- Gear pump (Correct answer)
- Mixed-flow pump
Correct answer: Gear pump
A gear pump traps a fixed volume of fluid between the gear teeth and housing and displaces it with each rotation, making it a positive-displacement pump. Centrifugal, axial-flow, and mixed-flow pumps are all kinetic (dynamic) pumps that use impeller velocity to transfer energy to the fluid.
Question 3: What does Net Positive Suction Head Available (NPSHa) represent?
- The maximum discharge pressure the pump can develop
- The absolute pressure energy at the pump suction minus the vapor pressure of the liquid (Correct answer)
- The total head loss in the suction piping due to friction
- The minimum inlet pressure required by the pump manufacturer
Correct answer: The absolute pressure energy at the pump suction minus the vapor pressure of the liquid
NPSHa is the absolute pressure at the pump suction inlet (accounting for atmospheric pressure, static head, and friction losses) minus the vapor pressure of the liquid at the pumping temperature. If NPSHa falls below NPSHr (required), the liquid flashes to vapor, causing cavitation.
Question 4: When installing lip seals on a pump shaft, what is the correct orientation of the seal lip?
- The lip should face away from the fluid being retained
- The lip should face toward the fluid being retained (Correct answer)
- The lip direction does not matter as long as the seal is fully seated
- The lip should always face the atmosphere side to prevent dirt ingress
Correct answer: The lip should face toward the fluid being retained
A lip seal works by system pressure pressing the flexible lip against the shaft, improving the seal under load. For fluid retention, the lip must face toward the fluid so that internal pressure forces the lip tightly onto the shaft surface. Installing it backwards causes rapid leakage.
Question 5: What is the primary reason mechanical seal faces must be kept flat and smooth to a very fine surface finish?
- To reduce the rotational torque required to drive the seal
- To allow the thin fluid film between faces to provide hydrodynamic lubrication without excessive leakage (Correct answer)
- To prevent heat buildup in the seal gland during start-up
- To make it easier to set the correct spring compression
Correct answer: To allow the thin fluid film between faces to provide hydrodynamic lubrication without excessive leakage
Mechanical seals rely on two extremely flat, lapped faces running against each other with only a micro-thin controlled fluid film between them. This film lubricates the faces and prevents dry contact, while the flatness limits leakage to an acceptable (near-zero) rate. Surface irregularities destroy this film and cause rapid face wear or leakage.
Question 6: A centrifugal pump is operating well to the right of its best efficiency point (BEP) on the pump curve. What condition is most likely occurring?
- The pump is developing excessive discharge pressure
- The pump is running with very low flow and high head, risking recirculation
- The pump is handling more flow than designed, leading to low head and possible cavitation (Correct answer)
- The pump speed is too low for the installed impeller diameter
Correct answer: The pump is handling more flow than designed, leading to low head and possible cavitation
Operating to the right of BEP means the pump is delivering higher-than-designed flow at lower-than-designed head. This can cause suction recirculation, increased NPSH requirement, turbulence at the impeller eye, and potential cavitation — as well as elevated vibration and accelerated wear on the impeller and casing.
Millwright Certification Pumps, Seals, and Packing 1
What is the purpose of a lantern ring (seal cage) in a stuffing box?