Millwright Certification Pumps, Seals, and Packing 1

Question 1

What is the purpose of a lantern ring (seal cage) in a stuffing box?

Accepted Answer

To allow flush liquid to be introduced between packing rings for cooling and lubrication

Answer

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?

Accepted Answer

Gear pump

Answer

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?

Accepted Answer

The absolute pressure energy at the pump suction minus the vapor pressure of the liquid

Answer

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?

Accepted Answer

The lip should face toward the fluid being retained

Answer

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?

Accepted Answer

To allow the thin fluid film between faces to provide hydrodynamic lubrication without excessive leakage

Answer

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.

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Blog Practice Test

Millwright Certification Pumps, Seals, and Packing 1