Pumps

Question 1

How to prime a centrifugal pump

Answer 1

Ensure discharge v/v is closed
Use an air eductor to remove air
Start p/p and open discharge v/v slowly

Question 2

How does a centrifugal pump work?

Answer 2

It uses Bernoulli’s principle to convert kinetic energy to pressure energy (velocity to pressure) by using centrifugal force to force liquid outwards. As it travels outwards the area is expanding and slowing the velocity while increasing pressure as a result.
The outwards movement of the liquid creates a low pressure area at the eye where more liquid will be attracted due to the pressure in the line/tank being higher.

Question 3

How does a gear pump work?

Answer 3

• Suction (inlet): as the gears rotate the teeth become unmeshed where they create an expanding volume (LP area) that the oil is drawn in to
• The oil is then sent around the casing with the teeth
-Discharge (outlet): as the teeth mesh the volume is then reduced and the oil is pushed out by the HP created

Question 4

What is a key feature of axial pumps?

Answer 4

Axial pumps are reversible so they can be used for heeling systems

Question 5

Bilge system piston pump fault finding

1 - High suction no discharge pressure

2 - No suction no discharge pressure

3 - High suction high discharge pressure

Answer 5

1 - strum box or strainer blocked

2 - hole in pipe, damaged strainer gasket, empty bilge or other bilge well open, suction v/v not reseating

3 - Discharge v/v closed

Question 6

A pumps gauges are showing high discharge and high suction, what may be wrong?

Answer 6

The discharge valve is closed

Question 7

Positive Displacement Pumps
(Characteristics, types, systems)

Answer 7

Characteristics:
• Self priming
• Delivers a fixed volume per revolution regardless of pressure changes
• Efficient with high viscosity liquids
• Does not emulsify (combine) the liquid like a centrifugal pump
• Will have a relief/safety v/v on discharge line

Types: Screw, gear, diaphragm, piston, lobe

Used for bilges, sludge, FO, LO

Question 8

What is cavitation?

Answer 8

Cavitation is when the liquid goes below its vaporisation pressure point then increases in pressure causing it to implode

Causes pitting on the impeller, vibration and noise, loss of flow and pressure and damage to bearings.

Question 9

Why is there not a relief valve on the discharge of a centrifugal pump?

Answer 9

Because a centrifugal pump cannot generate more pressure that it’s designed to

Question 10

Pump not getting suction from double bottom tank

Answer 10

Hole in line
Filter gasket
Another tank suction is open
Tank is empty (faulty level sensor)

Question 11

Why use a CF p/p for water systems and emergency bilge p/p?

Answer 11

They can move greater volumes than +ve displacement p/ps as they are for flow rather than pressure

Question 12

How would you change the gland packing on a SW p/p?

Answer 12

Check spare parts + packing
Inform duty
LOTO motor + p/p
Check tools such as extractor tool + lifting equipment
Take out using correct lifting techniques and take to workshop
Open the gland cover + take out using extractor tool
Remove gland packing
Cut new gland packing to correct size and put in
Rotate shaft with hand to ensure it is free
Close packing cover
Put back and open inlet v/v
Start pump and open outlet
Monitor

Question 13

Purpose of the pump coupling and types

Answer 13

The coupling between the pump and the motor transmits the drive and torque of the motor to the pump with minimal losses

Types: spider, flexible, gear

Question 14

Non-positive displacement

Answer 14

Centrifugal & axial flow
Used in water systems: fire, ballast, FW, SW, heeling

Works on Bernoulli’s principle where kinetic energy is converted to pressure energy (velocity to pressure)

Question 15

Checks on centrifugal pump during overhaul

Answer 15

• Check impeller diameter size & check for pitting caused by cavitation
(Cavitation is when the liquid goes below its vaporisation pressure point then increases in pressure causing it to implode and create pitting)
• Check volute casing for abrasive wear
• Check shaft is true
• Check key is there & not broken
• Check gland packing/mechanical seal for wear & replace if necessary
• Check wear ring diameter size and signs of wear and always replace