Introduction
The gravity disc is one of the most critical components in a marine fuel oil purifier. Even though it is small in size, selecting the wrong gravity disc can cause serious operational problems such as oil loss through the water outlet, water contamination in clean oil, or poor separation efficiency.
Every marine engineer must understand how to choose the proper gravity disc based on fuel properties and operating conditions.
What Does the Gravity Disc Control?
The gravity disc controls the position of the oil–water interface inside the purifier bowl.
It decides:
- where clean oil flows
- where water collects
- how separation takes place
If the interface shifts from its correct position, separation fails.
Thus, gravity disc size directly affects purification performance.
Why Correct Size is Important?
Using the wrong disc size may result in:
- oil loss through water outlet
- water mixing with clean oil
- unstable interface
- frequent alarms
- reduced efficiency
- increased fuel consumption
Hence, correct selection is not optional — it is essential.
Step-by-Step Gravity Disc Selection Procedure
Follow this practical method onboard.
Step 1 – Note the fuel density
Check bunker delivery note or lab report.
Find:
Specific gravity or density at 15°C
Example:
0.991 or 991 kg/m³
Step 2 – Heat the fuel to operating temperature
Normally:
98–104°C
Heating reduces viscosity and slightly changes density.
Step 3 – Refer to maker’s gravity disc chart
Each purifier manufacturer provides a gravity disc selection table.
The chart shows:
Fuel density → recommended disc diameter
Match your fuel density with chart value.
Step 4 – Select correct disc diameter
Gravity discs are available in different hole sizes.
Example:
- 68 mm
- 70 mm
- 72 mm
- 74 mm
Choose the size recommended in the manual.
Step 5 – Install and test run
After fitting:
- start purifier
- check discharge clarity
- observe water outlet
- monitor oil loss
If separation is stable, selection is correct.
General Selection Rule (Quick Guide)
If maker’s chart is not immediately available, remember:
- Higher density fuel → smaller disc
- Lower density fuel → larger disc
This keeps the interface stable.
But always confirm with manufacturer data.
During watchkeeping:
- check discharge clarity regularly
- monitor oil losses
- change disc when fuel grade changes
- keep spare discs ready
- never guess disc size blindly
Correct selection saves fuel and prevents trouble.