Gravity Disc

Oils containing water can only be de-watered in a perfect manner if the bowl is accurately adjusted to the difference in densities of oil and water. The gravity disc with proper inner diameter i.e. with the diameter that corresponds to the difference in densities of the oil-water mixture to be treated should, therefore, be inserted in the bowl, This disc can be chosen from the set of disc provided with the separator.

The inner diameter of the disc to be chosen can be determined by:

1.  Calculation 
2.  Experiment

The general rule is :

• Small diameter gravity disc when treating heavy oil
• Large diameter regulating ring when treating light oil

Determining the size of gravity disc by calculation:
For a given separating temperature, the inner diameter of the gravity disc and if the desired density of the oil can be determined from the diagram, provided that the density of the oil at a temperature ranging 15℃ and 90℃ is known.
For example;
Given: Density of oil at 20℃            ρ oil 20℃=0.875 kg/d３
            Separating temperature         t=70℃

To be determined:
Inner diameter of gravity disc d=?
Density of oil at 70℃ ρ oil 70 ℃=?

Determined:
Inner diameter of gravity disc according to diagram d=70mm
The density of oil at 70℃ according to diagram ρ oil 70 ℃=0.838kg/dm３ 

If no gravity disc of the determined inner diameter is available to check by experiment whether the gravity disc available is suitable for perfect de-watering. If that is not the case machine a smaller diameter gravity disc to the required dimension. 

Determining the size of gravity disc by experiment:
To avoid emulsification begin the test by inserting a large diameter gravity disc and then change over to the next smaller one. If the inner diameter of the gravity disc is too large the water discharging from the water outlet will contain oil. If the diameter of the ring is too small, the oil will tend to emulsify, or the oil discharging from the clean oil outlet will contain water. 
The milky or dirty appearance of water is quite normal, it results from the oil washing process.
During the operation, a separation zone between the light and heavy liquid will form in the bowl.
The middle of this zone referred to as separation line or e-line, should be close to the large diameter of the top disc.
The separation line between water and oil can mostly be seen on the underside of the separating disc.

Correctly chosen gravity disc
The gravity disc has been chosen correctly when the separation line between water and oil is close to the large diameter of the discs. The position of the separation line is to be on the underside of the separating disc. 

Effects:
The discharging clean oil is free of water.
The discharging water is free of oil.

Inner diameter of disc too large
The inner diameter of the gravity disc proves to be too large, if separation line between water and oil is found to be outside of the disc stack and too close to the large diameter of the separating disc. 

Effects:
The discharging clean oil is free of water.
The discharging water contains oil or the oil breaks the water seal.

Inner diameter of disc too small
The inner diameter of the gravity disc proves to be too small, when the separation line between the liquid components instead of being near the large diameter of the disc is found to be nearer to the small diameter of the discs. 

Effects:
The discharging clean oil contains water
The discharging water is free of oil.