Desalter Q & A

1.  What is the purpose of desalter?

Ø Removal of salts dissolved in water.

Ø Removal of silts and sediments.

Ø Removal of water

2.  What are the harmful effects if desalting is not done?

Ø Corrosion of equipments.

Ø Plugging / chokage of tubes, tray valves.

Ø Chances of over pressurisation due to water.

Ø High heat load requirement in furnace due to presence of water.

3.  What are the major steps in desalting?

Ø Addition of demulsifier.

Ø Increasing temperature

Ø Addition of wash water.

Ø Mixing of Crude and Water through PDV.

Ø Coalescence of water in the electric field.

Ø Settling and removal of brine.

4.  One of the purposes of desalting is to remove water from crude. Then why additional water is added to desalter?

Water present in the crude is about 0.2 to 0.5 vol%. These small quantities of water are very finely dispersed in the crude. On application of electric field, these water particles takes longer time to coalescence since the inter particle distance is large. On addition of extra water in the crude, the inter particle distance between the small water droplets decreases and they easily coalesce to produce larger droplets to settle out of crude. Hence additional water is added in crude.

5.  Desalter could have been installed at the Inlet of Preheat Train-I. Why desalter is installed after few exchangers in the Preheat Train-I?

As per stokes law settling velocity of a particle in a viscous liquid is inversely proportional to viscosity of the fluid. Viscosity of the fluid is inversely proportional to the temperature of the fluid. Hence the settling velocity is directly proportional to the temperature of the fluid.

At the inlet of Preheat Train-I the temperature of the crude is 30 Deg C, while at end of the Preheat Train-I outlet the temperature of the crude is ~ 125-130 Deg C. Hence the viscosity of the crude is lower at that point. Hence Desalter is installed at outlet of Preheat Train-I. 

6.  Why desalter operating pressure is 20 kg/cm2. Why not lower pressure?

Desalter pressure is at 20 kg/cm2 is to maintain the crude in a liquid state at a temperature of 125-130 Deg C. Desalting operation can be carried out at any lower pressure as long as lighter components are not vaporised from crude. If lighter components are evaporated then it will hinder in the settling of the water particles. If it is operated at a lower pressure than 20 kg/cm2 then there is a chance of crude vaporisation post desalter exchangers to V-003 which will cause 2 phase flow in the heat exchangers in Pre-Heat Train-II.

7.  What is the makeup water rate in desalter (in terms of % of crude flow)? Why makeup water is preheated?

Makeup water rate is ~ 8% of the crude flow. Makeup water is heated upto the crude temperature so that the final crude temperature doesn’t drop below the desalting temperature at desalter.

8.  What is the purpose of the mixing valve? What is the effect of too high or too low DP across the mixing valve?

The purpose of the mixing valve is to properly mix crude with water. Too high Delta P across mixing valve will cause stable emulsion while, too low DP will cause improper mixing. Proper mixing occurs in the range of 1.0-1.75 kg/cm2 delta P.

9. If voltage across the electric field in desalter is decreases, how will it affect the desalting efficiency? Why?

If voltage across the grid is decreased, the polarisation of water particles will be lower which in turn affect the coalescence of water droplets. Hence lower electric field will adversely affect the desalter efficiency.

Schematic Diagram of a Desalter Internal: