Hi,
Actually you have provided very little information in your query, but still I will try to answer.
1. Desalter Dehydrator:
Desalter is provided in oil & gas industry to remove salts from the crude (salts cause corrosion). Please note that the salts are not dissolved in the oil, salts are present in the water associated with the crude mixture (i.e. Oil-Gas- Water Mixture). For more information about operation of the Desalter just GOOGLE and you will get loads of information.
2. Surge Drum:
Surge Drum is an additional storage space, such drums are provide when the volume of the crude is high. The surge drum will allow to operate the plant for a limited period of time in case there is some problem at the upstream system of the surge drum.
The desalter and surge drum are provided in the second system, because the crude is wet (removal of salts) and to operate the plant (for a limited period of time) in case there is upset in the upstream system.
Hope that answers your query
Revert in case of any doubt.
Edited by Chemitofreak, 30 September 2015 - 03:12 AM.