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I'm new to the refining field. We have eletrostatic desalter with three transformers (each of 22KV). and I have questions about this:
1. Why is a single heavier transformer is not used instead of three?
2. We have 2 PSVs on desalter. What is the reason for two PSVs?
3. Why the water injection before desalter is maintained 4.5% of crude charging?
4. What is the function of grills (grids) inside the desalter?
 
Answers
24/11/2015 A: GAURAV BURDE, BHARAT OMAN REFINERIES LIMITED, burdegaurav88@gmail.com
Your questions are well answered, however I would like to amend on PSVs. Two PSVs with/without a bypass is a common practice in refineries so that if one is under calibration/maintenance you can use the other. (During normal opn one will be in line, but this depends on company's safety policy). Usually Desalter PSV outlets are lined up to CDU column only, not to flare.
10/08/2010 A: Christopher Turner, Snamprogetti Ltd, chris.turner@snampro.co.uk
1) Three phase supply - three transformers , one on each phase balances the load. Also if one breaks, you still have some treatment.
2) Not sure - what are the set points and design pressure? Are there two separate PSVs with different set points? Are you sure its a PSV andnot something else?
3) Ensures the dilution is sufficient to reduce the salt content to spec
4) The grids are connected to the high voltage outlets of the transformers, the oil passes between them, the water droplets become polarised, coalesce and travel towards the grids and are removed. Try google books for a longer explanation.
22/12/2009 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
1. Desalter manufacturer will have to answer the first question. My guess is that it's so that if a transformer fails, you don't lose all the electric field.
2. I don't believe that there is a fundamental reason why a desalter would need 2 PSVs. If it's a large vessel, you might have a very large fire load so that the required load exceeds the capacity of a single PSV or there could be a large blocked in liquid relief load if the vessel is not designed for the blocked in head of the charge pump.
3. Wash water is added to enhance the desalting by improving contacting of salts with water, diluting chlorides in water, and enhancing water coalescing (more water means water droplets are closer together, which makes it easier for them to coalesce & settle). 4-5% is a common amount of water for lighter crudes. If you are processing a heavy crude you might need 7-10% wash water to help with the desalting.
4. The grids create the electrical field used to coalesce the water droplets. The size of the transformer and the spacing between the grids set how strong of electric field can be generated for coalescing the water.