Design Challenges of Crude De Salter Unit on Onshore Plant EPC Contractor's Perspective
Authors: Shukla Bimlesh , Poreddiwar Nitesh , Singh Bhupinder , Singh Harendra , Kamal Faris Ragheb , Takieddine Oussama 
Published: November 2020

Desalting of Crude oil is a critical unit operation as improper De-salter design can leads to significant corrosion in downstream facilities and longer shutdown. Design of De-salter is mainly influenced by physical properties/emulsion tendency of crude oil, salt specification and desalting technology. Open ended FEED design is always challenging task for EPC Contractor to deliver project on schedule and it needs to be address during FEED stage to provide benefit to all stakeholder. This paper discusses impact on the design of de-salter due to late changes in design parameters at detailed engineering stage and highlight experience gained through real project which will be useful to all stakeholders.

Crude oil produced in oil fields is accompanied by produced water and normally contains dissolved salts, principally chlorides of sodium, calcium, and magnesium. The salt can cause various operating and maintenance problems in the downstream facilities (e.g. corrosion, fouling in equipment etc.). The purpose of crude oil desalting is to remove these undesirable salts to an acceptable range (typical range, 5-10 PTB) before sending to further treatment in refineries.

In desalting process, crude oil is heated to decrease its viscosity (water-oil demulsification), and then the crude oil is mixed with wash water. The water-in-oil mixture is then sent to the pressurized horizontal desalter vessel, where a high-voltage electrical field is applied in order to enhance separation of the produced water with laden salt and crude oil.

Technology for de-salting of crude oil is proprietary and several proven supplier can offer different technology. Selection of de-salting technology is mainly governed by crude type and process specifications. In one of the onshore project, new de-salter to be installed to meet desalted crude oil specification of 5 lb salt/1000 BBL oil. During detailed engineering EPC Contractor has finalized the design of desalter based on followings: Selection of desalting technology (single polarity AC or Dual Polarity AC/DC)Fluid Sample analysis and finalization of fluid properties (viscosity and density)

In the Detailed engineering, analysis of sample collected from the plant is carried out to derive the actual composition. Based on revised composition, physical properties mainly viscosity increases marginally. However, this marginal increase in viscosity significantly increases the size of the de-salter. Due to proprietary design, de-salter with higher sizes is required to be implemented imposing additional challenge to accommodate it in limited space of plant.

The approach for designing of desalter package, where design is not finalized during FEED stage (i.e. fluid properties, electrostatic desalter technology, availability of wash waters etc.), can impose challenges for EPC Contractors in term of plant layout, cost, procurement and meeting project schedule. This paper discusses measures to be considered by EPC Contractor which can useful to minimize potential risk.