• For a desalter system with low performance, we made an RCA that revealed multiple issues to the desalter hardware, including that the equipment is undersized and electro-coalescne not happening, while grid is still ON. Could anybody advise equipment retrofitters to shift the existing desalter almost electrodynamic type to low velocity type ?



  • Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

    Well, we have two types of electrostatic treaters which are known as low velocity type and high velocity type. The low velocity type is normally applied in platforms or in upstream assets aiming to promote the water separation from the produced crude oil, in this system the emulsion is fed in the top of the separation vessel along the length of the vessel and the emulsion is fed in the aqueous phase which due to the density gap presents an upward flow in direction to the electrodes which are normally positioned above of the centre line of the separating vessel. In these dessalters the crude oil is dispersed in the aqueous phase and suffers a kind of wash which remove salts and solid particles present in the oleous phase along of this upward flow in direction of the electrodes there is a small coalescence effect due to the low intensity electric field between the lower electrode and the water-oil interface. When the emulsion reaches the high intensity electric field occurs the remaining coalescence. These dessalters can suffer with poor performance along their lifecycle due to the raise in the water concentration in the crude oil along the time due to the depletion of the crude oil well and according to the strategy applied to improve the flowrate production of the well, and this can be one of the sources of the issue mentioned in the question.

    In this case, the revamp of the desalting system for a high velocity desalter can be thought as a solution. In these dessalters, the emulsion is fed directly between the electrodes.

    The limitation here is precisely the amount of water in the emulsion, for water concentration above 10 % the protection system can trip the desalting system due to the high electric current level and this limitation can be prohibitive to upstream assets due to the raise of water concentration in the crude oil along the time as explained above. For this reason, it's important to consider the service that you are planned to your desalting system, if the system is operating in an upstream asset, the revamp for a high velocity desalter can be a bad idea considering the trend of elevation in the water content in the crude oil along the time.

    My suggestion would be to analyze the installation of a third electrode grid as well as consider the installation of a mud wash system in order to remove the sediments in the separating vessels bottom which can significantly reduce the residence time of the emulsion in the dessalter and lead to a poor performance of the whole system.