To understand this phenomenon, it is required to remind how a desalter is designed.

The water gravity separation is driven by the Hadamard–Rybczynski equation that gives the terminal velocity of a slowly moving spherical bubble through a fluid. Assuming that the coalescence process is not affected by the type of crude and that the droplet size remains the same whatever the crude, this formula shows that the settling velocity is proportional to both oil density and viscosity.
As a result, in order to compensate this modification of crude properties, the flow rate should be reduced or the desalter volume should be increased, depending on whether the refinery wants to occasionally take advantage of opportunity crudes or if the refinery wants to implement a long term strategy of treating heavier crudes.

Another phenomenon that exacerbates this situation is due to the dipolar attractive force between droplets. This force, which produces coalescence on an AC field, depends on crude oil conductivity. Heavy crudes are usually more conductive, leading to a reduction in the voltage gradient within the desalter and thus affecting the coalescence process.

All these aspects should be taken into account during the desalter design phase to ensure that performances are maintained with all the specified crudes.

Some solutions can be tested to partly compensate the use of heavier feed and to help the desalting process. Desalter transformers are usually equipped with a tap changer that permits easy modification of the transformer voltage. Another action to be tested is to increase the wash water injection ratio in order to increase the water population distribution. This might improve the coalescence process by reducing the distance between water droplets and then increasing the attraction between charged water particles, even if it will not compensate a properly designed desalter.

To conclude on this question, it is important to keep in mind that a desalter is designed to perform for a specific range of crude feed and operational conditions. Hence, any changes in those conditions need to be addressed in coordination with the desalter supplier which can support the end user to tune its unit and assess specifically the impact on downstream equipment.