Method for two-step separation of water, salt and particles from a hydraulic fluid

A method for two-step separation of water, salt and particles from a hydraulic fluid in a subsea application, wherein

• the hydraulic fluid is carried into a return oil tank with an electrostatic coalescer and a connected high-voltage transformer,
• the return oil tank is pressure-compensated by a pressure compensator bellows against surrounding sea water pressure, and a body of the return oil tank is provided with an opening towards the pressure on the outside of the pressure compensator bellows of the return oil tank,
• the electrostatic coalescer is supplied with a DC voltage of up to 30,000 VDC with changing polarity on two electric grids which are mounted internally in the return oil tank, and the hydraulic fluid is exposed to an electrostatic field, so that water droplets in the hydraulic fluid coalesce into larger droplets which are separated as free water and settle into a collecting tank below the return oil tank,
• a level instrumentation arranged in the collecting tank detects when the collecting tank has been filled,
• when free water has been separated and removed from the hydraulic fluid, the hydraulic fluid is circulated via a circulation circuit to a filter module for any remaining water, salt and particles to be removed, and
• the filter module is established from several filter elements in filter housings, connected in parallel, with hygroscopic cellulose fibres which swell until saturation, and the swelling is measured by sensors measuring the degree of saturation, or alternatively instrumentation measuring conductivity.