Centrifuging

As the density of the oil approaches that of water (above 991kg/m³ ) the hydraulic equilibrium in the bowl becomes unstable, and a gravity disc, which is the method used to maintain the oil/water interface, as in a LO purifier, will no longer maintain a water seal.

To overcome this problem, Alpha Laval has developed the Alcap system, and Westfalia the Unitrol system, the principles of which are illustrated.

The Alcap system using the FOPX separator.

The FOPX separator comprises a frame containing in its lower part a horizontal drive shaft with friction clutch and brake, worm gear and vertical bowl spindle. The worm gear is placed in an oil bath. The bowl is fixed on the top of the spindle inside the space formed by the upper part of the frame and the frame hood which also carries the feed and discharge systems. Instead of a range of gravity discs, the FOPX separator bowl is supplied with one permanently fitted flow control disc Paring disc pumps (centripetal pumps) are built-in to the separator bowl for discharge of clean oil and separated water under pressure.

Oil is fed into the high speed rotating bowl which basically operates as a clarifier, but water and solids are separated and are thrown to the outside of the bowl by centrifugal force.

At regular intervals a sludge cycle will take place. Water is admitted into the bowl to soften the sludge and displace the oil in the bowl. When a transducer in the oil discharge line detects water the bowl is opened and the sludge and water discharged. The bowl opens and closes very rapidly and oil loss is minimal.

If the fuel contains water it will build up in the bowl and start to be discharged with the clean oil. The transducer in the discharge line will detect this and if this occurs after the minimum sludge cycle time, a sludge cycle will be initiated; if the water is detected before the minimum sludge cycle time, then water discharge valve will open.

Link to Alfa Laval website: http://www.alfalaval.com

The OSB type separator using the Unitrol system for either Water Monitoring WMS or Sludge Space Monitoring (SMS)

The separator can either be set up to monitor water content in the oil discharge (Water Monitoring System) or a sludge build up in the separator (Sludge Space Monitoring System). The sketch opposite shows a separator set up for the Water Monitoring System. This system is essential when density differences between the oil and water are very small. A small amount of oil is diverted via the separating disk and pumped out using a centripetal pump (This is the sensing liquid). As long as the conductivity sensor registers oil, the oil flows back to the purifier inlet., and the separator is operating as a clarifier. If the sensor registers water (95% by vol), then the recirc valve closes and the water discharge valve opens. The separator is now acting as a purifier until the water is no longer detected and the valves change back over.

At regular intervals the separator will go through a discharge cycle. Oil is displaced by water before the sludge ejection which takes place over a very short time. Westphalia claim that these total ejections keep the disk stack clean and increase TBOs to 6 - 8000 hours.

The separator can also be set up to monitor the sludge space for solids. In this case the separating disk is exchanged for one without any holes. The control unit is adjusted to monitor the pressure in the sensing liquid flow. If the sensing liquid flow is interrupted due to an accumulation of solids at the periphery of the bowl, the pressure switch registers a drop in pressure and a solids ejection program is initiated.

Two separators can be operated in series as two purifiers (for a high water content) or as a purifier/clarifier arrangement (shown below).

Sludge and water sensing combined This is achieved in the C generation separators (OSC type) with throughputs of up to 43100 l/h.