Page 170 - Introduction to Marine Engineering

156 Fuel oils, lubricating oils and their treatment

Continuous operation
Modern wide-bowl centrifuge designs enable continuous operation over
a considerable period of time. This is achieved by an ejection process
which is timed to discharge the sludge at regular intervals. The sludge
deposits build up on the bowl periphery as separation continues, and the
ejection process is timed to clear these deposits before they begin, to
affect the separation process. To commence the ejection process the oil
feed to the centrifuge is first shut off and the oil remaining in the bowl is
removed by admitting flushing water. Water is then fed into the
hydraulic system in the bottom of the bowl to open a number of
spring-loaded valves. This 'operating' water causes the sliding bowl
bottom to move downwards and open discharge ports in the bowl
periphery. The sludge is discharged through these ports by centrifugal

Figure 8.3 Sludge discharge

force (Figure 8.3). Closing 'operating' water is now fed in to raise the
sliding bowl up again and close the discharge ports. Water is fed into the
bowl to remake the liquid seal required for the separation process, the
oil feed reopened, and separation continues.
The complete ejection cycle takes only a few seconds and the
centrifuge is in continuous operation throughout. Different bowl
designs exist for various forms of sludge discharge, e.g. total discharge,
controlled partial discharge, and so on. With controlled partial discharge
the oil supply is not shut off and not all of the sludge is discharged. In
this way the separation process is not stopped. Whatever method is
adopted the centrifuge can be arranged so that the discharge process is
performed manually or by an automatic timer.

Maintenance
The bowl and the disc stack will require periodical cleaning whether or
not an ejection process is in operation. Care should be taken in stripping