The conventional approach to combating the build up of varnish is to treat the symptoms by addressing the systems issues.
Such methods include the introduction of:
Ultra-fine Filtration, Electrostatic Precipitation and Balance Charge Agglomeration systems are installed by some operators to control the level of total insolubles produced in the oil by the normal process of oxidation which takes place in the oil in service. Such processes are installed with the aim of removing semi-soluble and fine particulate material from the oil as it is formed.
Whilst such systems can be effective to a degree in removing this material, they are often unable to do so at a sufficiently fast rate to maintain insolubles at an acceptably low level.
Fitment of bypasses or spill plates to servo-valves and lagging of cool areas of the oil circuit pipe-work is good practice in eliminating 'dead-legs' from the system and reducing the thermal cycling to which the oil is subject. However, these actions in isolation are not normally sufficient to prevent varnishing of servo-valves.
Cycling of valves in operation is a good practice which is carried out by some operators to increase the flow of oil in the control circuit. This is beneficial in reducing the 'dead-leg' effect, which can allow soluble and semi-soluble material to drop out of solution and form varnish deposits, particularly in base load applications where certain control valves may not otherwise be exercised for long periods.
Valves are replaced with either new or refurbished units on a scheduled basis at an interval shorter than the ‘mean time between failures'.
This is seen as a relatively low cost means of ensuring reliable operation and reducing trips caused by varnished servo-valves. However, in cases of severe varnishing this may result in significant cost of new valves or of overhaul and may impact production due to additional outages whilst valves are exchanged.