How to prevent foaming and air trapped in gearbox oil?

Foaming and air retention in the oil are two processes that must be prevented from occurring in turbine gearboxes to achieve optimum performance.

There are many causes that lead to the generation of foam and trapped air, but this mainly occurs because of mechanical problems or due to the condition of the oil.

A mechanical problem may be associated with excessive agitation or a low oil level. If foam disappears quickly when the machine stops, it may be that there is a mechanical problem or that the anti-foam additive has run out.

To prevent foaming the following factors must be taken into account:

• Method of filling. The filling of the tanks is critical. It is advisable to add the oil with a hose pushed right into the tank to avoid introducing air. When a new tank is filled, air can be trapped in the dead zones, so purging should be carried out. The correct procedure consists in emptying the tank with a vacuum pump and maintaining the vacuum while the tank is filled.
• Preventative-proactive maintenance programmes. The recommended preventive maintenance tasks are: checking gaskets and seals, visual inspections, checking oil levels and filters. In addition, it is advisable to monitor cleanliness, eliminate moisture, filter the oil well and remove contaminants. If, despite following good maintenance practices, the foaming persists, you must locate the point at which the air is entering and prevent this from occurring.
• Tank design. Tank design is usually the most important factor contributing to the foaming of oil. Among the main problems are the following:
  -The return line is above the oil level
  -The tank is small
  -The tank should have a capacity of between 5 and 10 times the flow rate of the pump, or in                splash-lubricated systems 0.4 l/kW is recommended
  -The oil level in the tank is low
  -The moving parts introduce air into the system
• Lubricant selection. The optimal oil has to be highly resistant to the entry of air and should not retain bubbles after these form. The oils that best comply with this model are synthetic fluids and hydrocracked oils.