6.8.1   Oils

 

The basic turbine lubricating oils are to British Standard 489: 1983; CEGB Standard 207001 covers lubricants for use by the CEGB and Table 2.1 is an extract which covers turbine lubricating oils. The specific grade used on the modern 3000 r/min tandem turbine-generator is TO-32.

Tabie 2.1 Turbine lubricating oil requirements

Type prefix ISO VG grade designation (BS4231)
Viscosity, kinematic at 40°C, mm /s (cSt)
Viscosity index Flashpoint, closed, °C, Pensky-Martens Pour point, °C Total acidity, mg KOH/g Copper corrosion, classification (3 hours at 100°C) Rust-preventing characteristics
Demulsification number, s Foaming tendency: Foam, ml, at 24°C at 93.5°C at 24°C after test at 93.5°C Foam stability after 300 s Foam, ml, at 24°C at 93.5°C at 24°C after test at 93.5°C Air release properties, minutes to 0.2% air content at 50°C Oxidation characteristics: Total oxidation products (TOP) with sludge limited to 40% of the determined TOP No catalyst, %                л Solid copper catalyst, %          duration of tests 164 hours Soluble metal catalysts, %

min max min min max max max
max
max max max
max max max
max max max

TO
32
28.8 35.2 70 168 -6 0.20 2 rusting absent 300
450 50 450
Nil Nil
Nil 5
0.1
1.0 1.0

TO
46
41.4 50.6 70 168 -6 0.20 2 rusting absent 300
450 50 450
Nil Nil Nil 6
0.1 1.0 1.0

TO
68
61.2
74.8 70 168 -6 0.20 2 rusting absent 360
600 100 600
100
25 100 7
0.1 1.0 1.0

TO
100
90 110 70 168 -6 0.20 2 rusting absent 360
600 100 600
100 25 100 10
0.1 1.0
1.0

Additives are now used to inhibit oxidation, corrosion and foaming. An oxidation inhibitor is added to stabilise the oil against oxidation and to passivate the metals which act catalytically to increase oxidation. These inhibitors maintain a low neutralisation value (acidity) of an oil over many years of service. A rust inhibitor in the oil protects carbon-steel surfaces from rusting when in contact with water or moist air entrained within the oil circulating system.

Detergent additives are used to inhibit high temperature oxidation, the formation of low temperature sludge and the deposition of contaminants.

Viscosity index improvers reduce the fall in viscosity with temperature rise. Pour-point depressents have the opposite effect and reduce the temperature at which the oil becomes immobile.

Anti-foaming agents are added to suppress the tendency of aerated oil to foam and assist in the release of air from the oil.

The acidity (total) of new oils is normally within the range 0.02 to 0.10 mg of KOH/g, or slightly higher in additive-conditioned new oil. In service, oils tend to oxidise to organic acids and this is accompanied by an increase in total acidity. The acidity level of the oil is therefore a useful guide to the condition of the oil and the need for purification or conditioning.

Water ingress into the turbine oil is the most common problem with oil systems. The presence of excess water in oil, particularly if stationary for any length of time, may result in bacterial and fungal contamination of the oil systems. This can reveal itself as a yellow/black stringy grease-like material. This growth can take place in sediment in oil tanks or other plant items. It can be particularly difficult to clear from tube nests. The basic precautions are:

  • To ensure that the water content of the oil in operation is kept low by regular use of the oil purifier; the water content should not exceed 0.05%.
  • To remove sludge deposits from the oil system plant-sumps on a regular basis.

If bacterial/fungal growth becomes a problem, the addition of the correct biocide to the oil will kill the bacterial/fungal growth.

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