Recommendation of an MPC test
The lab reports of the semi-annual trend analyses of circulation oil from a steam turbine were always marked OK with a green tick. However, in the last lab report we received an urgent recommendation to conduct an MPC test or change the oil. What is behind the MPC test?
A high degree of efficacy, short start-up periods and flexibility under changing operating conditions – today, turbines must meet a variety of requirements. Increasingly, they depend on their turbine oils to operate safely.
These must cope with:
- very long oil change intervals
- increased oil circulation temperatures
- more compact oil circulation system structures with shorter circulation times.
Although the standard specifications for turbine oils are constantly being tightened, they often do not sufficiently cover individual strain scenarios. Accordingly, the focus is increasingly on practical performance tests as a prerequisite for the recommending of oils by the OEM.
The long-term use of turbine oils is influenced by several factors.
- Oxidation: accelerated by the oil's reaction with atmospheric oxygen in connection with increased oil circulation temperatures of over 60°C
- Thermal disintegration of base oil and additive package: through localised extreme temperatures during electrostatic discharges or local spontaneous combustion of compressed, undissolved air bubbles
- Contaminants: water, dust, wear particles and mixtures with other oils.
There are various consequences for the turbine oil: significant consumption of antioxidants, potential filtering of anti-foaming additives, deterioration of air output behaviour, decomposition of the wear protection package, reaction products in the form of undissolved oil components, deposits (sludge), varnish formation, dark colouration and/or oil turbidity.
As well as the deterioration of lubricant performance, the deposits constitute a risk for the system above all. The solubility of the additives in the base oil play a key role in their formation. Today, the manufacturers of turbine oils are increasingly using base oils in groups II (hydrotreated), III (hydrocrack) and IV PAO (poly-alpha olefins).
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