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Phosphate ester fluids have been used to control
thousands of steam turbines over the last 60
years. With this much user experience, one
would assume that their operational challenges
would be well-understood and that power plants would
be able to maintain the quality of these fluids with ease.
On the contrary, phosphate ester fluids are often the
Achilles' heel of most lubrication programs.
Degradation Modes
e primary degradation modes of phosphate ester
fluids are hydrolysis and microdieseling. ese fluids
may also degrade due to oxidation, thermal degradation
and the formation of metal soaps. e cause and effect
of phosphate ester fluid degradation in steam turbine
electro-hydraulic control (EHC) systems is illustrated
in Figure 1.
How to Monitor and Maintain
Phosphate Ester Fluids
CONTAMINATION CONTROL
By Greg Livingstone and Jatin Mehta, Fluitec International
Phosphate
ester fluids
are often
the Achilles'
heel of most
l
ubrication
programs.
Figure 1. Degradation modes of phosphate ester fluids in steam turbine EHC systems.
Phosphate Esters
Degradation Cause
Microdieseling
Oxidation
Hydrolysis
Leached
Metals
Phosphate Esters
Degradation Eect
Soot
Varnish
Acids
Metal
Soaps
Hydrolysis
Hydrolysis is the most common source of
degradation in phosphate esters. High operating
temperatures, intermittent steam leaks and the
hygroscopic nature of the fluids provide an ideal
environment that favors hydrolysis reactions. e
saturation limit of phosphate esters at operating
temperature is approximately 3,000 to 5,000 parts
per million (ppm) of water. During hydrolysis,
the fluid produces a stronger and a weaker acid.
e stronger acid undergoes a series of reactions,
forming subsequent phosphoric acid derivatives
and alkylphenols.
Temperature, water and fluid are the three prin-
cipal parameters controlling hydrolysis reactions,
with the consumption of water the limiting factor.
ese reactions are autocatalytic and initiate at
