Steam Turbine Control Fluids (EHC)

Phosphate Ester Fluid

What is a phosphate ester electro hydraulic control (EHC) Fluid?

Phosphate ester fire resistant fluids have been in use in the control systems of steam turbines for over 50 years. There were needed because of fires with mineral oils, the growing use of higher pressure systems, and the need for fewer fires with nuclear stations. Plus, as hydraulic fluids they have good oxidation resistance, offer wear protection and have low compressibility.

Phosphate ester fire resistant fluids have:

  • High flash and fire points
  • High autoignition temperatures
  • Low heats of combustion
  • High hot manifold temperatures
  • Most importantly, phosphate ester fluids are self-extinguishing.

Consequently, they are less likely to catch fire, less likely to spread flaming streams or form flaming pools of oil and any consequential damage can be less. Other fluid types do not have all these advantages.

Triaryl phosphate types are the most common for EHC fluid and can have all three organic groups the same as in tricresyl or trixylenyl phosphate or they might be different as in isopropylphenyl diphenyl phosphate. The early phosphate ester EHC fluids were reportedly tricresyl phosphate ester (TCP) and trixylenyl phosphate (TXP). For a while they were mainly TXP.

TCP and TXP are sometimes called “natural” phosphate esters because the cresols and xylenols used as raw materials were derived from petroleum oil or coal tar. “Synthetic” versions were developed in the late 1960’s. These had reduced handling concerns and lower production costs.

Phosphate ester control fluids tend to be an ISO VSG 46 while ISO VG 32 versions are more common as bearing lubricants in compressors for pipelines. While the viscosities might be like those for mineral oil-based steam turbine lube oil, they are different.

  • Higher specific gravity – 1.13 vs 0.86
  • Lower viscosity index – 0 vs 90 or more
  • Good wear protection without additives
  • Good plasticizers so different elastomer compatibilities
  • Good oxidation resistance but can have variable hydrolytic stability.

The characteristics can depend on which phosphate ester is used and blends are very common as well. In fact, a blend is the most commonly used. One other difference from other fluids, is that these triaryl phosphate ester fluid can be purified in-situ using kidney loop systems.

They can give decades of trouble free but many of the lessons learnt in the early days are being forgotten. This is unfortunate because there is now better purification media, better filtration, better remediation options and better condition monitoring testing.

Information is available on the website and fluid or system issues often have one or more of the following root causes;

  • Purification Media Not Changed Soon Enough.
  • Purification Flowrate is Wrong.
  • Purification Media is Wet or dry and/or fouled.
  • Purification Housings Are Air Bound.
  • Defective Valves.
  • Wrong Purification Media.
  • Wrong Fluid is Being Used.
  • Overstressed Fluid (hot spots, low levels, etc.)
  • Material Incompatibility, Unsuitable Part or Maintenance Substitutions.
  • Plus, it is not possible for many older and some newer plants to adequately maintain the fluids and prevent operational problems following the turbine OEM’s original procedures and/or with the original equipment. Use current info.

Help and solutions are available.

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