One of the main reasons we use phosphate ester fluid such as Fyrquel™ is its fire-resistant properties, such as the fact that it is self-extinguishing. For this reason it is often used in the hydraulic control system of steam turbines. This control system governs the rotational speed of the turbine and problems in the hydraulic fluid that could cause the turbine to overspeed and destroy itself.
Problems in the EHC Fluid
The servo and directional valves of a steam turbine control system, such as the trip-throttle valve, are extremely sensitive to the presence of particulate matter because of their tight clearances. For instance typical servo valve clearances are between 1µ and 4µ, while directional valve tolerances are between 2- and 8-micron. Sludge and silt particles with typical sizes between 1- to 5-micron enter the valve clearances and degrade performance through wear and sticking of valve components. Field samples have indicated that over 80% of the solid particles measured were in the range of 2- to 5-micron. This indicates that solids filtration to an ISO 14/11 should be maintained. We have found that a 5-micron filter element will suffice because it is only 5-micron for a short time. Very quickly the layer of solids that accumulates on the membrane will make it filter finer.
Phosphate esters are hygroscopic, i.e. they attract water. This causes a hydrolytic reaction that creates phosphoric acid. This reaction is self-perpetuating, i.e. it creates more acid as time passes. Obviously this corrodes the control system internals severely, sometimes with disastrous consequences.
The manufacturer recommends the following EHC fluid parameters:
- Viscosity at 40 °C to be between 37.8 and 46.2 cSt
- The resistivity to be a minimum of 4 Gohm.cm
- The TAN to be 0.3 mg KOH.gm of fluid maximum
- Water content to be between 1500 and 2000 ppm
- Particulate count to be ISO 14/10 to 16/13 maximum
- Chlorine content to be 150 ppm maximum
If you believe these parameters you will be buying a lot of new EHC fluid and the manufacturer will like you very much. Your manager will probably fire you.
Why?
Consider, for instance, resistivity. Below 4 or even below 5 or 6, electro-kinetic etching of the valve internals begins. We found in our refineries that we had to keep resistivity at a safe level of 20 with frequent sampling.
Secondly, consider a TAN of 0.3 mg KOH/gm of fluid. Again our practice has been to keep the TAN at or below 0.05 mg KOH/gm of fluid. At 0.3 the fluid is beginning to reach the point of no return even when using fuller's earth or ion exchange.
Thirdly, consider a water content of 1500 to 2000 ppm. This is absolutely insane. First of all it neglects to consider the saturation value of the fluid at the operating temperature. That water level could be very close to the saturation point and could be on the verge of dropping out as free water. We found that we had to keep the water level no higher than 10 ppm with at least a 30% safety margin between the water concentration and the saturation point.
A high chlorine content indicates internal corrosion and any value higher than 10 mg/l is a danger sign.
The Solution
There are two ways to keep your EHC fluid in good condition. One way is complicated, expensive, and messy and may backfire on you by making the fluid more contaminated. The other is economical in the long run: neat, clean, and proven to be highly effective.
Method 1: The Expensive, Messy, and Dangerous One
It consists of absorbing water with water adsorbing filter cartridges.
These cartridges hold a relatively small amount of water and simultaneously filter out solids to about 2-micron. They are expensive and usually have to be replaced quite often. Subsequent filtration consists of fuller's earth, activated alumina, or ion exchange cartridges. These are supposed to absorb the acid in the EHC fluid. However, if the acidity rises above a TAN of 0.2, all these media will start leaching various metal salts into the fluid, thereby further degrading it. Eventually the fluid will have to be discarded if the acidity has progressed beyond 0.5 mg KOH/gram of oil. A rigorous sampling and testing program in this case is vital.
Method 2: The Proven Effective Method of Filtration and Vacuum Distillation
An Allen Vacuum Distillation System such as the Allen AOC Series Oil Conditioner; will eliminate solids by filtration, evaporate moisture and gases by the process of vacuum distillation, and will continue to do so reliably for a very long time. This makes the system much more cost-effective in the long run when compared to the continued cost of expensive elements.
The Vacuum Distillation System is a compact, skid-mounted system consisting of a pre- and post-filter, a heater, and a vacuum vessel. The vacuum vessel is filled with randomly packed Raschig Rings™ which provide a large surface area for the oil to flow over in a thin film. The vacuum and the heat provide the necessary mass transfer to remove the moisture and other contaminants as a vapor, which is trapped in a condensate collection tank before it can reach the vacuum pump.
It purifies the EHC oil to the following parameters:
- Moisture down to 5 to 10 ppm total water
- Viscosity to as-new condition or better
- Solids down to 0.5-microns or less
- TAN at or below 0.05 mg KOH/gm of fluid
- Zero concentration of dissolved or entrained gases
- Resistivity > 20 Gohm.cm
Case History
In a large overseas oil refinery a steam turbine driver had not had its control oil sampled for at least one year. One fine day, the turbine experienced a variance in load. The servo valve system did not function and the trip-throttle valve did not shut off the steam supply. The turbine sped up uncontrolled and went into overspeed. Pieces of the turbine casing were found up to one mile from the refinery. Fortunately no one was hurt.
Upon examination it was found that the entire servo system including the trip-throttle valve stem was corroded to the point of immobility. A rigorous sampling program was instituted and a trailer-mounted Allen Oil Conditioner was purchased specifically to purify the control oil in ten large steam turbines. Later the Oil Conditioner was used to purify all lubrication oil throughout the refinery.
