Aging Systems Causing Reliability Issues
As the fleet of large steam turbine-generators continues to age, forced outages are becoming an increasing problem for the electric utilities.
Most large steam turbine-generators utilize EHC (Electrohydraulic Control System) high pressure fluid to position their governing and tripping valves. On most modern large steam turbines the main turbine bearing oil is not used for EHC control but, a separate specialized fire resistant EHC fluid is used for both the turbine valve actuators and the turbine trip system. Over the past five years many turbine-generators have been forced out of service due to problems with the EHC high pressure fluid system.
Investigations into these forced outages have revealed a common recurring cause: EHC fluid contamination.
According to NERCGADS for the category of “turbine trip system”, there is a 100% probability of at least one forced outage occurring in the next 12 months (EPRI Turbo X computer software). The” turbine trip system” consists of the electronic relays within the turbine control system, the hydraulic fluid system and the trip valves.
The “synthetic phosphate esther”, fire resistant EHC fluid is not a normal petroleum based lubricant. Contamination issues regarding the EHC fluid can be complex and difficult to solve. The root cause for “synthetic phosphate esther” fluid contamination can be one of many possibilities. Two common sources of EHC fluid contamination are excessive particulates and high acidity.
High particulates in the fluid can cause premature wear of critical components within the operating mechanisms of the governing system. In addition, excessive particulate contamination of the fluid can cause pluggage of the fine mesh inlet strainers of servo valves which in turn cause their operating cylinders to close requiring either replacement of the strainers, or in most cases, replacement of the servo valves.
Another common type of fluid contamination of the hydraulic fluid system is high acid content. Acids in the hydraulic fluid (synthetic phosphate esther) is caused by water being introduced into the fluid. The water causes the fluid to catalyze which results in the formation of Acid Phosphates. The Acid Phosphates cause etching of yellow metals within the hydraulic fluid system. The etched metals cause problems in operation of the components which they control.
Where do the particulates come from?
The cause of particulate build up in turbine high pressure fluid systems comes from one of six sources:
- Particles within the high pressure fluid system generated by wear of the internal components of the operating system (i.e. pumps, filters, servo valves, solenoid valves, and trip valves).
- Particles being introduced into the hydraulic fluid from an external source such as during maintenance inspections of the air breather assemblies attached to the hydraulic fluid
- Varnish particles are generated as the synthetic fluid ages and is exposed to temperature and pressure transients. If significant changes in fluid acidity occur, varnish can be removed from system components.
- New fluid that is added to the EHC system often has particulate concentrations several times greater than the existing system.
- As synthetic phosphate esther fluid ages, the fluid can breakdown resulting in particulates generated by the fluid.
- The EHC system fluid neutralization filter (typically Fullers Earth) can also contribute to significant particulate contamination.
The internal filtering system is not capable of maintaining clean fluid over a long period of time. As particles are introduced into the system, the primary particulate filter becomes less effective in removing particles based upon the filters low “multi pass filter efficiency” rating. Filter manufacturers rate their filters based upon the number of passes the fluid must take through the filter in order to remove particulate. The higher the efficiency number the more effective the filter is. Since the primary particulate filters are “full flow” filters, they must be capable of filtering the fluid at the same time they are maintaining a continuous flow of fluid to the system. In order to assure both conditions are met, the filters must have a low “multi pass filter efficiency” rating.
A side stream filtering system is therefore provided to continuously filter the fluid in the system before the pump suction. In most cases the side stream fluid filtration system’s primary purpose is to reduce acid content in the fluid not to remove particles.
The cause of the particles has to be eliminated or reduced significantly in order to assure continued reliable operation of the turbine hydraulic fluid system. If externally introduced particles and varnish are eliminated, the wear of internal components would be significantly reduced. In order to stop, or at least reduce externally introduced particles from entering the system, extreme care should be taken any time the hydraulic fluid system is exposed: such as when changing out a servo valve, cleaning the hydraulic reservoir or doing maintenance on the hydraulic actuators. The work area should be isolated by covering 100% of the work space with plastic in addition to thoroughly cleaning the area around the device prior to performing maintenance. Every time the fluid system is exposed to the elements, it becomes contaminated. Any maintenance that requires exposing the fluid system to the air should be conducted under tight control and the time the system is exposed should be minimized or the openings should be covered immediately with “lint free” rags.
As the EHC system ages, varnish is deposited on the inner walls of the piping, valves, actuators and system components. The varnish is generally a very hard, tenacious deposit that is accumulated over time. The only way to remove the varnish is to increase the fluid acidity and dissolve the varnish back into the fluid. This is normally accomplished by completing an EHC fluid flush. The EHC flushing fluid is a compatible phosphate esther fluid with a high acidity.
It should be noted that varnish can also be removed by acidity variations in the system during normal operation. As the fluid experiences temperature, pressure and flow transients, varnish particles can be formed in the fluid. It is very common to find varnish particles in the EHC system components. Most systems were not designed with particulate filtration and therefore do not remove the particles in the system. However, EHC systems can be upgraded with online fluid filtration and particulate control systems that maintain low particle concentration.
What causes the acid contamination of the fluid?
When water is introduced into the hydraulic fluid (phosphate ester), even small amounts, the fluid catalyzes causing the buildup of acid phosphates. If the acid phosphates are not eliminated, even though the introduction of water into the fluid has stopped, the fluid will continue to self catalyze causing the acids to further increase.
It is important to not only eliminate the cause of water into the system but also to have a reliable filtering system to neutralize any acid built up in the fluid.
The acid removal filters should be sized and fluid flows adjusted for optimum filter efficiency. Generally the flow through these filters should not exceed 1 gpm. In addition, these filters should be changed out at least once every six months. It has been noted that if the acid removal filters remain in the system for more than six month salts are released into the fluid causing the fluid to become gelatinous.
In order to assure a clean and reliable fluid system, periodic fluid samples should be taken. These fluid samples should be analyzed by a laboratory experienced in handling “synthetic phosphate esther” fluid. It is important that the fluid samples are taken using proper procedures in order to assure accurate results. In addition samples should be taken from at least two different locations :preferably the bottom of the reservoir (pump suction) and downstream of the pump discharge particulate filters.
Laboratory Tests Needed To Monitor EHC Fluid
The following tests are very important in monitoring the condition of the system and the fluid on a regular basis.
|Particle Count||ISO 4406||Particle Size And Distribution|
|Water Content||ASTM D 1744||Amount Of Water In Parts Per Million|
|Total Acid Number||ASTM D 664||TAN Measures Oxidation Of Fluid|
|Chlorine Content||XRAY||Checks For Solvent Contamination|
|Mineral Oil||GE METHOD||Checks For Fluid Contamination|
|Specific Gravity||ASTM D 1298||Checks For Fluid Contamination|
|Viscosity||ASTM D 445||Fluid Quality And Contamination|
|Spectroanalysis||SPECTROMETER||Fine Wear Particles And Additives|
|Color||ASTM D 1500||Oxidation And /Or Contamination|
|DC Resistivity||ASTM D 1169||Resistance or Conductance of Fluid|
|AC Conductivity||ASTM D 1125||Conductance of the Fluid|
By monitoring these basic tests of the system, a greater understanding of the fluid condition can be gained. In addition to the above tests many more advanced methods can be employed to deal with specific system problems. With tests such as filter residue analysis, FTIR, and other disciplines, greater insight can be gained on these systems.
The EHC fluid system is one of the most critical systems controlling the production of electricity today. Power plants that expect to achieve low forced outage rates and high availability should pay close attention to the fluid quality in the EHC systems. Proper fluid analysis, system upgrades and flushing during outages are a good start to a healthy EHC system.