Hydraulic and lube oil systems are critical to the safe, reliable operation of nearly all power plants. Steam turbine lubrication systems, generator sealing systems and boiler hydraulic devices are just a few of the many power plant components that rely on hydraulic and lubrication fluids. Insufficient attention to the cleanliness of these fluids can lead to increased component wear and increased operating and maintenance costs. This article examines the importance of fluid purity and describes the use of an electrostatic fluid cleaning device for power plant applications.
EHC COMPONENT DAMAGE
According to a study conducted by Dr. E. Rabinowicz at the Massachusetts Institute of Technology, surface deterioration was responsible for the loss of equipment usefulness in 70 percent of the instances observed. In the hydraulic and lubricating systems that were analyzed as part of this study, the majority of failures were due to mechanical wear and/or corrosion.
Abrasive wear is the primary wear mechanism in components associated with hydraulic and lubrication systems (Figure 1). Particles enter the clearance space between two moving surfaces,
bury themselves in one of the surfaces, and act like cutting tools to remove material from the opposing surface. The size of particles causing the most damage are those equal to and slightly
larger than the clearance space of the device. Most hydraulic components have operating (dynamic) clearances less than 5 microns. To protect opposing surfaces from abrasive wear and
fatigue, particles of approximately this clearance size must be removed from the fluid.
Abrasive wear brought about by particulate ingression has a significant impact on equipment operation. Wear can lead to dimensional changes, which modify fluid flows, leading to increased leakage and lower efficiency. In the case of a pump, for example, more power would be needed to generate the required flow and/or pressure, leading to higher operating costs. The particulate
abrasion also begins a domino effect, as the abraded particles lead to more abrasion.
Abrasive wear has specific detrimental effects on power plant equipment. Silt contamination in spool valves, for example, can result in slow response and instability, spool jamming/stiction, surface surface erosion, solenoid burn-out and failure of safety systems. Silt contamination has similar effects on hydraulic actuators. Rod seal wear can lead to loss of oil through leakage, bronze
bushing wear can lead to loss of rod alignment, piston seal wear can lead to loss of cylinder speed, and piston bearing wear can lead to a loss of holding characteristics and loss of alignment. Based on the close dynamic tolerances shown in Table 1 for common power plant equipment, effective fluid cleaning is essential to minimize the possibility of oversize particles accelerating wear mechanisms.
Cylinder rod and seal systems are notorious contributors to contaminant ingression. Evidence points to higher ingression rates, with seal systems having the lowest leakage. This reinforces the need for effective silt filtration.
Bearing surfaces are subjected to failures as a result of repeated stressing caused by particles trapped by the two moving surfaces. In the beginning, the surfaces are dented and cracking is nitiated. These cracks spread after repeated stressing by additional particles. Eventually the surface fails. Contamination reduces bearing life significantly through fatigue, abrasion, and roughening of operating surfaces. The operating or dynamic clearance of a bearing is not equal to its machine clearance, but depends upon the load, speed and numerous other factors (Figure 2). Ideally, the rotor would be centered and there would be equal clearance around the device (Figure 2a). In reality, the weight of the rotor imposes a load that creates the clearance pattern in Figure 2b. As motion is added, without lubricant, the rotor is offcenter, but still impacts the bearing cavity (Figure 2c). By adding a lubricating film between the two surfaces (Figure 2d), a clearance is produced, but the clearance is not equal around the circumference because of the rotor weight. The variations in dynamic clearance shown by this example reinforce the importance of fluid cleaning.
Most hydraulic fluid systems in use at power plants incorporate in-line full flow mechanical filters to reduce particulate contamination. These mechanical filters are generally capable of successfully removing particulate down to 2-4 microns in size. Since the clearances of many of the hydraulic components are less than 1 micron, the long-term effect of component wear using only mechanical filters is increased.
Until recently, mechanical filters provided the only reliable method for successfully removing particulate from operating fluids. The decision-making process that guided filter sizing for a given system was greatly influenced by the equipment manufacturer’s installed cost, which ignored longterm maintenance and reliability issues.
Further, the cost associated with using mechanical filters capable of removing fine particulate was very high. Not only did finer filters cost more, but they also had to be replaced more often ecause they plugged up more quickly.
Electrostatic particulate removal is not a new technology. Many industries, including injection molding, pulp and paper, steel, and automobile manufacturing, have been using electrostatic particulate removal for years to ensure proper fluid purity. Electrostatic fluid cleaning (EFC) has had little application in the power generation industry, however, primarily because of a lack of awareness.
Electrostatic fluid cleaning significantly differs from all of the current mechanical fluid cleaners by utilizing forces generated by an electrostatic field. All products not totally soluble within the
fluid (contaminants) are capable of being removed. Additives, typically the anti-wear type, are not removed because they are totally soluble within the fluid.
In theory, EFC can remove all particulate contaminants from the fluid. In practice, it easily achieves NAS grades 3 or 4 (National Aerospace Standard 1638) and ISO standards 11/9 to 12/10 for fluid cleanliness. When used in conjunction with current in-line mechanical filters, the sidestream EFC is capable of maintaining an ultra-clean fluid system, thereby significantly reducing component wear and filter replacement costs.
The heart of an EFC unit is the collecting chamber, where contaminants are removed. The chamber consists of a circular reservoir of 10, 25, 50, 100 liters capacity, dependent upon machine size, in which plate electrodes are located mately 30 mm apart. A set of pleated collectors made up of non-conducting material are placed between the electrodes (Figure 3). A high DC voltage (low
current) is used to generate a f o r c e f i e l d between the electrodes. Fluid is supplied to the bottom of the collecting chamber by a small pump/motor assembly and passes the fluid via a diffuser vertically upwards and parallel to the collectors through the force field. The ultra-clean fluid then exits via the top of the collecting chamber, leaving the contaminant on the collectors retained by the electrostatic force.
Clean at La Cygne
Kansas City Power & Light’s La Cygne Station is home to the first U.S. power plant application of EFC technology. Two EFC units are installed on the phosphate ester hydraulic fluid systems of the plant’s 700 MW General Electric and 750 MW Westinghouse steam turbines. The EFC unitsdraw 1.5 gpm from the turbines’ fluid reservoir, clean it, and return it to the reservoir.
KCP&L initiated a fluid management review program about four years ago to identify and address various problems attributed to hydraulic fluid and lube oil contamination, including increased pump wear and noise, filter pluggage, and servo valve failures. The steam turbines’ hydraulic fluid circulation systems, which were equipped with OEM filters, suffered from varnish build-up and high particulate loads. In a few instances, the severity of the fluid contamination had caused valves to stick, tripping the units. A consultant recommended evaluation of electrostatic fluid cleaning. After careful consideration, KCP&L decided to install an EFC unit on one of the turbines to gauge its performance. About one year later, satisfied with the ability of the EFC to remove varnish and maintain effective cleanliness levels, KCP&L installed EFC on the second steam turbine. Since installation, both EFC units have been maintenance-free and La Cygne has not experienced a single unit trip attributable to high fluid contamination levels. Based on this success, La Cygne is evaluating the installation of EFC units on various other mineral oil lube systems over the next few years.
The collector design is important (Figure 4). Pleats configured at an acute angle are used to greatly multiply the potential collecting area. The collector surface area can hold up to ten pounds of contaminants, compared with about one-tenth of a pound for a conventional mechanical filter. Fluid naturally flows along the line of least resistance which is also closer to the strongest area of attraction. The force field is distorted due to the collectors and also the pattern of the contaminant build-up. Particles, depending on their composition, can be either negatively or positively charged. As a result, both sides of the collectors attract contaminant. The collector’s acute pleating and unique surface finish which is deliberately rough, enable it to hold contaminant on all the surfaces and not just in the bottom of the valley.
Due to the gravitational effect, there is a natural attraction between particles when they are very close or touching one another. This phenomena, together with the rough surface texture of collectors and the stickiness of much of the contaminant, ensures that contaminant build-up on the collector does not fall off once the electrostatic field is broken.
The electrostatic fluid cleaning process has been well proven on more than 30,000 units throughout the world. The end users enjoy many benefits from the certainty afforded with having an ultra-clean fluid:
- Fluid Conservation – Almost without exception, end users have not had to change the hydraulic fluid since the introduction of EFC. Normal wastage through leakage ensures that sufficient top-off fluid is made on a regular basis to replenish the additives which naturally deplete.
- Preventive Maintenance – As an offline cleaning system, the EFC process is an ideal preventive maintenance tool and it is accepted that removal of all levels of contamination reduces wear greatly and extends the life of components such as pumps, valves, etc. In cases of sophisticated equipment such as servo valves, the reliability and integrity of the complete machine is greatly enhanced.
- Higher Productivity – Good preventive maintenance procedures bring the benefits of minimum downtime and the associated losses in production. With ever-increasing overhead costs, this is a major benefit.
- Restrictions – The majority of hydraulic systems still operate on mineral-based fluids, for which the EFC process is ideally suited. The process will not operate on any water-based fluids or fluids with an additive to improve conductivity such as Skydrol or Hy-Jet. However, the process will work on synthetic fluids such as phosphate ester, polyester, silicone oil and many industrial solvents.
EFC units cost from $7,000-$27,000 depending on the size of the associated fluid reservoir and the type of fluid being treated, phosphate ester or mineral oil. For a typical 600-gallon reservoir, the EFC unit will only draw about 500 W of power. The unit is also compact, measuring 28 inches in height.