In the power generation industry, phosphate ester fluid is generally used in electro hydraulic turbine governing control (EHC) systems of steam turbines because of its good stability, exceptional fire retardant and lubricating properties. Electro hydraulic power units supply hi-pressure fluid to control and power the turbine steam valves and trip mechanism. Normally the EHC System consists of following components: single or parallel lines, main pumps (screw pump or any other type, booster pumps (centrifugal pump or any other kind), relief valve, fluid cooler, acidity removal media, low pressure filter (LP), high pressure filter (HP), silica gel filter, accumulator, pressure control valve, servo valves, trip plungers etc..
Phosphate ester is a delicate fluid and in the presence of high water content (>1500ppm) and high temperatures, it is susceptible to hydrolytic breakdown and therefore need special attention. Hydrolysis is a process in which breakdown of molecular structure occurs at a slow rate producing acidic degradation products. Predictive maintenance of phosphate ester fluids requires periodic sampling to monitor for ISO particulate contamination, water content, acid number (AN), trace element, resistivity, mineral oil content, chlorine content, ASTM color and viscosity. This is to ensure that the fluid is within the in-service specifications as prescribed by the fluid manufacturer or industry standards.
As noted above the breakdown of phosphate ester fluid results in the formation of acidic compounds. To overcome this, conditioning medias are used to maintain the acid number within acceptable limits. Commonly fuller’s earth, activated alumina and ion exchange resins are employed to scavenge acidic compounds formed in the fluid. These scavengers have certain limitations to work effectively.
Impact of Contaminants and Fluid Degradation Products on Phosphate Ester Performance
Reference from Dr. Dave Philips paper on improved maintenance and life extension of phosphate ester.
Case History: High water content at Pickering Nuclear Generation Station B governing control system
Pickering B Station uses Reolube Turbofluids 46 XC manufactured by Chemtura. It is a triaryl phosphate based on a selected Xylenol distillate. It is high performance fire resistant hydraulic oil with good oxidation stability. There is monthly predictive maintenance (PdM) task requiring oil analysis tests at vendor’s facility: viscosity@40°C, ISO particle count, ASTM color, acid number, trace elements, mineral oil content, water content, electrical resistivity, appearance, chlorine content, and inorganic phosphate. Also yearly PdM task to perform all of the above tests plus air release, foam tendency, foam Stability and flash point tests by an independent lab.
On 24-Jul-2009, oil analysis results received from the vendor laboratory had indicated hi-water level (3595ppm) in the Pickering Unit5 EHC system. Last results from June2009 were at 691ppm. Two more samples were taken from Unit5 turbine governing control system and the moisture level was found at 4000 ppm. Ontario Power Generation Nuclear chemistry limits are: action level1=800ppm, action level 2=1000ppm (critical limit).
After the confirmation of elevated water level, a resolution team was formed to resolve the following challenges:
As per engineering recommendation, feed and bleed action was performed from the reservoir (four drums of fresh phosphate ester were injected and similar volume was removed). Oil samples were obtained from different location to find the sources of water. Oil sampling was scheduled twice per 12 hours shift. Water content and acid numbers were closely analyzed.
A vacuum dehydration unit (VDU) was purchased few years before but was never incorporated on the system. There was no procedure available and nobody was trained on its operation. There were concerns with the quick connects and hoses considering the pressure boundary requirements. With the help of local vendor HydraFab, it was commissioned over the weekend. The VDU was put into operation for 7-10 days but it had effectively reduced the water content to less than 800 ppm after 3 days. Unit 5 oil results were trended by station lubrication specialist. The Unit5 turbine control system servo valves operation was closely monitored. Varnish and sludge kind of deposits were observed on some components.
The source of water in EHC system for this event was attributed to gland steam entry into emergency stop valve relay. The gland steam valves to high power (HP) and low power turbine glands isolated immediately after unit tripped on 25th June 2009. Gland steam system root isolating valve was not included in the above temporary change request. Steam leak was observed at the HP steam chest area during the forced outage period as mentioned by operations. Condensing steam leaking from the turbine emergency stop valves’ gland can enter under gravity to the upper chamber of the power piston, which is connected to the fluid drain. Barriers like drip shield and relay lip seal are not designed for complete protection against condensate ingress but to reasonably prevent foreign materials to get into phosphate ester.
With an effective predictive maintenance oil analysis program at Ontario Power Generation, a 14 days of Unit (530MW) forced outage was avoided which saved our company approximately $ 12.5 million Canadian dollars. An expensive system flush and the fluid replacement costs had been avoided ($ amount is not available).
Khalid Malik, CLS, OMA I & II, is Senior Technical Officer of Nuclear Engineering Services at Ontario Power Generation. His contact information can be found in the membership database.
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