Chemical supervision and maintenance guidance for phosphate ester fire-resistant hydraulic oil in power plants

Acid value is an important indicator for evaluating the deterioration or hydrolysis deterioration of fire-resistant oil, and a high acid value indicates that the fire-resistant oil has deteriorated or has hydrolysis reactions. The lower the acid value, the more conducive to the stability of fire-resistant oil. Oil with high acid value has corrosive effect on metal parts. Since the speed governing system is made of stainless steel, acid corrosion is not the main problem, but the main problem is the increase of acid value, which indicates that the oil has deteriorated or hydrolyzed, and there are deterioration products in the oil. These deterioration products will affect the resistivity, particle size, foam and air release value of the oil to varying degrees, This further affects the accurate, stable, and efficient operation of the speed control system. Fire-resistant oil, like mineral oil, undergoes oxidation reactions and produces acidic compounds when exposed to oxygen and high temperatures. In addition, fire-resistant oil has an additional chemical reaction compared to mineral oil, which is hydrolysis reaction. After encountering water, fire-resistant oil undergoes hydrolysis, which generates acidic substances such as phosphoric acid or hemiphosphoric acid. The oxidation reaction and hydrolysis reaction increase the acid value of fire-resistant oil faster than mineral oil. The molecular structure of fire-resistant oil itself determines its own deterioration characteristics, especially in high temperature and other environments, where the chemical structure changes to produce acidic substances and hydrocarbon substances that affect the acid value. To control the change in the acid value of fire-resistant oil, it is necessary to first control the water content in the fire-resistant oil and prevent its hydrolysis as a necessary condition. Secondly, it is necessary to control the oil temperature of the fire-resistant oil within the range of 35-55 ℃ required by the power standard to avoid high-temperature aging of the fire-resistant oil. It is recommended that the chemical supervision of the power plant test the acid value index of the fire-resistant oil once a month.

2.3 Moisture
Phosphate ester fire-resistant oil is prone to hydrolysis by reacting with water, and can generate acidic phosphate diesters, acidic phosphate monoesters, and phenolic substances. The acidic substances produced by hydrolysis have a catalytic effect on the further hydrolysis of oil, accelerating its deterioration and deterioration, resulting in an increase in acid value and a decrease in electrical resistivity, leading to electrochemical corrosion problems. Excessive moisture may also cause emulsification and foam of fire-resistant oil. Excessive moisture content in fire-resistant fuel can also reduce its viscosity and affect the establishment of system oil pressure. The lower the water content, the more conducive it is to the stability of oil quality, which in turn affects the operation of the system. The moisture in fire-resistant fuel mainly comes from the moisture in the air sucked in from the top breathing port of the fuel tank. When the oil absorbs the moisture, the moisture will increase. When the unit is shut down for major repairs, some condensed water will enter the fire-resistant oil. If there is a significant increase in moisture in the fire-resistant fuel, it may be caused by leakage from the oil cooler. The sealing ring of the oil cooler should be checked in a timely manner. During operation, a respirator with desiccant should be installed at the breathing port of the fuel tank, and attention should be paid to frequent replacement to keep the desiccant dry. Conditional power plants should be used in conjunction with vacuum oil filters. It is recommended that the chemical supervisor inspect the hydraulic oil moisture index every month.

2.4 Resistivity
Resistivity is an important oil quality control index for phosphate ester fire-resistant fuel, which reflects that the index is a key indicator for determining whether the components of the speed control system can undergo electrochemical corrosion. Low resistivity can cause severe electrochemical corrosion of the servo system of the speed control components, which affects system control. Controlling the resistivity within a high range is the key to ensuring stable, accurate, fast, and safe operation of the speed control system. If the electrical resistivity of the oil is low during operation, it may cause electrochemical corrosion of the system speed control system, especially in the servo system due to changes in oil flow rate and oil flow morphology, which is prone to electrochemical corrosion. It can cause issues such as servo jamming, internal leakage, and increased load current of the oil pump, which may affect the performance of the unit regulation system and in severe cases, endanger the safe operation of the unit. The lower the resistivity, the more severe the electrochemical corrosion. Electrochemical corrosion is an irreparable damage to components. On the servo, it can be seen that the damaged surface is gray black with uneven pits. The result of electrochemical corrosion is that it is necessary to frequently replace components that have been corroded and their performance cannot meet the requirements, such as servo replacement. The use of diatomaceous earth filter elements in some power plants can also release metal ions, as well as metal powders generated by mechanical surface wear in the system, which can increase the conductivity of fire-resistant fuel and reduce its resistivity. The oil temperature in the fire-resistant fuel system is generally controlled at 35-55 ℃, but the oil temperature in the servo system is much higher when the hydraulic motor is close to the steam valve. Its resistivity changes rapidly with temperature. The experiment on triaryl phosphate ester showed that when the oil temperature increased from 20 ℃ to 90 ℃, the resistivity decreased from 1.2X1011 to 6X108. Due to environmental, equipment, or personnel errors, overheating still occurs from time to time, and the presence of local overheating greatly accelerates the deterioration rate of fire-resistant fuel. It is recommended that the chemical supervision of the power plant should test the resistivity index once a month.

2.5 Particle size
The solid particles in fire-resistant oil mainly come from external pollution and wear of internal parts. The particles in the fire-resistant fuel can cause wear on the moving surfaces of precision components such as pumps in the EH system, as well as blockage of throttle holes, jamming of servo nozzles and baffles, which can affect significant fluctuations in system oil pressure, failure of steam valve control, and delayed door opening and closing. The solid particles in the oil can also accelerate the aging of the fire-resistant fuel. There are two types of particles in oil: one is hard particles such as metal shavings, sand particles, dust, and metal oxides; One is soft particles such as sludge and gel. If hard particles in the oil continue to increase, it indicates that rotating parts such as the oil pump are worn or metal parts are corroded, and metal salts are produced after the deterioration of the resistant fuel; If the number of soft particles continues to increase, it indicates that the oil quality may have deteriorated significantly, resulting in the formation of oil sludge. So, the particle size index in oil has a significant impact on the entire system and should be strictly controlled. Controlling the particle size index can reasonably arrange filters in the system; New oil can only be added to the system after passing the filtration by the oil filter. It is recommended that the chemical supervisor test the particle size index once a month.

2.6 foam characteristics
The characteristics of foam will change the compression coefficient of the oil, which will lead to the misalignment of the electro-hydraulic control signal and endanger the safe operation of the steam turbine unit. Under high pressure, bubbles in the oil break, causing pressure fluctuations in the oil system, causing noise and vibration. Bubbles entering the oil pump can cause cavitation and damage to the oil system equipment. The high energy and oxygen in the gas generated during the rupture of bubbles under high pressure can cause oxidation and degradation of the oil. The foam in the oil is easy to cause false oil level in the oil tank, which may lead to oil leakage accidents in serious cases. The new fire-resistant oil contains Anti Foam ingredients, so the new oil can hardly produce foam. The deterioration of foam characteristics of running oil is generally caused by aging, hydrolysis and deterioration of oil or oil pollution. During operation, avoid introducing compounds containing calcium and magnesium ions into the oil, because the saponification generated by the interaction of calcium and magnesium ions with acid products generated by oil degradation will seriously affect the air release characteristics and anti foam characteristics of the oil. In a few cases, the foam exceeds the standard due to the contamination of the fire-resistant oil (such as the detergent residue containing foaming agent used to clean the oil system). There are two methods to solve the problem that the foam of the running fire-resistant oil exceeds the standard. One is that when the foam of the running oil does not exceed the standard seriously, some new oil can be added to the running oil, and the anti foam ingredients contained in the new oil are dispersed into the running oil, so that the foam can be eliminated. When the foam of the operating oil exceeds the standard seriously, the only way to solve the foam problem is to add defoamer to the oil, which can be added in the operating state without affecting the normal operation of the unit. The other is to configure molecular polar adsorption regeneration dehydration device to remove polar substances and metal salts produced by the degradation of fire-resistant oil and eliminate the causes of foam exceeding the standard, so as to eliminate the production of foam.

2.7 Chlorine content
Excessive chlorine content in phosphate ester fire-resistant fuel can cause corrosion to servo valves and other oil system components, and may damage certain sealing materials, causing system failure. In daily chemical supervision, it is necessary to prevent chloride ions from entering the system and do not use chlorine containing solvents to clean system components. It is forbidden to use chlorine containing spray cleaner to clean the internal and external surfaces of all components of the control oil system. If it is found that the chlorine content in the operating oil exceeds the standard, it indicates that the phosphate ester fire-resistant oil may be contaminated with chlorine containing substances. The cause should be identified and measures should be taken to deal with it.

3. Summary
Phosphate ester fire-resistant fuel is expensive, and it is not only widely used in power plants, but also has a long service life. It is necessary to continuously improve the understanding of the properties of the oil product and improve the level of supervision and management in order to properly solve the oil quality problems that occur during operation and extend the service life of the oil product. The characteristics of phosphate ester fire-resistant fuel determine that we must do a good job in supervision, maintenance, and management in the long term. This requires cooperation and joint efforts from maintenance personnel, operating personnel, and oil supervision personnel, to ensure the quality of each link and provide guarantees for the safe and economic operation of the unit.