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Research and Application of New Techniques for Checking Safety Valves of Power Station Boilers

Posted by: steel world 2021-10-14 Comments Off on Research and Application of New Techniques for Checking Safety Valves of Power Station Boilers

The provisions of the relevant standards for the calibration of power station safety valves are based on the “Steam Boiler Safety Technical Supervision Regulations issued by the Boiler and Pressure Vessel Supervision Bureau of the State Quality Supervision Bureau: “The safety valve of the boiler in use shall be checked at least once a year”; The Vessel Supervision Regulations ((DL612-1996) 9.1.13) stipulates: “After the boiler installation and overhaul and the safety valves are overhauled, the seating pressure of the safety valves should be checked” (and clearly stipulates the scope of supervision of the regulations: f) Thermal system pressure vessels: high and low pressure heaters, deaerators, various expansion vessels, etc.”; the “Twenty-Five Key Requirements for Prevention of Major Accidents in Electric Power Production” promulgated by the State Power Corporation stipulates: “Various pressure vessels The safety valve should be regularly checked and discharged.” These regulations are very clear on the regular verification of power station safety valves. 
  The electric power industry standard “Supervision Regulations for Boiler and Pressure Vessels in the Electric Power Industry” (DL612-1996) 9.1.13 clearly stipulates the instrument calibration safety valve: “Pure mechanical spring-type safety valves can be calibrated and adjusted by hydraulic devices, generally 75% ~80% of the rated pressure. The safety valve adjusted by the hydraulic device should be checked for actual seating at least for the safety valve with the lowest seating value.” 
    1 Safety valve inspection method At present, the safety valve calibration of the power system is common The methods used include offline measurement and online measurement. 
  Offline measurement needs to remove the safety valve and send it to the test bench when the equipment is stopped, which is time-consuming and laborious, and this kind of measurement method cannot solve the influence of temperature on the opening pressure of the safety valve. Valves, etc.) is even more powerless. On-line measurement is divided into real jump measurement and instrument measurement. The actual jump measurement will cause unnecessary damage to the safety valve, and the noise pollution and the safety of the measurement have attracted more and more attention. On July 9, 1999, the Jinzhou Power Plant in Liaoning Province was in the safety valve school. The tragedy of five deaths and three injuries during the inspection should not be forgotten. 
  The instrument measurement method is currently a more advanced and commonly used method, such as the practical product of TREVITEST developed by the British FURMANITE company in the 1980s. The instrument uses a pneumatic motor to drive a hydraulic pump as the power source, and uses a two-channel written test recorder with sensors as the measurement and recording unit. The recorded curve of the applied force during the opening process of the safety valve is manually identified by the operator. The final result is manually calculated. The entire measurement process requires manual participation. Therefore, the consistency of the measurement results is greatly affected by human factors, and the measurement accuracy cannot be guarantee. In addition, the price of this kind of instrument is expensive (the quotation reached 1.25 million yuan in 2000), and it is difficult to promote domestically. Safety valve manufacturers in Japan, the United States and other countries have also successively developed similar products in the 1990s, but such products can only be used in safety valves produced by their own manufacturers, and they are incapable of domestic and other safety valves. Since the 1990s, some imitation models have also appeared in China, and there is not much breakthrough in technology, and there are still shortcomings of poor measurement consistency and low measurement accuracy. 
  NSH series safety valve online constant pressure instrument is a new type of portable instrument developed by Beijing Bangbei Electromechanical Technology Co., Ltd. and Beijing Labor Safety Technology Equipment Center. Its basic working principle is equivalent to the TREVITEST safety valve detection system of FURMANITE in the United Kingdom. There is a major breakthrough. The NSH safety valve online constant pressure instrument is a new patented product for verifying power station safety valves. It is recommended by the Boiler and Pressure Vessel Supervision Bureau of the Ministry of Labor and the Power Station Valve Standards Committee. This product has passed the inspection of the Chinese Academy of Metrology. At present, this instrument has been used in safety valve calibration of more than 50 power plants nationwide. Practice has proved that this instrument only needs 2-5 if the NSH safety valve online calibrator is installed in the case of unit shutdown or operation. The second opening time can complete the online performance test and working parameter test of the safety valve, and can also carry out online tuning of the safety valve that has been unqualified. It can measure and record the opening pressure of the safety valve and the system medium pressure at the location of the safety valve. .  
    2 Design principle 
  The opening and closing action of the spring safety valve relies on the change of the medium pressure at the inlet end and the spring pre-tightening force to make the spool automatically open and close. When the medium pressure (internal pressure) rises to the point that the lifting force is greater than the spring preload force, the valve core automatically opens against the spring preload force, vents the excess medium, and reduces the internal pressure. When the pressure drops to a safe value, the spool automatically closes and the relief stops.
According to this principle, it can be assumed that when measuring or adjusting the safety valve online (also known as the hot state), if an upward additional force is provided from the outside, when the sum of the medium pressure and the additional force just overcomes the spring preload, the valve The core will also open, even in the offline (also called cold state) condition without the action of the medium, the valve core can also be opened when the external additional force overcomes the spring preload force alone. Figure 1 clearly shows the force Balance the relationship. 
    Obviously, we can see that the valve body is opened condition: 
    the FT = PL + FW × S 
    where: FT– spring preload 
    FW– additional external force 
    PL– medium pressure acting 
    S– valve sealing surface area of 
    the The opening pressure P is: P = PL + FW / S. 
    When it is cold, P = FW / S. 
    Obviously, if the additional external force FW can be accurately measured, it can be easily obtained based on the known spool area S and the system working pressure. Get the opening pressure P of the safety valve. This is the design basis and principle of safety valve online testing technology. 
    3 Technical basis 
  According to the analysis of the opening action characteristics of the spring safety valve, it can be concluded that under the action of the applied force FW, the change of the external additional force FW during the entire process of a safety valve from closing to opening, and from opening to closing According to the law, when the additional force gradually increases from zero, and the sum of the internal pressure PL×S is exactly the spring preload, the valve opens slightly, increasing the medium action area S, so as to overcome the internal pressure of the spring preload The force increases sharply, and as a result, the external additional force is instantly reduced. Thus the first characteristic peak A appears. When the external additional force gradually decreases and reaches the closing point, due to the sudden decrease of the medium action area, in order to maintain the balance relationship of the force, the external additional force will rise instantly, that is, the second characteristic peak point B. The above two characteristic peak points A and B are the technical basis for detecting the opening pressure and return pressure of the safety valve under online conditions. 
  Before the valve is opened, the external additional force overcomes the static rigid force of the spool. When the opening point is reached, the external additional force is changed to overcome the elastic force of the spring. The slopes of the two change with time are different, and the first inflection point C appears. In the same situation, another inflection point D will appear when the valve is closed. These two inflection points respectively correspond to the opening and re-seat of the valve, which are the technical basis for testing the opening and re-seat pressure of the valve in the cold state. From the above analysis, it is not difficult to see that the key to the safety valve online testing technology is how to correctly and quickly find the corresponding opening feature points. 
    4 System composition The 
  online inspection system is composed of three parts: mechanical fixture, hydraulic power unit and data acquisition and processing unit, which are relatively independent of each other. They are interconnected by two 10-meter-long hydraulic hoses and two 10-meter-long five-core shielded cables. Into a complete safety valve test system. It has the remarkable characteristics of small size, light weight, modular components, computerized equipment, simple operation, stable and reliable, etc. Its performance exceeds the level of similar foreign instruments in many aspects. 
    4.1 Fixture: Ensure that the safety valve to be tested is clamped and positioned, and provides an environment for applying external force to the hydraulic power unit. It adopts a combined structure and is very convenient for disassembly. (See Figure 3) 
    4.2 Hydraulic power unit: Provides adjustable hydraulic output and flow, with a maximum output of 10MPa and a maximum lifting force of 50KN to control the additional lifting force and lifting speed. 
    4.3 Data processing unit: It is the core of the test system and directly determines the reliability and accuracy of the system. 
  Among them, the force sensor adopts a spoke-type structure with extremely high sensitivity and an accuracy of 0.05%. The pressure sensor adopts a high-temperature sensor with a working temperature of 200 to 250 degrees. Both sensors are of high output type, with built-in amplifiers, and their linearity, repeatability and anti-interference ability are extremely strong. 
  The test system adopts two-channel low-gain, high-precision amplifier circuit, intelligent A/D conversion and data acquisition circuit, which can collect two parameters of force and pressure at the same time. The core part uses the current advanced notebook computer (CPU PⅢ600) on the market. Curves can be drawn and test results can be printed, with Chinese-characterized man-machine dialogue function, the data of each measurement channel can be displayed on the screen, in the case of over-range, to ensure the safety of the system and safety valve, all hardware adopts modular structure , Easy to repair and debug.
In addition, in order to ensure that the host can work normally and continuously in a high temperature environment of 50 ℃, a special auxiliary equipment-computer low temperature workbench is specially designed. 
    Operation steps The 
    operation steps are as follows: Figure 4, select the safety valve to be tested, according to the system installation drawing, the installation and connection of the NSH components can be easily completed; click “Load Parameters” on the main control window of the computer to select the safety valve to be tested (See Figure 5) Select the pressure sensor measurement method and enter the basic pressure value; after checking it is correct, click “Re-line constant pressure”, at this time the system prompts: “Testing! Please wait!” Turn on the power switch of the hydraulic unit to ” Open” position, adjust the “system pressure adjustment” knob to the value indicated by the computer “required oil pressure”; turn the reversing control knob to the “lift” position, so that the cylinder piston drives the valve stem of the tested valve to move upward; by observing the valve When the rod rises or hears the sound of medium discharge, after confirming that the valve has been opened, turn the reversing control knob back to the “reset” position and turn off the power; the computer will automatically enter the result display window (see Figure 6) to display the test curve and test time , Opening pressure, adjustment height and other parameters; adjust the constant pressure nut according to the adjustment height prompted by the computer; click the “exit” button to restart the test program, and obtain a new test curve and result. Repeat the above steps until the difference between the opening pressure and the setting pressure is not greater than the “measurement allowable error”, at this time the computer displays “adjustment completed”, indicating the end of the measurement. 
    5 Error analysis 
    According to the selection of the MSC-5101 sampler, the accuracy of the sensor and the design accuracy of the amplifier, the measurement error of the opening pressure of the test system is analyzed as follows. 
    Among them: f——physical quantity (force, pressure) 
    V1V2——intermediate electricity 
    N——digital quantity (force, pressure) 
    K1, K2, K3——conversion coefficients at all levels 
    according to error transmission theory, absolute error and relative error are respectively: 
    Substitute actual deviation: 
    (Maximum error of force sensor ) Accuracy 0.05% 

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