Preface
The esterification and polycondensation reactions of polyester instrument devices must be carried out under high vacuum conditions. Once air enters the reaction system during the production process, it will not only cause insufficient margin of the remote magnetic flap level gauge, and difficulty in adjusting viscosity and load. , And the air leaking into the system may have an oxidative degradation reaction with the material, affecting the color value, viscosity and other product indicators. The process steam drawn by the remote magnetic flap level gauge contains organic matter such as THF, which may explode when mixed with air under high temperature conditions. Therefore, the leak detection work before the device starts production is particularly important. The leak detection operation of the PBT instrument device means that the device performs a leak test operation according to different system segments before starting up. Under a certain pressure (positive pressure or negative pressure), check whether there is leakage in the valves, welds, flanges, etc. , Is the general inspection of equipment installation quality. The leak detection operation is generally divided into 2 stages:
Cold test and hot test.
Leak detection test methods are mainly bubble method, halogen leak detector detection, helium leak detector detection and so on. Halogen leak detector detection and helium leak detector detection can find the leaks more accurately, but the price is higher, and the procedure is cumbersome during the measurement process and the leak detection time is longer. Using the bubble method to detect leaks is low cost, and if the plan is detailed and carefully operated, the airtight leakage rate requirements of the device can be fully achieved. This article focuses on the leak detection operation of the bubble method.
1 Preparation before leak detection
1. 1 Defining the scope of the leak detection test, confirming the isolation method and the leak detection qualification standard. According to the installation conditions, four independent closed systems are designated for leak detection, including the esterification system, the prepolymerization system, the first line of final polymerization, and the second line of final polymerization. The system is isolated by closing the valve and installing a blind plate. The valve status of the melt pipeline must be confirmed in place before the system is shut down to cool down to avoid damage to the valve during operation after the valve temperature is lower than the melt melting point temperature. The blind plate should be installed with a thin stainless steel plate with a good surface finish, instead of aluminum skin or asbestos plate. The position and number of blind plates installed during system leak detection must be carefully recorded to prevent pipeline blockage due to omissions during system recovery. According to the process control conditions of each system and the size of the system, different leak detection qualification standards are set.
1. 2 When the equipment and meters are installed in place, when the air-tight leak detection is started, the installation and maintenance of the equipment is nearing completion. The equipment accessories and meters within the air-tight leak detection range must be installed in place, and the flange gasket used must be a formal gasket. Do not use gaskets made of asbestos or rubber. The mechanical seal lubricating fluid of the agitator should be filled and the fluid level should be marked. According to the system pressure required for leak detection, select a remote pressure gauge with a suitable range, which is convenient for judging the progress of the system leak detection work through the pressure change curve in the control room. Other detection instruments such as bubbling level gauges and pressure gauges need to be guided. The pressure pipeline is brought into the leak detection range, reducing the probability of leakage during the operation of the device.
1. 3 Preparation of air-tight leak detection tools. The tools needed for air-tight leak detection are relatively simple: soapy water, watering can, flashlight, reflector and so on. The process control card is used to record the time of charging and decompression, and to register the process of discovering and handling leaks, so as to summarize experience.
2 Cold state detection
2. 1 Soapy water leak detection After the preparation work is completed, use nitrogen or compressed air to pressurize the system to 90 KPa, and the pressure shall not exceed 100 KPa to prevent damage to the equipment. After the system stops charging, if the pressure drops quickly, it needs to be re-checked on the spot to see if the valve isolation is not in place, the equipment is not installed in place, and the sealing surface is not tightened. If the pressure drops slowly after the charging is stopped, the leak detection team uses soapy water to check the system flange connections, threaded connections, valve seals, welds, and other possible leak locations. Focus on checking the locations related to the maintenance items. After the leaks are found during the inspection process, mark them. After the system inspection is completed, they will be processed uniformly until all the leaks are found and processed.
The process of leak detection requires carefulness and patience. When no external leakage is found in the investigation and the leakage rate is still unqualified, it is necessary to analyze whether there is an internal leakage of the valve. Isolate the valve that may have internal leakage by installing a blind plate. The internal leakage of the valve can be analyzed and judged by the change of the leakage rate. For valves that are often opened during normal production, such as BDO spray pipeline valves, it does not need to be repaired if the internal leakage is not serious. , But if it is the internal leakage of the nitrogen valve, the vacuum margin of the remote magnetic flap level gauge during production may be insufficient, and the valve needs to be repaired or replaced immediately. Valves with confirmed internal leakage must be registered in the process control card after the blind plate is installed, and an isolation blind plate should be installed in the next leak detection preparation stage to improve the efficiency of leak detection.
2. 2 Cold state pressure keeping When the leak point processing is over, when the leak detection result of the system is judged by the change trend of the pressure curve, the system pressure is adjusted to a certain pressure, and the pressure is maintained for a period of time to calculate whether the leak rate is qualified. The formula for calculating the leakage rate is R = P 10 Vt. In the formula: R is the leakage rate, kPa L /S; P is the pressure difference before and after the pressure holding, K V is the volume of the system, and Lt is the pressure holding time, s. The cold state holding pressure is charged to 90 KPa (gauge pressure) for 24 hours. After the leakage rate is qualified, vacuum is applied for negative pressure holding, and the system pressure is pumped to 5 KPa (absolute pressure) through a liquid ring pump. Keep the pressure for 24 h to calculate the leakage rate. The leakage rate calculation needs to consider the influence of temperature changes. According to the ideal gas state equation, select 2 to 3 representative temperature measurement points in the system, and calculate the pressure compensation value through the temperature changes before and after the start of the pressure hold, and modify the pressure Pressure leakage rate results. But even if the compensation value temperature is included, the influence of temperature still exists. The leakage rate can be calculated by extending the holding time and selecting two time points with little temperature change. If the leakage rate reaches the qualified standard, the cold state test ends.
3 Thermal ultimate vacuum test The thermal ultimate vacuum test is a process in which the PET device detects the pumping capacity of the remote magnetic flap level gauge before feeding. After the cold leak detection rate of the device is qualified, the blind plate can be removed, and the heat medium heating system can be started after checking the reinstallation of the equipment to start the device heating up. During the heating process, the heating needs to be carried out according to the heating rate set by the device, and when the temperature is raised to a certain temperature, the equipment flanges, manholes and other positions are heated and tightened at a constant temperature. Since new leaks may appear during the heating process, the ultimate vacuum test can not only test the pumping capacity of the remote magnetic flap level gauge, but also infer whether there are new leaks through the test results.
After the secondary heat medium is heated to 225 ℃, the remote magnetic flap level gauge is used for the ultimate vacuum test. Once the vacuum pump unit is in the process of pumping the ultimate vacuum of the system, the pressure drops slowly and the target value is far from the target value. First, check whether each link of the vacuum pump unit is working properly, and then analyze whether there are new leaks. You can check the thermal tightness of each flange surface again, check whether there are any missing items, check whether the equipment and key valves meet the requirements of start-up preparation, these inspections have clear and detailed confirmation sheets in advance, and arrange for technicians to confirm twice to ensure that there is no Omission. After the ultimate vacuum test is completed, the remote magnetic flap level gauge is normally put into use, and the low pressure in each polycondensation reactor is maintained to enter the countdown of the system feeding and starting.
4 matters needing attention
(1) Before leak detection, the water, BDO and other liquids inside the system should be drained, especially the residual water after equipment washing and instrument calibration should be thoroughly purged to avoid affecting the measurement results during the negative pressure holding test of the system.
(2) During the shutdown and start-up of the device, the temperature rise and fall rate of the system design should be strictly followed to avoid damage to the equipment due to excessive speed or excessive temperature difference of the equipment. Excessive changes in the internal stress of the equipment may cause flange and weld leakage.
(3) Due to long-term use of various valves, there may be packing damage and leakage, and the stirring and sealing of the scraper condenser and the reactor may also leak. It can be judged by the change of the sealing liquid, and the sealing of the melt pipeline valve and other positions may occur. For leakage, install and tighten as required according to the characteristics of various types of seals of PET instrument devices.
(4) Pay attention to the leak detection of the pressure guiding tube of the instrument. The leak point on the pressure guiding tube not only affects the measurement of the instrument, but also easily causes the pressure guiding tube to be blocked during production. Most of the measuring instruments of the reactor are installed on the instrument pipe rack (aircraft pipe), and the root valve can be closed to judge whether the instrument interfaces on the instrument pipe rack are leaking through the pressure drop trend.
5 Conclusion Through the above methods, each system can complete the leak detection operation within one week at the same time, and the test result esterification leakage rate is less than 2 Kps 0. 1 kPa L /S, pre-polymerization and final polymerization leakage rate <1 kps 0. 1 kPa × L /S fully meets the design requirements. Leak detection operations need to be serious and meticulous, with steps and focus, pay attention to data sorting and analysis, and constantly sum up experience to ensure the completion of leak detection at low cost and high efficiency.
Post time: 10-12-21