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Key words: PV-09207A; Preload; Small opening; Accuracy; Valve core; Seat; Balance point 1 PV-09207A Importance of urea in the process PV-09207A urea plant High pressure system to low pressure system gas valve gate, carrying the high voltage stabilizing system pressure in a urea production process taking into account the pressure in the system pressure urea smooth operation, the slightest mistake will result in a low pressure urea system disorder.
2 operating status of the original design of the valve as a high pressure angle valve , due to long-term vibration, corrosion erosion has caused serious distortion of the flow characteristics of the valve spool, the original design of the percentage is now gradually close to the quick opening characteristics, spring stiffness changes, insufficient preload , Has been operating since the construction of the plant operation, long-term small opening 7% of the following operation, has never been replaced, due to prolonged operation in the high-pressure equipment, process operators operating even with 0.1% opening adjustment is difficult to find a balance Point, to stabilize the system high pressure. Slightly worse pool will result in high pressure is too low, medium pressure overpressure, severe load will be reduced, especially in recent years, more and more frequent this situation, although each time overhaul to be grinding, but for being washed After changing the flow characteristics is a drop in the bucket, so far the valve is still sick run, so the formation of small flow is too large now uncontrollable situation.
3 to take measures According to the actual operation of the scene, developed a variety of external compensation methods, and ultimately established two effective programs, one is to increase the membrane spring preload, the second is to shorten the valve stroke.
Pay special attention not to over-compress the spring when increasing the preload, because the spring service period is too long, once the compression exceeds the spring limit, it will cause irreversible loss, so in actual operation, the first valve to normal operation of the small Opening, and gradually increase the amount of preload, until the valve does not fluctuate.
Active valve positioner is a mechanical positioner, the valve position can not meet the 0.1% amplitude of the small opening of the operation, you must replace the high precision positioner, taking into account the intelligent positioning device positioning features, decided to use intelligent positioner to improve the valve small When the opening degree of accuracy, the finalizer is completed after the completion of the valve stroke to shorten the calibration, due to the aging valve and adjust the media pressure is too high double factors, the valve position will still fluctuate after the calibration of atmospheric pressure can not meet the normal operation Demand, we must find another way to check, combined with the actual situation after the decision to adopt online calibration method, online calibration there is a certain risk, should first be combined with the depth of technology personnel, with process stop valve to control the pressure, pressure exceeds the normal value Be prepared to reduce load, ready to do, the valve hand wheel trip from the original 4.0CM pressure to 1.5CM, then began calibration, calibration process always pay attention to pressure instructions, can not allow the device over-pressure, to be calibrated After the completion of the process slowly exit the globe valve, the master operator and field operations staff, close observation of pressure indicating hand Regulating valve door.
After the combination of the two methods, the effect is very significant within 2 years, which temporarily solves the problem that the pressure can not be stabilized. For safety reasons, it is suggested that the process operator control the valve by using the handwheel limit of the valve and the manual linkage.
4 solution Although temporarily solve the adjustment problem, but failed to fundamentally solve the spool defects caused by the impact. After each overhaul, the valve originally controls the setting of high and medium pressure system valve position will change, to find a balance point for a long time, affecting the device driving load, the process operator is also a headache, but also afraid of parking error caused by parking, although Now by the hand wheel to participate in manual adjustment together, due to prolonged operation in high-pressure equipment, this operation violates the operating procedures, the device there is a big security risk, with the use of time, the increasing security risks.
After long-term site visits and expert discussions, the final decision to replace the valve seat, the original design of the original spool for the same percentage flow characteristics, and now due to the long-term erosion of the valve spool flow characteristics change, taking into account the problem of high cost, Do not change the valve body and accessories in the case of only partial modifications of the valve core and valve seat, the static inspection of the old valve body, flaw detection, check the body to determine whether there are defects and repair valve on the valve components Carry on the surveying and mapping processing, and replace the suitable valve plug and its matching valve seat which can meet the requirements of the process and production control to restore 100% of the original design flow characteristics, let the valve work under the small opening degree, and test the performance of the valve actuator Confirm its stability and reliability, the overall assembly of the valve and do static debugging to ensure the overall operation is stable and reliable, the test valve sealing performance qualified, after the completion of the installation valve and control center DCS interlock debugging.
The transformation was completed on January 16, 2016. After the transformation, the valve not only meets the current production control requirements, but also ensures the stable operation and eliminates potential accidents of overpressure accidents, laying a solid foundation for the stable and well-functioning urea plant. .
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[3] Zhu Bingxing. Instrumentation test questions. Field Instrument Volume. Chemical Industry Press, 2002.5
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About the author: Zhao Jianhui (1987-), male, Han nationality, Henan Puyang people, undergraduate, assistant engineer, Zhongyuan Dahua Group Instrument Maintenance Engineering Company, engaged in instrument maintenance and management work, several times to solve major problems since construction.
Abstract: This article describes in detail the role of PV-09207A in urea, current operating conditions, and significant hazards to production. It provides a detailed on-line solution for the continued service of important valves in the event of a change in flow characteristics. Solve similar problems to broaden the train of thought, and carried out in-depth discussion of the future transformation.