Zuo Aiwu Xiao Jun Abstract: In-depth and systematic analysis of the causes of leakage of AS system plate heat exchangers, and targeted improvement from the equipment itself and production operation methods, to solve the problem of frequent leakage of plate heat exchangers in the past, to ensure the entire process Normal production.
(Wuhan Pingmei Wuhan Iron and Steel Coking Co., Ltd., Wuhan 430082, China)
Key words: AS system; plate heat exchanger; leakage
CLC number: TK172 Document code: B Article ID: 1008-4371(2010)01-0043-02
The plate heat exchanger is an important process equipment for the AS desulfurization system in the recovery workshop. It bears the role of heat exchange between the deacidified and lean liquid and the rich liquid. The working condition of the plate heat exchanger plays an important role in the working conditions of the entire AS system. The role. The AS system plate heat exchangers of the second recycling workshop of Wuhan Pingmei WISCO Joint Coking Company have been leaking frequently in recent years, which not only pollutes the environment of the production site, but also harms the physical and mental health of employees, and more importantly, directly affects the deacidification The heat exchange between the rich liquids, which in turn affects the deacidification effect of the deacidification tower, and eventually causes the H2S index to exceed the standard, which seriously affects the normal production.
1 Introduction to AS system deacidification process
The AS system deacidification process is produced in two systems, each with one fixed ammonia tower, one volatile ammonia tower and one deacidified ammonia tower. In normal production, the rich liquid (about 35 °C) sent from the hydrogen sulfide scrubber is divided into three deacidified lean liquids from the bottom of the deacidification tower, stripping water from the bottom of the volatile ammonia tower, and the bottom of the fixed ammonia tower. The three heat mediums (about 98 °C) such as ammoniated wastewater are heat exchanged through a plate heat exchanger, and then enter the three towers for distillation. The bottom of the medium (ie, deacidified lean liquid, stripped water and ammoniated wastewater) It is heat-exchange cooled with a cold medium (ie, rich liquid), and then further cooled to about 35 ° C by a spiral plate cooler, and then sent to a hydrogen sulfide washing tower to wash the hydrogen sulfide in the gas to become a rich liquid. This cycle is repeated and continuous production. The plate heat exchanger is responsible for the heat transfer.
2 Plate heat exchanger structure principle and characteristics
The plate heat exchanger is a new type of high efficiency heat exchanger which is assembled from a series of metal sheets with a certain corrugated shape. A thin rectangular channel is formed between the various sheets, and heat exchange is performed through the half sheets. The plate heat exchanger is mainly composed of two parts: a frame and a plate. The plate is made of various materials and pressed into various shapes of corrugations by various types of abrasive tools, and at the four corners of the plate. An angled hole is provided for the flow path of the medium. The periphery of the plate and the corner hole are sealed with a rubber gasket. The external structure diagram and internal working principle diagram of the plate heat exchanger are shown in Figure 1 and Figure 2 [1], respectively. Plate heat exchangers have been widely used in chemical production due to their high heat transfer efficiency, light weight, small size, easy cleaning, small heat loss and easy change of heat exchange area. The trend of heat exchangers.
3 Analysis of leakage causes of plate heat exchanger
For the leakage problem of the plate heat exchanger in the AS system, after several careful observations, the following characteristics were found.
1) It is often only leaked from the rubber gasket, and the plate rarely leaks.
2) It is often easy to leak when opening, stopping or switching operations, especially when it is stopped, and normal production is basically leak-free.
3) The case of leaking cold medium is more than that of heat medium. According to these phenomena, combined with the structural principle of the AS system process and the plate heat exchanger, it is concluded that the main causes of leakage of the plate heat exchanger are thermal expansion and contraction and pressure fluctuation. Take the shutdown operation as an example: if the operator first closes the lean liquid (heat medium) inlet and outlet valves, and then closes the rich liquid (cold medium) inlet and outlet valves, after the heat medium stops circulating, the cold medium still circulates inside the heat exchanger. The temperature of the heat exchanger sheet and the rubber gasket of the heat exchanger is drastically lowered, shrinkage occurs, and the rubber gasket is punched out of the sealing groove under the pressure of the cold medium, thereby causing leakage of the cold medium.
4 improvement plan
Based on the above analysis, improvements have been made in terms of equipment structure and production operations.
4.1 Equipment structure improvement plan
Since the root cause of the leakage of the plate heat exchanger is thermal expansion and contraction, it is possible to add a high-strength, high-compensation elastic disc spring to the several screws of the fixed plate heat exchanger from the structure of the device. Increase the temperature compensation capability when the temperature fluctuates greatly. The structure and principle are shown in Figure 3.
4.2 Production operation improvement plan
4.2.1 Driving steps and precautions
1) Before driving, strictly check whether the inlet valve of the cold and hot medium is closed and whether the outlet valve is open.
2) Start the pump of cold and hot medium first, open the inlet valve of the cold medium slowly, and then open the inlet valve of the heat medium to slowly flow the medium into the heat exchanger to avoid the temperature being too high.
3) Slowly raise the temperature while measuring and calculating whether the process requirements are met. Once satisfied, you can enter normal operation.
4.2.2 Shutdown operation steps and precautions
1) The pump must be stopped before stopping, and the pump should be stopped in the order of reheating medium before cooling medium.
2) After stopping the pump, first slowly close the cold medium inlet valve and then close the inlet valve of the heat medium. Finally, the outlet valves of the two media are closed.
3) If a vent valve is installed on the line, the vent should be opened.
5 Conclusion
After the improvement, the plate heat exchanger has not experienced new leakage within half a year, which not only greatly reduces maintenance and maintenance costs, reduces production costs, but also reduces environmental pollution, reduces damage to employees' physical and mental health, economic and social benefits. Both are very significant.
[references]
[1] Wang Jian. Introduction to plate heat exchanger and its application [J]. Nitrogen fertilizer design, 1994, 32 (2): 25-28.