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Our company currently has two urea production systems. The urea production unit of the \"8·13\" system, which was built at the end of 1998 using an ADB loan, employs the ammonia stripping process developed by Snam of Italy, while its DCS system uses the RS-3 distributed control system from Rosemount of the United States. Following expansion and renovation, the system now has the capacity to produce 120 kt of synthetic ammonia and 200 kt of urea per year. The following is a brief introduction to several modifications made to the control system of the 8·13 urea plant and their effects. 1. Step-controlled urine temperature in the medium-pressure decomposer. The medium-pressure decomposer is one of the key devices in the urea production process; its function is to heat and decompose the mixed solution containing ammonia, carbon dioxide, water, and urine that comes from the high-pressure system, thereby further improving the purity of the urine. The two streams of steam originally designed to supply heat to the medium-pressure decomposer are each controlled by two relatively independent control circuits: the 2.5 MPa steam is controlled by the HV9307 control valve, while the 0.45 MPa steam is controlled by the TV9301 control valve. In actual production, due to changes in load and fluctuations in the steam pressure at 2.5 MPa (with the 2.5 MPa steam being supplied from outside), adjusting the temperature requires modifying both control valves simultaneously, and these valves need to be coordinated with each other. As a result, it is difficult to control the medium-pressure decomposition temperature properly, leading to significant temperature fluctuations. Such large fluctuations in medium-pressure temperature inevitably affect the stability of the low-pressure and evaporation systems, thereby hindering the long-term stable operation of the urea production system. To change this situation and optimize the operations, our company has switched the two steam control valves HV9307 and TV9301, which regulate the medium-pressure decomposition temperature, to proportional control. The advantage of this control method is that once automatic control is engaged, when the medium-pressure decomposition temperature is low, TV9301 (low-pressure steam) is first opened. If the temperature still does not rise even after TV9301 has been fully opened, then HV9307 (medium-pressure steam) is turned on ； When the medium-pressure decomposition temperature is high, HV9307 is first reduced in size; once HV9307 is fully closed and the temperature remains high, TV9301 is gradually reduced in size. The modified steam control process is shown in Figure 1. After its implementation, the effects were significant: it not only kept the temperature of the urine at medium pressure within the specified range and stabilized the operation of the low-pressure and evaporation systems, but also enabled full utilization of low-pressure steam, thereby saving medium-pressure steam and achieving energy savings. At the same time, it reduced the need for manual adjustments, lowering the workload for operators. http://www.nmtech.com.cn/jishuwang/upload/060205838582694.jpg 2 Reform of the temperature control method for the stripper. The stripper is the only equipment used for high-pressure decomposition; poor control of the temperature at the bottom of the stripper has a significant impact on medium- and low-pressure decomposition. Therefore, the temperature control at the bottom of the stripper directly affects the stable operation of the entire system under high loads over long periods of time. The opening degree of the 2.5 MPa steam control valve (TV9210), which provides heat to the stripping tower, determines the level of the bottom temperature. Before the modification, steam pressure served as the controlled parameter, and the valve opening degree changed according to the level of steam pressure. Due to the inappropriate selection of this controlled parameter, when the pressure reached the required value but the temperature at the bottom of the stripping tower still did not meet the specifications, manual adjustment of the steam supply was the only option. If the specified steam pressure is increased, the stripping tower is prone to overheating, resulting in overpressure in the high-pressure section. After the modification, the temperature at the bottom of the stripping tower was used as the controlled parameter; the amount of medium-pressure steam was adjusted accordingly based on the temperature readings, thereby achieving automatic control of the stripping tower temperature and ensuring that it remained within the specified range at all times. Practice has shown that this renovation has had a very significant effect on stabilizing the entire system, particularly the operation of the medium and low pressure systems. 3 Automation upgrade of the liquid level in the medium-pressure inert gas scrubber (LC9303). The function of the medium-pressure inert gas scrubber is to absorb free ammonia from the exhaust gases in the medium-pressure system; thereafter, this ammonia is pressurized by an ammonia water pump and sent back to the medium-pressure absorption tower, thereby achieving recycling. The flow rate of the absorption liquid in the medium-pressure inerting tower is controlled by the FC9303 control valve. In the original design, FC9303 served as a flow control element; therefore, whether FC9303 is operated in automatic or manual mode, the water inflow to the medium-pressure inerting tower remains constant, and it does not adjust according to the level of liquid in the tower. The liquid level control of the medium-pressure scrubber is managed by the control valve LC9303 at the outlet of the ammonia water pump. Once LC9303 is set to automatic control, in order to maintain a stable liquid level in the medium-pressure scrubber, the opening degree of this control valve must change according to the level of the liquid. As a result, the flow rate of ammonia water entering the medium-pressure absorber (C101) also changes, which leads to fluctuations in the liquid level, temperature, and composition of the absorber. This reduces the stability of the system and increases various types of consumption. Now, the control valve LV9304, which regulates the amount of ammonia water flowing into the medium-pressure absorption tower, has been changed to remote control operation. As a result, the flow rate of ammonia water passing through LV9304 remains constant. The liquid level in the medium-pressure scrubbing tower serves as the controlled parameter, and together with the regulation of the water inflow rate to this tower (LV9303), it forms a control loop. The water inflow rate is controlled through automatic adjustment of the liquid level via LC9303, thereby achieving automatic control of the liquid level in the medium-pressure scrubbing tower. The automation reform of the liquid level control in the medium-pressure scrubber tower facilitates the control of the liquid level, temperature, and composition in the medium-pressure absorption tower, stabilizes production, and reduces consumption. The self-regulating liquid level control process for the modified medium-pressure inert gas scrubber is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload/060205839364489.jpg 4 Software configuration and hardware upgrades: The input modules, output modules, control modules, I/O connection modules, as well as the software for alarm and PID parameters of the aforementioned 3 control loops were reconfigured. Twelve new trend displays were added, enabling process operators to quickly view real-time trends; the flowchart interfaces for those 3 control loops were also modified accordingly. “The 8·13” urea system was contracted out in its entirety to the Italian company Snam, while the DCS control system is based on the RS-3 system provided by the American company Rosemount. Due to the limited spare parts provided with the system, it is difficult to ensure its proper operation. During the inspection of our hardware resources, we found that the distribution of nodes on the FIC cards and multi-channel detection cards is not optimal; the parameters are fairly scattered, with many empty nodes, and the utilization rate of these hardware resources is still very low. To this end, when carrying out technical upgrades to the hardware of the control system, our company re-planned 24 card modules and 13 detection points as well as control points, making significant adjustments and software configurations. After the modification, a total of 11 FIC cards and 1 multi-channel detection card were saved, enabling full utilization of hardware resources and achieving good results. 5 Conclusion Through the aforementioned modifications to our company’s “8·13” system, the process parameters were optimized, process operations were improved, production stability was enhanced, efficiency was increased, consumption was reduced, and energy was saved. This led to a reduction in the workload for operators, ensured the safe, stable, and long-term operation of the urea production system, and yielded good economic and social benefits.