1 Air compressor control system
Normally, the air compressor operates under the constant pressure and flow rate. When the pressure or flow rate is too high, through adjusting the PV valve and adjusting the discharge flow of the disperse tube FP valve, to maintain certain pressure and flow and avoid surge. When the product demand changes, the flow, pressure, temperature and so on of the air compressor need to be controlled. Generally, under the condition of ensuring the constant liquid production, the flow control range of the air compressor under changing working condition is 75% ~ Between 108%. At present, the relatively practical automatic anti-surge control is designed from the starting point of automatically opening the vent valve when centrifugal compressor flow is too small and pressure is too high. UY introduces the air compressor outlet pressure and air compressor outlet flow, and combines the surge curve provided by the air compressor manufacturer to control the vent regulating valve to avoid surge. For the sake of safety, FO valve should be selected as the release valve FP. When surge occurs, ensure rapid evacuation. In the case of considering the convenience of on-site maintenance, the release valve FP should be as close as possible to the main pipe, and the distance between the release pipe and the silencer should be shorter. At the same time, the discharge pipe should also be considered to be thicker to make the road convenient.
2 anti-surge control of booster
Surge phenomenon is caused by the structural characteristics of centrifugal compressor. When the air flow through the booster is reduced to a minimum flow value, the performance of the compressor becomes unstable. During surge, the sharp fluctuation of pressure and flow will increase the vibration of the whole booster, which should be avoided as much as possible. Load reduction is the main reason for the surge of the compressor, so a anti-surge control system is needed to ensure that at any rotation speed, the limit of the compressor flow Ql will not be less than the limit flow Qp corresponding to the surge limit line, and it is hoped that the normal operating point of the booster should be as far away as possible from the operating point of surge, so as to ensure a relatively large range of compressor flow regulation. Therefore, the circulating flow method can prevent entering the surge area. Let the flow through the compressor always be greater than a certain value of flow. When the suction amount Q1 of the booster is greater than Qp, the bypass valve will close; when Q1 is smaller than Qp, the bypass valve will open, increase the air flow through the booster to greater than Qp, as a result, the air supply to the pipe network system will be reduced to meet the need of load reduction.
Qp is a certain value, and the correct selection of Qp is the key. Qp usually selects the surge limit flow at the maximum rotating speed as the given value of Fc. But there is a problem, when the booster is running at a low speed, although the booster does not enter the surge area, the inlet air volume may be less than the set value (the set value is set according to the maximum speed flow ), the result is that the bypass valve is opened, and part of the gas flows back, causing energy waste. Therefore, when the load changes frequently, this method is not suitable. In addition, sometimes the throttling device can not be installed on the inlet pipe of the booster, because the flow measurement on the inlet pipe will cause pressure drop, therefore, the throttling device should be installed on the outlet pipe of the booster, the corresponding traffic Ql in this project is slightly greater than the surge limit traffic Qp by 5% ~ 10%. The hydrofining ratio adjustment control system reacts with hydrogen from the crude argon tower in the oxidation furnace to remove the oxygen in the crude argon. The flow of crude argon at the entrance of the oxidation furnace multiplies with the detected oxygen content, so as to calculate the actual volume flow of oxygen in the crude argon. Meanwhile, considering the excess hydrogen (about 1%) after the reaction in the oxidation furnace ), the actual volume flow of hydrogen to be added can be calculated.
After reaction, make the oxygen content in process argon less than 3ppm and the excess hydrogen content less than l %. If the oxygen content of the crude argon in the crude argon Tower is more than 4.5%, or the inlet temperature is relatively low, then stop the crude argon compressor. Because the oxygen content of crude argon is too high, it indicates that there is a problem in the distillation process, which will affect the output of oxygen. Meanwhile, it will be difficult to reduce the oxygen content through the oxidation furnace.
In this control system, hydrogen and crude argon sent into the oxidation furnace respectively have their own flow regulation circuits. The hydrogen flow loop is set from the hydrogen-oxygen ratio calculator. At the same time, when the main load of crude argon is lifted and reduced, the hydrogen input is also tracked and lifted, and the ratio of the two is unchanged. The measurement accuracy of hydrogen flow is relatively important here, and this project adopts the measurement of small pore plate.
3. Temperature control system of recycled nitrogen
The dirt nitrogen and steam in the system are sent into the steam heater. After the sewage nitrogen is heated, it is sent to the molecular sieve purification system to regenerate the molecular sieve absorbents. In the gas engineering of Guangdong and Hong Kong, the temperature of the recycled sewage nitrogen is stabilized by adjusting the vapor flow. But it can be seen that this control system has defects. If the sewage nitrogen flow into the heater changes, it will affect the temperature of the sewage nitrogen at the outlet of the heater; meanwhile, because the temperature detection quantity changes slowly, there is a lag problem in using it to control the volume of steam. Therefore, if feedforward control is adopted to introduce the pilot interference of the change of sewage and nitrogen flow into the control circuit, the response speed can be improved and the temperature can be stabilized. However, in order to ensure the precision of feedforward compensation, the control valve is strictly required. The feedforward is formed by overlaying the output of feedforward controller and the output of temperature controller as the given value of steam flow controller. cascade control will make the effect better.
4-Concluding Remarks
Nowadays, the air separation technology is increasingly developing towards the direction of specialization, scale and standardization. Under the premise of ensuring the maximum profit, trying to reduce energy consumption is the main work faced by this technology. Being good at discovering the problems existing in the system and improving the flow control scheme in practical work will surely further improve the level of automation control technology of air separation.
