Air compression in 20000nm3h air separation unit o

Automatic control of air compressor in Tonggang 20000nm3/h air separation unit introduction:

Tonggang 20000nm3/h oxygen production project began to be implemented in early 2005. The process flow adopts external compression, and its air turbine compressor is supplied by manturo (model: rikt90 – 1+1+1, motor driven, four-stage compression). The process and technical requirements are provided by manturo, and China air separation equipment company completes the configuration design and on-site start-up commissioning

1: control principle

the basic control requirement of the turbine compressor is to make full use of the allowable working area of the compressor on the premise of ensuring safe and stable operation, so that the unit can work within the range of pressure and dosage required by the tensile strength and elongation process of the main test data (such as the adjustment of the pressure and dosage during variable load and the adjustment of the pressure and dosage during the purging of the air separation unit). The working condition is stable and reliable, the operation is convenient, and the degree of automation is high. The working state of the compressor shall be open to the operators as much as possible to facilitate fault analysis and operation, and there is a long historical trend to check

2: control scheme

the compressor control adopts the following scheme: the compressor is only equipped with necessary field instruments, vibration, displacement probes and temperature sensors, and there is no local instrument panel. The central control room DCS (Honeywell TPS-3000 system) controls the whole unit, so as to give full play to the functions and advantages of DCS system

3: main control contents of the compressor

3.1 basic control

according to the process requirements, the compressor control adopts constant pressure (design value of outlet pressure 0.52mpa gauge pressure), which is the most basic process control of the unit. Other controls, such as anti surge control, motor overload protection control, automatic loading start and accident interlocking protection stop control, are set for the safe operation of the unit. The air circuit diagram of the air compressor is shown in Figure 1:

3.2 anti surge protection control

during the operation of the compressor, if the compressor is operated improperly or the equipment pipeline fails, the compressor will surge, which may seriously damage the unit. Therefore, anti surge control is an important safety control. The purpose is to keep the working point of the compressor within the limited range without entering the surge area, so as to ensure the safe operation of the unit at a more advantageous price. Its protection range is the whole working range of the unit. Generally speaking, the object of compressor anti surge control is the vent valve

the anti surge line of the compressor was measured during the on-site commissioning. Considering the dynamic characteristics of the system, surge occurs very quickly. The control system must be able to identify whether the surge limit line has been crossed, and there are high requirements for the scanning cycle of the control system. Therefore, a consensus was reached with the commissioning personnel of the manufacturer on site, and a control line was set on the right side of the surge line, whose shape is consistent with the surge line, However, the flow range value 10% away from the control line (the smaller the distance is, the less chance the vent valve will open, and it should be tightened in time; the less the energy loss is, but the higher the response time of the control system and valves is, the greater the distance is, the greater the chance the vent valve will open, the better the safety of the unit can be guaranteed, but the greater the energy loss), and at the same time, in order to better protect the unit, Add a safety line between the control line and the surge line (the distance from the surge line is 5% of the flow range value)

anti surge valve is the only actuator of anti surge control, and it is also an important actuator of interlock protection. Therefore, the response speed of the anti surge valve is required to be fast. When the solenoid valve loses power, the time from full close to full open of the valve is required to be less than 1s ~ 2S. A pneumatic amplifier is designed in the air circuit of the anti surge valve to ensure the rapid response of the valve

3.3 motor overload protection (refer to figure 3)

in order to prevent motor overload, motor overload protection is set. When the motor current exceeds the rated value, the motor overload protection regulator (ic2311) begins to stack with the original outlet pressure regulator (pic1051) at the same time to adjust the opening of the compressor population guide vane to protect the motor from overload

3.4 high outlet pressure protection control (refer to figure 3)

for the compressor controlled by constant pressure, the adjustment of the opening of the inlet guide vane can not stabilize the outlet pressure at the design value and continue to rise. When it reaches a certain value, the high outlet pressure protection regulator (pc1051) starts to work, and together with the anti surge regulator (pdc1055), adjust the opening of the vent valve to protect the outlet pressure from rising

3.5 start interlock logic (refer to Appendix 1)

after the relevant process or mechanical conditions are all met, the compressor start logic conditions are met, and the starting signal is sent to the field motor excitation cabinet. This logic includes the oil pressure interlock logic, that is, the automatic start logic of the auxiliary oil pump and the release of the shaft vibration interlock during the starting process. During the starting process, the vibration of the unit applies the alarm triple multiplication function of the vibration meter, so that the vibration and displacement of the unit can not only be within the safety monitoring range, but also smoothly cross the critical area to complete the starting process

3.6 shutdown interlocking or unloading logic (refer to Appendix 1)

when any important process parameter (such as vibration or displacement, bearing bush temperature, etc.) of the unit exceeds the limit or the unit fails, it will stop urgently to ensure the safety of the unit. It must be confirmed manually before starting after accident shutdown. The result of Shutdown Logic is that the motor stops, the vent valve is quickly and fully opened, and the compressor population guide vane is fully closed up to now. When the precooling system or molecular sieve purification system of the air separation unit fails, the interlocking action is unloading, that is, the vent valve is fully opened, the compressor population guide vane is in the minimum position (set at 10%), and the motor does not stop

the unloading, pressure relief and shutdown operations of the normal shutdown of the unit are realized by the operator on the DCS air compressor flow chart in the central control room

4: loading process (refer to figure 3) and unloading procedure (refer to figure 4)

in order to avoid misoperation and reduce operation intensity, the unit manufacturer requires to use the way of automatic starting the unit. Manual operation is not allowed during automatic startup. Only after the startup is completed or the unit has been put into normal operation, the operator can unload and relieve pressure, that is, the population guide vane can only be turned down on the basis of the original opening (set the inlet guide vane safety manual operator hc1051), and the vent valve can only be turned up on the basis of the original opening (set the anti surge valve safety manual operator HC)