Double-machine parallel load distribution principle of air compressor

Turbine power (KW): 5772 (NORM) - -6445 (RATED) C204: Compressor manufacturer: SIEMENS low pressure cylinder model D12MH6B high pressure cylinder model D08MV6B turbine speed (RPM): 7436 (NORM) - 7808 ( MAX) Turbine power (KW): 8140 (NORM) - -9786 (RATED) The original design is intended to adopt the manual parallel mode, that is, the C204 unit of the cracking gas compressor is started first, so that the outlet pressure and flow rate reach the rated value. The cracking gas compressor C201 unit is restarted, so that the outlet pressure and flow rate are close to the rated value. According to the operator's experience, the outlet pressure and flow rate of the C204 unit are appropriately reduced and manually incorporated into the C201 unit. The main problems of this parallel mode are: First, relying too much on the operator's experience, a slight mistake, easy to cause two compressors to surge, stop, severely damage the machine; Second, the expansion can reach 200,000 tons / year After that, the C204 unit of the cracking gas compressor is operated separately to reach the rated working condition, and the load of the unit is only 70%. When the C201 unit is combined with the cracking gas compressor, the flow rate of C204 is reduced, which easily causes the ethylene compressor C401 and the propylene compressor C402 to breathe. Vibration stop. Therefore, with the approval of China National Petroleum Corporation, it was decided to adopt a two-machine parallel control mode.

After investigation, at present, the capacity of the two compressors in the international cracking unit using the two-machine parallel control system is the same. There is no precedent for the parallel control mode of the unit with the same power and flow rate, the high pressure steam inlet of the two turbines, and the intermediate pumping unit sharing the same line.

2 Parallel optimization control system Introduction The existing C204 control system of the two cracking compressor units adopts the PLC-5 system of AB Company; the C201 unit adopts the RX7I system of GE Company. The transformation of the two-machine parallel control system needs to be carried out by adding the series5DuplexVanguard system of Compressor Controls Corporation (CCC) based on the existing two control systems.

2.1 Dual-machine parallel load distribution principle In this two-machine parallel system, it will include: * One main controller MPIC-1; * Anti-surge controller for each compressor section (FIC-201/FIC-215; FRCA- 01/FRCA-02); * One load distribution controller (LSIC-01/LSIC-02) equipped with each compressor is implemented.

System basic control algorithm principle as shown in the figure 2.2 Specific control scheme Description Parallel load distribution system has a main controller MPIC-1, used to control the pressure of the parallel compressor inlet manifold PT-1SX. Each compressor has four sections, respectively Use two anti-surge controllers. The anti-surge controller FIC-201 is used to control the I, II, and III segments of the C-201; the anti-surge controller FIC-215 is used to control the IV segment of the C-201. The anti-surge controller FRCA-01 is used to control the I, II, III segments of C204; the anti-surge controller FRCA-02 is used to control the IV segment of C204. A C-204 anti-surge control system is retained in the ABPLC system; the anti-surge controllers in the ABPLC and CCC control systems are switched by switches.

A single load distribution controller is used for each cracking gas compressor. The load distribution controller LSIC-01 is used to control C-201, and the load distribution controller LSIC-02 is used to control C-204. Each cracking gas compressor has a speed and an extraction controller. The speed and extraction controller of the C-201 is implemented in the GE control system. The C204's speed and extraction controllers are each available in the WOODWARD and CCC control systems, with bumpless switching via toggle switches. The start and stop of the C-204 is controlled by the WOODWARD governor. The C-204 is controlled by the CCC speed and the extraction controller within the normal speed range. Below are the settings, control objectives, and control functions of the CCC controller.

(1) Feedforward load control inlet manifold pressure The main controller MPIC-1 adjusts the pressure of the compressor inlet manifold by increasing/decreasing the turbine speed. The input variable of MPIC-1 is compressor inlet manifold pressure PT-1SX (0-210kPag). MPIC-1 has only local operation mode, and its local setting range is 0 to 200kPag. MPIC-1 directly adjusts to LSIC-01 and The output of the LSIC-02, the speed setpoints for C-201 and C-204, mediates the pressure at the compressor inlet manifold. The relationship between MPIC-1 and LSIC-01 and LSIC-02 is feedforward control.

(2) When both compressors are running, the small deviation Dev of the anti-surge circuits of the two compressors is controlled to the same level by the LSIC-01 and LSIC-02 (compressor operating point and asthma) The distance between the vibration control lines) to optimize the operation of the compressor. When the MPIC-1 receives the Dev variable of each compressor (select the minimum Dev variable in the two anti-surge controllers of each compressor), calculate the average of the Dev variables of the two compressor deviation values, and then The load distribution controller (LSIC-01 and LSIC-02) is sent to each compressor as the set point of its load balancing circuit, and the latter two load balancing controllers increase or decrease the speed according to the corresponding SP and PV variables. Set value.

(3) Limit control At the same time, in order to ensure the safety and stability of the device and equipment, some limit control functions are adopted in the anti-surge controller to better ensure that the safety valve does not jump when the machine is in parallel, and the compressor inlet and outlet pressure is within the normal range. . The CCC anti-surge controller limits the control settings.

3 Conclusion After the completion of the dual-machine and control system, the cracker was started on December 23, 2007. The 21C204 compressor unit was started first and the outlet pressure (P=35kg/cm 2 ) reached the normal working condition. Into the C201 compressor unit, without any disturbances, greatly shortening the incorporation time compared to manual. After the two units were operated in parallel, they stopped three times due to fluctuations in the equipment. The parallel control system played a good control function during the on-off period. At the same time, under the process disturbance, the advanced control such as decoupling control, feedforward control and pressure override control of the optimized control system also increases the stability of the process, reduces unplanned parking, and improves product output.

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