Frekvenčné meniče

Application of SLANVERT Inverter in Refinery

I  General
Frequency-conversion speed regulation is a kind of effective speed regulating manner, with high accuracy, wide range of speed regulation, and easy operation. Further, the self-contained RS485 serial communication interfaces on inverters can realize communications with control system so as to easily monitor work situations of the system and benefit operation of normal production. In 2002, our company carried out automation transformation for the extraction system of liquefied gas by propane compressors in the refinery. After this transformation, not only the process operating level is enhance greatly, but energy-saving is increased by about 20%, and economic benefits are very significant.

II  Main Features of SLANVERT SB61 Inverter
The propane is the essential equipment of this system and has certain requirements for reliability, starting torque, and dynamic response of inverters. You can select the SB61 All-round Inverter produced by our company. It uses advanced vector control algorithm, have complete functions, and can be freely applied in various industries. It is available to select V/f open-loop, V/f close-loop, vector control without PG, and vector control with PG; available to select operating modes of key board, external terminals, and functional terminals; and there are RS485 serial communication interfaces that can facilitate computer networked control. In design, it is able to carry out reliability design, electromagnetic compatibility design, and stability analysis etc.

III  Process Flow
The process system for extracting liquefied gas is shown as below block diagram.

Figure 1 Block Diagram for LPG Extraction Process System

In the diagram:
P0—inlet pressure, i.e. external air supply pressure
P1—air inlet pressure, i.e. the pressure of air inlet valve after regulation, required to be less than 0.6Mpa
P2—outlet pressure of compressors, required to be: 1.2Mpa-1.7Mpa
P3—pressure of separation tower, required to be 1.08Mpa
L1—inlet flow, i.e. suction capacity of compressor
L2—outlet flow, i.e. exhaust capacity of compressor
Aperture of valve: production requires the aperture of air inlet valve to be 30%-50%.
Aperture of outlet valve: 80%-100%.

Process flow:
The gas generated from oil refining is condensed as liquid after being pressurized by compressor and enters the separation tower. After through other processes such as absorption, LPG is extracted.

It is required the inlet pressure P1 shall be less than 0.6MPa and the compressor outlet pressure P2 may reach the range of 1.2MPa-1.7MPa, and the separation tower pressure P3 ensured after through outlet valve regulating shall be 1.05MPa. If the separation tower pressure increases for some reasons, for safety, open the pressure relief valve to let the liquid in the separation tower flows and returns to the inlet through pipes.
The aperture of regulating valves shall make the P1 indicated pressure be less than 0.6MPa and the P3 pressure be about 1.08MPa. Operators carry out regulation according to the indicated values of P1, P2, and P3. As the inlet pressure P0 is generally not stable, in order to control it accurately, relationships between P0 and P1, P2, P3 shall be known, listed in Table (1).

Table (1)

When the P0 is unchanged, inlet valve is regulated, and the change relationships between its aperture and P1, P2, P3 are listed in Table (2).

Table (2)

IV System Composition
From Table (1) and Table (2) for changes of P1, P2, P3 and L1, L2 varying with that of P0 and inlet valve aperture, we can see: increasing or decreasing of inlet P0 will result in corresponding changes of P1, L1, and L2. To remain L1 and L2 unchanged, the inlet valve aperture can be regulated to change inversely with that of P0, namely P0 increases while the inlet valve aperture decreases, or P0 decreases while the inlet valve aperture increases.

In order to ensure stability of production, the propane flow into separation tower shall have not a very big fluctuation. There are two control methods. One is flow control method that uses flow transmitter to collect flow signals and feed them back to control system so that the system forms a flow close-loop system. In this way, no matter what changes the P0 has, the flow can be ensured unchanged basically. Another way is to use P1 signals as feedback signals to keep flow unchanged only by keeping P1 with stable flow. Therefore, when the inlet pressure P0 fluctuates, the compressor speed is changed to keep P1 unchanged so as to achieve unchanged flow. So the pressure signal P1 is used as feedback signals to form a pressure close-loop system. Here, P1 signals lie in input pipes of the compressor, but changes of P1 are results of speed regulation for the compressor and are feasible to be used as feedback signals for the close loop system. This indicates the regulating for the compressor speed and for inlet valve aperture can reach the same effects in process. The compressor motor power of this system is 160kW. When the power frequency operates, the flow changes are regulated by valve throttling, so a big amount of energy is wasted and mechanical wear cannot be reduced, which have impact on service life of the machine unit. In order to realize automation of production process, one SB61 series 160kW and one pressure transmitter P1 are used to form constant pressure control. The system is shown as Figure 2.                                                                                        

Figure 2 Inverter Control System

In order to improve automation level of the whole system, the IPC (Industrial Personal Computer) is introduced for monitoring. The block diagram of the monitoring system is shown as Figure 3.                                                       

Figure 3 Block Diagram for System Control

In this system, the data collecting card collects pressure, flow, valve aperture and other analog quantities and outputs analog quantities for regulating the aperture of motor valves; it also collects alarming and other switch signals and outputs start/stop signals for standby systems. The IPC carries out control for inverters through RS485. The P1 pressure is collected by data collecting card and processed by the IPC before the RS485 sends control signals to inverters to control frequency increasing/decreasing for inverters and to get constant P1 so as to achieve basically unchanged flow. Besides, the start/stop of inverters shall be controlled, and operating parameters of motors shall be sent to IPC for monitoring as the same time. The IPC is one EVOC IPC of CPU PⅢ, 128M memory, and 20G hard disk and equipped with data collecting card, temperature signal processing card, switch signal wiring board, RS485 communication card, RS485 communication modules and so on.

The analog data collecting card coverts analog signals such as pressure, flow, valve aperture, motor temperature, and pipe temperature to 12-digit digital signals, and one way of signals occupy one analog channel on the card, so they do not influence each other, with collecting cycle of 300ms. The SB61 inverter supports RS485 half-duplex serial communication mode, which is to use IP function or VB’s MSCDMM communication controls to prepare serial communication programs on the IPC so as to realize communication between the IPC and inverters. This system is prepared with the communication controls of VB. The collected data signals are subject real-time analysis and treatment before they are combined to monitor images and to monitor real-time data and history data transmitted on site, and equipment operating conditions, analysis of history curves, real-time reports, printing of history reports, and audible and visual alarms etc.

V System Advantages
For original system, the P1 pressure is investigated manually to change the aperture of inlet valve to meet process requirements. When the P0 fluctuates frequently, the intensity of manual work is very big. After putting into operation of the control system, manual intervention is not required generally, and monitoring can be carried out remotely. After the compressor is dived with inverters, energy-saving effects are obvious, with about 18%-22% energy saving based on calculations by users. The start and operating currents of the motor are under the rated current, which reduces the impact on the power grid. The motor speed is not always working under the power frequency, this prolongs the service life of equipment and system safety is also improved.