From a process point of view, the compressor is now actually operating at point 3 of the unstable zone because the process only processes the flow rate V 2 , but in fact the compressor runs on point 2 of the control line and passes The bypass valve bypass flow balance (V 2 - V3) is still in a stable zone. In order to ensure that the compressor does not enter an unstable area, a certain safety margin must be maintained for the surge line. The surge line depends on different factors (such as gas type, temperature, pressure, molecular weight, etc.).
The bypass line is parallel to the surge line and the safety margin is approximately 5% to 8% of the design value. Compressor flow from 0% to 100% is controlled by the bypass valve and speed. The expected drop in reduction by compressor speed is at the design pressure ratio (PR = 3.47 - 7. 3M pa( a) / 2. 1M pa( a) - approximately 33%.
2 Controller Description Mixed gas operation Executes the following controller: PIC 013037 lift controller PIC 013022A pressure controller, acting on bypass valve FV 013041 FIC 013036 Surge controller Surge controller actual value PV is calculated based on the following measured values Dimensionless variables: dpflow orifice pressure difference mbar ( PDT 013012) dpstage interstage pressure difference mbar ( PDT 013014) f (speed) speed correction factor, depending on the drive speed F scale factor (100 or 1000) calculation based on the following formula :PV = dp flow dp stage xFxf (speed) The safety margin between surge line and recirculation is included in f (speed).
When the compressor is not running, the process value PV is set to "zero".
The set point SP of the surge controller FIC 013036 is fixed at 100 and cannot be changed.
The surge controller compares SP and PV with the difference of error ER = SP - PV. The controller always tries to achieve a "0" error.
A negative error (ie SP < PV) causes the controller output signal to decrease (bypass valve FV 013041 is off).
A positive error (ie SP> PV) causes the controller output signal to increase (bypass valve FV 013041 is open).
The output signal of the surge controller is passed to the high selector 11.
Surge controllers are only effective during extreme process conditions.
In order to ensure a fast interaction, the output signal of the high selector 11 is used as a feedback of the low clamp YL of the surge controller by the subtraction function. This causes the output of the surge controller to be at least 5% lower than the output of the high selector. The pressure controller PIC 013022, manual signal HIC 013022 and ramp function page act on the high selector 11.
The output signal of the high selector 11 is passed to the bypass valve FV 013041 via the inverse function 10 and the calculation A 2 = E1 + ( E2 x ( 100- E1) ) / 100. This calculation is necessary because the bypass valve is stopped/driving The maximum open position (100% open) and maximum opening (ie 80% open during operation) are required during this period. This maximum open position means that the bypass valve does not immediately respond to changes in the output signal of the high selector 11. The calculated output is passed to the bypass valve FV 013041.
On the maximum output signal of the high selector 11 (output 100% corresponds to 100% open), the bypass valve FV 013041 must be open. In order to ensure that this is in line with the operation of the bypass valve, an inverse function is required. The safe position of the bypass valve FV 013041 is open. All the above control is automatically realized by PLC, which greatly improves the control level and ensures the safe and stable operation of the unit to the utmost extent.
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