What do the horizontal and vertical axes of the compressor anti-surge curve represent?

It’s already been clearly explained. The key issue is that you may not yet fully understand the principles behind surge; the occurrence of surge, with the most severe moment of gas backflow, happens over a certain process. It’s best to first understand the principle and phenomena of surge, and then looking at this curve will make it clear at a glance.

I have a general understanding of the principle of surge, but what I don’t understand is what the horizontal axis H represents. Some say it is the flow rate, while others say it is the pressure difference at the throat; I’m not sure exactly what it means. What is the meaning of its ratio to the inlet pressure Ps?

The traffic is correct. The relationship between flow rate and pressure. It’s easier to understand by referring to the diagram box on the right. Turbine compressors actually have two types of surge phenomena. They all relate to surge, which occurs when the pressure at the compressor outlet is high enough to overcome the force of centrifugal force, causing the airflow to flow back instantaneously. There is also another type of surge that results from an excessively low suction pressure (too small an opening angle of the guide vanes), leading to the sudden entry of suction airflow into the compressor; this as well is a periodic phenomenon and constitutes a form of surge.

a. Pd refers to the compressor outlet pressure, PS refers to the compressor inlet pressure, and h refers to the percentage of the compressor inlet flow rate. The surge condition of the unit is predicted based on the measured outlet pressure, inlet pressure, and inlet flow rate. b. When the operating point is to the right of the control line, there is a fixed difference between the setpoint of the surge controller and the current flow rate. When the operating point moves toward the surge region at a faster speed than the tracking point moves, the proportional output quickly opens the counter-acting valve, causing the setpoint to decrease at the same rate, until the counter-acting valve opens and a new operating point is established. c. Surge prevention: If surge occurs in the unit, the surge controller activates the anti-surge control valve to open rapidly until the surge is eliminated, after which it closes slowly; this valve features rapid opening and slow closing. d. Proportional output: The system features a pure proportional control function that forces the backflow valve to open, regardless of the action of the controller at that time ； As the operating point moves toward the surge region and reaches a certain set range, the anti-surge valve begins to open; when the operating point reaches the surge line, the anti-surge valve opens 100%. That is: when the operating point moves to the left, the backflow valve is opened in proportion to the movement. e. During normal operation, the output of the anti-surge valve controller and the proportional output are compared; the anti-surge valve accepts the signal from the one with the higher value. f. During normal operation, it is set in semi-manual mode; both the semi-manual controller and manual control can be used to adjust the anti-surge valve, although the manual adjustment cannot be set lower than the value output by the controller. With manual control, the operator adjusts the anti-surge valve manually based on the operating conditions. The automatic mode is generally not recommended due to issues with instrument reliability. g. Each time surge occurs, the anti-surge control line and the margin line shift to the right by 2% each time; the amount of each shift is accumulated with each occurrence. At the same time, record the surge count and the change in color of the surge indicator ； By pressing the reset button, the anti-surge control line and the margin line return to their state before surge occurred.