Prevent surging of the main fan
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Methods to prevent surging in the main fan – a surge prevention valve? Increase the main air flow? Or reduce the regeneration pressure? Are there any other better methods? What is the simplest method? Everyone is welcome to discuss and learn from each other. Thank you
There are many measures to prevent reverse surging in the main fan, but the simplest method in a catalytic cracking unit is to vent at the main fan outlet. Centrifugal main fans operating at a constant speed should be equipped with a fixed-value anti-surge flow control system, and the set value of this flow regulator should be 7% to 10% higher than the flow rate at the surge point. For centrifugal main fans that operate at variable speeds and large axial-flow main fans, a feedback-based anti-surge flow control system is advisable. This system automatically adjusts the set value of the anti-surge flow regulator according to the different operating conditions of the main fan (compression ratio, outlet pressure, speed), along the anti-surge line. The outlet vent valve is opened in advance, before the main fan reaches the surge point, thereby preventing surge from occurring. To prevent surge, the oscillation line is generally set below the surge line, leaving a surge prevention margin of 5% to 10%. The anti-chaos lines for centrifugal main fans that operate at variable speeds and for axial flow main fans with adjustable static blades that operate at constant speeds are surfaces, making them relatively complex to handle. But under conditions where the rotational speed does not change much, it can still be simplified to a straight line or a broken line. For anti-surge flow control of large axial-flow main fans, a dual vent valve scheme is recommended. One vent valve is directly controlled by the anti-surge flow regulator, while the other vent valve is controlled by a self-protection signal.
The excerpt on the 2nd floor contains the perfect answer from the book
Methods to prevent surge in the main fan: The surge prevention line for centrifugal compressors is a straight line (representing flow rate), which is relatively simple. The surge prevention line for axial flow compressors is a broken line (with the vertical axis showing the percentage of the ratio of outlet to inlet pressure, and the horizontal axis showing the percentage of inlet flow rate). The most direct and fastest method is to increase the inlet flow rate, by quickly opening the anti-surge valve.
1. Increase inlet flow 2. Open the main fan outlet vent 3. Reduce regeneration pressure
The main fan outlet vent is usually not activated; it’s only used in case of an accident. So the simplest way is to make adjustments through the control panel, right?