Why is it necessary to open the counter-rotation valve first when starting the air compressor?

First, let’s clarify a concept: flying motion refers to surge, while reverse flying motion means anti-surge. The anti-rotation valve is opened first to ensure a smooth start-up and to prevent the unit from rotating uncontrollably.

Backflow occurs due to low inlet flow and high outlet pressure; at the beginning of operation, there is very little gas phase, and the inlet flow to the compressor is low. Therefore, backflow is induced to increase the inlet flow and stabilize the compressor’s operation

“The flying” and “surge” of a compressor are the same concept expressed in different terms. When starting the machine, it is necessary to open the anti-surge valve first; this is done to prevent surging in the compressor due to a sudden drop in pressure during startup, as that could cause damage to the unit.

At startup, the inlet valve of a centrifugal compressor is fully open, while the outlet valve is fully closed. During the startup of the unit, the processing capacity increases gradually; therefore, the volume of gas is relatively low at the beginning. To ensure a certain flow rate at the compressor inlet, the check valve also needs to be opened. As the gas volume increases, gradually close the backflow valve. Generally, the pressure at the outlet is adjusted using a flare on the side of the air compressor outlet, thereby ensuring smooth startup and preventing uncontrolled fluctuations in pressure

When the compressor starts, the inlet valve is fully open, the outlet valve is fully closed, and the outlet is in flare regulation mode. Since the processing capacity gradually increases during the commissioning of the device, the amount of gas is relatively low at first ； To ensure a certain flow rate at the compressor inlet, it is necessary to adjust the outlet flare to regulate the compressor outlet pressure, and to activate the anti-rotation valve in order to increase the flow rate at the compressor inlet promptly, thus preventing the compressor from entering a rotating state. This ensures a smooth startup and prevents shaking.

The anti-rollback valve is nothing but another name for the surge valve of a pneumatic compressor; it is activated first at startup in order to maintain an adequate flow rate in the centrifugal compressor, preventing too low a flow rate that could cause surging and damage to the compressor.

The reverse flow valve is used to ensure a certain flow rate at the inlet during startup, enabling the system to start under load and reducing fluctuations.

When the compressor is in operation, there is a minimum flow rate at a certain speed; if the flow rate is below this level, flight can occur. At the beginning of operation, the amount of gas is still small, so the flow rate at the compressor inlet is low. It is necessary to increase this flow rate in order to prevent flight and ensure normal operation of the compressor. Flight and surge are different names for the same phenomenon; they both represent special conditions that occur in compressors when there is interaction between the airflow and the compressor’s structure as well as its piping system. Generally, this phenomenon is referred to as surge in the case of main fans, while it is called flight in the case of compressors. Last edited by FloatHead on 2009-3-17 15:57]

Stall is referred to as surging in the main fan, while flying is called unsteady operation in the compressor; these are actually two different names for the same phenomenon, and both represent operating conditions unique to centrifugal compressors.

At present, the vast majority of gas compressors used in catalytic cracking units are centrifugal compressors. According to the performance curve of a centrifugal compressor, there is a flying point at a certain speed – that is, the point of minimum flow rate at a given pressure. Equal to or below this minimum flow rate triggers flutter, entering the flutter region. If the flow rate remains constant and the rotational speed increases, once the pressure reaches a certain value, the compressor will reach its critical point and start to oscillate. Therefore, in actual operation, whenever the operating conditions meet the above criteria, runaway rotation will occur. For example: (1) a sudden increase in the outlet pressure of the gas compressor (a sudden rise in the pressure of the absorption/desorption tower > At this point, since the compressor speed remains constant, the flow rate must decrease, and once the runaway condition is reached, the compressor will start to rotate uncontrollably. As the rotational speed increases, the flow rate rises until it reaches the critical point, at which point flight occurs. (2) The inlet flow rate of the gas compressor drops suddenly; the rotational speed remains unchanged, while the outlet pressure rises to the flutter point, resulting in flutter. (3) Changes in the properties of the gas components can also cause the unit to run uncontrollably.

The second floor’s points are concise and I completely agree.

Flyaway and surge are terms used respectively for compressors and main fans; flyaway refers to the vibrations that can cause damage to the machine, resulting from a low inlet flow rate and a high outlet flow rate. Surge refers to the vibrations that can cause damage to the machine, resulting from a large inlet flow rate and a small outlet flow rate. The feeding conveyor is activated before starting up the machine in order to prevent damage to it caused by the situations mentioned above.