46. What are the reasons for runaway during the operation of a compressor? How to handle it?

Reason: (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 time, since the compressor speed remains constant, the flow rate must decrease; once it reaches the runaway point, runaway occurs. As the rotational speed increases, the flow rate rises until it reaches the critical point, resulting in flutter. (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. The backflow control for catalytic cracking rich gas compressors generally employs single-parameter regulation, that is, the minimum flow rate specified for the compressor is used as the protective minimum flow rate. When the inlet flow rate of the rich gas compressor falls below this value, the backflow valve is opened to increase the backflow flow rate, ensuring that the inlet flow rate remains above the minimum flow rate at which backflow occurs. Handling: 1. The pressure at the outlet of the compressor, that is, the pressure in the reabsorption tower of the absorption and stabilization system, must be kept stable; any sudden increase in this pressure should be avoided. 2. The inlet flow rate to the compressor must be kept constant; pay attention to adjusting the backflow momentum. If a decrease in the amount of rich gas flowing in or an increase in the pressure at the compressor outlet is detected, it is necessary to contact the relevant personnel to determine the cause and take appropriate measures. The most common causes are loss of control over the pressure in the reabsorption tower and failures in the backflow flow control; followed by a decrease in the turbine speed or malfunctions of the governor. 3. Changes in the composition of the rich gas, particularly a decrease in its molecular weight; for example, when the temperature in the oil-gas separator is too low or when there is an increase in hydrogen content in the rich gas, resulting in a molecular weight significantly lower than the design parameters of the compressor. Under such circumstances, with a fixed number of compressor stages, the discharge pressure at the outlet becomes limited and fails to reach the rated value. Meanwhile, while the pressure in the reabsorber remains normal, the rich gas cannot be discharged. In this situation, the compressor may also exhibit “surge phenomena”. The measures to take should be to increase the temperature of the oil-gas separator, thereby increasing the molecular weight of the rich gas, or to reduce the pressure in the reabsorption tower so that the compressor can operate at its normal displacement. In emergency situations, temporary measures such as venting from the compressor outlet can also be taken to ensure proper control of the pressure in the counterabsorption system, after which the operating conditions of the absorption and desorption towers, as well as those of the compressor, can be adjusted further. Another emergency measure is to install a flare at the compressor inlet, isolate the compressor from the system and shut it down, and then restart it after making the necessary adjustments. It is generally better to adopt the former.

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 surge 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 flight into the flight zone. If the flow rate remains constant and the speed increases, once the pressure reaches a certain value, the flying point is reached and the compressor will also experience flying. Therefore, in actual operation, whenever the operating conditions meet the above criteria, runaway rotation will occur. For example: (1) a sudden increase in the pressure at the outlet of the gas compressor (a sudden rise in the pressure in 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, resulting in flutter. (2) The inlet flow rate of the gas compressor drops suddenly; the rotational speed remains unchanged, while the outlet pressure rises to the critical value, resulting in runaway. (3) Changes in the properties of gas components can also cause the unit to run away.

Reasons for flying: 1. High absorption and desorption pressure, 2. Low reaction depth, low gas-phase load