Production Technology [Weekly Topic]: Q&A on Production Practices (10-12-13~12-19)

When a centrifugal compressor experiences surge, the rotor and stator components are subjected to alternating dynamic stresses. Imbalances in the pressure between stages cause intense vibrations, which can damage the seals and bearings; in severe cases, it may even lead to collisions between the rotor and stator components, leakage of the compressed gas, and explosions. Therefore, centrifugal compressors must never operate in the surge region. The fundamental cause of surge is low flow rate. There are many factors that lead to low flow rates during operation, which can be summarized as follows: (1) The pressure at the compressor outlet increases, and when the system pressure exceeds this outlet pressure, the gas flow rate drops to the surge flow rate. The high pressure in the system causes pressure buildup at the compressor outlet, leading to gas flowing back into the compressor and resulting in a low flow rate of gas inside the compressor. (2) The inlet flow is below the specified value, and the backflow control valve has failed. At a certain rotational speed and gas density, a specific pressure can be maintained. Low gas flow rates during startup or shutdown, or an excessively wide opening of the flare valve, are the most common causes of low inlet flow to the compressor. (3) Changes in gas density: at a certain rotational speed, the centrifugal force decreases, resulting in a drop in outlet pressure and flow rate; this is often mistakenly considered to be a pumping action. (4) Unstable operation of the distillation system causes oil to be present in the gas entering the compressor (for example, due to malfunction of the gas tank level or interface sensors), allowing liquid components to enter the compressor. (5) The steam pressure of the turbine is low or of poor quality (low temperature), resulting in the unit operating at full load while its speed decreases. (6) The speed control system fails, auxiliary systems malfunction, vacuum efficiency decreases, and the unit is unable to perform at its rated capacity.

1. Formation of surge: The engine is the heart of an aircraft, and its proper operation ensures the safety of the plane. Engine surge is the most hazardous of all engine failures. This section provides a brief analysis of the formation of surge, the conditions under which it occurs, preventive measures, as well as matters to be considered during operation and maintenance. 2. Formation of surge: Compressor surge is a phenomenon of low-frequency, high-amplitude oscillations that occur in the airflow along the axis of the compressor. This type of airflow oscillation, characterized by low frequency but high amplitude, represents a significant source of excitation forces. It can cause severe mechanical vibrations in the engine components as well as overheating at the hot ends, leading to serious damage to these components in a very short time. Therefore, it is not allowed for the compressor to operate in the surge zone under any circumstances. 3. The phenomena that occur during surge are ； The sound of the engine changed from a high-pitched whine to a low rumble ； The vibration of the engine has increased ； Large fluctuations in total pressure and flow rate at the compressor exit ； The rotation speed is unstable, the thrust drops suddenly, and there are significant fluctuations ； The exhaust temperature of the engine rises, resulting in overheating ； In severe cases, detonation occurs, the airflow is interrupted, resulting in engine shutdown. Therefore, once the above phenomenon occurs, immediate action must be taken to bring the compressor out of a surge condition. 4. The root cause of surge is an excessive attack angle, which causes the airflow to separate on the back side of the blades; this separation then spreads severely throughout the entire blade row passage. The physical process of surge is as follows: as the air flow rate decreases, the attack angle of the airflow increases. When the flow rate drops to a certain level, the attack angle of the airflow flowing onto the moving blades exceeds the designed value; as a result, flow separation occurs on the back side of the moving blades. The greater the decrease in flow rate, the larger the area of separation

Typical phenomena that occur when a centrifugal compressor experiences surge include: (1) the outlet pressure of the compressor first rises, then drops sharply, exhibiting periodic and significant fluctuations; (2) the flow rate of the compressor drops sharply and also fluctuates greatly, and in severe cases, air may even flow back into the suction pipeline; (3) the current and power readings of the electric motor driving the compressor become unstable and fluctuate significantly; (4) the machine experiences intense vibration, accompanied by abnormal airflow noise. Causes of surge: 1. Surge caused by changes in rotational speed; 2. Surge caused by a decrease in the molecular weight of the gas; 3. Surge caused by throttling in the compressor outlet pipeline; 4. Compressor surge resulting from inlet throttling (low inlet pressure)

　The periodic oscillations of the fluid in fluid machinery and its pipelines are mechanical vibrations that occur as a result of the fluid being periodically drawn in and discharged. For example, the surge phenomenon that can occur during the operation of a pump or compressor is as follows: when the flow rate drops to its minimum value, the outlet pressure suddenly falls, and the pressure inside the pipeline becomes higher than the outlet pressure; as a result, the medium being transported flows back into the machine until the outlet pressure rises again, allowing the medium to be sent back into the pipeline ； When the pressure in the pipeline returns to its original level, the flow rate decreases again, causing the fluid in the pipeline to flow backward, and this cycle repeats itself. The occurrence of surge is related to the characteristics of fluid machinery and pipelines; the larger the capacity of the pipeline system, the more severe the surge and the lower its frequency. Fluid machinery products are generally provided with pressure-flow characteristic curves, based on which the surge point, surge boundary line, or surge region can be determined. Surge in fluid machinery disrupts the regular flow of the medium inside the machine, generates mechanical noise, causes severe vibration of the working components, and accelerates the wear and damage of bearings and seals. Once surge causes resonance in the pipes, machinery, and their foundations, it can also lead to serious consequences. To prevent surge, the fluid machinery must be operated outside the surge region. In compressors, anti-surge control systems of the minimum flow type, flow-speed control type, or flow-pressure difference control type are commonly used. When multiple machines operate in series or parallel, they should each have their own anti-surge control device.

Phenomenon: It is easy to tell from the sound, and in addition, the volume and pressure fluctuate, with the electrical power also changing as a result. Cause of surge: Excessively high outlet pressure or too low flow rate

Reply to 1# shmily327: The symptoms that occur when a centrifugal compressor experiences surge are as follows: (1) The outlet pressure of the compressor first increases, then drops sharply, showing periodic and significant fluctuations; (2) The flow rate of the compressor drops sharply and fluctuates greatly, and in severe cases, air may even flow back into the suction pipeline; (3) The current and power readings of the electric motor driving the compressor become unstable and fluctuate significantly; (4) The machine experiences intense vibration, along with abnormal airflow noise. The fundamental cause of surge is low flow rate. There are many factors that lead to low flow rates during operation, which can be summarized as follows: (1) The pressure at the compressor outlet increases, and when the system pressure exceeds this outlet pressure, the gas flow rate drops to the surge flow rate. The high pressure in the system causes pressure buildup at the compressor outlet, leading to gas flowing back into the compressor and resulting in a low flow rate of gas inside the compressor. (2) The inlet flow is below the specified value, and the backflow control valve has failed. At a certain rotational speed and gas density, a specific pressure can be maintained. Low gas flow rates during startup or shutdown, or an excessively wide opening of the flare valve, are the most common causes of low inlet flow to the compressor. (3) Changes in gas density: at a certain rotational speed, the centrifugal force decreases, resulting in a drop in outlet pressure and flow rate; this is often mistakenly considered to be a pumping action. (4) Unstable operation of the distillation system causes oil to be present in the gas entering the compressor (for example, due to malfunction of the gas tank level or interface sensors), allowing liquid components to enter the compressor. (5) The steam pressure of the turbine is low or of poor quality (low temperature), resulting in the unit operating at full load while its speed decreases. (6) The speed control system fails, auxiliary systems malfunction, vacuum efficiency decreases, and the unit is unable to perform at its rated capacity.

What are the phenomena of surge in centrifugal compressors? (1) The pressure at the compressor outlet increases; the system pressure exceeds the outlet pressure, causing the gas flow rate to drop to the surge flow rate. The high pressure in the system causes pressure buildup at the compressor outlet, leading to gas flowing back into the compressor and resulting in a low flow rate of gas inside the compressor. 4 @- a( Q( S* l$ h. m (2) The inlet flow rate is below the specified value, and the backflow control valve is malfunctioning. At a certain rotational speed and gas density, a specific pressure can be maintained. Low gas flow rates during startup or shutdown, or an excessively wide opening of the flare valve, are the most common causes of low inlet flow to the compressor. (3) Changes in gas density: at a certain rotational speed, the centrifugal force decreases, resulting in a drop in outlet pressure and flow rate; this is often mistakenly considered to be a pumping action. & T6 R8 P1 and/or C5 h6 I: Unstable operation of the fractionation system causes oil to be present in the gas entering the compressor (for example, due to malfunctions in the liquid level or level sensing devices), allowing liquid components to enter the compressor mechanism. (5) The steam pressure of the turbine is low or of poor quality (low temperature), resulting in the unit operating at full load while its speed decreases. ( T9 i' O8 x/ y E G! b （6）The speed control system fails, auxiliary systems malfunction, vacuum efficiency decreases, and the unit is unable to operate at its rated capacity. What are the causes of surge? Low speed, high head, low flow rate, high head

Typical phenomena that occur when a centrifugal compressor experiences surge include: 1\ The outlet pressure of the compressor first increases, then drops sharply, exhibiting periodic and significant fluctuations; 2\ The flow rate of the compressor drops sharply and also fluctuates greatly. In severe cases, air may even flow back into the suction pipeline; 3\ The current and power readings of the electric motor driving the compressor become unstable and fluctuate significantly; 4\ The machine experiences intense vibration, along with abnormal airflow noise. The fundamental cause of surge is low speed and high pressure head； Low flow rate, high head pressure. The operation is simple: increase the inlet flow, reduce the outlet head pressure, and raise the rotational speed. The basic method to prevent and eliminate surge is to increase the inlet flow rate of the compressor to a level above the surge flow rate for that operating condition. To prevent compressor surge, the following should be observed during operation: the anti-surge control system must be set to automatic mode; if a fault is detected in this system or it cannot be switched to automatic mode, it must be addressed promptly, otherwise the compressor cannot be brought online. When the compressor starts up, and before it stops due to pressure increase or load reduction, it is necessary to properly control the speed of the unit as well as the anti-surge valve of the compressor. The principle that speed should be increased first when raising pressure, and pressure should be decreased first when reducing speed, must be strictly followed. The outlet check valve must be inspected regularly to ensure its reliable and secure operation. Under non-emergency conditions, it is prohibited to shut down the compressor while it is under load; strict control must be exercised over changes in the compressor’s medium parameters, ensuring that they remain within the designed range at all times. Specify the contact and operational procedures for starting up and shutting down to prevent errors and similar issues

1. Vibration and noise 2. Decreased inlet flow and pressure or increased outlet pressure

The fundamental cause of surge is low flow rate. There are many factors that lead to low flow rates during operation, which can be summarized as follows: (1) The pressure at the compressor outlet increases, and when the system pressure exceeds this outlet pressure, the gas flow rate drops to the surge flow rate. The high pressure in the system causes pressure buildup at the compressor outlet, leading to gas flowing back into the compressor and resulting in a low flow rate of gas inside the compressor. (2) The inlet flow is below the specified value, and the backflow control valve has failed. At a certain rotational speed and gas density, a specific pressure can be maintained. Low gas flow rates during startup or shutdown, or an excessively wide opening of the flare valve, are the most common causes of low inlet flow to the compressor. (3) Changes in gas density: at a certain rotational speed, the centrifugal force decreases, resulting in a drop in outlet pressure and flow rate; this is often mistakenly considered to be a pumping action. (4) Unstable operation of the distillation system causes oil to be present in the gas entering the compressor (for example, due to malfunction of the gas tank level or interface sensors), allowing liquid components to enter the compressor. (5) The steam pressure of the turbine is low or of poor quality (low temperature), resulting in the unit operating at full load while its speed decreases. (6) The speed control system fails, auxiliary systems malfunction, vacuum efficiency decreases, and the unit is unable to perform at its rated capacity. To effectively prevent surge, it is necessary to control the vent valve so that its flow rate remains at a level not lower than that specified by the set pressure. Additionally, specific measures must be taken during operation: (1) When the source of compressed gas changes, the frequency should be adjusted appropriately and the counter-rotational momentum increased; the flare should not be used during startup or shutdown. When the gas flow rate at the compressor inlet is low, the counter-rotational momentum needs to be increased. During startup, gas should also be transferred from the stabilization system to the distillation system to maintain the gas flow rate at the inlet and ensure it stays within the specified range. (2) Strengthen the regulation of system pressure to prevent overpressure. (3) Strengthen the control of the liquid level and interface level in the oil-gas separator of the distillation system, and enhance dehydration. (4) Strengthen the condensate removal at the inlet and outlet of the compressor; it is absolutely necessary to prevent oil from being carried in the gas. (5) Ensure that the steam pressure of the turbine remains stable and is not lower than the design value. (6) When the reaction pressure is high, the inlet flare valve can be opened; when the compressor outlet pressure is high, the outlet flare valve can be opened. However, care should be taken to ensure that the inlet and outlet flare valves are not opened simultaneously.