Twenty Frequently Asked Difficult Questions about Compressors
Compressors serve as critical equipment in many production systems. Below is a collection of common questions encountered in daily operation, packed with practical technical knowledge.
1. What are the characteristics of centrifugal compressors?
A centrifugal compressor is a type of turbocompressor. It features large gas handling capacity, compact footprint, simple structure, stable operation, convenient maintenance, oil-free gas delivery, and compatibility with multiple drive forms.
2. Working Principle of Centrifugal Compressors
The core objective of boosting gas pressure is to increase the number of gas molecules per unit volume, namely reducing the distance between gas molecules. Centrifugal compressors adopt aerodynamic methods to achieve this goal. High-speed rotating impellers (power components) apply work to gas. Under centrifugal force, the gas pressure rises and its kinetic energy increases significantly. Subsequently, this kinetic energy is converted into static pressure energy inside diffuser passages to further raise gas pressure. This describes the working principle of centrifugal compressors.
3. What are the common prime movers for centrifugal compressors?
Common prime movers include electric motors, steam turbines, and gas turbines.
4. What auxiliary equipment is equipped for centrifugal compressors?
Normal operation of the main compressor unit relies on stable auxiliary systems, including:
(1) Lubricating oil system
(2) Cooling system
(3) Condensate system
(4) Electrical instrument system (control system)
(5) Dry gas sealing system
5. Classification of centrifugal compressors by structural features
By structural design, centrifugal compressors are categorized into axially split type, radially split (barrel) type, isothermal compression type, and combined type.
6. Components of the rotor
The rotor consists of main shaft, impellers, shaft sleeves, shaft nuts, spacer sleeves, balance disc and thrust disc.
7. Definition of a Stage
A stage is the basic unit of a centrifugal compressor, composed of one impeller and a set of matching stationary components.
8. Definition of a Section
A section consists of one or multiple stages arranged between one suction inlet and discharge outlet.
9. Definition of a Casing
A casing of centrifugal compressor contains one or several sections. One casing can accommodate a minimum of one stage and a maximum of ten stages.
10. Definition of a String
High-pressure centrifugal compressor units sometimes require two or more casings. A string refers to one or several casings arranged on the same shaft. Different strings operate at different rotational speeds; high-pressure strings run faster than low-pressure strings. For coaxial strings with identical rotational speed, impellers of the high-pressure string have larger diameters than those of the low-pressure string.
11. Functions and classification of impellers
The impeller is the sole component that transfers energy to the gas medium inside a centrifugal compressor. Driven by the high-speed rotating impeller, gas rotates together with the impeller and acquires kinetic energy. Part of this kinetic energy is converted into pressure energy inside the diffuser. Under centrifugal force, gas is discharged from the impeller outlet, flows through the diffuser, bend and return channel into the next-stage impeller for further pressurization, and finally exits from the compressor discharge port.
Based on structural characteristics, impellers fall into three types: open impeller, semi-open impeller and closed impeller.
12. What is the maximum flow operating condition of a centrifugal compressor?
The operating condition at maximum achievable flow rate is defined as the maximum flow condition. It arises under two scenarios:
Gas flow reaches critical state at the throat of a passage within a stage. At this point, the volumetric flow rate of gas reaches its upper limit. Further reduction of backpressure cannot increase flow rate. This condition is also known as choking condition.
No critical flow or choking occurs in the passage. However, under large flow conditions, internal flow losses inside the compressor become substantial. The deliverable discharge pressure is extremely low, barely sufficient to overcome pipeline resistance to maintain such high flow. This constitutes the maximum flow operating condition of the compressor.
13. What is surge of centrifugal compressors?
During operation, centrifugal compressors may suddenly generate severe vibration. The flow rate and pressure of the gas medium fluctuate drastically, accompanied by periodic low rumbling noise. Gas pulsations propagate through the pipeline network and produce heavy huffing sounds. This phenomenon is called surge.
Compressors must not operate under surge conditions for extended periods. Once surge occurs, operators shall take immediate regulating measures, reduce outlet pressure or increase inlet/outlet flow to pull the compressor out of the surge zone and restore stable operation.
14. Characteristics of surge
When surge occurs in a centrifugal compressor, the unit and pipeline network display the following features:
(1) Drastic fluctuation of gas outlet pressure and inlet flow; reverse gas flow may occur. Gas flows backward from compressor outlet to inlet, which is an extremely hazardous condition.
(2) The pipeline network exhibits periodic vibration with large amplitude and low frequency, accompanied by periodic roaring noise.
(3) Intense vibration of the compressor body, casing and bearings, together with strong periodic airflow noise. Severe vibration destroys bearing lubrication, burns bearing pads, may twist the shaft, trigger friction and collision between rotor and stator, and cause severe damage to sealing components.
15. Methods for anti-surge regulation
Surge causes severe damage and cannot be eliminated completely by design. Measures must be adopted during operation to avoid surge conditions. The core anti-surge principle is to increase compressor flow promptly before surge initiates to move operation away from the surge zone. Three practical methods are available:
(1) Partial gas venting method
(2) Partial gas recirculation method
(3) Compressor speed adjustment method
16. Causes of operation below the surge limit
(1) Excessively high outlet backpressure
(2) Throttling of the inlet pipeline valve
(3) Throttling of the outlet pipeline valve
(4) Defective or improperly adjusted anti-surge valve
17. Regulation methods for centrifugal compressor operating conditions
Process parameters inevitably vary during production, requiring manual or automatic regulation of compressors to adapt to variable working conditions and stabilize the whole production system.
Regulation modes are divided into constant-pressure regulation (adjust flow under fixed backpressure) and constant-flow regulation (adjust discharge pressure under fixed flow). Five specific regulation approaches:
(1) Outlet flow regulation
(2) Inlet flow regulation
(3) Speed variation regulation
(4) Rotatable inlet guide vane regulation
(5) Partial venting or recirculation regulation
18. Influence of rotational speed on compressor performance
Rotational speed changes the compressor performance curve while maintaining efficiency. Therefore, speed adjustment is the optimal regulation method for centrifugal compressors.
19. Definitions of constant-pressure regulation, constant-flow regulation and proportional regulation
(1) Constant-pressure regulation: Maintain constant compressor discharge pressure while adjusting gas flow rate.
(2) Constant-flow regulation: Maintain constant gas delivery flow rate while adjusting discharge pressure.
(3) Proportional regulation: Maintain fixed pressure ratio (e.g., anti-surge regulation), or maintain fixed volumetric flow percentage of two gas media.
20. What is a pipeline network? What are its constituent elements?
A pipeline network is the piping system for gas transportation of centrifugal compressors. Piping upstream of the compressor inlet is the suction line; piping downstream of the compressor outlet is the discharge line. The combination of suction and discharge pipelines forms a complete pipeline system generally referred to as the pipeline network.
A pipeline network mainly consists of four elements: pipelines, pipe fittings, valves and equipment.
1. What are the characteristics of centrifugal compressors?
A centrifugal compressor is a type of turbocompressor. It features large gas handling capacity, compact footprint, simple structure, stable operation, convenient maintenance, oil-free gas delivery, and compatibility with multiple drive forms.
2. Working Principle of Centrifugal Compressors
The core objective of boosting gas pressure is to increase the number of gas molecules per unit volume, namely reducing the distance between gas molecules. Centrifugal compressors adopt aerodynamic methods to achieve this goal. High-speed rotating impellers (power components) apply work to gas. Under centrifugal force, the gas pressure rises and its kinetic energy increases significantly. Subsequently, this kinetic energy is converted into static pressure energy inside diffuser passages to further raise gas pressure. This describes the working principle of centrifugal compressors.
3. What are the common prime movers for centrifugal compressors?
Common prime movers include electric motors, steam turbines, and gas turbines.
4. What auxiliary equipment is equipped for centrifugal compressors?
Normal operation of the main compressor unit relies on stable auxiliary systems, including:
(1) Lubricating oil system
(2) Cooling system
(3) Condensate system
(4) Electrical instrument system (control system)
(5) Dry gas sealing system
5. Classification of centrifugal compressors by structural features
By structural design, centrifugal compressors are categorized into axially split type, radially split (barrel) type, isothermal compression type, and combined type.
6. Components of the rotor
The rotor consists of main shaft, impellers, shaft sleeves, shaft nuts, spacer sleeves, balance disc and thrust disc.
7. Definition of a Stage
A stage is the basic unit of a centrifugal compressor, composed of one impeller and a set of matching stationary components.
8. Definition of a Section
A section consists of one or multiple stages arranged between one suction inlet and discharge outlet.
9. Definition of a Casing
A casing of centrifugal compressor contains one or several sections. One casing can accommodate a minimum of one stage and a maximum of ten stages.
10. Definition of a String
High-pressure centrifugal compressor units sometimes require two or more casings. A string refers to one or several casings arranged on the same shaft. Different strings operate at different rotational speeds; high-pressure strings run faster than low-pressure strings. For coaxial strings with identical rotational speed, impellers of the high-pressure string have larger diameters than those of the low-pressure string.
11. Functions and classification of impellers
The impeller is the sole component that transfers energy to the gas medium inside a centrifugal compressor. Driven by the high-speed rotating impeller, gas rotates together with the impeller and acquires kinetic energy. Part of this kinetic energy is converted into pressure energy inside the diffuser. Under centrifugal force, gas is discharged from the impeller outlet, flows through the diffuser, bend and return channel into the next-stage impeller for further pressurization, and finally exits from the compressor discharge port.
Based on structural characteristics, impellers fall into three types: open impeller, semi-open impeller and closed impeller.
12. What is the maximum flow operating condition of a centrifugal compressor?
The operating condition at maximum achievable flow rate is defined as the maximum flow condition. It arises under two scenarios:
Gas flow reaches critical state at the throat of a passage within a stage. At this point, the volumetric flow rate of gas reaches its upper limit. Further reduction of backpressure cannot increase flow rate. This condition is also known as choking condition.
No critical flow or choking occurs in the passage. However, under large flow conditions, internal flow losses inside the compressor become substantial. The deliverable discharge pressure is extremely low, barely sufficient to overcome pipeline resistance to maintain such high flow. This constitutes the maximum flow operating condition of the compressor.
13. What is surge of centrifugal compressors?
During operation, centrifugal compressors may suddenly generate severe vibration. The flow rate and pressure of the gas medium fluctuate drastically, accompanied by periodic low rumbling noise. Gas pulsations propagate through the pipeline network and produce heavy huffing sounds. This phenomenon is called surge.
Compressors must not operate under surge conditions for extended periods. Once surge occurs, operators shall take immediate regulating measures, reduce outlet pressure or increase inlet/outlet flow to pull the compressor out of the surge zone and restore stable operation.
14. Characteristics of surge
When surge occurs in a centrifugal compressor, the unit and pipeline network display the following features:
(1) Drastic fluctuation of gas outlet pressure and inlet flow; reverse gas flow may occur. Gas flows backward from compressor outlet to inlet, which is an extremely hazardous condition.
(2) The pipeline network exhibits periodic vibration with large amplitude and low frequency, accompanied by periodic roaring noise.
(3) Intense vibration of the compressor body, casing and bearings, together with strong periodic airflow noise. Severe vibration destroys bearing lubrication, burns bearing pads, may twist the shaft, trigger friction and collision between rotor and stator, and cause severe damage to sealing components.
15. Methods for anti-surge regulation
Surge causes severe damage and cannot be eliminated completely by design. Measures must be adopted during operation to avoid surge conditions. The core anti-surge principle is to increase compressor flow promptly before surge initiates to move operation away from the surge zone. Three practical methods are available:
(1) Partial gas venting method
(2) Partial gas recirculation method
(3) Compressor speed adjustment method
16. Causes of operation below the surge limit
(1) Excessively high outlet backpressure
(2) Throttling of the inlet pipeline valve
(3) Throttling of the outlet pipeline valve
(4) Defective or improperly adjusted anti-surge valve
17. Regulation methods for centrifugal compressor operating conditions
Process parameters inevitably vary during production, requiring manual or automatic regulation of compressors to adapt to variable working conditions and stabilize the whole production system.
Regulation modes are divided into constant-pressure regulation (adjust flow under fixed backpressure) and constant-flow regulation (adjust discharge pressure under fixed flow). Five specific regulation approaches:
(1) Outlet flow regulation
(2) Inlet flow regulation
(3) Speed variation regulation
(4) Rotatable inlet guide vane regulation
(5) Partial venting or recirculation regulation
18. Influence of rotational speed on compressor performance
Rotational speed changes the compressor performance curve while maintaining efficiency. Therefore, speed adjustment is the optimal regulation method for centrifugal compressors.
19. Definitions of constant-pressure regulation, constant-flow regulation and proportional regulation
(1) Constant-pressure regulation: Maintain constant compressor discharge pressure while adjusting gas flow rate.
(2) Constant-flow regulation: Maintain constant gas delivery flow rate while adjusting discharge pressure.
(3) Proportional regulation: Maintain fixed pressure ratio (e.g., anti-surge regulation), or maintain fixed volumetric flow percentage of two gas media.
20. What is a pipeline network? What are its constituent elements?
A pipeline network is the piping system for gas transportation of centrifugal compressors. Piping upstream of the compressor inlet is the suction line; piping downstream of the compressor outlet is the discharge line. The combination of suction and discharge pipelines forms a complete pipeline system generally referred to as the pipeline network.
A pipeline network mainly consists of four elements: pipelines, pipe fittings, valves and equipment.
