Summary of Q&A on Compressor Knowledge
1. What are the characteristics of centrifugal compressors?
A centrifugal compressor is a type of turbocompressor. It features large gas handling capacity, compact size, simple structure, stable operation, convenient maintenance, zero oil contamination of process gas, and supports multiple drive forms.
2. What is the working principle of a centrifugal compressor?
Generally speaking, the core objective of raising gas pressure is to increase the number of gas molecules per unit volume, namely reducing the distance between gas molecules. To achieve this goal, gas dynamics principles are adopted. A mechanical working element (high-speed rotating impeller) applies work to the gas. Under centrifugal force, the gas pressure rises and its kinetic energy increases significantly. Afterwards, part of the kinetic energy is converted into static pressure energy inside the diffuser passage to further boost gas pressure. This describes the working principle of centrifugal compressors.
3. What are the common prime movers for centrifugal compressors?
Common prime movers for centrifugal compressors include electric motors, steam turbines, gas turbines, etc.
4. What auxiliary equipment is matched with centrifugal compressors?
Stable operation of the centrifugal compressor main unit relies on normal operation of auxiliary systems, which are listed as follows:
(1) Lubricating oil system
(2) Cooling system
(3) Condensate system
(4) Electrical, instrumentation and control system
(5) Dry gas seal system
5. How are centrifugal compressors classified according to structural features?
Based on structural characteristics, centrifugal compressors are classified into horizontally split type, vertically split type, isothermal compression type, combined type and others.
6. What components make up a rotor?
A rotor consists of the main shaft, impellers, shaft sleeves, shaft nuts, spacer sleeves, balance disk and thrust disk.
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
All stages between one suction port and discharge port form a section. One section contains one or multiple stages.
9. Definition of a Casing
A compressor casing consists of one or multiple sections. A single casing can accommodate a minimum of one stage and a maximum of ten stages.
10. Definition of a String
High-pressure centrifugal compressors sometimes require two or more casings. One or several casings arranged on the same shaft form a string of the centrifugal compressor. Different strings operate at different rotational speeds; the high-pressure string runs faster than the low-pressure string. For strings sharing the same shaft and rotational speed, impellers of the high-pressure string have larger diameters than those of the low-pressure string.
11. What is the function of an impeller? What types are classified by structural features?
The impeller is the only component that transfers work to the gas medium in a centrifugal compressor. Driven by centrifugal force from the high-speed rotating impeller, the gas rotates together with the impeller and gains kinetic energy. Part of this kinetic energy is converted into pressure energy inside the diffuser. The gas is discharged from the impeller outlet under centrifugal force, flows through the diffuser, bend and return channel to the next-stage impeller for further pressurization, and finally exits from the compressor discharge port.
By structural form, impellers fall into three categories: 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 volumetric flow is defined as the maximum flow condition. It occurs in two scenarios:
First, the gas flow at the throat of a passage within the stage reaches critical state. At this point, the volumetric gas flow reaches its upper limit. No further flow increase can be achieved even if the compressor backpressure drops. This operating condition is also known as the "choke condition".
Second, critical flow state (choke) does not occur in the passage. Nevertheless, under high flow rates, internal flow losses inside the compressor become substantial, and the achievable discharge pressure becomes extremely low, nearly zero. The generated pressure can only overcome resistance inside the discharge pipeline to sustain such high flow. This is the maximum flow operating condition of a centrifugal compressor.
13. What is surge of a centrifugal compressor?
During operation, centrifugal compressors may suddenly experience severe vibration, dramatic fluctuations in gas flow and pressure, accompanied by periodic low rumbling noise and heavy huffing sound generated by flow oscillation in the piping network. This phenomenon is defined as the surge condition of centrifugal compressors.
The compressor must not operate continuously under surge conditions. Once surge occurs, operators shall immediately implement adjustment measures to reduce discharge pressure or increase inlet/outlet flow, so that the compressor quickly exits the surge zone and resumes stable operation.
14. Characteristics of surge phenomenon
When surge occurs in a centrifugal compressor, the unit and piping network exhibit the following features:
(1) Sharp variation of gas discharge pressure and inlet flow; reverse gas flow may occur, where gas flows backwards from the compressor discharge to the inlet. This represents a hazardous operating condition.
(2) Periodic vibration of the piping network with large amplitude and low frequency, accompanied by periodic roaring noise.
(3) Severe vibration of the compressor body, casing and bearings, together with strong periodic airflow noise. Intense vibration damages bearing lubrication, leads to thrust bearing burnout, shaft fracture, friction and collision between rotor and stator, and severe failure of sealing components.
15. Methods for anti-surge regulation
Surge causes severe damage, yet it cannot be eliminated by design. Measures must be adopted during operation to avoid surge conditions. The anti-surge principle is as follows: when surge is impending, rapidly increase the compressor flow to pull the unit away from the surge zone. Three anti-surge control methods are available:
(1) Partial gas venting method
(2) Partial gas recirculation method
(3) Rotational speed adjustment method for the compressor
16. Causes of operation below the surge limit
(1) Excessive discharge backpressure
(2) Throttling of the inlet pipeline valve
(3) Throttling of the discharge pipeline valve
(4) Defective or improperly adjusted anti-surge valve
17. Regulation methods for centrifugal compressor operating conditions
Process parameters inevitably fluctuate during production, so manual or automatic regulation of the compressor is frequently required to adapt to variable operating conditions and stabilize the production system.
Regulation of centrifugal compressors is divided into two types: constant pressure regulation (adjust flow under constant backpressure) and constant flow regulation (adjust discharge pressure under constant gas flow). Five specific adjustment approaches are listed below:
(1) Discharge flow regulation
(2) Inlet flow regulation
(3) Rotational speed adjustment
(4) Inlet guide vane rotation adjustment
(5) Partial venting or recirculation regulation
18. Impact of rotational speed on compressor performance
Changing rotational speed shifts the compressor performance curve without altering efficiency. Therefore, speed adjustment is the optimal regulation method for compressors.
19. Definitions of constant pressure regulation, constant flow regulation and proportional regulation
(1) Constant pressure regulation: Adjust gas flow while maintaining constant compressor discharge pressure.
(2) Constant flow regulation: Adjust discharge pressure while maintaining constant gas delivery flow.
(3) Proportional regulation: Maintain a fixed pressure ratio (e.g., anti-surge regulation), or maintain a fixed volumetric flow percentage of two gas media.
20. What is a piping network? What are its constituent elements?
A piping network is the pipeline system for gas transportation of centrifugal compressors. Pipelines upstream of the compressor inlet are suction pipelines; pipelines downstream of the compressor outlet are discharge pipelines. The combination of suction and discharge pipelines forms a complete piping system generally referred to as the network.
A piping network mainly consists of four elements: pipelines, pipe fittings, valves and equipment.
21. Hazards of axial thrust
A high-speed rotating rotor is persistently subjected to axial thrust directed from the high-pressure end to the low-pressure end. Under axial thrust, the rotor produces axial displacement, triggering relative sliding between journal and bearing shell. This may scratch the journal or bearing shell. Worse still, rotor displacement causes friction, collision and mechanical damage between rotor and stator components.
Axial thrust risks friction, wear, collision and even catastrophic failure of equipment. Effective balancing measures must be implemented to improve unit operational reliability.
22. Balancing methods for axial thrust
Axial thrust balancing is a critical consideration in the design of multistage centrifugal compressors. Two mainstream balancing methods are widely adopted:
(1) Opposed impeller arrangement (back-to-back arrangement of high-pressure and low-pressure sides of impellers)
The axial thrust generated by a single impeller points toward the impeller inlet (from high pressure to low pressure). If multistage impellers are arranged sequentially, the total rotor axial thrust equals the sum of axial thrust of each stage, resulting in substantial thrust load.
When multistage impellers are arranged oppositely, impellers with opposite inlet directions generate axial thrust in reverse directions, realizing mutual balance. Opposed arrangement is the most common axial thrust balancing method for multistage centrifugal compressors.
(2) Installation of balance disk
The balance disk is a widely used axial thrust balancing device for multistage centrifugal compressors, normally mounted at the high-pressure side. Labyrinth seals are installed between its outer rim and casing to maintain a specific pressure difference between the high-pressure side and the low-pressure side connected to the compressor inlet. The axial thrust generated by this pressure difference acts opposite to the thrust produced by impellers, offsetting the impeller-induced axial force.
24. Causes of rising thrust bearing temperature
(1) Improper structural design: insufficient bearing area of the thrust pad, exceeding allowable load per unit area.
(2) Failure of interstage seal: gas leaks from the outlet of the rear-stage impeller to the preceding stage, increasing pressure difference on both sides of the impeller and generating excessive thrust.
(3) Blocked balance pipe: pressure in the secondary pressure chamber of the balance disk cannot be released, disabling the balance disk.
(4) Failed balance disk seal: operating chamber pressure cannot be maintained normally, reducing balancing capacity and transferring excess load to thrust bearings leading to overload.
(5) Small throttling bore of thrust bearing oil supply: insufficient cooling oil flow, unable to fully remove frictional heat.
(6) Water or impurities mixed in lubricating oil, preventing formation of a complete liquid lubrication film on thrust pads.
(7) Excessively high temperature of bearing inlet oil, deteriorating operating conditions for thrust bearings.
25. Solutions for excessively high thrust bearing temperature
(1) Verify pressure load on thrust pads; appropriately expand bearing area to keep thrust load within standard range.
(2) Disassemble and inspect interstage seals, replace damaged sealing components.
(3) Inspect balance pipes and remove blockages to timely release pressure in the secondary pressure chamber of the balance disk and restore its balancing function.
(4) Replace balance disk sealing strips, improve sealing performance, maintain pressure inside the balance disk working chamber for reasonable axial thrust balance.
(5) Expand the bore of bearing oil supply throttling orifice to increase lubricant flow and timely carry away frictional heat.
(6) Replace with new qualified lubricating oil to guarantee lubrication performance.
(7) Open cooling water inlet and return valves wider, increase cooling water flow and reduce oil supply temperature.
26. Operating response of combined compressor operators during severe overpressure of the synthesis system
(1) Notify field operators of the synthesis section to open PV2001 for pressure relief.
(2) Notify patrol operators of the combined compressor to open the manual vent valve at the second-stage discharge of the compressor for pressure relief (in emergency conditions). Operators shall be supervised and equipped with anti-toxic protection gear.
27. How to establish circulation of the synthesis system using the combined compressor?
Before startup of the synthesis system, nitrogen charging and heating shall be performed under specific pressure. The syngas compressor needs to be started to build circulation for the synthesis system.
(1) Start the steam turbine of the syngas compressor following normal startup procedures and operate at no-load until reaching rated speed.
(2) Maintain gas recirculation from the outlet of the anti-surge cooler to the first-stage suction. The recirculation flow shall not be excessive, and over-temperature shall be avoided.
(3) Control gas flow and pressure entering the synthesis system via the circulation-section anti-surge valve, and stabilize the temperature of the synthesis reactor.
28. Operation of the combined compressor during emergency gas cut-off of the synthesis system (compressor remains running)
Implementation steps for emergency gas cut-off of the combined compressor:
(1) Report emergency gas cut-off to the control room; switch primary seal gas to medium-pressure nitrogen. Vent the combined compressor outlet at the purification section outlet while maintaining pressure.
(2) Close XV2683, XV2681 and XV2682. The compressor operates under no-load condition; the synthesis system releases pressure while preserving temperature and pressure.
(3) Open the fresh gas section anti-surge valve to reduce fresh gas flow; open the circulation-section anti-surge valve to reduce recycle gas flow.
(4) Open the vent valve PV2620 at the second-stage discharge of the compressor and release unit pressure at a rate ≤0.15 MPa/min.
(5) After accident handling of the synthesis system, charge nitrogen from the combined compressor inlet to purge the synthesis system and establish circulation.
29. Methods for increasing fresh gas flow
Normally, the inlet valve XV2683 remains fully open. Fresh gas flow is regulated solely via the fresh-gas section anti-surge valve downstream of the anti-surge cooler. Closing the first-stage anti-surge valve reduces recirculation flow and increases fresh gas supply.
30. Control of space velocity via the compressor
Space velocity of the syngas reactor is adjusted by increasing or decreasing recycle gas flow. Under fixed fresh gas flow, raising the synthesis recycle gas flow increases space velocity, which imposes certain impacts on the methanol synthesis reaction.
31. Control method for synthesis recycle gas flow
Regulated by throttling of the circulation-section anti-surge valve.
32. Reasons for insufficient increase of synthesis recycle gas flow
(1) Low fresh gas flow. When the reaction proceeds efficiently, volume contraction causes rapid pressure drop at the reactor outlet, resulting in low outlet pressure. Space velocity needs to be raised to control reaction rate.
(2) Excessive venting (purge gas flow) of the synthesis system, with excessive opening of PV2001.
(3) Excessively large opening of the recycle gas anti-surge valve, leading to massive gas recirculation.
33. Interlock logic between synthesis system and combined compressor
(1) Low liquid level interlock of the steam drum: level ≤10% triggers interlock of the combined compressor; XV2683 closes to prevent dry-out of the steam drum.
(2) High liquid level interlock of the methanol separator: level ≥90% triggers trip interlock of the combined compressor; XV2681, XV2682 and XV2683 close to prevent liquid entering the compressor casing and damaging impellers.
(3) High limit interlock of synthesis reactor hot spot temperature ≥275°C triggers trip of the combined compressor.
34. How to deal with excessively high temperature of synthesis recycle gas
(1) Monitor the recycle gas temperature of the synthesis system. If the temperature exceeds the index, reduce the recycle flow or inform the control room to increase cooling water pressure or lower water temperature.
(2) Check whether the return water temperature of the anti-surge cooler rises. A temperature rise indicates excessive gas recirculation leading to poor cooling effect, and the recycle flow shall be increased accordingly.
35. Alternate flow increase of fresh gas and recycle gas during synthesis system startup
During synthesis startup, low gas temperature leads to low catalyst hot spot temperature and restricts the synthesis reaction. Flow adjustment shall prioritize stabilizing the catalyst bed temperature. Therefore, recycle gas flow shall be increased before fresh gas flow (generally, recycle gas flow is 4~6 times the fresh gas flow). Flow increase shall be carried out slowly with proper intervals, subject to whether the catalyst hot spot temperature can be maintained and show an upward trend. When the gas flow reaches a certain level, the synthesis section can reduce startup steam.
Reduce the opening of the fresh gas section anti-surge valve to increase fresh gas flow.
Reduce the opening of the circulation-section anti-surge valve to increase recycle gas flow.
36. Heat preservation and pressure maintenance of the synthesis system using the compressor during startup and shutdown
Charge nitrogen from the inlet of the combined compressor to purge and pressurize the synthesis system, and establish circulation between the combined compressor and the synthesis system. System venting is determined according to the synthesis system pressure. Space velocity is used to maintain the outlet temperature of the synthesis reactor, and startup steam is turned on to supply heat. The synthesis system operates under low pressure and low-speed circulation for heat preservation.
37. Pressure rise operation of the synthesis system during startup and the control rate
Pressure rise of the synthesis system mainly relies on increasing fresh gas flow and recycle gas pressure. Specifically, reducing the opening of the fresh gas section anti-surge valve increases fresh gas supply; reducing the opening of the circulation-section anti-surge valve controls the synthesis system pressure. During normal startup, the pressure rise rate of the synthesis system is generally controlled at 0.4 MPa/min.
38. Control of synthesis reactor heating rate by the combined compressor and control index
During heating, startup steam is turned on to supply heat and drive furnace water circulation to raise the reactor temperature. Meanwhile, the combined compressor is started to circulate gas in the synthesis system via gas feeding and discharging of the circulation section, control heat and stabilize the heating amplitude of the reactor. Therefore, the heating rate is mainly adjusted by changing the recycle gas flow. The control index of heating rate is 25°C/h.
39. Flow regulation of anti-surge gas for fresh gas section and circulation section
When the compressor operating condition approaches the surge zone, anti-surge regulation shall be implemented. To avoid excessive fluctuation of system gas flow before adjustment, judge and confirm which section is close to surge first, then properly open the anti-surge valve of this section to eliminate surge. Attention shall be paid to gas flow fluctuation (stabilize gas flow entering the reactor as much as possible). Two anti-surge valves shall not be opened simultaneously to eliminate surge.
40. Causes of liquid entrainment at compressor inlet
(1) The process gas delivered by the upstream system has high temperature and is not fully condensed. The long gas transmission pipeline leads to liquid condensation inside the pipeline.
(2) High temperature of the process system causes low boiling point components in the gas medium to condense into liquid.
(3) Excessively high liquid level of the separator results in gas-liquid entrainment.
41. Treatment measures for liquid entrainment at compressor inlet
(1) Contact the upstream system to adjust process operation.
(2) Appropriately increase the liquid draining frequency of the separator in this system.
(3) Lower the liquid level of the separator to prevent gas-liquid entrainment.
42. Causes of performance degradation of the combined compressor unit
(1) Severe damage of compressor interstage seals, reduced sealing performance and increased internal gas recirculation.
(2) Severe impeller wear, degraded rotor performance and insufficient kinetic energy obtained by gas medium.
(3) Blockage of the steam filter screen of the steam turbine, obstructed steam flow, low flow rate and large pressure difference, which reduces the output power of the steam turbine and unit performance.
(4) Vacuum degree lower than the required index, obstructing exhaust of the steam turbine.
(5) Steam temperature and pressure parameters lower than operating indexes, insufficient internal energy of steam failing to meet production operation requirements.
(6) Occurrence of surge condition.
43. Main performance parameters of centrifugal compressors
The main performance parameters of centrifugal compressors include flow rate, discharge pressure or compression ratio, power, efficiency, rotational speed, head, etc.
Equipment performance parameters are basic data characterizing structural features, handling capacity and operating conditions of equipment, serving as important guiding materials for equipment procurement and planning.
44. Definition of efficiency
Efficiency represents the utilization rate of energy transferred from the centrifugal compressor to gas. Higher utilization rate means higher compressor efficiency.
Gas compression includes three processes: polytropic compression, adiabatic compression and isothermal compression. Accordingly, compressor efficiency is divided into polytropic efficiency, adiabatic efficiency and isothermal efficiency.
45. Definition of compression ratio
The compression ratio refers to the ratio of compressor discharge gas pressure to suction gas pressure, also known as pressure ratio.
46. Composition of the lubricating oil system
The lubricating oil system consists of an oil supply unit, overhead oil tank, connecting pipelines, control valves and measuring instruments.
The oil supply unit comprises oil tank, oil pump, oil cooler, oil filter, pressure regulating valve, various measuring instruments, oil pipelines and valves.
47. Function of the overhead oil tank
The overhead oil tank is one of the safety protection measures for the unit. During normal unit operation, lubricating oil enters from the bottom and flows back to the tank directly from the top. In case of power failure and shutdown accident where the auxiliary oil pump cannot start oil supply timely, lubricating oil from the overhead oil tank flows through each lubrication point along the oil supply pipeline and returns to the tank, ensuring lubrication requirement during unit coast-down.
48. Safety protection measures for the combined compressor unit
(1) Overhead oil tank
(2) Safety valve
(3) Accumulator
(4) Quick-closing valve
(5) Other interlock devices
49. Sealing principle of labyrinth gland
Convert potential energy (pressure) into kinetic energy (flow velocity), then dissipate the kinetic energy in the form of vortex flow.
50. Function of thrust bearing
The thrust bearing has two functions: bearing rotor thrust and providing axial positioning for the rotor. The thrust bearing bears the residual rotor thrust unbalanced by the balance piston and thrust transmitted from the gear coupling. The magnitude of these thrusts mainly depends on the steam turbine load. In addition, the thrust bearing fixes the axial position of the rotor relative to the casing.
51. Why should the unit pressure of the combined compressor be released rapidly during shutdown?
Long-term shutdown under pressure will cause damage to seals if the primary seal gas pressure cannot exceed the compressor inlet pressure. Unfiltered process gas inside the unit will penetrate into the seals and damage them.
52. Function of seals
For stable operation of centrifugal compressors, a certain clearance must be reserved between rotor and stator to avoid friction, wear, collision and damage. Meanwhile, the clearance inevitably causes interstage and shaft end leakage. Leakage not only reduces compressor efficiency, but also leads to environmental pollution and even explosion accidents. Therefore, leakage is prohibited.
Seals are effective measures to prevent interstage and shaft end leakage while maintaining proper clearance between rotor and stator.
A centrifugal compressor is a type of turbocompressor. It features large gas handling capacity, compact size, simple structure, stable operation, convenient maintenance, zero oil contamination of process gas, and supports multiple drive forms.
2. What is the working principle of a centrifugal compressor?
Generally speaking, the core objective of raising gas pressure is to increase the number of gas molecules per unit volume, namely reducing the distance between gas molecules. To achieve this goal, gas dynamics principles are adopted. A mechanical working element (high-speed rotating impeller) applies work to the gas. Under centrifugal force, the gas pressure rises and its kinetic energy increases significantly. Afterwards, part of the kinetic energy is converted into static pressure energy inside the diffuser passage to further boost gas pressure. This describes the working principle of centrifugal compressors.
3. What are the common prime movers for centrifugal compressors?
Common prime movers for centrifugal compressors include electric motors, steam turbines, gas turbines, etc.
4. What auxiliary equipment is matched with centrifugal compressors?
Stable operation of the centrifugal compressor main unit relies on normal operation of auxiliary systems, which are listed as follows:
(1) Lubricating oil system
(2) Cooling system
(3) Condensate system
(4) Electrical, instrumentation and control system
(5) Dry gas seal system
5. How are centrifugal compressors classified according to structural features?
Based on structural characteristics, centrifugal compressors are classified into horizontally split type, vertically split type, isothermal compression type, combined type and others.
6. What components make up a rotor?
A rotor consists of the main shaft, impellers, shaft sleeves, shaft nuts, spacer sleeves, balance disk and thrust disk.
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
All stages between one suction port and discharge port form a section. One section contains one or multiple stages.
9. Definition of a Casing
A compressor casing consists of one or multiple sections. A single casing can accommodate a minimum of one stage and a maximum of ten stages.
10. Definition of a String
High-pressure centrifugal compressors sometimes require two or more casings. One or several casings arranged on the same shaft form a string of the centrifugal compressor. Different strings operate at different rotational speeds; the high-pressure string runs faster than the low-pressure string. For strings sharing the same shaft and rotational speed, impellers of the high-pressure string have larger diameters than those of the low-pressure string.
11. What is the function of an impeller? What types are classified by structural features?
The impeller is the only component that transfers work to the gas medium in a centrifugal compressor. Driven by centrifugal force from the high-speed rotating impeller, the gas rotates together with the impeller and gains kinetic energy. Part of this kinetic energy is converted into pressure energy inside the diffuser. The gas is discharged from the impeller outlet under centrifugal force, flows through the diffuser, bend and return channel to the next-stage impeller for further pressurization, and finally exits from the compressor discharge port.
By structural form, impellers fall into three categories: 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 volumetric flow is defined as the maximum flow condition. It occurs in two scenarios:
First, the gas flow at the throat of a passage within the stage reaches critical state. At this point, the volumetric gas flow reaches its upper limit. No further flow increase can be achieved even if the compressor backpressure drops. This operating condition is also known as the "choke condition".
Second, critical flow state (choke) does not occur in the passage. Nevertheless, under high flow rates, internal flow losses inside the compressor become substantial, and the achievable discharge pressure becomes extremely low, nearly zero. The generated pressure can only overcome resistance inside the discharge pipeline to sustain such high flow. This is the maximum flow operating condition of a centrifugal compressor.
13. What is surge of a centrifugal compressor?
During operation, centrifugal compressors may suddenly experience severe vibration, dramatic fluctuations in gas flow and pressure, accompanied by periodic low rumbling noise and heavy huffing sound generated by flow oscillation in the piping network. This phenomenon is defined as the surge condition of centrifugal compressors.
The compressor must not operate continuously under surge conditions. Once surge occurs, operators shall immediately implement adjustment measures to reduce discharge pressure or increase inlet/outlet flow, so that the compressor quickly exits the surge zone and resumes stable operation.
14. Characteristics of surge phenomenon
When surge occurs in a centrifugal compressor, the unit and piping network exhibit the following features:
(1) Sharp variation of gas discharge pressure and inlet flow; reverse gas flow may occur, where gas flows backwards from the compressor discharge to the inlet. This represents a hazardous operating condition.
(2) Periodic vibration of the piping network with large amplitude and low frequency, accompanied by periodic roaring noise.
(3) Severe vibration of the compressor body, casing and bearings, together with strong periodic airflow noise. Intense vibration damages bearing lubrication, leads to thrust bearing burnout, shaft fracture, friction and collision between rotor and stator, and severe failure of sealing components.
15. Methods for anti-surge regulation
Surge causes severe damage, yet it cannot be eliminated by design. Measures must be adopted during operation to avoid surge conditions. The anti-surge principle is as follows: when surge is impending, rapidly increase the compressor flow to pull the unit away from the surge zone. Three anti-surge control methods are available:
(1) Partial gas venting method
(2) Partial gas recirculation method
(3) Rotational speed adjustment method for the compressor
16. Causes of operation below the surge limit
(1) Excessive discharge backpressure
(2) Throttling of the inlet pipeline valve
(3) Throttling of the discharge pipeline valve
(4) Defective or improperly adjusted anti-surge valve
17. Regulation methods for centrifugal compressor operating conditions
Process parameters inevitably fluctuate during production, so manual or automatic regulation of the compressor is frequently required to adapt to variable operating conditions and stabilize the production system.
Regulation of centrifugal compressors is divided into two types: constant pressure regulation (adjust flow under constant backpressure) and constant flow regulation (adjust discharge pressure under constant gas flow). Five specific adjustment approaches are listed below:
(1) Discharge flow regulation
(2) Inlet flow regulation
(3) Rotational speed adjustment
(4) Inlet guide vane rotation adjustment
(5) Partial venting or recirculation regulation
18. Impact of rotational speed on compressor performance
Changing rotational speed shifts the compressor performance curve without altering efficiency. Therefore, speed adjustment is the optimal regulation method for compressors.
19. Definitions of constant pressure regulation, constant flow regulation and proportional regulation
(1) Constant pressure regulation: Adjust gas flow while maintaining constant compressor discharge pressure.
(2) Constant flow regulation: Adjust discharge pressure while maintaining constant gas delivery flow.
(3) Proportional regulation: Maintain a fixed pressure ratio (e.g., anti-surge regulation), or maintain a fixed volumetric flow percentage of two gas media.
20. What is a piping network? What are its constituent elements?
A piping network is the pipeline system for gas transportation of centrifugal compressors. Pipelines upstream of the compressor inlet are suction pipelines; pipelines downstream of the compressor outlet are discharge pipelines. The combination of suction and discharge pipelines forms a complete piping system generally referred to as the network.
A piping network mainly consists of four elements: pipelines, pipe fittings, valves and equipment.
21. Hazards of axial thrust
A high-speed rotating rotor is persistently subjected to axial thrust directed from the high-pressure end to the low-pressure end. Under axial thrust, the rotor produces axial displacement, triggering relative sliding between journal and bearing shell. This may scratch the journal or bearing shell. Worse still, rotor displacement causes friction, collision and mechanical damage between rotor and stator components.
Axial thrust risks friction, wear, collision and even catastrophic failure of equipment. Effective balancing measures must be implemented to improve unit operational reliability.
22. Balancing methods for axial thrust
Axial thrust balancing is a critical consideration in the design of multistage centrifugal compressors. Two mainstream balancing methods are widely adopted:
(1) Opposed impeller arrangement (back-to-back arrangement of high-pressure and low-pressure sides of impellers)
The axial thrust generated by a single impeller points toward the impeller inlet (from high pressure to low pressure). If multistage impellers are arranged sequentially, the total rotor axial thrust equals the sum of axial thrust of each stage, resulting in substantial thrust load.
When multistage impellers are arranged oppositely, impellers with opposite inlet directions generate axial thrust in reverse directions, realizing mutual balance. Opposed arrangement is the most common axial thrust balancing method for multistage centrifugal compressors.
(2) Installation of balance disk
The balance disk is a widely used axial thrust balancing device for multistage centrifugal compressors, normally mounted at the high-pressure side. Labyrinth seals are installed between its outer rim and casing to maintain a specific pressure difference between the high-pressure side and the low-pressure side connected to the compressor inlet. The axial thrust generated by this pressure difference acts opposite to the thrust produced by impellers, offsetting the impeller-induced axial force.
24. Causes of rising thrust bearing temperature
(1) Improper structural design: insufficient bearing area of the thrust pad, exceeding allowable load per unit area.
(2) Failure of interstage seal: gas leaks from the outlet of the rear-stage impeller to the preceding stage, increasing pressure difference on both sides of the impeller and generating excessive thrust.
(3) Blocked balance pipe: pressure in the secondary pressure chamber of the balance disk cannot be released, disabling the balance disk.
(4) Failed balance disk seal: operating chamber pressure cannot be maintained normally, reducing balancing capacity and transferring excess load to thrust bearings leading to overload.
(5) Small throttling bore of thrust bearing oil supply: insufficient cooling oil flow, unable to fully remove frictional heat.
(6) Water or impurities mixed in lubricating oil, preventing formation of a complete liquid lubrication film on thrust pads.
(7) Excessively high temperature of bearing inlet oil, deteriorating operating conditions for thrust bearings.
25. Solutions for excessively high thrust bearing temperature
(1) Verify pressure load on thrust pads; appropriately expand bearing area to keep thrust load within standard range.
(2) Disassemble and inspect interstage seals, replace damaged sealing components.
(3) Inspect balance pipes and remove blockages to timely release pressure in the secondary pressure chamber of the balance disk and restore its balancing function.
(4) Replace balance disk sealing strips, improve sealing performance, maintain pressure inside the balance disk working chamber for reasonable axial thrust balance.
(5) Expand the bore of bearing oil supply throttling orifice to increase lubricant flow and timely carry away frictional heat.
(6) Replace with new qualified lubricating oil to guarantee lubrication performance.
(7) Open cooling water inlet and return valves wider, increase cooling water flow and reduce oil supply temperature.
26. Operating response of combined compressor operators during severe overpressure of the synthesis system
(1) Notify field operators of the synthesis section to open PV2001 for pressure relief.
(2) Notify patrol operators of the combined compressor to open the manual vent valve at the second-stage discharge of the compressor for pressure relief (in emergency conditions). Operators shall be supervised and equipped with anti-toxic protection gear.
27. How to establish circulation of the synthesis system using the combined compressor?
Before startup of the synthesis system, nitrogen charging and heating shall be performed under specific pressure. The syngas compressor needs to be started to build circulation for the synthesis system.
(1) Start the steam turbine of the syngas compressor following normal startup procedures and operate at no-load until reaching rated speed.
(2) Maintain gas recirculation from the outlet of the anti-surge cooler to the first-stage suction. The recirculation flow shall not be excessive, and over-temperature shall be avoided.
(3) Control gas flow and pressure entering the synthesis system via the circulation-section anti-surge valve, and stabilize the temperature of the synthesis reactor.
28. Operation of the combined compressor during emergency gas cut-off of the synthesis system (compressor remains running)
Implementation steps for emergency gas cut-off of the combined compressor:
(1) Report emergency gas cut-off to the control room; switch primary seal gas to medium-pressure nitrogen. Vent the combined compressor outlet at the purification section outlet while maintaining pressure.
(2) Close XV2683, XV2681 and XV2682. The compressor operates under no-load condition; the synthesis system releases pressure while preserving temperature and pressure.
(3) Open the fresh gas section anti-surge valve to reduce fresh gas flow; open the circulation-section anti-surge valve to reduce recycle gas flow.
(4) Open the vent valve PV2620 at the second-stage discharge of the compressor and release unit pressure at a rate ≤0.15 MPa/min.
(5) After accident handling of the synthesis system, charge nitrogen from the combined compressor inlet to purge the synthesis system and establish circulation.
29. Methods for increasing fresh gas flow
Normally, the inlet valve XV2683 remains fully open. Fresh gas flow is regulated solely via the fresh-gas section anti-surge valve downstream of the anti-surge cooler. Closing the first-stage anti-surge valve reduces recirculation flow and increases fresh gas supply.
30. Control of space velocity via the compressor
Space velocity of the syngas reactor is adjusted by increasing or decreasing recycle gas flow. Under fixed fresh gas flow, raising the synthesis recycle gas flow increases space velocity, which imposes certain impacts on the methanol synthesis reaction.
31. Control method for synthesis recycle gas flow
Regulated by throttling of the circulation-section anti-surge valve.
32. Reasons for insufficient increase of synthesis recycle gas flow
(1) Low fresh gas flow. When the reaction proceeds efficiently, volume contraction causes rapid pressure drop at the reactor outlet, resulting in low outlet pressure. Space velocity needs to be raised to control reaction rate.
(2) Excessive venting (purge gas flow) of the synthesis system, with excessive opening of PV2001.
(3) Excessively large opening of the recycle gas anti-surge valve, leading to massive gas recirculation.
33. Interlock logic between synthesis system and combined compressor
(1) Low liquid level interlock of the steam drum: level ≤10% triggers interlock of the combined compressor; XV2683 closes to prevent dry-out of the steam drum.
(2) High liquid level interlock of the methanol separator: level ≥90% triggers trip interlock of the combined compressor; XV2681, XV2682 and XV2683 close to prevent liquid entering the compressor casing and damaging impellers.
(3) High limit interlock of synthesis reactor hot spot temperature ≥275°C triggers trip of the combined compressor.
34. How to deal with excessively high temperature of synthesis recycle gas
(1) Monitor the recycle gas temperature of the synthesis system. If the temperature exceeds the index, reduce the recycle flow or inform the control room to increase cooling water pressure or lower water temperature.
(2) Check whether the return water temperature of the anti-surge cooler rises. A temperature rise indicates excessive gas recirculation leading to poor cooling effect, and the recycle flow shall be increased accordingly.
35. Alternate flow increase of fresh gas and recycle gas during synthesis system startup
During synthesis startup, low gas temperature leads to low catalyst hot spot temperature and restricts the synthesis reaction. Flow adjustment shall prioritize stabilizing the catalyst bed temperature. Therefore, recycle gas flow shall be increased before fresh gas flow (generally, recycle gas flow is 4~6 times the fresh gas flow). Flow increase shall be carried out slowly with proper intervals, subject to whether the catalyst hot spot temperature can be maintained and show an upward trend. When the gas flow reaches a certain level, the synthesis section can reduce startup steam.
Reduce the opening of the fresh gas section anti-surge valve to increase fresh gas flow.
Reduce the opening of the circulation-section anti-surge valve to increase recycle gas flow.
36. Heat preservation and pressure maintenance of the synthesis system using the compressor during startup and shutdown
Charge nitrogen from the inlet of the combined compressor to purge and pressurize the synthesis system, and establish circulation between the combined compressor and the synthesis system. System venting is determined according to the synthesis system pressure. Space velocity is used to maintain the outlet temperature of the synthesis reactor, and startup steam is turned on to supply heat. The synthesis system operates under low pressure and low-speed circulation for heat preservation.
37. Pressure rise operation of the synthesis system during startup and the control rate
Pressure rise of the synthesis system mainly relies on increasing fresh gas flow and recycle gas pressure. Specifically, reducing the opening of the fresh gas section anti-surge valve increases fresh gas supply; reducing the opening of the circulation-section anti-surge valve controls the synthesis system pressure. During normal startup, the pressure rise rate of the synthesis system is generally controlled at 0.4 MPa/min.
38. Control of synthesis reactor heating rate by the combined compressor and control index
During heating, startup steam is turned on to supply heat and drive furnace water circulation to raise the reactor temperature. Meanwhile, the combined compressor is started to circulate gas in the synthesis system via gas feeding and discharging of the circulation section, control heat and stabilize the heating amplitude of the reactor. Therefore, the heating rate is mainly adjusted by changing the recycle gas flow. The control index of heating rate is 25°C/h.
39. Flow regulation of anti-surge gas for fresh gas section and circulation section
When the compressor operating condition approaches the surge zone, anti-surge regulation shall be implemented. To avoid excessive fluctuation of system gas flow before adjustment, judge and confirm which section is close to surge first, then properly open the anti-surge valve of this section to eliminate surge. Attention shall be paid to gas flow fluctuation (stabilize gas flow entering the reactor as much as possible). Two anti-surge valves shall not be opened simultaneously to eliminate surge.
40. Causes of liquid entrainment at compressor inlet
(1) The process gas delivered by the upstream system has high temperature and is not fully condensed. The long gas transmission pipeline leads to liquid condensation inside the pipeline.
(2) High temperature of the process system causes low boiling point components in the gas medium to condense into liquid.
(3) Excessively high liquid level of the separator results in gas-liquid entrainment.
41. Treatment measures for liquid entrainment at compressor inlet
(1) Contact the upstream system to adjust process operation.
(2) Appropriately increase the liquid draining frequency of the separator in this system.
(3) Lower the liquid level of the separator to prevent gas-liquid entrainment.
42. Causes of performance degradation of the combined compressor unit
(1) Severe damage of compressor interstage seals, reduced sealing performance and increased internal gas recirculation.
(2) Severe impeller wear, degraded rotor performance and insufficient kinetic energy obtained by gas medium.
(3) Blockage of the steam filter screen of the steam turbine, obstructed steam flow, low flow rate and large pressure difference, which reduces the output power of the steam turbine and unit performance.
(4) Vacuum degree lower than the required index, obstructing exhaust of the steam turbine.
(5) Steam temperature and pressure parameters lower than operating indexes, insufficient internal energy of steam failing to meet production operation requirements.
(6) Occurrence of surge condition.
43. Main performance parameters of centrifugal compressors
The main performance parameters of centrifugal compressors include flow rate, discharge pressure or compression ratio, power, efficiency, rotational speed, head, etc.
Equipment performance parameters are basic data characterizing structural features, handling capacity and operating conditions of equipment, serving as important guiding materials for equipment procurement and planning.
44. Definition of efficiency
Efficiency represents the utilization rate of energy transferred from the centrifugal compressor to gas. Higher utilization rate means higher compressor efficiency.
Gas compression includes three processes: polytropic compression, adiabatic compression and isothermal compression. Accordingly, compressor efficiency is divided into polytropic efficiency, adiabatic efficiency and isothermal efficiency.
45. Definition of compression ratio
The compression ratio refers to the ratio of compressor discharge gas pressure to suction gas pressure, also known as pressure ratio.
46. Composition of the lubricating oil system
The lubricating oil system consists of an oil supply unit, overhead oil tank, connecting pipelines, control valves and measuring instruments.
The oil supply unit comprises oil tank, oil pump, oil cooler, oil filter, pressure regulating valve, various measuring instruments, oil pipelines and valves.
47. Function of the overhead oil tank
The overhead oil tank is one of the safety protection measures for the unit. During normal unit operation, lubricating oil enters from the bottom and flows back to the tank directly from the top. In case of power failure and shutdown accident where the auxiliary oil pump cannot start oil supply timely, lubricating oil from the overhead oil tank flows through each lubrication point along the oil supply pipeline and returns to the tank, ensuring lubrication requirement during unit coast-down.
48. Safety protection measures for the combined compressor unit
(1) Overhead oil tank
(2) Safety valve
(3) Accumulator
(4) Quick-closing valve
(5) Other interlock devices
49. Sealing principle of labyrinth gland
Convert potential energy (pressure) into kinetic energy (flow velocity), then dissipate the kinetic energy in the form of vortex flow.
50. Function of thrust bearing
The thrust bearing has two functions: bearing rotor thrust and providing axial positioning for the rotor. The thrust bearing bears the residual rotor thrust unbalanced by the balance piston and thrust transmitted from the gear coupling. The magnitude of these thrusts mainly depends on the steam turbine load. In addition, the thrust bearing fixes the axial position of the rotor relative to the casing.
51. Why should the unit pressure of the combined compressor be released rapidly during shutdown?
Long-term shutdown under pressure will cause damage to seals if the primary seal gas pressure cannot exceed the compressor inlet pressure. Unfiltered process gas inside the unit will penetrate into the seals and damage them.
52. Function of seals
For stable operation of centrifugal compressors, a certain clearance must be reserved between rotor and stator to avoid friction, wear, collision and damage. Meanwhile, the clearance inevitably causes interstage and shaft end leakage. Leakage not only reduces compressor efficiency, but also leads to environmental pollution and even explosion accidents. Therefore, leakage is prohibited.
Seals are effective measures to prevent interstage and shaft end leakage while maintaining proper clearance between rotor and stator.
