What components constitute the lubricating oil system of a centrifugal compressor? What are the characteristics of centrifugal compressors? What is the working principle of a centrifugal compressor?
What components constitute the lubricating oil system of a centrifugal compressor?
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 an oil tank, oil pumps, oil coolers, oil filters, pressure regulating valves, various measuring instruments, oil pipelines and valves.
What is the function of the overhead oil tank?
The overhead oil tank serves as one of the safety protection devices for the unit. During normal operation of the unit, lubricating oil enters from the bottom and flows out from the top to return directly to the oil tank. In case of power failure and shutdown where the auxiliary oil pump cannot start oil supply in a timely manner, lubricating oil from the overhead oil tank flows through each lubrication point along the oil supply pipeline and returns to the oil tank, ensuring lubrication requirements of the unit during coast-down.
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, oil-free gas, and multiple available driving modes.
Working principle of centrifugal compressors
Generally speaking, the core objective of increasing gas pressure is to raise the quantity of gas molecules per unit volume, namely reducing the distance between gas molecules. To achieve this goal, gas dynamics are adopted: high-speed rotating impellers (power-working components) apply work to gas. Under centrifugal force, gas pressure rises, and kinetic energy increases significantly. Afterwards, such kinetic energy is converted into static pressure energy inside diffuser passages, further boosting gas pressure. This is the working principle of centrifugal compressors.
What are the common prime movers for centrifugal compressors?
Common prime movers for centrifugal compressors include electric motors, steam turbines, gas turbines, etc.
What auxiliary equipment does a centrifugal compressor have?
Normal operation of the main compressor relies on stable auxiliary equipment, which includes the following systems:
(1) Lubricating oil system
(2) Cooling system
(3) Condensate system
(4) Electrical instrument system (control system)
(5) Dry gas seal system
Classification of centrifugal compressors by structural features
Centrifugal compressors are classified into horizontally split type, vertically split (barrel) type, isothermal compression type, combined type and others based on structural characteristics.
Components of the rotor
The rotor consists of main shaft, impellers, shaft sleeves, shaft nuts, distance sleeves, balance disk and thrust disk.
What is surge of a centrifugal compressor?
During operation, centrifugal compressors may suffer severe sudden vibration, drastic fluctuation of gas flow and pressure, accompanied by periodic dull roaring sounds and heavy "huffing" noise caused by airflow oscillation in the pipeline network. This operating condition is defined as compressor surge.
A compressor cannot run under surge conditions for a long time. Once surge occurs, operators shall take immediate adjustment measures to reduce discharge pressure, or increase inlet/outlet flow, so that the unit quickly exits the surge zone and resumes stable operation.
Characteristics of surge
When surge occurs in a centrifugal compressor, the unit and pipeline network exhibit the following features:
(1) The discharge pressure and inlet flow of gas fluctuate violently; gas backflow may occur where gas flows from the compressor outlet back to the inlet, representing a hazardous operating condition.
(2) The pipeline network generates periodic vibration with large amplitude and low frequency, accompanied by periodic roaring noise.
(3) Intense vibration occurs on the compressor body, casing and bearings, together with loud periodic airflow noise. Severe vibration damages bearing lubrication, burns bearing shells, even fractures the shaft, triggers friction and collision between rotor and stator, and causes severe damage to sealing components.
How to implement anti-surge regulation?
Surge brings enormous hazards and cannot be eliminated completely in design. Operation must avoid entering surge conditions. The principle of anti-surge control is to increase compressor flow immediately when surge is impending to pull the unit out of the surge zone. Three common anti-surge methods are listed below:
(1) Partial gas venting method
(2) Partial gas recirculation method
(3) Compressor speed adjustment method
Causes of liquid entrainment at compressor inlet
(1) Process gas delivered from upstream systems has high temperature and incomplete condensation. Long transmission pipelines lead to liquid condensate inside the gas.
(2) High temperature of the process system causes low-boiling components in the gas medium to condense into liquid.
(3) Excessively high liquid level inside the separator leads to gas-liquid entrainment.
Causes of compressor operation below the surge limit
(1) Excessive discharge backpressure
(2) Throttling of the inlet pipeline valve
(3) Throttling of the outlet pipeline valve
(4) Defective or improperly adjusted anti-surge valve
Regulation methods for operating conditions of centrifugal compressors
Process parameters inevitably fluctuate during production, so manual or automatic regulation is often required to adapt the compressor to variable operating conditions and stabilize the production system.
Two major regulation modes are available: constant pressure regulation (adjust flow with unchanged backpressure) and constant flow regulation (adjust discharge pressure with unchanged flow). Specifically, there are five adjustment approaches:
Outlet flow regulation
Inlet flow regulation
Speed variation regulation
Inlet guide vane rotation regulation
Partial venting or recirculation regulation
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 constant pressure ratio (such as anti-surge control), or maintain constant volumetric flow ratio of two gas media.
What is a pipeline network? What are its constituent elements?
A pipeline network is the piping system for gas transportation of centrifugal compressors. Piping before the compressor inlet is the suction pipeline; piping after the compressor outlet is the discharge pipeline. The combination of suction and discharge pipelines forms a complete pipeline system generally called the pipeline network.
A pipeline network generally consists of four elements: pipelines, pipe fittings, valves and equipment.
Hazards of axial force
A high-speed rotating rotor is continuously subjected to axial force directed from the high-pressure end to the low-pressure end. Driven by axial force, the rotor generates axial displacement, leading to relative sliding between journals and bearing shells. This may scratch journals or bearing shells. Worse still, rotor displacement causes friction, collision and mechanical damage between rotor and stator components.
Since axial force may trigger friction, abrasion, collision and equipment failure, effective balancing measures shall be adopted to improve unit operational reliability.
Methods for axial force balancing
Axial force balancing is a key consideration in the design of multi-stage centrifugal compressors. Two widely adopted methods are as follows:
(1) Opposed arrangement of impellers (back-to-back arrangement of high-pressure and low-pressure sides of impellers)
The axial force generated by a single-stage impeller points to the impeller inlet (from high pressure to low pressure). Sequential arrangement of multi-stage impellers results in total axial force equal to the sum of axial force of each stage, leading to extremely large rotor axial thrust.
Opposed arrangement of multi-stage impellers generates counter-directional axial forces from impellers with opposite inlet directions, achieving mutual balance. Therefore, opposed arrangement is the most common axial force balancing method for multi-stage centrifugal compressors.
(2) Installation of balance disk
The balance disk is a widely used axial force balancing device for multi-stage centrifugal compressors, usually installed at the high-pressure end. Labyrinth seals are arranged between its outer rim and the cylinder to maintain a certain pressure difference between the high-pressure side and low-pressure side connected to the compressor inlet. The axial force generated by such pressure difference acts opposite to the axial force produced by impellers, balancing the thrust from impellers.
Purpose of rotor axial force balancing
The main objective of rotor balancing is to reduce axial thrust and relieve load on the thrust bearing. Under normal circumstances, approximately 70% of axial force is eliminated by the balance disk, and the remaining 30% is borne by the thrust bearing. Production practices verify that retaining appropriate residual axial force effectively stabilizes rotor operation.
Causes of rising thrust bearing temperature
(1) Improper structural design: insufficient bearing load area leading to excessive load per unit area.
(2) Failed interstage seal causes gas leakage from the outlet of the rear-stage impeller to the preceding stage, increasing pressure difference on both sides of the impeller and generating large thrust.
(3) Blocked balance pipe prevents pressure relief in the secondary pressure chamber of the balance disk, disabling the balance disk.
(4) Failed balance disk seal cannot maintain normal pressure in the working chamber, reducing balancing capacity and transferring extra load to the thrust bearing, resulting in overload operation.
(5) Small throttling bore for bearing oil supply leads to insufficient cooling oil flow, failing to fully remove friction heat.
(6) Water or impurities mixed in lubricating oil prevent formation of a complete liquid lubricating film on the thrust bearing.
(7) Excessively high temperature of bearing supply oil deteriorates operating conditions of the thrust bearing.
Solutions for excessively high thrust bearing temperature
(1) Verify bearing pressure load, appropriately expand the thrust bearing load area to keep load within standard range.
(2) Disassemble and inspect interstage seals, replace damaged sealing components.
(3) Inspect balance pipes and remove blockages to release pressure in the secondary pressure chamber of the balance disk timely and ensure normal balancing performance.
(4) Replace balance disk sealing strips, improve sealing performance, maintain pressure in the balance disk working chamber and realize reasonable axial thrust balance.
(5) Expand the throttling bore of bearing oil supply to increase lubricating oil flow and timely remove friction heat.
(6) Replace with qualified new lubricating oil to guarantee lubricating performance.
(7) Fully open the inlet and return water valves of the oil cooler, increase cooling water flow and reduce oil supply temperature.
Treatment for liquid entrainment at compressor inlet
(1) Contact upstream systems to adjust process operation.
(2) Increase the frequency of liquid draining from separators within this system.
(3) Lower the liquid level of separators to avoid gas-liquid entrainment.
Causes of performance degradation of turbine-driven compressor units
(1) Severe damage to compressor interstage seals reduces sealing performance and increases internal gas recirculation.
(2) Severe impeller abrasion impairs rotor function; gas cannot acquire sufficient kinetic energy.
(3) Blocked steam filter screen of the turbine obstructs steam flow, resulting in low flow and large pressure drop, reducing turbine output power and unit performance.
(4) Vacuum degree below specification hinders turbine exhaust.
(5) Steam temperature and pressure parameters below operating standards reduce steam internal energy and fail to meet production requirements.
(6) Occurrence of surge conditions.
Main performance parameters of centrifugal compressors
Main performance parameters include flow rate, discharge pressure or compression ratio, power, efficiency and rotating speed. These basic data reflect structural features, handling capacity and operating environment of equipment, serving as important guidance for equipment procurement and planning.
Definition of efficiency
Efficiency represents the utilization rate of energy transferred from centrifugal compressors to gas. Higher utilization rate means higher compressor efficiency. Three compression processes exist for gas compression: polytropic compression, adiabatic compression and isothermal compression. Accordingly, compressor efficiency is divided into polytropic efficiency, adiabatic efficiency and isothermal efficiency.
Definition of compression ratio
Compression ratio refers to the ratio of compressor discharge gas pressure to inlet gas pressure, also known as pressure ratio.
Function of seals
For stable operation of centrifugal compressors, certain clearances must be reserved between rotors and stators to avoid friction, abrasion, collision and damage. However, such clearances inevitably cause interstage and shaft-end leakage. Leakage not only lowers compressor efficiency, but also triggers environmental pollution and even explosion accidents and thus must be avoided.
Seals are effective devices to prevent interstage and shaft-end leakage while maintaining appropriate clearance between rotors and stators.
Classification of sealing devices by structural features and selection principles
Different seal structures are adopted according to operating temperature, pressure and hazards of process gas media.