This is a comprehensive guide covering centrifugal compressor working principles, structural analysis, performance metrics, surge control, and maintenance tips.
What is a centrifugal compressor?
A centrifugal compressor is a type of turbine compressor (dynamic compressor) in which gas flows radially perpendicular to the compressor shaft. It features large gas-handling capacity, continuous operation, easy maintenance, oil-free gas compression, and multiple drive options.
How does a centrifugal compressor work?
The primary goal of increasing gas pressure is to raise the number of gas molecules per unit volume by reducing the distance between molecules. To achieve this, a centrifugal compressor uses gas dynamics. The mechanical working component (a high-speed rotating impeller) imparts energy to the gas.
Under centrifugal force, the gas gains pressure and kinetic energy. This kinetic energy is then converted into static pressure in the diffuser, further boosting the gas pressure. And this is the centrifugal compressor working principle.
(KOA Series Centrifugal Compressors)
What drives a centrifugal compressor?
Common prime movers include electric motors, steam turbines, and gas turbines.
What supporting systems does a centrifugal compressor need?
A centrifugal compressor relies on several support systems:
- Lubrication system for bearings and gears.
- Cooling system to manage heat from compression.
- Condensate drains for moisture removal.
- Control systems for monitoring and adjustments.
- Dry gas seals to prevent gas leaks.
What are the design types of centrifugal compressors?
There are four main structural designs:
Horizontally split (horizontal compressors have their shell split along the axis for easy access).
(Compression component of centrifugal compressor)
Vertically split (vertical compressors have their shell split perpendicular to the axis for high-pressure use).
Isothermal types (optimized for heat management).
Combined designs (custom configurations for specific needs).
What parts make up the rotor of a centrifugal compressor?
The rotor assembly includes:
The main shaft.
Impellers (mounted on the shaft).
Spacers and sleeves to hold components in place.
Balance discs to counteract axial forces.
Thrust discs to handle axial movement.
What is a stage?
A stage is the basic unit of a centrifugal compressor, comprising one impeller and its associated stationary components.
What is a section?
A section refers to the group of stages between the inlet and outlet. It may consist of one or multiple stages.
What is a casing?
A casing houses one or more sections. The number of stages in a casing ranges from one to as many as ten.
What is a train?
High-pressure multi-stage centrifugal compressors may require multiple casings arranged along the same axis to form a train. Different trains may operate at different speeds, with high-pressure trains typically running faster than low-pressure ones. Casings within the same train (same shaft) have larger impeller diameters for high-pressure stages.
What are the key performance parameters of centrifugal compressors?
Key parameters of a centrifugal compressor include flow rate, outlet pressure (or compression ratio), power, energy efficiency, and rotational speed. These parameters define the compressor’s structural characteristics, capacity, and operating conditions, guiding equipment selection and planning.
What causes performance degradation in centrifugal compressors?
Common causes include severe interstage seal damage (increased gas reflux), impeller wear (reduced rotor efficiency), clogged steam filters in steam turbines (lower output power), low vacuum levels, suboptimal steam parameters (temperature/pressure), and surge conditions.
What is surge in centrifugal compressors?
Surge is a sudden violent vibration during operation, accompanied by large fluctuations in gas flow and pressure, periodic low-frequency noises (“roaring”), and oscillations in the pipeline. Prolonged operation under surge can damage the centrifugal compressor. Operators must immediately adjust outlet pressure or increase inlet/outlet flow to exit the surge zone.
What are the characteristics of centrifugal compressor surge?
Surge manifests as drastic pressure and flow fluctuations (including reverse flow), low-frequency pipeline vibrations with loud noises, intense compressor vibration (damaging bearings, seals, and rotor-stator components), and potential mechanical failures.
How to perform anti-surge control on a centrifugal compressor?
Anti-surge strategies aim to increase flow before surge occurs. Methods include partial gas venting, partial gas recirculation, and adjusting compressor speed.
Why does a centrifugal compressor operate below the surge limit?
Possible reasons include high outlet backpressure, throttled inlet/outlet valves, or faulty anti-surge valve settings.
What are the methods for adjusting centrifugal compressor operating conditions?
Adjustments ensure stable operation of centrifugal compressors under varying process demands. The most efficient method is speed control. Other methods include outlet/inlet flow regulation, adjustable inlet guide vanes, and partial venting / recirculation.
What are constant-pressure, constant-flow, and proportional control of centrifugal compressor?
Constant-pressure control maintains discharge pressure while altering flow.
Constant-flow control maintains flow while adjusting discharge pressure.
Proportional control maintains a fixed pressure ratio (e.g., anti-surge) or volumetric flow ratio between gases.
What are the risks of axial force in centrifugal compressor?
In centrifugal compressor, axial force pushes the rotor toward the low-pressure end, causing displacement. This can damage bearings, induce rotor-stator friction, and lead to mechanical failure. Proper balancing is critical for reliability.
How to balance the axial force of a centrifugal compressor?
Common methods include:
Opposing impeller arrangement: Impellers are arranged back-to-back to offset axial forces.
Balance piston: Installed on the high-pressure side, it generates counteracting axial force via a pressure differential across a labyrinth seal.
