How should a chemical plant’s compressed air system be designed? Key considerations include air quality, piping, and safety controls.

Introduction

In chemical plant operations, compressed air is regarded as the second-largest power source, after electricity. The reliability and stability of the compressed‑air system directly affect the continuity of process operations and product quality. Consequently, the scientific and rational design of the compressed‑air system constitutes a critical component of the plant’s basic engineering design.

I. Key Design Considerations for Gas Quality

The quality requirements for compressed air vary significantly across different processes in a chemical plant; therefore, during design, it is essential to implement tiered treatment tailored to the specific points of air consumption.

• Dew Point ControlFor instrument air and process gases, the pressure dew point must be strictly controlled. Typically, the pressure dew point is required to be lower than the local historical extreme minimum temperature to prevent condensation within the piping, which could lead to instrument malfunction or process contamination.
• Oil Content and Particulate Matter ControlInstrument air and process air in contact with the product must be supplied by oil-free compressors or equipped with high-efficiency oil‑removal filtration units. Filtration accuracy must meet process requirements; typically, the oil content of instrument air must be maintained at an extremely low level, and particulate size must be strictly controlled.
• Principle of Tiered Processing: Avoid a one-size-fits-all approach to high‑standard treatment. Divide the plant’s compressed air requirements into instrument air, process air, and general plant air, and equip each stream with dedicated drying and filtration systems to reduce initial capital costs and operational energy consumption.

II. Key Considerations in Piping System Design

Pipeline network design not only affects transportation efficiency but also impacts the system’s overall pressure drop and service life.

• Pipe Material Selection: Instrument air and process air piping typically uses stainless steel or galvanized steel to prevent internal rusting and secondary contamination. For general plant air, carbon steel seamless pipes are generally suitable. All pipe inner surfaces must be smooth to minimize frictional resistance.
• Slope and Drainage DesignCompressed air pipelines must be installed with a slope of at least 0.3% in the direction of flow. Condensate drainage devices shall be provided at low points, pipe ends, and branch connections to periodically remove condensate, thereby preventing water hammer and pipeline corrosion.
• Flow Rate and Pipe Diameter Control: Properly controlling the flow velocity within the piping is key to minimizing pressure drop. The flow velocity in the main pipeline is typically kept at a relatively low level, while that in branch lines may be appropriately increased. Pipe diameter design should comprehensively account for maximum air consumption, the simultaneous‑use factor, and the allowable pressure drop, ensuring that the pressure at the terminal points meets process requirements.
• Ring network layoutFor large chemical plants, the main pipeline should be designed as a looped network. This not only balances network pressure and reduces local pressure drops but also ensures uninterrupted gas supply to other areas during localized maintenance.

III. Safety and Control Key Points

Compressed air systems fall under the category of pressure vessels and pressure piping, making safety design a top priority.

• Pressure Control and Interlocks: A pressure regulator and overpressure interlock protection shall be installed at the air compressor outlet. When the air receiver pressure reaches the set upper limit, the air compressor shall automatically unload or shut down; when the pressure drops to the lower limit, it shall automatically load or start up, ensuring that system pressure fluctuations remain within the allowable range.
• Safety relief device: Safety valves must be installed on gas storage tanks, dryers, and critical pipeline sections. The discharge capacity of the safety valve shall exceed the compressor’s maximum gas production rate, and the discharge outlet must be routed to a safe area to prevent injury from high-pressure gas or the occurrence of secondary accidents.
• Surge Prevention and Backflow PreventionWhen multiple air compressors operate in parallel, each unit must be equipped with a check valve at its discharge to prevent backflow and potential damage. Additionally, centrifugal compressors should be fitted with a surge‑prevention control system to ensure safe operating conditions.
• Online Monitoring and Alarm: Install online monitoring instruments for pressure, temperature, dew point, and oil content at critical process points. Integrate the data into the plant’s distributed control system, set alarm thresholds, and enable early warning and automated intervention in abnormal operating conditions.

Conclusion

The design of a compressed air system in a chemical plant is a complex, systems‑based undertaking that requires comprehensive consideration of process requirements, fluid dynamics, and safety standards. By precisely controlling air quality, optimizing pipeline routing, and implementing stringent safety measures, the system’s operational efficiency and reliability can be significantly enhanced, thereby providing a robust foundation for the plant’s long‑term, safe production.