This is true not just for new units, but also with respect to upgrades of existing units in the field. Applying better materials, component designs, fabrication techniques and other advances to existing machines gives compressors powerful new capabilities, usually in the very same footprint and often with no need to alter supporting utilities and infrastructure.

While many different types of industrial compressors exist, the most frequently employed compressors for pipeline use are centrifugal design. Axial compressors are generally used for air service and in mixed refrigerant trains of LNG.

Centrifugal compressors

Centrifugal compressors make use of the principle of centrifugal force. When gas enters the housing, a rotating impeller forces it outward to the impeller rim, thereby increasing the gas velocity and, in general, the energy content of the gas. From there a diffuser, with or without stationary vanes, converts the increased velocity to higher pressure.

In general, gas pumping is performed by centrifugal compressors driven by gas turbines, steam turbines or electric motors. They can be capable in flow from 1,000 cubic metres per hour up to 400,000 cubic metres per hour. Axial-flow compressors

Axial-flow compressors are high-efficiency units used where there is a requirement for high flow rates but low pressure. They employ rows of airfoils to progressively accelerate and compress gas flowing through the unit. Multiple airfoil stages are placed along a rotor, matching up with corresponding stages of stationary airfoils in the compressor housing. Axial-flow compressors are used in applications such as catalytic cracking plants, air service, air separation, LNG, nitric acid and gas-to-liquid applications.

Compressor components

Compressors have a number of key components in common, regardless of their design:

• Shaft end-seals: To eliminate or minimise leakage of compressed gas or the entry of air into the compressor casing (in case of subathmospheric applications). Different types are used depending on the application.
• Bearings: Hydrodynamic tilting pad radial journal bearings are generally used, normally equipped with thermocouples to monitor bearing temperature. Thrust bearings are usually double-acting, tilting pad bearings with an equalising device, fitted with thermocouples for temperature monitoring and sometimes with load cells in high-pressure applications to measure axial thrust.
• Oil and gas seal systems: Lube oil systems provide lube oil to the radial and thrust bearings, to the gear box, and to the driver, except for some gas turbines. A seal oil system supplies filtered oil to the liquid film rings or to mechanical seals at required pressures and temperatures. Seal oil systems may be combined with the lube oil system.
• Control systems: Many manufacturers offer proprietary control solutions based on programmable logic control technology. For example, GE offers a uniquely integrated driver-compressor control system. It provides protection and monitoring to maximise the security of the compressor against potentially harmful surge conditions, while enhancing process efficiency and availability. One system controls the entire compression train, auxiliaries and related process equipment.

Modern materials, designs, fabrication techniques and testing procedures have greatly improved the performance and efficiency of compressor components. Upgrades provide an economic, easy-to-implement and timely way to accommodate different operating conditions such as changed process conditions, production increases, higher available power from the driver, or to optimise performance.