Vertical Knockout Drum – Mist Elimination Internals

The reliability and service life of all compressors is directly related to the inlet gas cleanliness. To ensure optimum knockout drum performance, mist elimination devices are necessary to increase impingement surface area in the gas flow path. Reciprocating compressors are most sensitive to gas quality. It is highly desirable to reduce the amount of aerosol contaminants to the lowest practical concentration.

Liquid and solid aerosols entrained in the inlet gas are the prime contributors to premature valve failure, cylinder wear and corrosion damage. Although centrifugal compressors are more forgiving to entrained aerosols, they are still susceptible to efficiency losses due to particle induced fouling, erosion, and corrosion.

There are several methods commonly used in compressor suction service, each one capable of achieving a certain degree of removal efficiency. They are:

1. Vane mist eliminators
2. Wire mesh pads (demister pads)
3. High efficiency coalescer filters
4. Brownian diffusion mist eliminators

Figure P-2 illustrates the approximate capture efficiency for these impingement devices for a given range of particle sizes.

Although most installations use only one mist elimination device, a combination of two or more of the above devices will provide additional protection for almost all services. Selecting the combination depends primarily on the nature and volume of the liquids to be removed. Generally, as the viscosity of the liquid increases, the larger the entrained droplet size. Similarly, as liquid surface tension decreases, so does the droplet size.

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