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 IT HAPPENS in petrochemical plants, refineries, and anywhere else that the gas approaching a compressor is wet. Traces of aqueous or organic liquid escape the inlet knockout drum—often intermittently—and silently damage the compressor. Telltale signs include pitting corrosion, salt deposits, and diluted lubricants.
Instead of trying to repair symptoms, look for the root cause, which usually involves the mist eliminator in the knockout drum (Figures 1 and 2). Problems may include improper mist eliminator specifications, overloading, uneven velocity profiles, incorrect installation, high liquid viscosity, waxy deposits, liquid slugs, foaming, and several other possibilities.
The trouble may even be that no mist eliminator was provided in the first place—or perhaps no knockout drum at all. But wherever free liquid drops out in a suction drum, it generates some mist that can damage the compressor unless it is removed by a mist eliminator. Even in cases where the feed gas never has any free liquid, there are often fine mist droplets that coalesce into large drops on the walls of the inlet pipe or inside the compressor. For all but the driest gas, a compressor should be protected by an inlet mist eliminator. New high-capacity, high-efficiency mist eliminator technologies pay off the first time you avoid a shutdown.
For optimum separation performance, compressor knockout drums must be properly designed and sized with appropriate mist eliminator elements in correct configurations, taking into account many factors. In multistage compressor installations, the proper knockout drum design is seldom the same for all stages. To maintain good performance, the design of each drum should be reviewed whenever there are significant changes in the process, such as increases or decreases in throughput, shifts in composition of the gas or mist droplets, alterations of upstream equipment, or revisions of operating and control procedures. In addition, mist eliminator elements should be visually inspected occasionally (especially after major process upsets) to make sure they are intact and free of excessive solid deposits.
A thorough understanding of the relevant considerations will help you avoid common suction-drum pitfalls—and some not-so-common ones—that could severely damage your compressors due to liquid carryover. For detailed explanations of mist eliminator selection, sizing, and vessel design for a wide range of applications, see Amistco’s literature such as “Amistco Mesh & Vane Mist Eliminators,” Bulletin 106. This paper provides information that applies specifically to compressor inlet knockout drums.
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