Compressors’ Suction Knockout Drum
S Raghava Chari, Consultant
Compressors, depending on the type are used by almost all industries. Inadequate suction Knockout Drum (KOD) is a common cause of compressor failure. Ways to prevent compressor wrecks, associated production loss, failed deliveries, etc are discussed in this article.
Compressors - piston type, centrifugal and screw - serve almost all industries. Inadequate suction Knockout Drum (KOD) is a common cause of compressor failure. This article discusses ways to prevent compressor wrecks, associated production loss, failed deliveries and customer wrath. Introduction
Practically, any gas compressor in petroleum refineries, chemical process plants, power plants and for that matter, any plant handled gases, is wet with aqueous or organic liquids. Even traces of these escaping the KOD and silently entering can damage the compressor. From the pitting corrosion, salt deposits to diluted lubricants, the liquid ingress telltale signs point out the cause, viz. failed KOD. Only addressing the symptoms does not solve the problems; they persist. Hence, finding the root cause(s) and removing it (them) is the only way to get rid of debilitating compressors failures. Often, problems stem from KODs, especially their Mist Eliminator ME (fig 1) failing to trap the gas entrained liquids and letting these into the compressors – see case study 1 at the end. Usual problems are: improperly specified ME, KOD overloading, uneven velocity profiles, incorrect installations, high liquid viscosity, waxy deposits, liquid slugs, foam- ing, and several others. Often, KODs come with no mist eliminator. Even in feed gases of no free liquid particles, fine mist droplets often exist. These coalesce into large drops over the pipe wall inside and eventually reach the compressor inside. Hence, protecting the compressors with inlet ME is necessary in case of highly dry gases. Fortunately, present day economical high-capacity and high-efficiency ME technologies pay off, as even averted one shutdown is worth numerous times the KOD investment. Obviously, as with any process equipment, compressor KODs too, must be properly designed and sized for adequate flow margins. In addition, its mist eliminator (ME) elements shall be selected for optimum moisture separation and resulting long compressors on stream hours. Remember, individually engineering multi-stage compressors’ each stage in KOD is essential as each stage inlet conditions differ considerably. In addition, in case of operative conditions, e.g. throughputs increase or decrease, gas
composition and moisture content changes and operations and control procedure revisions, KOD adequacy needs checking and upgrading as needed. Periodically checking the mist eliminator elements, especially after any major process upsets and making sure they are intact and deposit free is necessary for their continued good performance.