Demisters for Knock-Out Drums and Scrubbers

In oil and gas and refining utilities, demisters protect compressors, reduce solvent losses and limit liquid carryover from drums and scrubbers.

Define the separator’s job first

“Knock-out drum” describes a vessel function, not one inlet condition. A flare drum receiving intermittent slugs, a steady compressor suction scrubber and a contactor overhead can have different droplet distributions, liquid loads and consequences of carryover. Before selecting media, state whether the demister is expected to remove routine entrainment, protect against an upstream upset, reduce solvent loss or meet a downstream liquid limit.

The complete separation train matters. Inlet devices reduce momentum and separate bulk liquid; the vessel provides disengagement and surge volume; the mist eliminator handles remaining droplets. A pad should not be used as a substitute for inadequate inlet distribution, level control or slug volume.

Typical services

• Knock-out drums and suction scrubbers
• Glycol contactors / still overheads
• Amine and other treating towers (case-by-case chemistry)
• Flare knock-outs (check fouling and fire considerations with the site)

Design emphasis

Velocity and liquid surge matter more than catalog density. Compressor protection duties often prioritize reliable drainage and mechanical integrity over chasing the highest claimed efficiency number.

Check gas and liquid properties at the actual pressure and temperature. Standard volumetric flow must be converted to the operating condition before calculating superficial velocity. Review minimum, normal, maximum continuous and credible upset rates separately; the controlling case may be reduced droplet capture at low gas rate, flooding or re-entrainment at high gas rate, or high liquid loading at any relevant gas rate.

The Souders-Brown method is useful for preliminary velocity screening when a project K value is documented for the separator construction and service. It does not model an inlet slug, prove droplet efficiency or account for a damaged inlet device. Use it together with vessel geometry, inlet momentum, liquid level, disengagement distance and outlet arrangement.

Mesh, vane or no simple replacement

• Knitted mesh is a reasonable first candidate for clean service, relatively fine droplets and moderate liquid load.
• Vanes can provide more open drainage paths where liquid load, fouling or intermittent slugs are important.
• Combined devices may be considered when fine-droplet agglomeration and high liquid handling are both required, but height and pressure drop must be checked.
• Process correction comes first when carryover follows foaming, failed level control, a damaged inlet device or throughput beyond the vessel basis.

Repeated carryover is not evidence that the pad must be denser. A denser pad can hold more liquid, raise pressure drop and flood earlier.

Materials

SS316L is a common starting candidate for many hydrocarbon/water services, but it is not an automatic default. Chlorides, acid gases and rich amine environments can require a different material. Always state the actual fluid, not only “natural gas.”

For H2S-containing production or gas-treatment service, identify the project sour-service requirements and environmental limits. A generic “NACE material” description is incomplete: the applicable specification, material form, hardness or condition controls, and required evidence need to be stated by the project. This cracking review does not replace the separate check for general, localized or galvanic corrosion.

Replacement reality

Many drums were built decades ago. Manway-segmented pads with clear section maps are usually more valuable than a theoretical one-piece design. Send vessel ID, manway size and any compressor trip history linked to carryover.

Before removal, record support-ring dimensions, pad elevation, section layout, wall gaps, hold-downs and the upstream face. Photograph deposit and deformation patterns before cleaning. If the old pad performed acceptably and duty is unchanged, build-to-drawing replacement may be suitable. If it flooded, plugged, corroded or allowed recurring carryover, the route should return to engineering review.

Buyer and engineer checklist

Freeze gas cases at operating conditions, liquid properties and loading, vessel geometry, inlet and outlet arrangement, minimum distance to liquid level, media type, material set, section map, support interface and packing. State whether the purchase is build-to-drawing or performance-guaranteed. For a performance route, add inlet droplet and loading basis, carryover criterion, allowable pressure drop and acceptance method.

The quoted QA scope should list agreed measurement points and limits, material or batch traceability, section marking, product and packing photos, and the packing list. Extra certification, testing or third-party inspection is optional and should be requested before procurement and production.

Use the compressor suction scrubber guide for critical rotating-equipment protection and the RFQ guide to separate process data from manufacturing evidence.

Engineering source and limits

Sulzer’s Gas/Liquid Separation Technology treats inlet devices, vessel separation and mist eliminators as one system and distinguishes mesh and vane duties by droplet and liquid-load behavior. It is a useful design framework; named proprietary performance is not a default for an unverified replacement.