When a downstream compressor, flare tip, or sample system suddenly sees liquid, the first ticket is often “mystery water.” The knockout drum that was supposed to catch it still shows a comfortable level. Operators shrug: the vessel is there, so the problem must be elsewhere.
It usually is not. This note is demister (mist-eliminator) integrity and liquid carryover from knockout / scrubber drums. Y-strainer DP, steam-trap condensate, and analyzer sample-system honesty are neighbors. They do not own the pad that sits in the vapor outlet.
A demister works by forcing fine droplets to coalesce on wire mesh, vane packs, or fiber beds so gravity can drain them. When the pad plugs with salts, polymers, or iron, open channels form and vapor velocity punches liquid through. When the pad was never sized for today’s throughput, the same thing happens on a clean mesh. Level transmitters that only see the boot do not confess either failure.
Carryover signatures worth trusting
Liquid in a compressor suction scrubber that “should have been dry,” sudden analyzer fouling after a rate increase, or flare-tip smoking that tracks a drum after a feed change all point upstream. So does a DP across the demister section that has climbed for months while nobody logged a pad change. Sites that only trend drum level invent calm.
Inspect and replace demisters on a chemistry- and velocity-aware interval, not on “vessel still looks empty.” Verify drains and siphon breaks so coalesced liquid can leave. After a rate step-up, walk the vapor path for the first wet evidence—do not wait for the compressor trip. A knockout drum without an honest demister is just a large pipe fitting with a level gauge.