Why is liquid carrying over past my demister pad?

The symptom

Liquid droplets reach equipment downstream of the demister. Possible effects include fouled compressor blades, off-spec overhead product and pipe corrosion. Carry-over can occur even when the pad appears intact at inspection.

Root causes

1. 01 Gas velocity outside the design range can reduce separation. High velocity re-entrains captured liquid. At low velocity, fine droplets can follow the gas around the wires.
2. 02 Gaps between pad segments or at the vessel wall allow gas to bypass the pad. Even a small bypass can reduce separation.
3. 03 Pad density or thickness may not suit the droplet distribution. A pad for coarse mist can pass fine droplets after an upstream process change.
4. 04 Polymer, salts or coke can block the wire structure. Local gas velocity rises and liquid may be re-entrained.

Suitable filter media

• ▸ Review the pad against current operating data. Density, thickness and free area must suit gas rate and droplet size.
• ▸ Limit bypass with suitable support grids, segment overlap and wall sealing.
• ▸ For fine mist, assess a higher pad density or a co-knit layer of wire and yarn matched to the fine-droplet fraction.
• ▸ For fouling service, consider a more open, thicker pad and provide cleaning access in the design.

Prevention

• ⊕ Record gas rate at every process change and check that it remains within the design range.
• ⊕ Inspect segment joints and wall clearances whenever the vessel is entered.
• ⊕ Specify the replacement as an assembly of pad, grid and sealing so the components fit the installation together.