Should I use densities at standard or operating conditions?

Operating conditions, always. Gas density changes dramatically with pressure, and it sits in the denominator of the equation — using a standard-condition figure can be wrong by an order of magnitude.

What pressure drop should I expect?

A clean 150 mm pad at design velocity typically runs around 2 mbar, rising as it fouls. Treat any calculated figure, including ours, as indicative — pressure drop depends on construction detail that only a vendor curve captures properly.

How do I know if an existing pad is undersized?

Carryover is the symptom. Calculate the actual velocity from current throughput and vessel area, compare it with V_max for your conditions, and if you are above about 85% the pad is the likely cause.

Sizing a demister pad with Souders–Brown

A mist eliminator is a simple object with an unforgiving specification. Size it above its capacity and it floods; below, and it is money spent on pressure drop.

7 min read · updated 30 October 2026

A knitted wire mesh demister pad removes entrained liquid droplets from a gas stream. Droplets impinge on the wire, coalesce, and drain back against the flow. It works until the gas is moving fast enough to tear the coalesced liquid off the wire again — at which point the pad re-entrains and carries over.

The equation

Souders–Brown

V_max = K × √((ρ_liquid − ρ_gas) / ρ_gas)

V_max in m/s; densities in kg/m³ at operating conditions, not standard conditions.

K is an empirical capacity factor. It is not a universal constant — it varies with orientation, pressure, and how dirty the service is. Published values cluster around 0.107 m/s for the standard case.

K factors

Condition	K (m/s)	Why
Horizontal pad, vertical flow	0.107	The standard reference case
Vertical pad, horizontal flow	0.082	Less effective capture area; drainage works against flow
Vacuum, below ~0.3 bara	0.061	Low gas density makes re-entrainment easier
High pressure, above ~20 bar	0.082	Denser gas carries more momentum
Fouling service	0.076	Allows for partial blinding in operation
Marine / floating installation	0.090	Vessel motion disturbs drainage

These are design values. Where two conditions apply, the reductions compound.

Choosing the pad itself

Once velocity is settled, three parameters remain: thickness, density and wire diameter. They trade efficiency against pressure drop and fouling resistance.

Target droplet	Wire Ø	Density	Thickness
≥ 3 µm	0.15 mm	192 kg/m³	200 mm
≥ 5 µm	0.23 mm	144 kg/m³	150 mm
≥ 10 µm	0.28 mm	112 kg/m³	150 mm
Coarse / fouling	0.28 mm	80 kg/m³	100–150 mm

Finer wire and higher density catch smaller droplets, at the cost of pressure drop and a greater tendency to blind.

Material

SS 304 — clean hydrocarbon and steam service at moderate temperature.
SS 316L — anywhere chlorides or amines are present, which covers most refinery and coastal duty.
Monel 400 — seawater and hydrofluoric acid, where 316L pits.
Copper — specific chemical duties and some cryogenic service.
Polypropylene — low temperature, aggressive aqueous streams.

A worked example

Hydrocarbon separator, Ø1800 mm vessel, clean service

Liquid density: 700 kg/m³
Gas density: 20 kg/m³ at operating pressure
K factor: 0.107 (horizontal pad, vertical flow, clean)
V_max: 0.107 × √(680/20) = 0.62 m/s
Design velocity at 80%: 0.50 m/s
Pad area: π × 1.8² / 4 = 2.54 m²
Selection: 150 mm, 144 kg/m³, 0.23 mm SS 304
Segments through a 500 mm manhole: 4

Segmentation

A pad has to pass through the manhole. Above roughly 1200 mm diameter it ships in overlapping segments, and the count follows from the manhole size rather than the vessel. Give us the manhole diameter with the enquiry and the segmentation is designed in rather than improvised on site.

When a pad is the wrong answer

Heavily fouling or waxing service, where any mesh will blind — use a vane pack.
Very high velocity duty that cannot be resolved by vessel diameter — use a cyclonic or vane separator.
Sub-micron aerosols, which mesh will not capture — use a coalescing filter element.

You do not have to do this by hand — we built the calculator that runs these numbers.

Run the sizing

Common questions

Should I use densities at standard or operating conditions?: Operating conditions, always. Gas density changes dramatically with pressure, and it sits in the denominator of the equation — using a standard-condition figure can be wrong by an order of magnitude.
What pressure drop should I expect?: A clean 150 mm pad at design velocity typically runs around 2 mbar, rising as it fouls. Treat any calculated figure, including ours, as indicative — pressure drop depends on construction detail that only a vendor curve captures properly.
How do I know if an existing pad is undersized?: Carryover is the symptom. Calculate the actual velocity from current throughput and vessel area, compare it with V_max for your conditions, and if you are above about 85% the pad is the likely cause.