How Oily Water Separation Works — Gravity, Coalescence and Why Turbulence Ruins It

1. The basic mechanism

Warning: Operating rule

The maker's manual and the approved operating procedure override these notes. This chapter explains behaviour; it does not set throughput limits.

An oil globule suspended in water is acted on by two things at once: it is buoyed upwards by the difference in density between oil and water, and it is resisted by the viscosity of the water as it tries to move. Separation happens when the upward (terminal) velocity is enough to carry the globule to the surface before the water carries it out of the vessel.

That terminal velocity depends on three things, and all three are under the operator's influence to some degree:

• the size of the oil globule — large globules rise much faster than small ones;
• the relative density difference between oil and water — a bigger difference means a bigger driving force;
• the viscosity of the oil — a thinner oil rises more readily than a thick one.

2. The design features that exploit the physics

A well-designed separator is a machine for making the flow behave. The features that do it:

• A large entrance area, so that the flow velocity is low and large slugs of oil can move to the surface quickly instead of being swept through. The low-capacity supply pump is part of this — a deliberately slow, gentle flow is the design intent, not a limitation.
• Alternation of the flow path in a vertical direction, which repeatedly brings oil near the surface. Weirs are used to reduce the liquid depth at those points, shortening the distance the globule has to travel.
• Angled surfaces on which oil accumulates and forms globules large enough to float away.
• Fine gauze screens, which act as coalescing surfaces — oil wets the gauze and builds up on it until the globule is big enough to rise.

3. Two stages, always

Every practical installation separates in two stages:

1. First stage — gravity separation. Coarse oil is floated off.
2. Second stage — coalescence filtration. The droplets that gravity could not lift are caught and grown until they can be lifted.

The second stage works because the filter elements remove small droplets of oil and hold them until they merge into larger droplets — they coalesce — and the enlarged droplets then rise to the oil collecting space and are drained off.

4. Three stages

Most installations in service are described as three-stage, and the arrangement is worth being able to sketch from memory.

The first stage is the automatic gravity separator. The second and third stages are coalescers. Effluent from the first stage enters the bottom of the second stage and passes up through the middle of the coalescer. Coalesced oil collects at the top of the coalescer, and the water discharges at the bottom and passes on to the next stage.

The reason for two coalescing stages rather than one is simply that a single pass cannot reliably take the effluent from the gravity stage all the way down to 15 p.p.m. over the whole range of inlet conditions the regulation demands.