Free Water Knockout Drum

Overview

A free water knockout drum, abbreviated FWKO, is a large horizontal vessel that removes the bulk of the water from a crude oil stream early in the surface facility. The term free water refers to water that separates from oil by gravity alone, as opposed to emulsified water that is chemically bound into droplets and needs heat or an electric field to break. By dropping out the free water first, the FWKO sends a much smaller liquid volume to the downstream heater treaters and dehydration equipment, which lets those units be smaller and use less fuel.

The vessel is placed after the first stage Three-Phase Production Separator or directly after the inlet manifold on high-water-cut fields. It often handles fluid that is 70 to 90 percent water at the inlet and reduces that to roughly 5 to 15 percent at the oil outlet. Because separating water from oil by gravity is slow, the FWKO is sized for a long liquid retention time, commonly 10 to 30 minutes, which makes it one of the larger vessels in the facility.

Why free water is removed first

Removing free water early has a strong economic payoff. Water has a high heat capacity, so any water that reaches the heater treater consumes fuel that does no useful work on the crude. Water also takes up volume in every vessel and pipe it passes through, so carrying it deeper into the process wastes capacity. The Pressure Vessel does the cheapest part of the job, gravity settling of the easy water, and leaves only the difficult emulsified water for the energy-intensive treating stages that follow.

How separation works

Fluid enters through the Inlet Section, where a diverter breaks the inlet momentum and a perforated distributor spreads the flow evenly across the vessel cross section. Even distribution matters, because channeling or jetting would short-circuit the long settling path and let water carry over with the oil. Once calmed, the liquid moves slowly down the length of the vessel in near plug flow, encouraged by the Separation Internals.

In the settling zone, water droplets fall under gravity toward the bottom while oil rises. A parallel plate coalescer pack gives the droplets short distances to settle and large surfaces on which to merge into bigger, faster-settling droplets. Perforated settling baffles keep the flow even and suppress turbulence. The de-watered oil flows over the Oil Outlet Section weir into the oil bucket and out to treating, while the separated free water collects at the bottom and leaves through the Water Outlet Section. A sand jetting header at the bottom flushes any settled solids, since high-water wells often carry formation sand.

Interface and level control

The key measurement in an FWKO is the oil-water interface, the level where the bottom water meets the oil above it. The Level Control system uses an interface transmitter, typically a capacitance probe or a displacer, to find that boundary and modulate the water dump valve so the interface stays at setpoint. If the interface rises too high, water carries over into the oil; if it falls too low, oil is lost into the water outlet. A second loop controls the oil level above the weir through the oil dump valve. Some designs set the interface with an external water leg, a vertical pipe whose height balances the pressure of the oil and water columns, giving a simple and reliable interface control without instrumentation. Reflex sight glasses and high and low level switches back up the automatic controls.

Pressure control and safety

The FWKO usually runs under a light gas blanket to keep oxygen out and to provide a stable operating pressure. The Pressure and Relief system holds that pressure with a gas back-pressure valve and protects the vessel with an ASME relief valve and a backup rupture disc. Pressure transmitters report vessel conditions to the facility safety system, and the level switches trip the inlet shutdown if a control loop fails. Because the operating pressure is modest, the wall thickness is driven more by the large diameter than by pressure.

Materials and maintenance

The shell and heads are normally SA-516-70 carbon steel. The lower water-wetted section sees the most corrosion because produced water is saline and may contain dissolved gases, so that section carries an internal corrosion coating and a generous corrosion allowance. Two manways, one at each end, give access to clean the coalescer pack, clear sand from the bottom and inspect the internals. Sand accumulation and fouling of the coalescer plates are the most common maintenance issues, and regular jetting keeps the settling performance from degrading.

Variants and use

A heated FWKO adds a fire tube or heat exchanger to warm the emulsion, which lowers oil viscosity and speeds water settling on heavier crudes. Vertical FWKO drums are used where plot space is limited, though horizontal vessels separate close-density fluids better because of their larger liquid surface area. The FWKO is most valuable on mature and high-water-cut fields, where it can remove the majority of the produced water cheaply and protect the downstream Crude Oil Desalter and treating train from being overwhelmed.