What is a Free Water Knockout (FWKO) Vessel?

A Free Water Knockout, abbreviated as FWKO, is a pressure vessel that uses the concept of residence time to separate water from the production process. Residence time refers to the quantity of water in a system or reservoir divided by either the rate of loss of water or the rate of addition to it. It is often the first step in an oil dehydration unit. Further, the water leaving the FWKO device is processed in vessels known as treaters to separate the contaminants from it so that the water can be used for a variety of purposes.

Free Water KnockoutHorizontal 3 Phase Separator

Types of Free Water Knockout

These vessels are either vertical or horizontal in shape. Primarily, there are two types of FWKO units:

  • Two-Phase Type – This type of pressure vessel works to separate water and oil.
  • 3-Phase Separator – As the name suggests, it is mainly used to separate three substances, water, oil, and gas.

What is the 3-Phase Separator?

It refers to a vertical or horizontal separator primarily used to remove any water. Water can cause problems such as forming compounds with carbon dioxide or any other substance, creating strong emulsions which are difficult to break, or causing corrosion. Since it can separate water, oil, and gas, a free water knockout vessel is generally referred to as a three-phase separator.

How Does A Free Water Knockout Vessel Work?

The FWKO works on the concept that the oily emulsion will get separated from the water because of the difference in their densities, rate of flow, and residence time. The speed depends on several aspects such as the temperature variation, the difference in specific gravities, and the viscosity between fluids. For light oil, low residence time is as short as 3-5 minutes, whereas heavy oil may have a residence time up to 30 minutes.

Since low and high flow rates of fluids adversely impact the ability of water and crude oil separation, the basic operation of the FWKO system is designed in a way which requires steady-state production conditions. The detailed working principle is as follows:

  • The high-pressure liquid portion of the fluid is allowed to flow towards the free water knockout pressure vessel.
  • The FWKO equipment is used to remove free water from crude oil-water emulsions and crude oil‚ using the variations in the densities of these substances.
  • The device is usually located at the Central Processing Facility‚ which is why, depending on the distance, pumping may be required to boost the liquid to the inlet joint of the equipment.
  • Based on the concept of residence time, as the liquid flows into the vessel, a large portion of the water gets separated from the oil‚ and gets collected in the lower part of the device.
  • The collected water is pumped away for re-use, and the partially processed oil reaches the Treater system.

Free Water Knockout Design & Size

The FWKO is designed in a way to provide low-velocity flow when fluids with different densities pass through it. Thus, when combined with a large oil or water interface area, it allows the maximum quantity of water to settle down in the vessel. The degree of separation depends on the following three factors:

  • The density of the fluids
  • Retention time
  • The temperature of liquids

When sizing an FWKO, adhere to the following:

  • Customize it for the highest expected flow rate that is reasonable because residence time increases the efficiency of the unit as the flow rate decreases.
  • For gases, it is best to consider a three-phase free water knockout vessel.
  • If you require extra nozzles in the vessel, make sure to add them during the initial fabrication stage, before the operations begin.
  • If you need to pump liquids upstream of the vessel, you should use a low shear pump for efficiency and best results.

Contact Aspire for Custom-Designed Free Water Knockout Equipment

Aspire Energy Resources Inc. is a leading multi-disciplined company with over 25 years of experience offering design, fabrication, and installation services to the energy industry. We place importance on accuracy and safety and ensure that all our products adhere to the required industry standards. We have FWKO vessels to suit your requirements, and our services include vessel alterations and repairs, pipe fabrication, structural skids, hydro testing, electrical & instrumentation installation, insulating, and more.

For further information about our products and services or to request a discussion about your project, you may reach out to our team of experts by calling 403-314-5422 or 1-800-993-9958.

Request a Project Discussion: 403-314-5422 or

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