Free Water Knockout

on . Posted in Pressure Vessel

Free water knockout, abbreviated as FWKO, is a pressure vessel that uses residence time to separate water from the production process.  It is usually the first part of an oil dehydration facility.  Removing unwanted water in the process stream can reduce processing steps by reducing the size of downstream equipment and lowers energy requirements.  Water leaving the FWKO is further processed to remove contaminants and then may be used for injection, boiler feed water or for sale.  Oil and emulsion is processed further until the oil is ready for sale.

Free Water Knockout Index

Free Water Knock Out SchematicFree Water Knock Out Schematic

Because the FWKO is usually the first piece of processing equipment after the oil well, it can collect solids that come up with the oil and water.  For very sandy wells, the FWKO can be automatically cleaning by pushing water through a series of nozzles at the bottom of the vessel to move sand that may have collected.  Some free water knock outs operate as a three phase (oil, water, gas) or as a fluid packed two phase (oil, water) vessel.

A FWKO works best when the process is as stable as possible.  Large swings oil or water may cause the process to upset.  To assist in separation, the incoming production stream might be preheated prior to the FWKO.  This is usually with a shell and tube heat exchanger utilizing heat that would otherwise have been wasted.

The free water knock works based on speed that the oily emulsion will separate from the water.  This speed depends on many things like the difference in specific gravities, the temperature and the viscosity between the fluids.  A low residence time is as short three to five minutes for light oil.  Heavy oil can have residence times as long as 30 minutes.  If it is a tight emulsion, chemicals may be introduced to the FWKO to help break the emulsion. 


Free Water Knockout Advantages and Disadvantages

  • FWKO primary function is to separate and remove free water from crude oil.  This is crucial for ensuring the quality of the extracted oil for further processing.
  • Removing water from crude oil helps reduce the risk of corrosion and scaling in pipelines and processing equipment, enhancing the overall quality of the oil.
  • Oil with lower water content is more likely to meet industry and environmental standards, making it easier to transport and refine.
  • Separating water at the source helps protect downstream equipment, such as pumps and compressors, from potential damage caused by the presence of water.
  • Removing water from crude oil reduces its volume, making transportation more efficient and cost effective.
  • Installing a free water knockout system involves a significant capital investment.  Smaller operators or projects with limited budgets may find it challenging to implement such systems.
  • FWKO systems require regular maintenance and inspections to ensure proper functioning.  This can add operational costs and downtime.
  • FWKO units can be large, requiring significant space.  In locations with space constraints, finding suitable areas for installation may be a challenge.
  • Operating a free water knockout may require energy, contributing to operational costs.  This is an ongoing expense that operators need to consider.
  • FWKO may not be as effective in separating emulsified oil and water mixtures.  Additional treatment processes may be required for such cases.
  • Proper disposal of the separated water is crucial to avoid environmental impacts.  Regulatory compliance for water disposal can add complexity.


Nozzle types Used

  • Production Inlet  -  This will be the largest nozzle because it contains the entire process.
  • Oil & Emulsion Outlet
  • Water Outlet
  • Gas Outlet  -  If operating as a three phase vessel or if the vessel is on a vapor recovery system.
  • Sand Cleanout  -  Series of nozzles at the bottom of the FWKO that direct water into predetermined locations of the vessel to clean the sand from the bottom of the vessel.
  • Pressure relief valve  -  Should be sized based on the design code the vessel is designed to
  • Manway  -  If there are multiple baffles, ensure there is access for an inspector.
  • Nozzles for control devices  -  High level, low level, interface, pressure and temperature.
  • Sample Box  -  A container or kit used in the oil and gas industry to collect, store, and transport samples of oil, gas, water, or other substances.

Free Water Knockout Typical Internals

  • Inlet Deflector or Baffle  -  Reduces disturbance from the inlet fluids and keeps them from creating a channel through the FWKO.
  • Calming Baffle/ Wave Beaker  -  If you’re using that experiences surges, a baffle will help dampen inlet surges.
  • Weir Box  -  A weir box may be used to separate ensure that the oil/ emulsion outlet does not get contaminated with water.
  • Vortex Breaker  -  Discharge nozzle to prevent the discharge from drawing from the gas blanket.
  • Jets  -  Jets to direct flow rate to help clean out sand.

WavebreakerWave breaker or calming baffleWeirOil wier in a vesselVortex Breaker 1Vortex breaker in a tank









Free Water KnockouT Design Considerations

  • When sizing a FWKO, size it for the highest reasonable expected flow rate.  As the rates decrease, residence time increases the efficiency of the unit.
  • For gassy services, consider using the FWKO as a three phase vessel.  Entrained gas carried through the water or oil outlet will cause flow meters to show an erroneous flow rate.  If the production stream is gassy, running the separator at higher pressures will keep the gas from breaking the gas out of the fluid.
  • As with all pressure vessels, it is easier (and cheaper) to add extra nozzles during the initial fabrication than it is after the vessel has been in operation.
  • If pumping is needed upstream of the FWKO, a low shear pump, such as a progressing cavity pump should be used.  A centrifugal pump (high shear) will cause the emulsion to tighten which will increase the amount of time needed to separate the oil and water.

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