Free Water Knockout Vessel?

Asked by josvanden 5 years, 2 months ago | 3 Answers

Can someone provide design guidelines and basic requirements for the design and selection of free water knockout vessel for general use in onshore and offshore oil production? Thanks in advance.

3 Answers

Normandy 5 years, 2 months ago

Cold Treating Free Water Knockout illustration.


franchaze 5 years, 2 months ago

I thought I would add Instrumentation requirements for the Free Water Knockout below.

Probes/Alarm Float Switches

  • If an oil-water interface control probe is used, it should be mounted vertically with an on-line removal fitting, including a full port isolation ball valve. Drexelbrook No. 508–25–6 probes have given good service in this application.
  • Two horizontally-mounted probes with on-line removal fittings, including a full-port isolation ball valve, should be used as oil-water interface redundant indicators, installed for high and low level alarms. Babbitt LS–8000–115VAC Halar coated probes have given good service in this application.
  • Both high and low liquid level alarm float switches (or probes) should be provided.  Where required by government regulations or job specifications, shutdown systems also should be supplied. Protection (shutdown) is desirable to prevent oil emulsion overflows into the gas system and gas blowby into downstream liquid systems.  This may vary from field to field, depending upon the equipment arrangement. Microprocessor control is generally the least expensive and most satisfactory system.  However, in some remote locations, where electric power is not available, mechanical interface detection and control is the only solution.


  • All solenoid valves should be 3-way with a manual override switch and 120 VAC coil.
  • Relief valves and level controls should be installed as specified per requirements.
  • All instrument tubing should be stainless steel.

Panel Enclosure

  • All wiring terminations and devices should be installed in a single panel enclosure, which should also contain all probe electronics.  Panel enclosure should be rated for the area classification, but should be NEMA 4X as a minimum.  The electronics should be 4–20 mA remote output and local indication.

Low Flow Alarm

  • The sand dump outlet line should be fitted with a low flow alarm. The low flow alarm shall indicate when the sand dump system is plugging.  This device can also be used to detect leaking sand dump valves.

Temperature and Pressure Control

  • Temperature indication should be displayed by one 5 inch dial face thermometer with a stainless steel thermowell.  
  • Pressure indication should be displayed by one 4½ inch dial face pressure gauge with a ¾ inch ball valve for isolation.  A bleeder valve is recommended.  
  • An instrument air header with adequate regulation for control, minimum NPS ½, should be provided. 

taddpham 5 years, 2 months ago

General Design Requirements:

  • The pressure vessel should be fitted with lifting lugs suitable for installation of the vessel.
  • All nozzles except those for instrumentation should be fitted with isolation firesafe ball valves composed of a carbon steel body, stainless steel ball and stem and TFE seats and ring.  Reduced port valves should be furnished unless otherwise specified.
  • All manways should be at least 20 inch internal diameter.
  • All sample piping should discharge into a sample box mounted on the outside of the vessel with a minimum 1 inin bottom drain connection.
  • Manually operated full-port isolation valves should be placed on each side of all solenoid-actuated, air-operated sand jet and drain valves. 
    • Both the air-operated and manually operated valves should be firesafe ball valves composed of a carbon steel body and stainless steel ball and stem and TFE seats and ring. 
    • Actuator specifications should designate the percentage of extra torque required to handle sand/dirty service. Jamesbury solenoid-actuated ball valves (Model No. 5150–31–2236MT) with 316 stainless steel ball and stem, carbon steel body and TFE seats and ring have given good service for sand jets and drains. A minimum of 150 percent extra torque is recommended for valve actuators in sandy/dirty service.
    • Ladders and platforms for relief valve and interface probe access should be provided.
    • A minimum of two 2-inch low point drains should be provided for vessel cleanout, in addition to sand flush drains.
    • The vessel should be sized based on specified retention times or from analysis of the fluid streams expected.  Sizing should consider chemical addition, expected emulsions, quality of oil and water outlets and oil and water downstream treating facilities.

Vessel Internals:

The vessel should have the following components, as a minimum.  All internals should be removable through manways.

  • An inlet momentum diverter baffle.
  • Sample pipes designed to sample vessel liquids over the entire liquid level range, starting at 6 inch from the bottom and at 6–12 inch intervals.  Sample pipes should be NPS 3/4" minimum.
  • A single oil weir.
  • A sand dump system consisting of  2 inch nozzles, sand troughs and draw-off pipes.  The sand troughs should be on inverted vee type, secured with J-bolts.  Maximum spacing of the nozzles should be 6 feet.
  • A sand jet system consisting of 2 inch exterior nozzles spaced 6 feet apart, with the internal headers with spray nozzles on 18 inch centers.
  • A water outlet spreader pipe with vortex breakers.
  • Clips for a gas outlet gas box/mist pad.  However, the gas box/mist pad should not be installed unless required.
  • Wave/distribution baffles should be spaced at 10–15 feet intervals.
  • An interface draw-off spreader pipe (perforated). The interface draw-off system is required to remove unbreakable emulsion layers sometimes formed at the oil/water interface.
  • Design and fabrication should include all required clips and distribution baffles for the installation of matrix packs.  The matrix pack should be installed. Distribution baffles and matrix packs should be 316L stainless steel.
  • All internal bolting should be 316L stainless steel.

Piping Connections:

  • Sample connections should be provided at the downstream end of the settling zone to allow physical examination of the cross section of the emulsion.  The number of points required will depend on the size of the vessel and its physical design.  Sample lines should be ¾ inch and should use double-block ball valves that are readily accessible.
  • Sand flush and dump headers should be NPS 4, with isolation ball valves at all inlet and outlet nozzles.  Also, solenoid-actuated, diaphragm-operated ball valves should function as header isolation valves. 



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