Should we still Adhere to API Designs for OWS System?

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Should we still Adhere to API Designs for OWS System?

Why are we still adhering to API, when it comes down to oil-water separator?

API design is an age-old technology, and in my opinion, its output or efficiency of the separator designed with API is not in line with present stringent environment norms. 

"The API method also makes the assumption of an inlet droplet size of 150 micrometers, which is inaccurate for environmental oil water separators because a spectrum of droplet sizes will be present, most of which are smaller than 150 μm"

Also in its own document, it sumps up in section 2.3 - Performance. The API 421 document (section 2.3) sums this up: “Based on the 1985 API refinery survey of 32 conventional separators, such separators should not be expected to achieve effluent oil levels lower than 100 parts per million.”

The snapshot is attached from API 421 docs.

I think it is high time, design engineers and EPC consultants should stop copy-pasting of old specifications for OWS in any project and if they still want to, then they should add one line, as other efficient technology which can provide required ppm level in discharge water are welcome and can also make their offer or quote for the same project. 

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5 Answers

  1. As refineries switch to heavier crude slates, API separator efficiency declined,as now happens,in many cases.

    No doubt secondary treatment technologies like DAF,Anaerobic Aerobic biological treatment,parallel plate processors,hydrocyclones etc are needed.

  2. API follow by polishing technologies such as DAF and the mentioned Nut filters all well process proven treatment chains.

  3. Agree with Rafael and Marco. The API calculation is only the simple approach and doesn't take elements like a coalescing filter, DAF, dosing of emulsion breakers etc. into the equation. Also the design of the feed pump can have an influence: high rpm vortex pumps tend to disperse droplets, where low rpm screw-type impellers can provoke some coalescing action in the connecting piping.

    1 Comment

    1. most of the mechanical separation system based on Stokes law, including hydrocyclone technology which takes only 1/10th of the space compare to API and works on single PD Pump and can separate oil droplet of up to 15 micron compare to API design which only separate more than 150 micron.

  4. Dear Deven,

    API 621 calculation method is based on the law of Stokes which is valid in general and can be applied to whatever size of oil droplets, nproviding that the density of oil is known and the viscosity of the water is estimated properly for the working temperature.

    I often design for oil droplets of 25 micron, which normally ensures a removal efficiency of more than 95% i.e. reduction from 1000 to 50 ppm.

    For better final performance a secondary treatment should follow such as Flotation or adsorption by NutShell Filters.

    1 Comment

    1. most of the mechanical separation system based on Stokes law, including hydrocyclone technology which takes only 1/10th of the space compare to API and works on single PD Pump and can separate oil droplet of up to 15 micron compare to API design which only separate more than 150 micron.

  5. Dear Deven,

    API 421 is a well balanced and proven design tool. Granted it has limitations, but the deviations could be reasoned on basis of the Stoke's law.

    API separators initially were designed  on basis of and for refinery waters.

    There are many other waters where the "ideal" OWS separator may be based on other principles of physics. I do not believe there is one single apparatus for all applications and it is the responsibility of the specifications writer to define the boundaries and objectives, not necessarily just calling on an API OWS.

    2 Comments

    1. most of the mechanical separation system based on Stokes law, including hydrocyclone technology which takes only 1/10th of the space compare to API and works on single PD Pump and can separate oil droplet of up to 15 micron compare to API design which only separate more than 150 micron.

    2. very true and that is exactly what I wanted to point out, most of the projects I have seen only API OWS. Copy Paste of old specification for OWS specs wont help with stringent environment discharge specs.

      Just an addition of one line " similar efficient equipment/technologies which can provide required discharge specs can also participate for design and quote"