Produced Water Treating

Skim Tank — Engineering Fundamentals

API 421 gravity-settling design: Stokes velocity, Hazen overflow vs retention-time methods, and vertical vs horizontal orientation.

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Design basis

API Publication 421

Gravity oil-water separation.

Design droplet

150 µm

API 421 default for bulk gravity removal.

Sizing methods

Hazen / retention

Overflow rate (vertical) or retention time (horizontal).

Contents

Use this guide when you need to:

  • Size a vertical skim tank from Hazen overflow rate.
  • Size a horizontal skim tank from retention time.
  • Set the 150 µm design droplet and Stokes rise velocity.

1. Role in the train

A skim tank is the simplest unit operation in the produced-water dewatering train — an atmospheric vessel sized for gravity oil-droplet rise per API Publication 421. It does the bulk-removal work: 5,000 mg/L → 500 mg/L is typical, capturing droplets ≥ 150 µm. Downstream stages (CPI, hydrocyclone, DAF, walnut-shell filter) progressively polish smaller droplets.

Skim tanks are also used to handle reject streams from cyclones and slop oil from upset events — anywhere a low-cost, low-OPEX gravity vessel is wanted.

2. Stokes settling for 150 µm

The 150 µm design droplet is the API 421 default — coarse enough that ΔSG of 0.05–0.20 yields rise velocity in the 0.5–5 ft/min range, fast enough to size a reasonable vessel in 30–60 min retention. Smaller design droplets push the vessel size up by d⁻² — going to 75 µm makes the vessel 4× larger for the same throughput.

Vt (ft/s) = 1.78 × 10−6 · dm2 · ΔSG / μw

3. Vertical (Hazen)

Vertical skim tanks use the Hazen overflow rate: water enters at the bottom and exits at the top through a weir, while oil droplets rise faster than the upward water velocity:

Areq = Q / Vt ; D = √(4·A/π)

Total height = retention volume / area + freeboard for oil layer (typically 2–3 ft). Vertical tanks have a smaller footprint than horizontal for the same volume but are limited to ~30 ft tall before transport / fabrication becomes awkward.

4. Horizontal (retention)

Horizontal skim tanks function like half-full FWKO vessels — sized for retention time at 1.42 · d² · L = Q · t (d inches, L ft, Q BPD, t min). They handle larger BPD throughput than vertical because the cross-sectional flow area is larger; trade-off is a much bigger plot.

5. References

  • API Publication 421 — Design and Operation of Oil-Water Separators.
  • Stokes (1851), Hazen (1904).
  • Manning & Thompson Vol 2 Ch 7.
  • Stewart & Arnold Vol 1 (FWKO retention-time method).

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Frequently Asked Questions

What job does a skim tank do in the water train?

It is the simplest, lowest-OPEX unit — an atmospheric vessel sized for gravity oil-droplet rise per API 421. It does the bulk removal, typically 5,000 mg/L down to about 500 mg/L by capturing droplets ≥ 150 µm, leaving CPI, hydrocyclone, DAF and walnut-shell stages to polish finer droplets.

Why is 150 µm the design droplet?

150 µm is the API 421 default — coarse enough that a 0.05–0.20 density difference gives a 0.5–5 ft/min rise velocity, sizing a reasonable vessel at 30–60 min retention. Because Stokes velocity scales with d², dropping to a 75 µm design droplet quadruples the vessel size for the same throughput.

Vertical or horizontal skim tank?

Vertical tanks size on Hazen overflow rate (A = Q / Vt) and give a smaller footprint, but are limited to about 30 ft tall. Horizontal tanks size on retention time like a half-full FWKO vessel and handle larger throughput because of the bigger flow area — at the cost of a much larger plot.