Skim Tank — Engineering Fundamentals
API 421 gravity-settling design, Stokes velocity, Hazen overflow vs retention-time methods, vertical vs horizontal orientation.
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.
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:
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).