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Engineering Tool — Rod Pump

Separator OD sizing calculator

The right separator OD isn't the biggest one that fits the casing — it's the one that balances two velocities against each other. Built for rod pump completions. Enter your well data below to see where each candidate OD lands.

Sizing a desander spiral instead? Use the desander spiral calculator.

Why two velocities, not one

Increasing separator OD grows the internal area around the dip tube, which slows the downward liquid velocity inside the separator — good, because gas bubbles need enough slip velocity to rise against that downward flow. A separator that's too small drives liquid down too fast, and gas gets carried into the dip tube instead of breaking away.

But the same OD increase shrinks the casing annulus around the tool, which speeds up the upward gas velocity in that annulus — bad, because too much velocity there causes liquid entrainment and misting before fluid ever reaches the separator. A separator that's too large chokes its own gas path.

The right OD sits in the window where both velocities stay under their critical thresholds — not at either extreme.

1. Casing

Determines actual drift/ID — heavier weight means a thicker wall and smaller ID. ID is used automatically once selected.

2. Production & downhole conditions

Self-assessed — "high" means different things on different wells. Drives the solids recommendation at the bottom, not the velocity calculation.
Converts surface Mcfd to actual gas volume at intake conditions. Leave at default if unknown.

3. Separator geometry & design limits

NPT nominal size vs. actual pipe OD — the actual OD is what drives the internal area calculation.
Field-derived screening thresholds (Echometer/SWPSC-style gravity separator design). Adjust if your team uses different limits.

4. Desander spiral — enter the pump, or leave blank and we'll use oil + water from step 2

Results — these are assumed limits as a starting place; every formation behaves differently, and actual critical velocities should be confirmed against your own field data

OD
Liquid velocity (internal)
Gas velocity (annulus)
Verdict
Fill in the well data above and click Calculate.

Send this design

This is a screening tool, not a final engineering design. It applies a simplified two-velocity model (downward liquid velocity inside the separator vs. upward gas velocity in the casing annulus) using an ideal-gas approximation to convert surface gas rate to actual downhole volume. It does not account for TAC OD, collars, or other restrictions above the separator — the narrowest point in the full string can control actual annular gas flow even when the separator body checks out. These are calculated results, and actual performance will vary with how the well produces. Confirm sizing with a Vannevar engineering representative before finalizing a completion design.