Processing and Transmission
Processing and Transmission tests gas conditioning, separation, dehydration, sweetening, NGL recovery, pipeline hydraulics, compression, and custody transfer. Expect unit-process selection and simple flow or pressure checks.
Core rules
- Separation is staged to stabilize liquids. A three-phase separator removes gas, oil, and water.
- Glycol dehydration lowers water dew point; TEG is common. Amine sweetening removes H2S and CO2, and a Claus unit converts H2S to elemental sulfur.
- Pipeline capacity is constrained by MAOP, pressure drop, and velocity. Two-phase flow can cause slugging and liquid holdup.
- Compressor stations add pressure; stage ratio and intercooling control discharge temperature.
- Custody transfer uses standard conditions, gas composition, and energy content, not just volumetric flow.
Worked example
A 50 MMSCFD stream at 1,000 psia and 560 R with z = 0.90 flows in a 12-in ID line. Convert to actual flow: 50,000,000 × (14.7/1,000) × (560/520) × 0.90 ≈ 712,000 ft3/d. The pipe area is 0.7854 ft2 for a 1-ft diameter. Velocity = 712,000/(86,400 × 0.7854) ≈ 10.5 ft/s. The reasoning is standard-to-actual conversion first, then v = Q/A.
Traps
Treating standard flow as actual flow, ignoring z and temperature, or using pipe OD instead of ID will misstate velocity and pressure drop. Do not treat gas as incompressible over a large pressure drop. Ignoring liquids, slugs, or hydrate conditions can make a hydraulically acceptable line operationally unsafe.
Key points
- Dehydration controls water dew point; sweetening removes acid gas.
- Pipeline capacity is limited by MAOP, pressure drop, and velocity.
- Convert standard flow to actual flow before sizing.
- Use ID, not OD, in velocity and pressure-drop calculations.
- Custody transfer requires standard conditions and composition.