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Gas Separators
Gas Separators covers vessels that take gas out of returning mud or well fluids. On a drilling rig that means the mud gas separator, also called a poor boy degasser, with its vent line and mud leg. It also includes two-phase and three-phase test separators used on well tests. A used mud gas separator is judged on its whole layout, because the mud leg and vent line set its capacity.
How a mud gas separator works
During well control, the kick is circulated out through the choke manifold. Mud and gas leave the choke at low pressure and enter the mud gas separator, a vertical atmospheric vessel. Gas breaks out by gravity as the mud falls over internal baffles. It leaves through a vent line, usually run to the top of the derrick. Mud drains through a U-shaped mud leg back to the shakers or trip tank. The mud leg is a liquid seal that stops gas reaching the mud system.
The vessel can only hold gas back up to the hydrostatic pressure of mud in the leg. In oilfield units, blow-through pressure in psi is 0.052 times mud weight in pounds per gallon times leg height in feet. A 20 ft (6.1 m) leg of 9.5 ppg (1.14 SG) mud holds about 9.9 psi (0.68 bar). If friction in the vent line raises vessel pressure above that, gas blows through to the shakers.
A test separator is different. It is a pressure vessel that splits well flow into gas, oil and water under level and pressure control, and meters each phase.
Configurations
- Vertical atmospheric mud gas separators with open-bottom or closed mud legs
- Pressurised mud gas separators for managed pressure drilling, with a back pressure on the gas outlet
- Two-phase test separators for gas and liquid
- Three-phase horizontal test separators for gas, oil and water
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Vessel diameter and height | Sets gas and liquid handling capacity | Inches or mm |
| Mud leg height | Sets the liquid seal | Feet or metres |
| Vent line bore and length | Sets back pressure at peak gas rate | Inches, feet or metres |
| Inlet size | Must match the choke manifold outlet | Inches, connection type |
| Design pressure | Low for atmospheric MGS, higher for test separators | psi or barg |
| Sour service | Needed for H2S wells | NACE MR0175/ISO 15156 |
Standards and certification
API 12J covers oil and gas separators, and most vessels are built to ASME Section VIII. A widely used sizing method for mud gas separators is MacDougall's, published by the Society of Petroleum Engineers. It checks vent line friction and separation against the worst-case kick. Your well control manual and operator standards set the design kick. Sour wells need materials to NACE MR0175/ISO 15156.
Failure modes and inspection
- Erosion at the inlet and impingement plate, where mud hits the wall at speed. Ask for an ultrasonic thickness survey with a grid at these points.
- Baffles that tear loose at their attachment welds. Inspect them through manways.
- Blocked or corroded mud legs and drains. Check that clean-out connections open.
- Pressure gauges or transmitters out of calibration, which hide a rising vessel pressure.
- Test separators with failed level controls, worn meters and out-of-date relief valves.
Documentation
- Data book with design code, calculations and material certificates
- Hydrotest report and any third-party certification
- General arrangement drawing with internals and nozzle schedule
- Sizing calculation for the original rig layout
- Wall thickness surveys and any repair records
- Meter calibration for test separators
Integration and compatibility
Check the inlet against your choke and kill manifolds outlet. Check the vent line route and length on your derrick, and the available height for the mud leg. A longer or narrower vent, or more bends, can cut the gas rate a used vessel handles. A vacuum degasser for entrained gas during normal drilling sits in mud treatment systems.
Refurbishment and recertification
Re-survey wall thickness, repair or replace baffles, renew gaskets and gauges, and hydrotest where the code requires it. Then rerun the sizing calculation for your rig's vent and leg. Unused surplus vessels from newbuilds or upgrades often only need a preservation check and the calculation. MI SWACO, an SLB company, is one well-known supplier of mud gas separators.
FAQ
Frequently asked questions
What is a poor boy degasser?
A poor boy degasser is another name for a mud gas separator. It is a vertical vessel downstream of the choke manifold that separates free gas from mud during well control. Gas leaves through a vent line, and mud drains back through a liquid-sealed mud leg to the shakers or trip tank.
How do you calculate mud gas separator blow-through pressure?
Blow-through pressure equals the hydrostatic pressure of mud in the mud leg. In oilfield units it is 0.052 times mud weight in ppg times leg height in feet. A 20 ft (6.1 m) leg of 9.5 ppg mud holds about 9.9 psi (0.68 bar) before gas blows through to the shakers.
What should I check on a used mud gas separator?
Check a used mud gas separator for erosion at the inlet and impingement plate with an ultrasonic thickness survey, and inspect the baffle welds. Also check the mud leg and drains, the pressure gauge calibration, the data book and whether the materials suit sour gas.
Does a mud gas separator have to be sized for the rig?
Yes, a mud gas separator must be sized for your worst-case kick and your own vent line. Vent line bore, length and bends add back pressure, and mud leg height sets the limit. Rerun the sizing calculation for your rig's layout before buying a used vessel.
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