Introduction
An early production facility has two competing instincts about its own gas. It wants to run on it — fuelling the gensets, the heating medium, the flare pilots — so it does not have to import diesel across an ocean. And it wants to not waste it — recovering the low-pressure flash and tank vapours that would otherwise be flared or vented, because flaring is lost revenue, a carbon liability, and increasingly a regulatory limit rather than a nuisance.
The fuel gas system and the vapour recovery unit are the two small process systems that serve these instincts. Neither is large or glamorous, but each has consequences out of proportion to its size: a fuel gas system that delivers wet, cold gas wrecks the gensets it feeds, and a vapour recovery unit sized for the wrong flash flow either flares anyway or trips on its own recycle. On a weight-limited MOPU or EPF, getting both right is what separates a facility that runs on its own resources from one that burns money at the flare tip.
This post covers what the fuel gas system has to deliver, how vapour recovery claws back the flash gas, and the bootstrapping problem of starting a facility that has no gas yet.
What the Fuel Gas System Has to Deliver
Fuel gas is taken off the process — usually from an HP or MP gas stream — and conditioned to a quality and pressure the consumers can tolerate. The consumers are fussier than they look:
- Gas engines / gas turbines for power generation — the largest and most sensitive user.
- Heating-medium heaters / reboilers — if fired.
- Flare pilots and purge — small but must never be starved.
- Instrument/utility gas — on some facilities.
The conditioning the fuel gas system performs:
| Step | Purpose |
|---|---|
| Scrubbing (KO drum / coalescer) | Remove liquids — the single most important duty; liquid carryover destroys engines and turbines |
| Pressure letdown + control | Deliver a stable supply pressure to each consumer |
| Superheat (heating) | Raise the gas above its hydrocarbon dew point so letdown (Joule-Thomson) cooling does not condense liquids downstream |
| Filtration | Remove particulates |
| Metering | Account for fuel consumption |
The two specifications that bite hardest:
- Dew-point margin / superheat. After pressure letdown, the gas cools by Joule-Thomson expansion. If it starts near its hydrocarbon dew point, that cooling condenses liquids right at the consumer inlet. The fuel gas system must deliver a healthy superheat margin (commonly 20–30 °C above the hydrocarbon dew point at the delivery pressure) so the gas stays dry all the way to the burner.
- Combustion quality — Wobbe index and methane number. Engines and turbines are designed for a fuel within a band of Wobbe index (heating value normalised by density), and reciprocating gas engines also care about methane number (knock resistance). A heavy, rich associated gas can fall outside the engine's tolerance and cause knock or derate — which can drive a conditioning or blending requirement.
Vapour Recovery — Clawing Back the Flash Gas
When crude is stabilised through multi-stage flash (see crude stabilisation), the low-pressure and atmospheric stages produce gas at pressures too low to enter the fuel or export header directly. The same is true of the vapour that breathes out of atmospheric storage tanks. Historically this low-pressure gas was flared or vented. A vapour recovery unit (VRU) captures it instead.
The VRU is a compression package that takes low-pressure flash and tank vapour, boosts it, and delivers it into the fuel gas or export system:
- Compressor type — screw and reciprocating compressors are common; liquid-ring compressors suit the very-low-suction, wet, variable service of tank-vapour recovery because they tolerate liquids and pressure swings well.
- Suction scrubber — protects the compressor from slugs of liquid carried over from the flash drums or tanks.
- Aftercooler and discharge — cools and routes the recovered gas to the fuel gas KO drum or the export compression suction.
- Recycle / capacity control — flash and tank-vapour flows are variable; the VRU must turn down and recycle gracefully as the source flow swings, without flaring on a spike or surging at low flow.
The payoff is threefold: recovered gas has value (fuel or export), flaring drops (carbon and regulatory), and a recovered LP stream can be exactly the marginal fuel that makes the facility self-sufficient.
The Fuel Gas Balance — Is There Enough?
A facility cannot assume it has spare gas to burn. The fuel gas balance — production of suitable gas versus consumption by the power plant and heaters — depends entirely on the field:
- Gas-rich / high-GOR field → surplus gas; the question is export or flare, and fuel is easy.
- Oil field with low GOR → fuel gas can be short. The associated gas may not be enough to run the power plant, especially early in field life before the GOR rises. The fallback is diesel or dual-fuel gensets, importing liquid fuel until (or unless) the gas balance turns positive.
The fuel gas balance also moves over field life as water cut and GOR change, so the power-generation fuel strategy is a lifecycle decision, not a single-point one. Dual-fuel capability is cheap insurance on a field whose gas balance is uncertain — exactly the situation on many early-production developments.
The Bootstrapping Problem — First Gas
A facility that runs on its own gas has a chicken-and-egg problem at startup: it needs power and fuel gas to start production, but it has no production gas until it starts. The fuel gas system has to be bootstrapped:
- Import fuel — diesel-fired (or dual-fuel) generation to provide power until the facility makes its own gas.
- Import gas — a temporary gas supply (bottled, or from a neighbouring facility) to light the flare pilots and feed initial fuel users.
- Fuel gas accumulator — a buffer volume to ride through transients and supply the pilots during brief upsets.
The startup fuel strategy is a real design input, not an operations afterthought. A facility designed to be gas-self-sufficient but with no credible first-gas plan cannot actually start.
Worked Example — Fuel Gas Conditioning Margin
Scenario: an EPF taking fuel gas off an MP stream at 25 bara, letting it down to a genset supply pressure of 6 bara, associated gas with a hydrocarbon dew point of about 5 °C at delivery pressure.
The J-T cooling check: letting down from 25 to 6 bara cools the gas by Joule-Thomson expansion — for a typical associated gas, on the order of 0.4–0.5 °C per bar, so roughly 8–10 °C of cooling across this letdown. If the gas entered the letdown valve at, say, 15 °C, it would arrive near 5–7 °C — right at the dew point, and any further cooling condenses liquid into the genset fuel.
The fix: a fuel gas superheater upstream of (or across) the letdown raises the gas temperature so that, after J-T cooling, it still sits 20–30 °C above the 5 °C hydrocarbon dew point — i.e. delivered at ~25–35 °C. An electric heater or a heating-medium coil does this; the duty is small but the consequence of omitting it is condensate in the engines.
The protection: a fuel gas KO drum with a high-level trip and a low-low supply-pressure trip protects the gensets from both liquid carryover and supply loss. Lose fuel gas pressure and the power plant must trip cleanly, not ingest a slug.
Common Pitfalls
- Inadequate superheat. The most common fuel-gas failure: gas delivered near its dew point condenses on J-T cooling and carries liquid into engines and turbines. Always check the superheat margin after letdown cooling.
- Underestimating liquid carryover. A marginal KO drum or coalescer lets fine liquid through to the most expensive, most sensitive equipment on the deck. The fuel gas scrubber is not the place to save weight.
- Assuming the gas balance is positive. Low-GOR oil fields can be short of fuel gas, especially early. Confirm the balance over field life and provide dual-fuel capability if it is uncertain.
- No first-gas plan. A gas-self-sufficient facility with no startup fuel strategy cannot start. Design the bootstrapping.
- Sizing the VRU for a steady flow. Flash and tank-vapour flows are variable; a VRU without graceful turndown and recycle either flares on spikes or surges at low flow. Size for the range, not a point.
- Ignoring Wobbe / methane number. A rich associated gas can fall outside the gensets' combustion tolerance, causing knock or derate. Check the fuel against the engine's fuel spec, not just "it's gas."
Conclusion
The fuel gas system and the vapour recovery unit are the two systems that let an early production facility live on its own resources — running the power plant on produced gas and recovering the flash and tank vapour it would otherwise burn. Both are small; both are unforgiving when wrong.
Deliver the fuel gas dry and superheated, with margin above the dew point after letdown cooling, and protected against liquid carryover and supply loss. Recover the low-pressure flash and tank vapour with a VRU sized for a variable flow. Confirm the fuel gas balance over field life and plan the first-gas bootstrap. Get this right and the facility runs on what it produces and flares almost nothing; get it wrong and you either import diesel forever or feed wet gas to engines that will not forgive it.
