This standard covers the design of fuel gas systems for oil & gas production installations. The system boundaries are illustrated in Figure 1.
The primary goal of a fuel gas system is to provide a **reliable and safe supply of fuel gas** of the required specification to the consumers under all operating conditions. This is achieved by proper selection of the source gas, conditioning, and distribution.
The fuel gas source is selected based on the required specifications. The gas is preferably taken from a process at a pressure higher than the required working pressure. Common off-take points include:
Off-take points should be duplicated where possible. A secondary source (e.g., export pipeline) should be chosen to allow for "black" startup.
The first step is to identify all consuming equipment (e.g., gas turbines, fired heaters) and determine the required range of gas demand and quality. Gas turbines normally have the most stringent specifications. Vendor specifications should be challenged if they are believed to be too strict.
| Property | Description |
|---|---|
| Wobbe Index (WI) | A measure of energy supplied (Gross Heating Value / √Relative Density). Fuels with a similar WI are reasonably interchangeable. Fluctuations should normally not exceed ±10%. |
| Dew Point | The system must be free from hydrocarbon and water condensation. As a rule of thumb, gas should arrive at the consumer at least 20°C above its HC and water dew points. |
| Pressure | Determined by the highest delivery pressure required. Often, two levels are used (HP for turbines, LP for blanket gas, etc.). The system should operate at the highest practicable pressure to provide a buffer on process trips. |
| Temperature | Must meet dew point requirements (e.g., 20°C above HC dew point or 0°C, whichever is higher). Should not exceed limits for seals (typically 70°C). |
| Impurities | Sulphur content generally should not exceed 1.3 wt%. Strict limits apply for metals (Sodium, Potassium). Solids should be less than 30 ppm. |
| User | Minimum Fuel Gas Pressure (barg) |
|---|---|
| Rolls Royce RB211B/G | 30 |
| Ruston Tornado | 17 |
| Solar Centaur | 13 |
| Ruston TB5000 | 11 |
| Low Pressure Users | 5 (typical) |
Note: These figures are illustrative only. The pressure is the minimum required at the inlet to the turbine's pressure regulating valve.
The conditioning system must deliver the required fuel gas quality in all operating conditions. The goal is to remove liquids and provide superheat.
A common method uses the **Joule-Thompson (J-T) effect** from pressure reduction:
The gas should only be cooled to a temperature typically 5°C above the expected hydrate formation point. Cooling below this point is not desirable and would require glycol injection.
A simple PFD showing the system boundary, including an optional Pre-Heater and Cooler, a Fuel Gas Scrubber, a final Heater, and distribution to users and flare.
The design methodology involves plotting the source gas phase envelope and the hydrate formation curve. The conditioning process (cooling, flashing, superheating) is then mapped out to ensure the final fuel gas remains outside the phase envelope and at least 5°C above the hydrate curve at all points in the distribution system.
A detailed P&ID showing a typical system with a source SDV, Pre-Heater, Cooler, Fuel Gas Scrubber, Electric Heater, and distribution to LP users and a Filter Type Separator for the turbine package.
A fuel gas scrubber shall always be provided. Its purposes are to:
An important sizing criterion is to provide sufficient gas volume for the time it takes to perform this changeover.
This is specified and provided by the gas turbine manufacturer and is usually part of the guarantee. It is located as close as possible to the turbine.
An automatically operated ESD (Emergency Shutdown) valve shall be installed on each fuel gas supply line. On shutdown, the line between the ESD valve and the user shall be automatically depressurised.
This table shows the automated shutdown logic. For example, a **High High Liquid Level** in the scrubber will: Close the Inlet SDV, Close the Outlet SDV, generate an Alarm, and initiate the Change Over to Diesel Fuel.
Materials of construction are specified in a project-specific document, which must account for CO₂, H₂S, chloride content, etc. Piping downstream of the filter type separator (see Figure 2) is typically Stainless Steel.