How once-Through oilfield steam generators work
Once-through steam generators for enhanced oil recovery produce wet steam, typically about 80% quality, which is injected into heavy oil reservoirs. Feedwater is pumped through a single tube circuit, first the convection section and then a horizontal radiant section, with no drum. Units are skid-mounted, commonly about 15 to 30 MW fired, and placed in remote oilfields. Fuel is natural gas, associated gas from the field, or crude oil at some sites. The burner fires along the radiant section, and the gas rises through the convection section to a short stack. Most units fire under forced draft only; an ID fan is added where a scrubber, air preheater or heat recovery follows the convection section.
Where the FD fan sits
- Inlet filter
- FD fan
- Air preheater, where fitted
- Burner
- Radiant section
- Convection section
- Stack
The FD fan sits at the burner end of the skid, often on the skid frame itself. It draws ambient air, which in desert fields can be very hot and carry sand. On forced draft units its pressure covers the burner, the radiant section, the convection section and the stack base, typically 200 to 400 mmWC. Where an air preheater is fitted, the fan pushes air through it first. The skid layout limits fan size and the length of inlet ducting.
Air at the FD fan
| Air temperature at the fan | Ambient, up to about 50°C in desert fields |
| Pressure at the fan outlet | Typically 200 to 400 mmWC on forced draft units |
| Dust | Blown sand and dust; inlet filtration needed |
| Air density | Lower at high ambient and elevation, reducing mass flow |
| Fuel | Gas or crude, with different excess air needs |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for once-Through oilfield steam generators
| Blade form | Backward curved, stable across the burner range |
| Material of construction | Mild steel, with wear protection at the inlet where sand load is high |
| Drive and arrangement | Direct coupled on the skid base |
| Bearings and cooling | Sealed pedestal or motor bearings rated for high ambient |
| Flow control | VFD or inlet vanes under burner management, with O₂ trim |
| Also specified | Inlet filter with differential pressure gauge, inlet silencer |
What this FD fan has to cope with
Sand ingestion
Without filtration, blown sand erodes the impeller inlet and blade leading edges and is carried into the burner. An inlet filter with a differential pressure gauge, and a fan selected with pressure margin for a partly loaded filter, protect both. Filter changes are scheduled from the gauge, not the calendar, because sandstorms load filters unevenly.
Capacity loss in summer
At 50°C air is about 10% lighter than at 20°C, so a fan sized at standard conditions delivers less mass flow and the steam generator cannot reach full firing rate on hot afternoons. The fan is selected at the highest ambient and the site elevation, with speed margin on the drive to recover full air.
Crude firing needs more air and pressure
Switching from gas to crude raises the excess air needed for clean combustion and fouls the convection section, so resistance rises between washes. The FD fan must cover the fouled crude case, or the unit has to be derated as the tubes foul and steam output falls short of the injection plan.
Energy and control
These units run steadily at injection rates that change slowly over months. A VFD sized for the real operating range saves power over inlet vane control, and O₂ trim holds excess air down, saving fuel that is often gas or crude the field could otherwise sell. At remote sites, lower fan power also eases the load on local generation.
Questions buyers ask
What is the FD fan pressure on a once-through steam generator?
On forced draft units the FD fan must cover the burner, radiant section, convection section and stack base, typically 200 to 400 mmWC. Where an ID fan is used, the FD fan covers only the air side. The steam generator maker's resistance data sets the exact figure.
How do you protect an FD fan from sand in desert oilfields?
An inlet filter with a pressure gauge, selected for the local dust load, stops most sand. The fan is selected with pressure margin for a partly loaded filter, and sealed bearings and wear protection at the inlet deal with what gets through. Filter changes follow the gauge reading.
Why does my oilfield steam generator lose output on hot days?
Hot air is less dense, so the FD fan delivers less air mass at the same speed, and the burner must be turned down to keep enough excess air. Selecting the fan at the maximum ambient, or a VFD with speed margin, restores the full firing rate in summer.
Information needed to quote an FD fan for once-Through oilfield steam generators
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Site conditions. Maximum ambient temperature, elevation and sand or dust load.
- Skid constraints. Space and weight available for the fan, motor and inlet filter.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.