How nuclear station auxiliary boilers work
A nuclear power station needs steam when its reactor is not producing it: for start-up heating and deaeration, turbine gland sealing, building and process heating, and some radioactive waste treatment systems. This is supplied by an auxiliary boiler, usually a packaged or field-erected oil-fired shell or water tube unit fired with light diesel oil or HSD, and sometimes several units in parallel. The boiler is conventional plant outside the nuclear island, but it supports outage work and start-up, so it is expected to start reliably after long standby. Most have a pressurised furnace with a forced draft burner and no ID fan. Many stations are on the coast, where salt-laden air affects all outdoor equipment.
Where the FD fan sits
- Inlet silencer
- FD fan
- Air damper
- Burner windbox
- Pressurised furnace
- Stack
The fan sits beside the burner front or on the boiler skid, drawing air from inside the boiler house or through an outside inlet with a silencer. As the furnace is pressurised, the fan overcomes the burner, furnace and convection pass resistance up to the stack, typically 300 to 600 mmWC depending on the boiler size and design. Many stations install two boilers or two fans so one can be maintained while the other stands by.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air quality | Salt-laden air at coastal sites |
| Dust | Clean air in most cases |
| Standby | Long idle periods between outages, with fast start-up when needed |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 nuclear station auxiliary boilers
| Blade form | Backward curved, for stable pressure and burner control |
| Material of construction | Mild steel with marine-grade paint system; stainless fasteners at coastal sites |
| Drive and arrangement | Direct coupled, arrangement 4 or 8, with redundancy where the station specifies |
| Bearings and cooling | Bearings protected against moisture during standby, with heaters in the motor |
| Flow control | Variable speed drive or inlet vanes linked to the burner management system |
| Also specified | Material test certificates, inspection and test plan, performance test per IS 4894 or BS 848, and balancing to ISO 21940-11 |
What this FD fan has to cope with
Corrosion during standby
Salt-laden humid air settles on idle fans, and corrosion develops on casings, shafts and fasteners. Marine paint systems, motor space heaters and periodic test runs keep the fan ready. Stainless fasteners avoid seized bolts when the fan is opened for maintenance.
Bearing damage from standing idle
A fan standing still for months can suffer false brinelling from nearby plant vibration. Periodic rotation, suitable bearing grease and bearing condition checks before start-up prevent failure on demand. The standby fan is run on a regular schedule so both units stay in proven condition.
Documentation and inspection
Nuclear stations expect full traceability of materials, inspection hold points and test records even for conventional plant. These are planned into the manufacture so the fan is accepted without delay. Weld procedures, material certificates and test results are compiled into a dossier handed over with the fan.
Energy and control
Auxiliary boilers run mainly during outages and start-ups, so their annual energy use is modest. A variable speed drive still helps by matching the burner at low fire and reducing start-up current. The main value of good fan selection here is reliability: a fan that starts first time and holds steady pressure across the burner range.
Questions buyers ask
Why does a nuclear power plant need an auxiliary boiler?
To supply steam when the reactor is not producing it, for start-up, deaeration, gland sealing, heating and some waste treatment systems. It is conventional oil-fired plant outside the nuclear island. It must start reliably after long standby, because it is needed when the reactor is shut down.
Does an auxiliary boiler at a nuclear station need an ID fan?
Usually not. Most are pressurised furnace units where the FD fan pushes the gas through to the stack. An ID fan is needed only on balanced draft designs. In that case the FD fan pressure covers the furnace, boiler passes and stack, and it is selected accordingly.
What testing is required for an FD fan at a nuclear station?
Usually material certificates, dimensional inspection, performance testing per IS 4894 or BS 848, and dynamic balancing to ISO 21940-11, with records per the station quality plan. The station's quality plan may add witness points, and these are agreed before manufacture starts so the schedule allows for them.
Information needed to quote an FD fan for nuclear station auxiliary boilers
- 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.
- Quality plan. The inspection and test plan, document requirements and any witness points.
- Site conditions. Coastal exposure and indoor or outdoor installation.
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.