How to Handle Both Forced Draft Fans Trip in Running Boiler

Boilers have forced draft (FD) fans to supply air for the combustion of fuel. In addition to this fan, there can be many other fans such as induced draft (ID), primary air fans (PA), seal air fans, scanner air fans, etc. In these the FD, ID, and PA fans are large capacity fans, while the others are smaller fans. In the case of a pressurized boiler, only an FD fan is present. Here the FD fan handles the full pressure drop of the whole boiler air and flue gas system. In the case of a balanced draft system, the ID fan evacuates the flue gas from the furnace and handles the pressure drop in the flue gas section. The PA fan is used in the case of solid fuels to carry the fuel to the furnace and give the primary air requirement to the fuel.

Trip in a running boiler

Any trip of major equipment in the boiler causes the boiler parameters to vary widely before it stabilizes depending upon the action taken by the operator. Boiler tripping can also be caused if some vital equipment trips or if some unsafe condition appears. The important factors to be understood in a trip of equipment in a boiler are mainly four.

  1. The specific cause of trip
  2. The plant response to the trip
  3. The immediate operator action required
  4. The immediate local operator action requirement

In the case of both FD fans tripping, the boiler will go for trip as the air supply to the fuel is cut off in full or excluding the primary air depending upon the system design. The PA fan also trip as soon as all the FD fans trip. Depending upon whether the boiler is for a process unit or power station, the action on the steam consumer end will vary.

The specific cause of the trip

For both FD fans to trip the main reasons can be as below.

  • 6.6 KV supply failure
  • 0.4 KV supply failure
  • Cooling water to motors failure

Plant response to the trip

The immediate plant response when both the FD fans trip will be boiler trip. The PA fans and mills also trip.

Immediate operator action required

The boiler will have to be prepared for light up immediately. Any delay in this action within a very short period can cause lot of problems and loss for the plant. The following will help any boiler operator to speed up his action to bring back the boiler on line.

  • Check feed pump, maintain drum level
  • Get clearance from connected group to start equipment in boiler
  • Start one ID fan, as soon as possible
  • Start FD fan
  • Maintain purge conditions
  • Complete purge
  • Introduce furnace probe
  • Light up boiler
  • Open start-up vent valve or bring to service LP / HP bypass depending upon the consumer
  • Quickly start PA fan / milling system and give steam to the consumer maintaining the required steam parameters
  • Maintain steam parameters and load the boiler as consumption of steam increases
  • Control the water level in drum
  • Maintain furnace conditions
  • Load to maximum load as required once all parameters stabilize

Local operator action requirement

The local operator of the boiler will have to perform a quick task of restoring things in the locale and communicate to the boiler control room. This set of activity is carried out by the respective groups in each area like boiler operating floor, mill area, etc. The actions required are:

  • See all fuel trip valves are closed
  • Check furnace has no flame
  • Check if air heaters are rotating – only for regenerative air heaters
  • Check fans for starting conditions and confirm to control room
  • Make ready the fuel oil system for light up of unit and inform control room
  • Check local boiler drum level and make sure
  • Check and make ready feeders and mills for coal firing
  • Make all electrical check and clear for starting the boiler

All the said actions are broad and for general understanding only. It is required to train and educate the operators for each design of the units. There are lot of changes taking place on the controls side and equipment design side. This brings in a need to understand the specifics of the system that is being handled.

About the Author

Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.

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