Control on Fans - Primary Air Fan

FUNCTION

The main function of the PA FAN (Primary Air Fan) is to convey the fuel mainly coal to the furnace with Air Pressure created by the Fan. The pressure normally maintained by this Fan is around 1000mmWC to overcome the pressure drop in Pulverisers and Coal Pipe. The component in the below figure in 20. Normally one independent thermal power plant comprises of 2-PA Fans.

 

 

The two fans run in parallel operation and severe critical conditions arises which leads to plant trip when any one Fan trips. This topic is considered of two major critical issue that can be taken care of:

 

• ONE FAN TRIPS DURING A PARALLEL OPERATION

Problem:-

During a parallel operation of the Fans the header pressure is maintained by both the Fans. And when suddenly one fan trips, the header pressure drops with a transition, which is very much ahead of the transition operated in the normal operation mode. So the controller tuned for normal operation fails to maintain the desired set value. And this results in insufficient pressure in the header which was in use to convey the fuel (coal) to the furnace.

 

Solution:-

The normal operating controller to be suppressed by a direct demand setter for the running fan with transition higher than the transition loss occurred due to the tripped fan.

 

• UNSTABLE CONTROL OF PULVERISER HOT AIR DAMPER

Problem:-
Hot Air Damper operated to maintain the Pulveriser Inlet Air Flow which is based on coal flow requirement. But due to the usage of coal poor than the coal for which the system is designed, the coal requirement increased. This increase in coal demand generates a demand for more air flow, and this extra air requirement make the Hot Air Damper to operate on zone, which may be beyond the Air Flow controllable zone. Because this type of dampers is controllable for certain operating zone.

Solution:-
The known range Hot Air Damper to be checked. And the Header Pressure (discussed above) set value to be reduced if the Hot Air Damper tends to operate beyond the controllable zone.

 Reference Books that can be followed:

 

• Power Plant Instrumentation
• Instrumentation in the Power Industry
• A Text Book of Thermal Engineering

Control on Fans - Induced Draft Fan

FUNCTION

The main function of the ID FAN (Induced Draft Fan) is to suck the flue gases coming out of the furnace via economizer, air heater and also it maintain the negative draft (pressure) inside the boiler approximately 10mmWC. The component in the below figure is 26. Normally one independent thermal power plant unit comprises of 2- ID Fans.

 

 

At present four different problematic operating zones are identified:

• DURING PARALLEL IN

Problem:-

When one is running in Auto mode, controlling the furnace draft and the additional fan is taken in Auto parallel, there could be the furnace draft get disturbed.

 

Solution:-

The additional fan should follow the running fan control signal with an optimum rate and the existing fan blade pitch should come down and match with the new one with action of PID controller. And similarly this should continue it number of fans are more than two. 

•  AFTER PARALLEL IN

Problem:-

Two fans are running at different load though operating in Auto.

 

Solution:-

Fan operator bias need to be adjusted to minimize the deviation between the load.

 

• ONE IN AUTO AND THE OTHER IN MANUAL

Problem:-

Auto control of the equipment works fast if tuned for multiple, furnace draft becomes unstable

 

Solution:-

The gain of the controller to be adjusted by implementing a separate gain adjustment logic based on number of equipment involved

 

• OTHER FANS STARTED LIKE FD FAN OR PA FAN

Problem:-

Furnace draft increases and makes control unstable

 

Solution:-

Advancement of FD/PA Fan blade pitch to be adjusted to ID Fan blade pitch, so that any increment in other fans make ID Fan blade pitch advance accordingly. So ratio wise Feed-forward signal to be incorporated to prevent the excursion of Furnace draft due to addition of Air considering start of other fans.

Reference Books that can be followed:

• Power Plant Instrumentation and Control Handbook: A Guide to Thermal Power Plants 
• Power System Stability and Control (EPRI Power System Engineering)
• Modelling and Simulation of Power Generation Plants (Advances in Industrial Control)
• Power System Operation

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