Steam boiler or simply a boiler is basically a closed vessel into which water is heated until the water is converted into steam at required pressure. This is most basic definition of boiler. Boiler or more specifically steam boiler is an essential part of thermal power plant.
Working Principle of Boiler
The basic working principle of boiler is very simple and easy to understand. The boiler is essentially a closed vessel inside which water is stored. Fuel (generally coal) is bunt in a furnace and hot gasses are produced. These hot gasses come in contact with water vessel where the heat of these hot gases transfer to the water and consequently steam is produced in the boiler. Then this steam is piped to the turbine of thermal power plant. There are many different types of boiler utilized for different purposes like running a production unit, sanitizing some area, sterilizing equipment, to warm up the surroundings etc.
Steam Boiler Efficiency
The percentage of total heat exported by outlet steam in the total heat supplied by the fuel(coal) is called steam boiler efficiency. It includes with thermal efficiency, combustion efficiency & fuel to steam efficiency. Steam boiler efficiency depends upon the size of boiler used. A typical efficiency of steam boiler is 80% to 88%. Actually there are some losses occur like incomplete combustion, radiating loss occurs from steam boiler surrounding wall, defective combustion gas etc. Hence, efficiency of steam boiler gives this result.
Industry Installed Boilers
Internal Parts/ Diagram of Boiler
Cut Away View of Steam Boiler
General Operation of Boiler:
- The boiler is supplied with water by a feed pump.
- Water enters the boiler through the feed check valve
- when the water reaches normal water level, which is approximately the middle of the water gauge glasses, the feed pump is stopped by the dual controller (on/off control) or closes the feed control valve (modulating feed control).
- The burner is bolded to the front casing. Air required for the combustion is provided by the forced draught fan which is integral with the burner unit.
- The furnace receives radiant heat from the flame and the hot gases from the flame give up heat to the tubes in the second pass as they travel to the front chamber. Gases turn in the front chamber and give up more heat to the tubes in the third pass as they travel to the rear chamber. The gases leave the boiler by the chimney at the rear of the boiler
- As the steam pressure rises, the pressure in the boiler is shown by the pressure gauge.
- When the pressure is sufficiently high, the main stop valve can be opened to allow steam to pass to the process or heating system.
- When steam leaves the boiler, the water level will drop until it reaches a point approximately 12.5mm below the normal level. The dual control will start the feed pump and more water will be fed into the boiler. When a modulating feed system is filled, the water level is controlled with in the operating band of the modulating feed valve.
- Two pressure switches are fitted to the boiler, one acts as a pressure limit control, the other is a pressure controller, which controls the firing rate of the burner.
- When the boiler pressure reaches the maximum required, the limit pressurestat shuts down the burner. The burner will restart automatically when the pressure falls to a preset point below the maximum.
- If the pressurestat fail to stop the burner, the safety valve will lift the excess pressure will escape to atmosphere, thus preventing any further rise in boiler pressure.
A typical control panel will contain the following lights and switches, dependent on the rating of the boiler and type of burner.
- Normal Firing Light:
This is illuminated when normal firing conditions exist.
- Lock-out Light :
This is illuminated when the flame failure or second low water conditions have occurred. On some boilers ratings the lock-out light also acts as a Reset push button. The lamp when pressed will reset the control panel after lockout.
- Low water light:
This is illuminated when the water level drops to the first low water level. An alarm will sound and the burner will shut down. The burner will restart automatically on restoration of the correct water level.
- Extra low water level:
This is illuminated if the water level falls to the second low water level. Both low water lights would be illuminated, an alarm is sounded and the burner proceeds to lockout.
The Burner will require to be manually reset after the water level I restored.
- Mute Alarm / Run switch:
This is a two position switch. During normal operation this switch would remains in the RUN position, only turning to MUTE ALARM to silent the alarm.
- Fault Switch:
This is a spring loaded alarm switch which is fitted to some boiler as an alternative to the mute alarm / run switch and the press-to –reset lockout light. This switch performs the same function, i.e. stopping the alarm bell when rotated through 30o anticlockwise, or will reset the control panel after lockout when rotated through 30o clockwise. It will always return to the vertical position immediately after rotation.
- Feed Pump Switch:
The water feed pump switch has three position, HAND/OFF/AUTO. When placed in the HAND position, it will keep the pump running continuously. When placed in the OFF position, it will stop the pump running. When placed in the AUTO position, it allows the pump to start and stop under the control of the dual control.
- Burner ON/OFF Switch:
When this switch is placed in the OFF position, the burner will not fire. In the ON position, the burner firing sequence will be initiated.
- Modulation HAND/AUTO Switch:
When placed in the HAND position, the burner is held at low fire. In the AUTO position, the burner will modulate automatically to meet load demands under the control of the modulating pressure controller.
- Firing Rate Hand Potentiometer:
This item is only operational when the HAND/AUTO modulation switch is in HAND position. The dial is rotated to increase or decrease the firing rate as required.
The boiler is fitted with a modulating pressure control which measures variation in the boiler pressure, which proportionately changes the position of the modulating motor controlling the burner, to increase or decrease the quantity of fuel and air meet load demands.
Water Level Controller:
Two controls are fitted to the boiler. The dual or modulating control operates the water feed pump and shuts down the burner at the first low water level. The second control operates as an independent over-riding control, which activates the extra low (lowest) water alarm, rendering the burner in a lockout condition.
Function of over-riding control
- Activates the extra low (lowest) water level alarm.
- Renders the burner in a lock-out condition.
- Optional high water alarm can be fitted.
Function of dual (ON/OFF) control
- Operate feed pump on/off.
- Activates the low water alarm.
- Shuts burner down on first low water.
Function of dual (Modulating) control
- Maintain the water level by controlling the throughput of the modulating level control valve.
- Activates the low water alarm.
- Shuts burner down on first low water.
Water feed Pump:
The water feed pump supplies the boiler with water. When the feed pump is operating normally under automatic control, it will be switched ON or OFF according to the signal given by the dual control. On boiler fitted with modulating feed control, the feed pump runs continuously.
Burner is consist of main Isolating valve, control valve, safety shut off valves, pilot and main gas governor, gas proving system and gas pressure sensors, modulating motors, main air blower, air pressure sensor.
In our Cochran boiler, the burner is the mono-block type unit comprising of a combustion head assembly, electronically operated multi-bladed air damper, forced draught fan, gas pilot ignition and flame monitoring equipment pre-wired to burner mounted junction boxes. The operation of the burner is controlled by the Cochran micro modulating system. At the heart of the system is the control module located in the front door of the main control panel. While in our Robey boiler, the burner operated with modulating controller.
- Combustion Head Assembly:
It comprises of the ignition assembly, the primary and secondary air assemblies, photocell, front and rear swirl plates, gas jets and burner lance assembly.
- Ignition Assembly:
The ignition assembly comprises of a pilot gas tube, an electrode and a photocell. The gas pilot flame is lit each time the burner stars by an electric spark, produced by the electrode and a high voltage transformer. The gas pilot flame is used to ignite the main fuel. When the main burner flame is established the ignition gas is automatically switched off.
- Damper Motor Drive Unit:
The air inlet damper is adjusted to suit the rate of firing by a servo drive motor directly connected to the burner damper shaft.
- Modulating System:
The modulating system is a programmable combustion management system.
The modulating system shows the angular position of the air damper, motor and gas valve motor.
The position of each servo motor is monitored by a voltage dividing system enabling digitized position information to be encoded into the control modules memory. The relative position of the air and the fuel motors are constantly checked by the system during normal operation.
- Pre-purge Sequence
It commences with the operation of the forced draught fan with the dumper driven to the high fire position.
Shortly after the fan motor starts, the air sensor on the fan casing must register an increase in air pressure. If an increase in air pressure is not detected, then the system locks out.
- Light Up sequence
On completion of pre-purge, the air damper is driven to the low fire on completion of which the ignition transformer is switched on, producing the ignition spark.
- Pilot gas ignition
Once the ignition spark is established, the pilot gas valve opens. Gas is ignited by the spark. Ignition transformer switches off to prove stability of the ignition flame. If no flame is detected by the flame detector, then the system locks out.
- Normal firing
During the normal running the burner will automatically modulate via a signal from the pressure transmitter to suit the boiler load condition.
The Cochran micro modulating system is interrupted if a lockout condition occurs or the burner control is switched off. Following the burner control switch being turned off the fan continues running for a short period of time to provide a post-purge sequence.
- Post-purge sequence
Following a controlled burner shutdown the main fuel valve will close and the air damper will drive towards the fully open position. This allows the flame to extinguish without lockout. This prevents the risk of build-up of explosive gases.
- Air Pressure Proving:
The burner is fitted with a self checking air sensor. The sensor will continually check itself against commissioned values during normal operation.
- Boiler Safety Parts:
Following parts are used in the boiler for safety purpose
- Low gas pressure switch / transducer
- High gas pressure switch / transducer
- Air Flow switch
- high steam pressure sensor
- high exhaust temperature
- Water Extra Low level
- Flame failure.
- Ignition limit switch.
- Boiler Drum With Insulation
- Reversing Chamber
- Pressure Regulators
- Over Pressure Cut-Off
- Manometer With Control Valve
- Water Level Regulator And 1st Low Water Cut-Off
- 2nd Low Water Cut-Off
- Control Panel
- Main Steam Valve
- Safety Valve
- Two Water Gauges
- Flue-Gas Connection
- Safety Flap
- Feed Pump(S) Complete With Fittings
- Base Frame
- Oil-, Gas- Or Dual Fuel Burner