There are many important parameters to be monitored during any
Power plant normal & emergency operations. Furnace pressure is one such very important parameter, which needs continuous control, protection & monitoring against any furnace explosion and implosion. NFPA 85: Boiler and Combustion Systems
Hazards Code are dedicated to fire and furnace explosion and implosion protection. The NFPA also requires some additional logic for the furnace pressure control loop to ensure adequate operating safety margins.
There should be high and low furnace pressure logic to block the ID fan from increasing or decreasing speed, as is appropriate. A high furnace pressure signal the fan should be blocked from decreasing speed and on a low furnace pressure signal it should be blocked from increasing speed.
A main fuel trip (MFT) there should be MFT kicker logic. An MFT occurs when the burner management system detects a dangerous condition and shuts down the boiler by securing the fuel per
NFPA and boiler manufacturer requirements
Boiler is a major equipment in any Thermal Power Plant, need various control & protection for safeguard of big & complex mechanical component. While designing the boiler great care have to be taken for safety of each component.
Control & protection system will take care of any operational mistake and process abnormality. Control system ensure the sequential & safe operation of the boiler. Important parameters to be monitored during any Power Boiler normal & emergency operations.
Furnace Pressure is one such very important parameter, which needs continuous control, protection & monitoring. This is very important from safety of the boiler. Why to maintain furnace pressure? In case of very low and very high pressure the furnace may subject to explosion and implosion, which may result in Boiler Structure and pressure parts, tubes damage and furnace deformation.
FURNACE EXPLOSION
Furnace explosion is rare, but very severe in nature. This situation exists because furnace is supplied with explosive accumulation. Just a minute part of those explosive charges receive sufficient ignition energy to actually cause an explosion.
The Furnace explosion requires both sufficient explosive accumulation and sufficient energy for ignition. The ignition requirements for an explosive charge are very small, making it possible to protect against all possible sources of ignition, such as electricity discharges, hot slag and hot furnace surfaces.
So practical means of avoiding furnace explosion is the prevention of an explosive accumulation.
The factor determining the magnitude of furnace explosion (change in composition, mass and reaction time) are related in explosion factors
Explosion Factor = (Mass/Furnace Volume) x Composition Change/Elapsed Time)
Each Furnace has a limiting explosion factor. If the conditions create an explosion factor exceeding this limit, a catastrophic explosion can result. Any lesser reaction will produce a furnace “puff” (sudden increase is pressure i.e nondestructive explosion.) or a temporary upset in furnace pressure.
To protect furnace from such incidences a safety system must ensure a minimum reactive mass accumulation and with minimum available composition charges and with maximum reaction time required. Only control of the composition of the furnace atmosphere offers complete coverage in minimizing the explosion factor.
After firing has begun, furnace always contain sufficient mass to have an explosion and control of time factor is not possible. Therefore, the composition change must be controlled to prevent furnace explosion.
The accumulation composition, which must be within the limits of flammability for that particular fuel, is formed in one or more basic ways. A flammable input into any furnace atmosphere (loss of ignition). A fuel rich input into an air rich atmosphere (fuel interruption)
An air rich input into a fuel rich atmosphere (air interruption) For positive furnace pressure, a value which would be the lesser of either +35”wg or FD Fan test block capability at ambient temperature.
For negative furnace pressure, a negative value whose absolute value would be lesser of either the absolute value of -35”wg or the absolute value of ID Fan test block capability at ambient temperature.
FURNACE IMPLOSION
Boilers with both induced and forced draft fans may become unbalanced especially if the forced draft unit becomes tripped and the induced fan unit remains in full operation.
The induced draft fan will produce an excessive draft in the furnace and create the real likelihood of furnace implosion. Negative pressure excursion of sufficient magnitude to cause structural damage.
In case of ID Fan is capable of producing more suction head than the boiler structure is capable of withstanding. Control malfunction and / or operator error, establish such conditions.
The other process called flame collapse or flameout effect. The negative pressure excursion following a fuel trip and loss of furnace flame, in order to realistically evaluate pit falls and prevention techniques.
The resident mass is automatically balanced by controlling the flue gas flow out of the boiler to maintain a given furnace pressure. When the fuel input is terminated, this balance no longer exists.
The flue gas being pulled out f the furnace by the ID fan is now being replaced only by preheated air rather than by the products of combustion in the firing zone.
The average temperature of the gases resident in the furnace (or other subsystem) at any given time following fuel trip will decrease rapidly. Because of the temperature drop, the pressure in the furnace starts to decrease.
Positive Draught System:
FD Fan is there which is used to suck air from atmosphere and push it into boiler through Air Preheater where it will be preheated with the help of flue gases and then it will helps in carrying the coal to the Boiler (PA Fan) and support for addition Combustion & Turbulence.
If we only used the FD & PA Fan means then in the boiler, pressure will be more than the atmospheric pressure
This is known as Positive Draught System.
If you are using a Two Pass Boiler means after it attaining the reheater region it won’t moves down and heat parts naturally because the density of flue gas will be lighter because it is hot.
So we have to apply more to drive it. i think may one of the disadvantage of the Positive Draught System.
Negative Draught System:
If only ID Fan is available means then it may suck the inside air or Combusted gases from the boiler and push it out through Chimney.
Due to that a Negative pressure may be maintained in the Boiler which may be less than the atmosphere.
Balanced Draught System:
if Both the FD & ID are available and a balanced draft is maintained means then it may be named as a Balanced Draft System.
Nowadays, everyone are using a Balanced Draft System, and maintaining a little low pressure below atmosphere (Negative Draft) that may give an advantage over the system such as explosions & implosion etc.