DL/T 435-2004 Code for the Prevention of Pulverized Coal Firing Furnace Explosion/Implosions in Power Plant Boilers
1. Scope
This Code specifies basic requirements on relevant equipments and system aspects for prevention of pulverized coal firing furnace explosions/implosions in power plant boilers, shows out guide for the on/off sequence and operation of equipments.
This Code is applicable to pulverized coal firing furnace in power plant boiler.
2. Terms
For the purposes of this national standard, the following terms apply.
2.1
Main primary air shut off gate
It is a kind of main air gate which can be closed tightly and prevent the primary air entering the burners; generally, it shall be mounted on the pulverized-fuel-free airflow, such as primary air fan outlet of positive pressure unit pulverized-coal system or coal pulverizer inlet of negative pressure unit pulverized-coal system and storage pulverized-coal system.
2.2
Flame detector
The equipment to detect the flame-free and provide signal
2.3
Directional blocking
A sort of interlock system moves to the likely-to-be enlarged deviation direction and all the relevant final control elements must be prohibited when the furnace pressure is tested to deviate from the normal value greatly.
2.4
Fan test block capability
Fan TB point for short; when selecting the fan, it is the corresponding points required by the flow-head curve, that is, when the boiler is in maximum continuous rating, it is the capability for the point on the fan curve correspondingly after adding certain allowance to the calculated air volume-pressure head.
2.5
Fan control device(s) override action
A kind of control action, namely, when the furnace pressure is tested with sudden great deviation, it is the action of draught fan control device that has precedence over the normal self-action, and it may be regulated rapidly to the position keeping the normal negative pressure for the furnace.
2.6
Master fuel trip ("MFT" for short)
Once the working conditions endangering the boiler safety happen, cutoff all the furnace-entering fuels, including fuel of ignitor, which is part of the primary functions of the furnace safety supervisory system.
2.7
Master fuel trip relay
It is the relay used for parallel off the required equipments simultaneously when master fuel trips.
2.6
Loss all flame
The fuel and air entering into the furnace cannot be burnt to be converted into burning products; all furnaces have no abnormal conditions of visible flame.
2.9
Partial loss of flame
Individual loss of flame in the furnace, while other burners still keep their firing phenomenon
2.10
Ignitor safety shutoff valve (fuel trip valve)
Response to the fuel trip command, automatically and completely switch off the shutoff valve for fuel entering the ignitor.
2.11
Ignitor
It can provide adequate ignition energy for igniting the permanent devices (including sparker, igniting oil and popgun) of the main burner fuel for an instant. The capacity of the ignitor as close relationship with the fire behavior of the main fuel, burner type and requirement for the ignitor function, generally, it consists of Class-I and Class-II ignitors.
Class-I ignitor: used to ignite the gas-pulverizer mixture entering the furnace from the main burner in any operating condition. This type of ignitor requires large capacity; generally, it shall be over 20% of the heat quantity of the main burner under full load.
Class-II ignitor: can only be used to ignite the gas-pulverizer mixture entering the furnace from the main burner according to the air volume and pulverized-fuel volume when igniting, its capacity is generally 10%~20% of the heat quantity of the main burner under full load.
The above two ignitors are both be used for combustion-supporting under part load or when the burning is instable, however, it may not be used as means for saving extinguishment when the furnace has been fire-extinguishing but not yet been purged according to the requirements.
2.12
Warm-up oil gun
It is the oil gun for warming up boiler, lifting pressure, washing pipe and with part load, while its thermal power hereof is larger than that of the ignitor (usually, its total thermal power is about 30% of the thermal power for the full loaded boiler).
2.13
Furnace safety supervisory system ("FSSS" for short)
It is a kind of security protection and sequential control equipment that ensures that the equipments of the boiler combustion system is safety on/off according to the specified working order and conditions, and that can rapidly cut off all the fuel (including igniting fuel) entering the furnace in critical operating conditions, so as to prevent destructive emergencies like deflagration and explosions.
Note: the primary functions of the international so-called "burner management system", "fuel-firing safety system" or "burner control system" are identical to FSSS defined by this Code.
2.14
Logic portion
The logic portion, a kernel of FSSS, may be consisted of relay or solid-state component or programmable logic controller. All the safety protection, operation, monitor and alarm function must go through logic operation; therefore the signal may be judged correctly.
2.15
Alarm
Display operation condition deviate from the standard limitation by sound and optical signal, or other deviant operating conditions
2.16
Interlock
It is a sort of control program, which is used to make equipment trip or shunt down when its operational parameter has been reached the limitation or deviated from the limitation; or it may be used to interrupt the program when the operation order is incorrect; or when some equipment trips, the direct correlated equipments must interlock trips therewith, lest emergency expansion.
2.17
Air rich
The furnace/burner-supplied air to fuel ratio is larger than that when the fuel is under optimal combustion, namely, the excess air is greater than the optimal value.
2.18
Fuel rich
The furnace/burner-supplied air to fuel ratio is less than that when the fuel is under optimal combustion, namely, the excess air is less than the optimal value.
2.19
Adjustable air damper (register)
A set of air handling damper loaded in front of the burner, which is used for the distributing the air capacity of burners entering the furnace according to the demand; the ventilation damper or vane loaded in the rotational flow burner is used for regulating the rotational flow of the air flow, to make the air and pulverized-fuel get their required mixing, and absorb high temperature flue gas in the boiler, so as to make the pulverized-fuel entering the furnace be on fire and burnt.
Foreword II
1. Scope
2. Terms
3. Relevant Requirements on the Prevention of Furnace Explosions
Appendix A (Informative) Causes likely for Furnace Explosions
Appendix B (Informative) Explanation on the Transient State Pressure-bearing Capability designed for the Furnace
Appendix C (Normative) Fuel Trip's Time of delay and Additional Alarm Project
