Thursday, December 11, 2014

Industrial Burners and Safety Systems - Part Three

CTi Combustion

Igniters

Igniters provide proven ignition energy to immediately light-off the burner. They are permanently installed. Igniters are classified as follows by NFPA:

  • Class 3 special: High energy igniter (HEI) capable of directly igniting the main burner fuel. Generally consists of a spark-rod, and power pack to deliver the high voltage pulse train, and required cabling. Operation time of igniter is no longer than required to light-off burner, within maximum allowed trial-for-ignition time.
  • Class 3: Low capacity igniter applied particularly to gas and oil burners. Ignites the fuel input to the burner under prescribed light-off conditions. The range of class 3 igniters generally do not exceed 4 percent of full load burner input. Operation time of igniter is not longer than required to light-off the burner, within the maximum allowed trial-for-ignition time.
  • Class 2: Medium capability igniter applied particularly to gas and oil burners to ignite the fuel input to the burner under prescribed light-off conditions. The range of class 2 igniters generally is 4 to 10 percent of full-load burner input. Class 2 igniters may remain in operation to support ignition under low-load or adverse operating conditions. Class 2 igniters cannot be used to extend main burner turn-down range.
  • Class 1: High capacity igniter used to ignite the fuel input through the burner. Supports ignition under any burner light-off or operating conditions. Its location and capacity provide sufficient ignition energy at its associated burner to raise any credible combination of burner inputs of both fuel and air above the minimum ignition temperature. Tests are to be performed with this ignition system in service to verify that the igniter furnished meets the requirement of this class as specified in its design. Class 1 igniters can be used to extend the main burner’s turndown, where they are in service and flame if proved.
The Ignition subsystem must be sized and arranged to ignite the main burner input within the limitation of the igniter classification. Many factors affect the classification and subsystem of igniters, including the characteristics of the main fuel, the furnace and burner design, and the igniter capacity and location relative to the main fuel burner.

Burner configuration is dependent on boiler furnace design and configuration. Some burner configurations in multi-burner boilers are:
  • Front fired. Burners are located in only one of the furnace walls, nominally the front of the boiler. Variations in arrangements of front-fired applications can include a single row of burners at one level, to multi-burners per level at many levels. Four levels of three burners (3 over 4), or three levels of six burners (6 over 3), etc. would be common multi-burner configurations.
  • Opposed fired. Burners are located in two of the furnace walls, opposite each other, firing toward the center of the furnace. The same variations in burner arrangements as in frontfired may apply to opposed fired burners.
  • Tangential fired. Where burners are located in the corners of the furnace, firing tangentially into the furnace. This creates a large fireball in the center of the of the furnace area. Tangentially fired boilers have four burners per level, and variations are in the burner decks.
By law, boilers must be operated with the installation and check of a flame safeguard and instrumentation system. Boiler control is divided into two groups — the combustion control system and the burner management system.

The combustion control system regulates the furnace fuel and air inputs to maintain air-to-fuel ratio. This ratio must be within the limits required for continuous combustion and flame stability throughout the operating range.

The burner management system must be a stand-alone system dedicated to boiler safety. It assists the operator in safe starting and stopping of burners while preventing operator error. A burner management system (BMS) includes the following components.
  • Interlock system.
  • Fuel trip system.
  • Master fuel trip system.
  • Master fuel trip relay.
  • Flame monitoring and trip system.
  • Ignition subsystem.
  • Main burner subsystem. 
This blog post is excerpted from the paper titled "Flame Safeguard Controls in Multi-Burner Environments" by Willy Vandermeer and courtesy of Fireye. The entire document may be downloaded here.