KST – KROMSCHRÖDER SYSTEM TECHNOLOGY
5.2.3.3 Air/gas fuel ratio
The air mass flow rate shall always be in a ratio with the gas mass flow in order to ensure safe ignition and that throughout the operating range, a stable and safe combustion is maintained at each individual burner. The ratio needs not be the same value at all operational conditions.
The design of the air/gas ratio control has to consider process conditions as well as fuel and combustion air properties. Defect or malfunction should effect that the system will tend towards higher excess air or proceed to lockout if the air/gas ratio results in an unsafe condition.
Pneumatic gas/air ratio controls shall comply with EN 12067-1 or EN 12078 if applicable.1)
Electronic gas/air ratio controls shall comply with EN 12067-2 if applicable.2)
To ensure their reliability, air/gas ratio controllers shall be used in conditions (temperature, pressure, flow rate) for which they have been designed. These conditions and instructions for maintenance shall be specified in the instruction handbook.
In case other methods/technologies are used for the ratio control and depending on the combustion air and fuel gas properties, additional protective measures shall be required (e.g. in case of frequency control of the combustion air blower, preheated combustion air, variable Wobbe index of the gas). The air/gas ratio control function shall meet the requirement of the protective system according to 5.7.2 and 5.7.3.3)
NOTE The combination of any ratio control (pneumatic or other) in combination with a flame supervision system that detects all possible incorrect ratios is generally considered to be sufficient protection.
The following chapters are printed with the permission of the DIN Deutsches Institut für Normung e.V. (German Institute for Standardization). For the implementation of the DIN Standard, the version with the most recent issue date is deemed authoritative. This is available from Beuth Verlag GmbH, Burggrafenstrasse 6, 10787 Berlin, Germany.
Our note on this Standard:
1) Conventional electronic air/gas ratio control (single-channel by mass flow rate measurements) does not comply either with the requirements of EN 12067-2 or the requirements made of a protective system in accordance with .
2) Use of a fail-safe PLC in conjunction with redundant measuring transducers allows implementation of a protective system up to SIL 2/PL d.
