KST – KROMSCHRÖDER SYSTEM TECHNOLOGY
22.214.171.124 Pre-purging of the combustion chamber
Except where specified below, start up or re-start after a lock-out (see 3.48) shall not be initiated until adequate steps have been taken to ensure that no combustible mixture is present in the combustion/processing chamber, connected spaces and flue products evacuation circuit (heat exchangers, dust extractors). This condition shall be achieved by means of a period of pre-purging immediately prior to ignition or within a time to be specified in the instruction handbook.1)
The pre-purge time shall be such as to ensure that the concentration of any combustible products in any part of the combustion chamber/connected spaces and flue duct is below 25 % of the LFL of the fuel gas; this being calculated with the combustion chamber/connected spaces and the flue duct assumed as being initially filled up with flammable gases.
In general, five complete air changes of the combustion chamber/connected spaces and flue duct will suffice2). The air flow rate used for a pre-purge shall be at least 25 % of the maximum combustion air flow rate. In the case of natural draught, the condition to achieve the above requirements shall be defined in the instruction handbook.
Inert or non flammable gases shall be used instead of air if required by the equipment or process. Other methods of ensuring that the combustion chamber and connected spaces do not contain flammable gases can be utilized providing that the equivalent level of safety is achieved.
The pre-purge time and purge procedure and or methodology shall be specified in the instruction handbook.
The system for ensuring correct pre-purge time and the airflow shall meet the requirements of a protective system according to 5.7.2 and 5.7.3.
In the following cases the pre-purge shall be omitted:
a) in applications where the presence of free oxygen can be a risk (e.g. flammable atmosphere) or shall affect the equipment (e.g. graphite crucible) or the product quality; additional precautions shall be taken to prevent gas leakage across the automatic shut-off valves by using two class A valves of EN 161 and a valve proving system;3)
b) when the combustion chamber is proved to be at a temperature above 750 °C (as defined for high temperature equipment);
c) when re-cycling a burner after shutdown for control purposes, pre-purging is not required in the following different cases:
- where the burner is fitted with an independently supervised permanent or alternating pilot;
- where the burner is fitted with two class A valves of EN 161, that close simultaneously and are equipped with a valve proving system. The valve proving system is not required for pulse fired burners;
- with pulse fired burners if the burner shut-off valve is certified by the supplier to be suitable for the increased number of cycles typical of pulse firing;4)
- in multiple burner systems when one or more burners remain alight provided that not more than one burner is extinguished due to flame failure;5)
- when the combustion chamber is proved to be at a temperature above 750 °C (as defined for high temperature equipment ) at any point where a flammable mixture coming from the burner(s) will ignite without delay.
d) in the case where one burner of a group of radiant tube burners has been locked-out maximum of one restart is permitted before lock-out (see 3.48) after a flame failure, if:
- each burner has an automatic burner control system; and
- the ignitable fuel-air-mixture inside the exhaust system is below 25 % of the LFL or the radiant tube, the burner and the connection to the exhaust are designed for the maximum pressure increase that is possible during ignition; and
- die the gas supply to each radiant tube burner is equipped with automatic shut-off valve class A of EN 161.6)
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) “Ignition” of the first burner, i.e. starting of the heating equipment, is relevant in the case of multiple burner systems.
2) Five complete air changes can be guaranteed through a combination of pre-purge time and flow rate. The protective system must ensure that the set pre-purge time is not undershot under any circumstances!
An orifice with differential pressure switch can be used to monitor the flow rate.
3) We always recommend pre-purging as the safest solution. Alternatively, pre-purging with inert gas is also possible.
4) See SIL (up to 96 switching operations per hour with two SIL 3 valves)
5) In the case of pulse firing, all burners of a control zone may be out of operation as the result of a controlled shut-down if applicable.
Standard EN 746-2 prescribes that pre-purging must be carried out if there is a fault with more than one burner. If there is a deviation from this specification, it must be recorded in a risk assessment what number of “failed” burners triggers the need for a pre-purge. Subclause must be complied with in any case.
6) Radiant tube burners must be considered as single burner systems (a separate combustion chamber per flame) and must be equipped with two valves.