THE COMTHERM ON-LINE TECHNICAL MANUAL

NOTES ON EN676

This standard is the European standard applying to fully automatic forced draught packaged gas burners.

The standard applies to gas burners intended to be marketed as a complete package.
Although primarily intended for application to packaged burners fitted to boilers and air heaters and requiring CE marking; it should be used in conjunction with EN746 when considering industrial and process applications.
The standard does state that it is not intended for burners for use in industrial processes.

Gas governors used (up to 200mbar) should comply with EN88
Automatic safety shut off valves should comply with EN161
Flame safety and ignition control units should comply with EN298
Pressure switches should comply with EN1854

Valve proving units should comply with EN1643

A automatic forced draught burner is defined as a burner in which combustion air is introduced by means of a fan and that is fitted with automatic ignition, flame monitoring and burner control devices and capable of being controlled on - off automatically by a remote switch.

The reference conditions used in the standard are 15C and 1013.25mbar.

The standard uses the following abbreviations :-

Gross calorific value - H_s
Net calorific value - H_I
Relative density - d
Gross Wobbe index - W_S
Net Wobbe index - W_I
Heat input - Q

The units used by the standard are :-

Heat - megajoules
Volume - metres
Pressure - millibars
Heat input rate - kilowatt

The standard defines the two types of burner lockout as follows :-

A non-volatile lockout is a safety shut down such that a restart can only be made by a manual reset of the system and by no other means.

A volatile lockout is a shut down of the system such that a restart can only be accomplished by either manual reset or an interruption of the main electric power supply and its subsequent restoration.

The following sequence times are referred to in the standard and require clarification :-

The first safety time is the period between the pilot/start gas valve opening and then closing because of the absence of a flame.
When there is no second safety time this is referred to as the safety time.
The second safety time is the period between the main gas valves opening and then closing due to the absence of a flame.

The following remarks concerning the gas soundness of the burner should be noted :-
Holes for screws, studs etc. Intended for fixing or assembly of parts, brackets etc. should not open into gasways.
The soundness of parts likely to be dismantled during maintenance should be achieved by metal to metal joints, gaskets or 'O' rings -
sealing materials such as tape, paste or liquids must not be used.

Every burner shall be fitted with an adjustable air damper for controlling the air flow; this device shall be adjustable only by means of a tool. The adjusting positions of the air damper shall be visible possibly by removing a cover. Ant damper should be capable of being set and sealed and the position noted.

All gas line components shall be suitable to accept the inlet pressure or be protected against excessive pressure by means of relevant safety devices.

A filter shall be fitted at the inlet of the safety shut off valve system; the maximum filter hole dimension shall not pass 1mm pin gauge.

The gas supplies must be under the control off a device e.g. Gas governor to keep the pressures within 5% of the specified control pressures.

The burner shall be fitted with a device e.g. Low gas pressure switch to shut off the burner when the supply pressure falls too low.

A high gas pressure switch must be fitted if no governor is fitted on the burner.
A high gas pressure switch when fitted must cause non-volatile lock-out.
Any high gas pressure switch should shut down the burner either when the heat input exceeds 15% more than the nominal heat input or the gas pressure exceeds 30% of the nominal required gas pressure.

An adjustment device (Gas governor is acceptable) shall be provided for setting of the gas pressure at the burner head.

The mounting of any flame sensor should be such that it cannot receive extraneous light.
When burners have separate pilot burners, the pilot burner should not remain in use during main flame operation i.e. no continuous pilots, or alternatively separate flame sensors should be fitted on the main and pilot flames.

The burner shall be fitted with a device e.g. combustion air pressure switch to prove combustion air availability; failure of such a switch should cause non-volatile lockout.

The air flow device shall also be proved in the no flow state prior to start up.

The combustion air pressure switch should operate before the pressure is less than 80% of the required pressure and the CO content of the combustion products exceeds 1% by volume. For multi-stage, (high-low) and modulating burners only one air proving device is necessary - the system must tend to excess air operation during increases or decreases of the firing rate.

Burner start up shall be possible only after all safe start interlocks have been completed, the flame detector has been checked for flame simulation, any valve proving system has completed its check successfully and the function of the combustion air switch has been proved.

Before ignition any combustion chamber must be purged; the pre-purge rate should be 100% of combustion air rate for a minimum of 20 secs. Or pro-rata.
A pre-purge must occur after every safety shut down.

The start gas rate shall not exceed 120kW or the value given by the t_s x Q_s = 100,
Where t_s is the safety time in seconds, and Q_s is the start gas rate as a % of main gas rate.

For burners with start gas rate above 120kw with the start gas taken from between the two safety shut off valves, the downstream valve must be closed prior to start up, the ignition safety time shall be determined by the equation t_s x Q_s = 100 but in no case should exceed 5 seconds.

Table B gives details of allowed ignition times related to start gas heat input.

A working diagram for each burner should be available i.e. a diagram showing burner thermal capacity and back pressure data.

Although not applicable to process gas burners the figures stated in this standard for emissions can be taken as a guide for good burner performance; particularly when firing combustion chambers.

Under normal operating conditions the CO content should not exceed 100mg/kWh
(93 ppm).
The standard does specify maximum CO contents for operation of the burner under fault conditions with combustion air shortage or excess gas.

NO_X levels should not exceed 170mg/kWh when using natural gas and 230mg/kWh on propane.

All burners shall be provided with two automatic safety shut-off valves in series as defined in table A and complying with EN161.
Where the main flame establishment is by means of a start gas flame, the start gas supply shall be under the control of the downstream gas safety shut-off valve incorporating a start gas limit or under the control of valves as specified in table A.

Burners should operate within the limits of this standard when supplied with an electric supply voltage of between 85% and 110% of the specified voltage.

Table A, Safety shut off valve requirements.

Table B. Maximum start gas heat inputs and safety times.

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