THE COMTHERM ON-LINE TECHNICAL MANUAL

NOTES ON EN746

This standard is the European standard applying to industrial fuel and thermo-processing equipment.
Part 2 of the standard refers specifically to combustion and fuel handling systems.

The standard applies to packaged gas burners and special burner systems installed on process plant.

This web page article does not pretend to cover the standard completely, but simply attempts to draw attention to facts of particular interest to Comtherm customers and personnel.

Process plant includes furnaces, kilns, ovens, salt baths, melting tanks, casting machines, ladle heaters etc. The standard applies to the handling of fuel immediately adjacent to the equipment but downstream of the plant fuel isolating valve.

The standard applies to all forms of gaseous, liquid and solid fuel.

Gas governorsused (up to 200mbar) should comply with EN88
Automatic safety shut off valves should comply with EN161
Manual ball and taper plug valves should comply with EN331
Flame safety and ignition control units should comply with EN298
Pressure switches should comply with EN1854
Valve proving units should comply with EN1643

The standard defines the dividing point between low temperature and high temperature equipment as 750C.

Screwed pipe-work connections should only be used on sizes up to DN25 up to 5bar pressure, sizes up to DN50 up to 2bar pressure and sizes up to DN80 up to 100mbar pressure.

Pipe fittings and joints above DN80 size should be flanged or welded joints.

Flexible connections when used should be as short as possible and have a directly accessible upstream valve; the screwed fitting on the end of the flexible should be an integral part of the flexible connection.

The gas circuit should be tested at 1.5 times the maximum working pressure with a minimum of 50mbar.

A filter shall be fitted at the inlet of the safety shut off valve system immediately downstream of the first manual isolating valve.

The safety shut off requirements for various burner sizes and applications are shown in table A.

Table A, Safety shut off valve requirements for forced and induced draught burners.

The safety shut off valve system should shut off the fuel if the following conditions occur :-

Flame failure
Insufficient gas flow
Gas pressure above a safe maximum
Insufficient combustion air flow
Failure of power supply
Failure of heat transfer fluid
Fume extraction malfunction
Excessive plant temperature.

If the outlet side of the gas governor or other burner components have not been approved for operation with the maximum supply pressure under fault conditions then a high gas pressure shut off valve shall be fitted upstream of the gas governor; in addition a safety relief valve shall be fitted downstream of the gas governor - this shall be vented to a safe area.

The governor, high pressure shut off valve and relief valve may be integrated in one apparatus.

Burners fitted with forced or induced combustion air must be fitted with a device to prove combustion air flow; this device shall be checked in the no flow state prior to start up.

Air flow may be monitored by static air pressure sensing where it can be shown to be a satisfactory method or by flow sensing.

A low gas pressure switch should be fitted when no flame safeguard system is installed; this switch should cause lockout.

A high gas pressure switch must be fitted in all circumstances except when :-

1. The pressure drop across the gas pressure governor is less than 30% of the normal operating minimum governor pressure.

2. Governor failure does not result in an unsafe start gas rate.

3. The equipment capacity is below 600kW and the supply pressure is below 100mbar.

A high gas pressure switch when fitted must cause lock-out.

When the gas supply for the ignition burner is taken from upstream of the main burner governor the ignition burner supply must be fitted with a suitable governor.

For safety requirements with regard to governors, filters, manual valves and safety shut off valves the ignition burner must be treated in a similar way to the main burner

. The ignition burner should be an integral part of the burner equipment and fixed in position.

Each individual burner should be fitted with a manual shut off valve.

The combustion or processing chamber must be pre-purged before ignition - re-start after a lockout condition shall commence with a pre-purge.

Pre-purge is not required when the combustion chamber temperature is proved to be above 750C.

In applications were free oxygen may affect the product the pre-purge may be omitted but the safety system must be fitted with two class A valves and a valve leak proving unit.

Pre-purge is not required on re-cycling after shutdown for control purposes where :-

1. The burner is fitted with an independently supervised permanent or alternating pilot.

2. The burner is fitted with two class A valves and a valve leak proving unit.

3. In multiple burner systems when at least one burner per zone stays alight.

4. When combustion chamber temperature is above 750C.

Pre-purge shall ensure that the concentration of combustibles in the chamber and flue space is below 25% of the lower flammable limit; this being calculated with the combustion & processing chamber and flue duct assumed as being 100% filled with flammable gases.

In general five complete air changes of the chamber and flue space will suffice.

The pre-purge air flow rate shall be at least 25% of the full burner air flow rate.

The energy released during start fuel flame ignition period shall be limited so that the maximum pressure rise from delayed ignition does not damage the plant.

In the case of 2nd family gases where the fuel:air ratio at start up exceeds 33% of the stoichimetric mixture the energy release should be limited to 53kJ.per m³ of total chamber volume for every 100mbar pressure rise that the chamber can withstand. (22% for other family gases.)

Burners with heat inputs up to 350kW may be ignited directly.

The ignition and extinction safety times that shall not be exceeded are given in table B.

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

In the case of forced draught burners with ignition rate below 33% of full rate but with full air flow (lean firing) the ignition safety time may be extended to 5 seconds.

If required for process reasons longer times can be accepted provided that they do not compromise plant safety.

For forced draught burners in any case safety times shall not exceed 10 seconds; although in the case of cross-ignited burners an extension of 1.5 seconds per metre of burner length, with a maximum of 10 seconds is acceptable provided the flame is monitored at the end of the burner remote from the ignition.

In the event of flame failure during start up the burner shall go to lockout; although in certain circumstances up to three re-cycles are allowed provided plant safety is not compromised.

In the case of flame failure during the run condition the burner shall go to lockout; although in some cases one attempt to re-light is allowed provided plant safety is not compromised.

The main flame and if applicable the ignition burner flame shall have a flame safeguard fitted, where the ignition and main burner have separate flame sensors, the ignition flame shall not influence the main flame sensor.

Where a burner is required to fire continuously in excess of 24 hours, the flame safeguard shall be of the self checking type or shall be regularly checked for correct operation.

Low temperature equipment fitted with a single burner shall always be fitted with a flame safeguard.

For low temperature equipment fitted with several burners, each burner shall be fitted with a flame safeguard.

With high temperature applications a flame safeguard shall be fitted to operate during start up whilst the chamber temperature is below 750C.

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