44
NATURAL GAS IGNITION AND ADJUSTMENT
N.B. See end pages for detailed description of gas supply adjustment depending on type of valve fitted to burner.
Before igniting, it is essential to discharge air in the pipes and check, if burner is three-phase, that motor rotation
direction is correct. Next, proceed as follows:
1) Ensure that combustion waste can be discharged without any hindrance (chimney gate open) and that there is
water in the boiler.
2) Open the combustion air regulator (see BT 8653/1) to the extent considered necessary and open to about one third
the air passage between blast tube and disk, using the flame disk adjustment screw.
3) Operate the relief valve and “first flame” regulators so that desired quantity (start-up flow rate) of gas in supplied.
N.B. See end pages for detailed description of gas supply adjustment depending on type of valve fitted to burner.
4) Switch off the second flame thermostat and switch on the burner by operating main and burner (natural gas setting)
switches. In this way the burner is cut-in and carries out the pre-ventilation phase. If the air pressure control
pressure switch reads pressure above its preset value, first switch on the ignition transformer and then the gas
valves (relief and first flame). The valves open fully is limited to the setting to which the flow rate regulator (built-into
the first flame valve (pilot) has been manually set. On first ignition, further “blocks” can occur due to insufficient air
clearance of gas pipes. There is therefore not enough gas to give a stable flame. A “Block” with flame present can
also be caused by flame instability due to incorrect air/gas ratio. This can be remedied by varying quantity of air/
gas supplied in order to find correct ratio. This can be remedied by operating the blast tube adjustment device,
closing (unscrew the blast tube position adjustment screw in the case of COMIST 36 and move forward the blast
tube fixing knobs in the case of COMIST 72 and COMIST 122) or opening (screw the blast tube adjustment screw
in the case of COMIST 36 and move backward the blast tube fixing knobs in the case of COMIST 72 and COMIST
122) more the air passage between blast tube and flame disk (see BT 8608/1 and BT 8608/3).
5) While burner is operating, adjust supply to desired value for “first flame” (natural gas = 8550 kcal/m
3
), taking a
reading at the meter. This flow rate can be modified by using the regulator built-into the valve, as described above.
6) Check that combustion occurs correctly (use the appropriate instruments). Maximum permitted level of carbon
monoxide (CO) is 0,1% whereas permitted range for carbon dioxide (CO
2
) is 8 ÷ 10%.
7) After adjusting, switch burner on and off several times to check that ignition is correct.
8) Switch off burner via the main switch to make a direct (bridge) connection between the terminals of the second
flame thermostat. Adjust combustion air to setting considered suitable for switch-on of second flame (see BT 8653/1).
Also open the gas flow regulator (built-into the second valve) to obtain a supply considered suitable for the main
flame and set the 1st and 2nd stage switch in “2nd stage” position.
9) Now switch on main switch to ignite burner. While burner is operating on second flame, check gas supply (meter
reading) as previously described. Depending on readings, adjust gas supply if necessary, to adapt it to actual value
required (boiler output). Using suitable instruments, check that combustion is correct (CO
2
= 8 ÷ 10 % for natural
gas - CO max. = 0,1%). To improve combustion and for good flame stability (no pulsations) it may be necessary to
operate the blast tube adjustment device. If burner operates on low fuel supply, the air passage between disk and
blast tube should be reduced (unscrew the blast tube position adjustment screw in the case of COMIST 36 and
move forward the blast tube fixing knobs in the case of COMIST 72 and COMIST 122). When the burner is operating
at a higher fuel supply level, this passage should be opened proportionately (screw the blast tube position adjustment
screw in the case of COMIST 36 and move backward the blast tube fixing knobs in the case of COMIST 36 and move
backward the blast tube fixing knobs in the case of COMIST 72 and COMIST 122). If flame disk position is modified,
it is usually also necessary first to correct the positions of the first and second flame air regulation gates.
Then, check that ignition is correct.
SAFETY CHECKS
After adjusting, always check the following:
1)
Burner stop: open air and gas thermostats and pressure switches.
2)
“Blocking”: shadow the photocell (UV).
To unblock, push the appropriate button.
