MIXTURE PRESSURE AND SAFETY HEAD MONITORING.
All MR-5000 Combustion systems include mixture pressure and safety head monitoring as standard in addition to the normal main gas safety shut-off system. This system includes safety heads incorporating a micro-switch installed in the mixture pipe work upstream from the burner manifolds on each side of each combustion zone, a low mixture pressure switch and a high mixture pressure switch installed in each individual mixer outlet and a solenoid shut-off valve in the gas supply inlet to each individual mixer. These components are all electrically connected to a mixture pressure and safety head monitoring control panel mounted on the combustion skid unit. The shut-off valve and pressure switches are pre-wired to the control panel on the combustion unit. The safety head switches must be wired from the safety heads mounted in the mixture pipe work to the control panel. Safety heads are installed to provide relief of the shock wave pressure caused by a backfire, temporarily opening to relieve the pressure and stopping the back fire and then immediately closing to re-seal the pipe work. This prevents damage to equipment upstream. Whilst the safety heads are very effective at relieving the pressure caused by the backfire they do not correct the cause of the back fire which must then be determined and corrected. European regulations state that indication and alarming is required when a safety head has operated. In the event of a backfire the safety head switch operates and indication and alarming is provided on the control panel and an additional alarm output signal is also provided for remote indication in a central supervisory temperature control system. In addition the gas supply to the individual combustion zone is shut-off. This allows the cause of the problem to be investigated safely whilst reducing the effect the problem would have on production if the main gas safety shutoff valves were closed shutting off the gas supply to the entire forehearth. The micro-switch in the safety head is a latching switch which must be re-set with a pushbutton switch on the safety head following examination of the condition of the safety head to ensure that it has closed and is sealing.
One primary cause of backfiring is the manifold mixture pressure being lower than the pressure inside the forehearth which then allows hot gases from the forehearth atmosphere to flow back into the burner nozzles igniting the mixture. Three safe guards against this occurring are already in place with the combustion actuator being calibrated to provide a minimum manifold mixture pressure of 1 inch (2·5 mbar) at the minimum control output position, the closed position limit switch inside the combustion actuator being set to provide a minimum manifold mixture pressure of 0·75 inches water gauge (1·875 millibar) and the physical stop on the mixer screw carrier being set to provide an absolute minimum manifold mixture pressure of 0·5 inches water gauge (1·25 millibar) during commissioning. However, a minimum mixture pressure switch is also installed in the mixture outlet of each mixer and set at typically 0·4 inches water gauge (1 millibar) to operate and shut-off the gas supply to the mixer with the solenoid valve if the manifold mixture pressure falls below 0·5 inches water gauge (1·25 millibar) further preventing a backfire from occurring. The cause of the low manifold mixture pressure would still the require investigation and could be due to a leak in the mixture pipe work or burners or incorrect setting of the mixer minimum physical stop screw.
The normal maximum manifold mixture pressure at maximum firing for each combustion zone is 20 inches water gauge (50 millibar). A maximum manifold mixture pressure switch is also installed in the mixture outlet of each mixer and set at typically 28 inches water gauge (70 millibar). In the event that the manifold mixture pressure exceeds this value the pressure switch operates to shut-off the gas supply to the mixer with the solenoid valve. The cause of the high manifold mixture pressure would then require investigation and could be due to a downstream valve in the mixture pipework having been inadvertently closed, blockage of the mixture pipework or burners or failure of the safety head to operate and relieve the pressure from the shock wave during a backfire.
Normally one common alarm is used from the mixture pressure and safety head monitoring control panel to the main temperature control panel and individual indication of alarm conditions for each combustion zone are provided on the mixture pressure and safety head monitoring control panel and individual pressure switches.
As a further enhancement to this system, individual zone alarms are available from the mixture pressure and safety head monitoring panel to the main temperature control panel to indicate that the gas has been shut-off to the particular zone due to an incorrect mixture pressure or operation of the safety heads in that zone. If normal temperature control continued under these circumstances then, as the temperature would fall due to the loss of gas supply, the control system would proceed to open the combustion actuator, possibly eventually to maximum, to increase the zone firing in an attempt to increase the zone temperature to set point. However, as only combustion air would be passing through the mixer this would actually result in a further, possibly more rapid, reduction in temperature. Under these circumstances the alarm signal from the mixture pressure and safety head monitoring panel for the individual combustion zone can be used to automatically put the particular control zone into manual with minimum firing and minimum combustion air to help to conserve the zone temperature until the problem can be corrected and the gas supply to the zone re-established.
Another cause of backfiring is blockage of the burner nozzles due to inadequate maintenance. This has recently been addressed by the introduction of ceramic burner nozzles which are much less likely to become blocked and require much less maintenance.
Leak detectors are valve proving systems that have been used by PSR for many years in the main gas safety shut-off systems as a start-up check to ensure that the main gas safety shut-off valves have completely shut-off and sealed before they can be re-opened. European regulations stipulate that automatic shut-off valves controlling capacities greater than 1,200 kW shall be equipped with a valve proving system. Although the majority of the combustion units supplied by PSR are below this capacity, we equip all our safety systems with a leak detector unit as standard.
DUTY AND STAND-BY COMBUSTION AIR FANS AND MAIN GAS SAFETY SHUT-OFF SYSTEMS.
Each forehearth and its associated Distributor or Alcove Section supplying it has duty and stand-by combustion air fans and gas safety shut-off systems installed as standard. These provide back up in the event of a combustion air fan or gas safety shutoff system component failure or for routine maintenance and testing. The advantage of this configuration is that a component failure on one system would only temporarily affect one production line on an installation with a distributor and several forehearths.