we are using Natural gas as fuel gas and the settings are 13.2 kg/cm2 low limit and 18.4 kg/cm2 as high limit. what is the criticality of fuel gas pressure apart from proper combustion and high pressure gas might turn off the flame?

 

For natural gas to flow into an engine its pressure must be higher than the pressure inside the engine; if the gas pressure is low then it may not be possible for enough gas to flow into the engine to maintain load and speed and the flame will eventually be extinguished even if the control valve is fully open.

If the natural gas pressure is too high the control valve may have to close considerably to maintain load and speed and sometimes when control valves are closed to very low values they don't regulate flow very well.

There is also a safety concern when gas fuel pressure is too high--piping and flanges and components (control valves; stop valve; fuel nozzles; etc.) have maximum pressure limits which if exceeded can result in rupture and or failure to operate properly (as in a stop valve might not close if the incoming pressure is too high.

Lastly, many fuel nozzles and injectors have a maximum pressure drop which if exceeded will result in improper fuel atomization and unstable combustion. Also, if the pressure drop across the nozzle is too low then incomplete combustion can also occur resulting in unstable operation. Damage to combustors can also occur if the pressure drop across the fuel nozzle is too high or too low, depending on the engine and the design of the combustor.

Hope this helps! Contrary to popular belief, the setpoints are not chosen to be a nuisance to operators; they are chosen to ensure safety as well as optimal performance while still allowing some range of variability. Designing an engine and components which would operate over a wider range of supply pressures would be much more expensive and probably much more complicated. For example, it might be necessary to add a compressor to increase the fuel supply pressure and the compressor and necessary controls would also require maintenance and spare parts which would increase the cost of operating the plant.

Or, a special pressure regulator might be necessary to limit pressure if it gets too high, and sometimes that even requires two or more regulators and auxiliary controls which also require tuning maintenance and additional spare parts.