Boiler panting

If a boiler was open ended to atmosphere then boiler panting would not occur. However it is not, instead combustion products must flow over a whole range of items all of which contribute to a pressure drop indicated as P drop. For example, screen tubes, generating tubes, superheater tubes, economisers etc. All of these items cause a pressure drop which varies according to the combustion variations.

Mechanism of panting

The system shown above is considered to be in steady state. The windbox pressure is at a slightly higher pressure than the furnace pressure which is at a higher pressure than atmospheric.

If there was a sudden disturbance to the plant, for example, poor combustion caused by say low atomising steam pressure then combustion of the fuel would be less efficient. The pressure in the furnace will drop, the P drop increases and the mass/volume of the furnace gases increases. The actual volume of the gas has however reduced.

The furnace pressure drop will then cause increased air flow from the windbox ( after some period allowing for inertia). The density of the air remains high and Pdrop remains high.

This in rush of air into the furnace aids the combustion process of the flame and also burns up any fuel products not completely combusted. This has the effect has the effect of reducing the density of the furnace atmosphere, increasing its volume, reducing Pdrop and increasing furnace pressure.

The flow of air from the windbox reduces as the pressure differential reduces. The poor combustion of previous is re-established and the whole process is repeated.

The cycle time will depend on the aggravating process i.e. in this case the poor combustion caused by the low atomising steam pressure., the volumes of the respective chambers as well as the size of the inlet for windbox air flow and also the amount of restriction caused by the elements forming the P drop.

This example only describes one possible scenario, in reality there may be many different sources all acting together or independently to cause the panting.

Probably the most common cause of panting is an uptake fire, others may be such as slagging of the tube stacks or even build up of the furnace floor on front fired boilers.

Effects

The effects of panting are too cause a low frequency ( governed by volume/ P drop criteria ) oscillation of furnace spaces repeated to a lesser extent in the windbox and flue gas spaces.

For membrane boilers which are by design air tight the effect can be to cause heavy mechanical loading on all points especially on the drum connections, placing unwanted tensile stressing on welds. Other no less important effects are poor combustion leading to inefficient operation and choking of the tube stacks.

Remedies and remedial action

Modern combustion control equipment by their design inherently act to prevent panting. When the drop in furnace pressure is detected by the air flow transmitter it is sent to the P+I controller as a reduced air flow measured value.The P+I controller acts to increase the air flow hence going some way to negate the cycling problem caused by the inertia of the air.

Should a boiler start panting during its life, the condition of the internal surfaces should be inspected and deposits removed.