Priming | Foaming | Carryover
Priming is the sudden discharge of water with steam from the boiler. It is just like the bumping of water that repeatedly accompanies rapid heating on a open vessel.
Priming is caused due to following reasons:
1. Improper designing of boiler.
2. Improper method of firing.
3. Overloading of boiler.
4. Sudden change in load.
5. Combination of two or more above factors at same instant.
Priming can be reduced by following ways :
(i) By installation of steam purifiers.
(ii) Lowering water level in boiler drum.
(iii) Maintain constant load (Avoid pulsating steam demand).
Foaming is the formation of small and stable bubbles into the boiler water.
Foaming is caused due to following reasons :
1. Higher percentage of dissolved solid.
2. Excessive alkalinity.
3. Presence of oil in boiler water.
Higher concentration of solids form small size of steam bubbles. This type of bubbles are much more stable in nature , do not burst easily. The foaming contaminates the steam with desired amount of boiler water which contains the corrosive salts.
Carryover /Water Hammering :
Boiler water solids are also carried over in the moisture mixed with steam even when there is no indication of either priming or foaming. This type of process known as carryover.
Carryover may be partly mechanical or partly chemical problem. Whereas mechanical causes as follows :
(i) Boiler design.
(ii) Higher water level in boiler drum.
(iii) Overloading in boiler.
(iv) Fluctuation in load on boilers.
(v) Possibility of carryover will be more if steam storage space are too small in boiler drum.
Normally steam from boiler carries 0.5 to 1.5% moisture in the form of mist or fog. In modern high pressure boiler carryover of moisture limited about 0.1%.