Floor studs are exposed to different conditions compared to studs in other areas of a Recovery Boiler. Under normal conditions, they operate covered by a thick layer of smelt. The temperature at the frozen layer is way below the melting point of any of the smelt's components freezing point. Floor studs are not exposed to air and tend to lose diameter mainly because the attack is even along the stud length. Due to this diameter loss, restudding them may require nipping close to the base to obtain a good contact area .

Sometimes some abnormal conditions occur and one or more of following deviations may happen.

Studs may operate uncovered by smelt

Total consumption of studs jeopardizes the buildup of a frozen layer

"Corner tubes" may operate as downcomers therefore they are not properly cooled

Steam buildup inside tube creates stresses between wet zone and upper part of tube

If one or more of the first three conditions occur, the fourth will probably take place and stresses due to overheating the upper part of the tube will amount fairly quickly. The tube portion reinforced by the membrane will not yield to the stresses of the upper portion of the tube to expand and circumferencial cracks with unpredictable results may result. To minimize the chances of exposure to these conditions, we have modified the design of floor studs. The new design presents studs with larger heat exchanging area, have better anchoring ability and will last considerably longer due to deeper chromium diffusion at their tips.

A boiler with composite tubes has less than 50% of the heat exchanging area of a studded tube. Additionally heat conductivity of the stainless outer tube is much lower than that of carbon steel. That by itself explains why composite tube boilers are more prone to present problems at the superheat or economizer level. Since late 80's a significant increase in the solids concentration was added to the problem. Not only more heat is brought into the furnace but also a significant part of the cooling effect granted by the water is also lost.

All of the above conditions has taken a toll on the floor studs, mainly on the smelt runs and also at the first line of studs right below the Rear Wall bend. Many users apply a refractory layer on the floor, using the studs as anchoring devices, provided the floor tubes are not composite.

The stud shown below can be either chromized or treated to withstand erosion if applied in the smelt runs. Its increased area significantly improves its adhesion to a refractory layer or to the char bed itself.

If the floor requires restudding, we nip off the original studs with the pneumatic tool shown below to achieve better contact area.

The life expectation of the stud is heavily dependant on the time it takes for the upper layer of chromium to be consumed. Normally after the "protective lid" goes, the stud still performs for twice the time taken to break through the lid before it calls for repair. Below we see the effect of lower stud density affecting the "protective lid".

Lessons taught by this effect led us to enhance the new design incorporating several benefits, as shown below,