Thermal Expansion of Wingwall Superheater and its Distortion

THERMAL EXPANSION OF WINGWALL SUPERHEATER AND ITS DISTORTION

By K.K.Parthiban, B. Tech (IIT-Madras), M.E Thermal Engineering- RECT Trichy

Distortion of wingwall superheater is experienced in many CFBC boilers. This article is about the cause and care to be taken in design, installation and operation of CFBC boilers, to prevent distortion. Photo 1 shows the distortion experienced in a CFBC installation.

The below drawing (photo 2) shows the expansion movements given in thermal expansion diagram by the boiler manufacturer for this case. Plant engineers may obtain this information from the manufacturer for their boilers.

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In this illustrated case, the wing wall superheater is anchored at the front waterwall panel at the bottom. The working point is at 17.851 m. The wing wall superheater outlet header is supported at 32.894 m. The absolute expansion of the wingwall SH outlet header is 110 mm upward. The downward expansion of the anchor point is 66.7 mm. The net expansion at the wingwall SH outlet header is 110 - 66.7 = 43.3 mm upward. The spring movement of the constant level hanger provided for supporting the wing wall superheater is designed for the upward travel of 43.3 mm. The length of the wing wall superheater panel is 15 m.

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Most CFBC boilers are provided with coal feed points in the front panel. During start-up of the plant, the combustion takes place in the upper furnace due to the inadequate bed material in the furnace. The required upper bed is formed only after sufficient fine ash is generated in the combustion process. Only after the sufficient fine ash is generated, the waterwall starts absorbing the design heat, at the design heat flux. Without the fine ash, the combustion takes place at the upper furnace. This is because there is no resistance available to prevent the transport of burning coal to the top. Hence the wingwall superheater receives high heat input and is subjected to dry heating up without adequate cooling steam. The thermal expansion of the wing wall superheater at start­up would now exceed the normal expected thermal expansion. This start-up criteria may not be envisaged by the designer, while procurement for the constant spring support for wingwall SH.

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There is no thermocouple provided in the upper furnace, below wingwall superheater, to monitor the gas side temperature during start up. This is not provided, as it could cause furnace wall tube failure due to preferential flow of furnace inventory during normal operation. One must observe the metal temperature of wingwall superheater outlet tubes only. The metal temperature can go to the maximum allowable metal temperature for the SH material, which is usually 585°C. We may note the rated pressure is achieved gradually after the furnace inventory increases in start-up process.

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During start-up, the waterwall is cold. The rated pressure is not achieved in the beginning. The saturation temperature currently would be less than the rated temperature. The downward thermal expansion at anchor point could be much less. However, the wingwall superheater must expand upwards as per the maximum permitted metal temperature - 585°C. This will cause a good amount of upward thermal expansion of the wing wall superheater outlet header. This would be in the order of 150 mm. However, the constant load hanger selected may not accommodate this much amount of expansion encountered during start up. The seal box must also be designed to accommodate an expansion of 150 mm and generally not designed so.

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The thermal expansion of the wingwall superheater must take place against gravity. The shape of the wing wall superheater is such that, it can easily buckle because of its less cross-sectional dimensions versus the height ratio. Hence it doesn’t expand as rigid member. Given a resistance to prevent thermal expansion, the panel buckles. This leads to a permanent deformation.

The worst distortion can be seen in photo 3. This is from another boiler. For this case, replacement is a must.

During every start-up, the distorted panel flexes at the location where the permanent deformation had taken place at the first time. This flexing of panel is now elastic in nature. In case of replacement wing wall superheater panels, the deformation would take place again at some location naturally. If this must be prevented, a constant load hanger, which can accommodate large deflections would be required. The current model selected by boiler maker does not take up large deflection. See photo 4 above. The Lisega (well-known manufacturer of spring supports) make spring supports are available for large deflections. See photo 5.

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What to do with the present distortion?

Distorted wingwall SH panels would be subject to higher erosion rate. Regular monitoring of erosion and thickness logging is a must. The risk carried would be similar to CFBC furnace panel erosion.

The other type of damage expected would be the cracks at the header stubs. The thermal forces would be partly transferred to the stubs at both inlet and outlet header. Dye penetrant test is advised to be carried out on every annual shutdown. There are cases wherein the thermal expansion forces act on the header stubs and caused leaks. See photo 6. The number of startup & shut down cycles would decide the initiation of stub / weldment crack at headers. This could occur after a long service.

A tube failure in wingwall SH would lead to furnace pressurization. The furnace trip protection on high furnace pressure should not never be bypassed. Regular inspection of header stubs is advised.

Conclusion

The wingwall SH distortion can be controlled by avoiding rapid start-up during cold and hot restart times. Even if the right type spring support is provided by the boiler maker, it is possible that the distortion can be caused by improper operation. In the case of the worst distortion seen in photo 3 immediate replacement of the SH is required. A retractable thermocouple can be arranged below wingwall SH. It would help the operator to monitor and prevent high gas side temperatures during cold start up and hot restarts.

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