Category: Engineering

The efficiency of a heat recovery system, based on natural circulation, is determined by assuring satisfactory boiling conditions across the section of each heat exchange modules There are four main aspects for optimization:

By definition, this is the mass water flow in circulation divided by the steam flow, which can be understood as the number of times the water flows around the evaporator section to be fully converted into steam, considering steady latent heat at constant pressure and temperature High circulation ratio corresponds to low steam fraction and vice versa Low volumes of steam fraction by volume prevent tubes from overheating

Another aspect of significant importance is to reach adequate wet wall flow to maintain nucleate boiling under any given condition. Water velocity optimization will enhance heat transfer by increasing turbulences while preventing from steam blanketing, overheating and deposition of entrained solids.

The reliability of any of the previous design aspects is directly linked to the criteria for selecting an appropriate chemical treatment program.
Chemistry related problems will affect the performance of the plant by altering the water velocity, the hydraulic stability and ultimately the heat transfer The chemistry conditioning depends on the quality of the makeup water, on the chemical treatment program selected for the cycle, on the performance of the drum internals and it is also linked to the blowdown system design The image represents similar conditions to those of water flow through a boiler tube at different heat inputs.

Photo source http :://isnps unm edu/research/facilities/

A simple modification in the engineering design may turn out to be of significant impact in the way systems perform and increase its functional reliability. This image presents a good example in which the blowdown drain system in a power plant was affected by severe flow-accelerated corrosion and fatigue-cracking. As a consequence, multiple fractures were found in many interconnections with the flash tank.

We run a computational simulation to better understand the nature of the fluid dynamics involved in the process and the results suggested the potential root-cause of the failure. Tilting 45º degrees all drain connections with regard to the main header would prevent the system from turbulences, flow divergences, vibrations and steam-hammer. All these process dysfunctionalities will ultimately affect the stability of the magnetite layer, leaving the system unprotected and subject to operational deterioration.

Heat-affected zone infiltration may be a potential cause of erosion/corrosion failures of carbon steel piping when exposed to a continuous stream of wet steam or feed water. The problem is associated with high-velocity and turbulences created by the presence of the welding penetration as a flow obstacle.

The deterioration is developed for two reasons:
1) The mechanical wear due to the increase of velocity (red color area) would abrade the welding;
2) The turbulences generated in the region downstream the welding (blue area) would affect the formation of the protective metal-oxide layer and erode the surface.
3) Localized turbulences may also contribute to release CO2 or other acid-forming anions contained in the water chemistry and develop weak acid attack (see microscope image)

As a result, carbon steel piping becomes vulnerable to general deterioration and wall thickness dissolution. Make sure to have a good field supervisor on board… and better welders!