Forensic Engineering
erosion corrosion

Erosion-Corrosion

Erosion-Corrosion

Introduction

Erosion degradation on the outside of cold reheat tubes are due to mechanical action, frequently by impingement of wet steam and/or high-velocity flow. Impingement can be controlled by placing baffle plate tray protections upstream the heat exchanger module; high velocity flow can be handled by oversizing inlet nozzle diameter. However, the effects depicted in the photo represent a more complex failure mechanism commonly encounter during the commissioning of a power plant.

Conclusions

A comprehensive investigation conducted revealed the presence of high amount of iron citrate deposits in most of the impingement cavities. Also it became apparent the existence of strong and general abrasion in the interface tube-baffle. The reason of this failure was due to a combination of an unsatisfactory chemical cleaning operation and an incorrect steam blowing design.

erosion corrosion
Commissioning
boundary layer

Steam Blowing Insights

Some Insights About Steam Blowing

Introduction

The science behind steam blowing involves the optimization of the drag force of the steam and the chemical conditioning for corrosion control.

Fludization Aspects
The fluidization potential (drag force) can be enhanced by minimizing the boundary layer thickness of the fluid, all along the surface of the pipe, as follow:

1.  Running at high Reynolds will increase turbulences, adjust the velocity profile and boost terminal velocity near the surface.
2.  Rising temperature will reduce the viscous layer, and so, the shear wall stress.
3. Increasing density will maximize the momentum (kg/m·s2) of the steam. This can be done by injecting water in the superheater outlet to blow with saturated steam.

Chemical Aspects
Chemistry also plays an essential role in the performance of steam blowing. The dynamic nature of this operation makes impossible to control carryovers, deposition of salts, oxides formation and diffusion of impurities. Shutdown periods for thermal cycles can also increase the concentration of O2 in the boundary layer. As a result, corrosion can be quite active during these stage of commissioning.

Suggestions

As suggestion, it is recommended to first carry out a chemical cleaning in the steam system. Then, start-up the steam generator for a few days to improve chemistry conditioning. Finally, proceed with the execution of steam blowing.

boundary layer
Commissioning

Common Boiler Deposits

Common Boiler Deposits

Description

Deposition in power plants is most likely the least understood aspect of the operational cycle chemistry applicable to these units during commissioning. Having a complete understanding of the subject is a key factor for the performance of an effective chemical cleaning and the chemical treatment program to be applied. This will represent the fundamental building blocks for the success of the steam conditioning process and further reliability of the chemistry cycle during operation.

For this matter, knowing the nature of the deposit is of significant importance aspect to take into consideration. Most prevalent deposits compounds found in power plants are corrosion products of iron oxides, cooper oxides and derivatives formed as a result of lack of conservation during construction, improper preservation procedures, and deficient water treatment programs.
The following table lists crystalline compounds and other materials that have been observed in water formed deposits of steam generation equipment

Commissioning
magnetite

Ineffective Cleaning?

Ineffective Chemical Cleaning?

Introduction

Ineffective chemical cleaning? Not really… we have encountered situations where engineers and operators complaint about the cleanliness of the boiler as they see a black liquid coming off the blowdown system during first fire.

It´s not difficult to get carried away by a first impression and put into question the effectiveness of the chemical cleaning.

Explanations

This dark watery constituent is in reality a very welcome natural corrosion-protection product called Magnetite and it is formed as the steel surface is exposed to water under certain conditions.

The formation of Magnetite is an electrochemical process controlled by a continuous diffusion of iron ions from the steel surface across the water boundary layer and its stability, morphology and porosity depends on the oxidation-reduction potential, pH, oxygen concentration, pressure, temperature and the influence of the make-up water chemistry (CO2, Cl, Si, Na, Ca, Mg…)

Solubility rises with temperature up to 150ºC, then decreases with a steep drop to 300°C. Pressure promotes the growth of magnetite and thickness Having said that, the reason of the dark appearance of the blowdown during a boiler ramp-up is mainly due to the deposition of low quality magnetite since steady conditions has not been reached yet.

magnetite
Forensic Engineering
molten salt corrosion

Molten Salt Corrosion

Molten Salt Corrosion

General Considerations

Molten salts, commonly used as heat transfer fluids in solar power plants, are essentially eutectic mixtures of sodium-potassium nitrates and the nature of these bonds provides strong ionic conditions and good chemical stability of the thermal solution against metal corrosion.

However, they are very hygroscopic and highly reactive with water molecules, which combined with its tend of thermal dissociation to form NOx, can produce high concentrated nitric acid and lead the system to a quite extreme acid attack.

The corrosion phenomenon can occur within a few hours and the effects are shown in the following photo. Make sure to dry the system after the pressure test and nitrogen purge is a must. We really mean it.

molten salt corrosion
Commissioning
chromatic changes

Chromatic Changes

Chromatic Changes

Recommendation

Chromatic interactions are a great indicator of the kinetic and chemical reaction throughout the course of a chemical cleaning.

It is of significant importance to collect samples from different part of the system during the treatment operation as color changes provide reliable information about the evolution of the reaction taking place.

Explanation

Inhibitive effects of sodium nitrite (among other passivation agents) are accepted due to the oxidation of Fe2+ ion to form a very thin layer (40-100 armstrongs) of gamma-Fe2O3 on the piping inner surface under alkaline solutions.
The color of this final compound is black or brownish-black and along with the redox potential measurement will determine the completion of the process.

The picture below displays from left to right color changes in all stages of an HF acid treatment methodology.

chromatic changes
Commissioning
Boiler Tube

Travelling Across Boiler Tube

Travelling Across A Boiler Tube

Ready for an amazing journey?

Take a minute to travel across a preheater boiler tube affected with severe under deposit corrosion and overall oxygen pitting at early stages of development. This journey shall help you to become aware of the importance of having a deep understanding of the natural chemical reactions involved in a power plant. Only this way you will be able to identify potential failures and take proactive actions. In this particular case, the internal corrosion pattern in the tube is mainly due to the effect of high dissolved oxygen concentration in the presence of excessive non-volatile constituents in the quality of the make-up water. Enjoy!!   click here

Boiler Tube
Commissioning
exfoliation

Exfoliation Corrosion

Exfoliation & Lamellar Corrosion

During Inspection

Severe leakages in a steam generator superheater were the leading cause of a major outage a few months after first operation of a power plant. In the course of a visual inspection, the first section of the superheater was observed to develop several sharp perforations, impingement and general abrasion on the baffle/tube interface.

Heavy deposits were also found all over the steam drum along with serious surface exfoliation.

Conclusions

Failure investigation was conducted to reveal that unsatisfactory chemical cleaning left a heterogeneous layer of calcium carbonate, sodium phosphate, silica and iron citrate.

This layer inhibited the formation of a consistent magnetite layer that was partially developed across the surface.

All these layers present dissimilar thermal coefficient with regard to the steel surface creating exfoliation and lamellar corrosion which was intensified with vibrations and temperature changes experienced during hot commissioning (see photo).

As a result, scales detached from surface and passed to steam side leading to significant damage in the superheater and dramatic consequences to the project.

exfoliation
Engineering
CFD Drain System

CFD in Drain System

CFD Drain System

Description

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.

Solution

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.

CFD Drain System
Engineering
heat affected zone

Heat-Affected Zone

Heat-Affected Zone

Circulation Ratio

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!

heat affected zone

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