August 2019 - Inerchem.com

Acid Cleaning Corrosion

Background

Acid cleaning corrosion is a key factor to considering during the chemical cleaning activities. A few months after the completion of a combined cycle, the plant was forced to shut down due to operational instabilities and loss of pressure in the steam generator.

An exhaustive inspection was conducted to find out multiple leaks in the tube side of the evaporator and an uncommon yellowish pigment at the bottom header. Some tube samples were taken and sent to laboratory to determine the metal morphology, surface chemical composition and the nature of the deposit.

Analysis

The spectroscopy X-ray results indicated the presence of a significant enrichment of carbon along with bonded oxygen and ferrous ions which suggests the presence of organic compounds.
X-ray diffraction revealed that most of the carbon and oxygen were combined in an aliphatic structure, suggesting the existence of citrates, which would explain the yellowish aspect.
The microscopy analysis showed an acid corrosion pattern with jagged, roughness, undercut pinholes and general pitting appearance

Conclusions

During pre-commissioning, the cleaning of the boiler was carried out by the execution of a traditional one-phase citric cleaning.

Throughout the acid pickling, citric acid bonds to iron oxide to form ferrous citrate which precipitates in the form of solid salt and it is afterward removed from the system in subsequent cleaning steps.

In this case, the system was not effectively washed out after the chemical cleaning, and great amount of deposits, with high content of ferrous citrate remained, in the evaporator.

The ferrous citrate, in combination with remaining water, created under deposit cells and regions of low pH which generated the corrosion damage shown in the photo.

Forensic Engineering

Sulphur Dew-Point Corrosion

Introduction

The main purpose of material selection is to reduce the cost of the design while meeting the operation performance target. However, due to the transient nature of commissioning, field activities can be significantly aggressive, and under some circumstances, can even exceed the operation conditions of in-line equipment. For this reason, it is very important to perform a pre-analysis of the commissioning strategy and identify the equipment that is going to see critical conditions, so that the appropriate material can be specified in each case.

This photo was taken from the bottom of a diesel heater stack and illustrates the dramatic effects of acid dew point corrosion.

Problem

When sulfur-bearing fuel is burned in the combustion chamber, sulfur is converted to its dioxide (SO2) and trioxide (SO3) structure in the presence of other compounds such as CO2 and H2O.

Sulfur trioxide combines with water vapor to form gaseous sulfuric acid (H2SO4g). If temperature drops below the acid dew point, liquid sulfuric acid will form and spread all over the metal surfaces of the equipment. Initially, sulfuric condensate won’t attack carbon steel.

According to the equilibrium curve H2O – H2SO4, this acid condensate is highly concentrated and its combination with iron will form a protective salt layer of iron sulfate, which prevents the development of corrosion.

The real problem comes when flue gas temperature falls below the dew point of water and dilutes the concentration of sulfuric acid. Under this condition, the acid dissociation evolves toward the formation of protons and will corrode the metal within a few hours.

Solution

From the engineering & design standpoint, this problem could have been avoided if the chemical composition of the fuel had been analyzed and therefore, a more resistant material for the stack had been selected.

Engineering

Boiler Circulation

Design Concepts

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:

Circulation Ratio

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

Water Velocity

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.

Hydraulic Stability

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/

Forensic Engineering

Coating Deficiences

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