Inspection of Boiler Bank Tubes | Internal Rotary Inspection System (IRIS)

by ircengg on May 28, 2021

Are you unable to carry out the Inspection of Boiler Bank tubes?

Are you regularly carrying out Capital Investment by purchasing tubes rather than finding only the defective Bank Tubes?

Here is the solution for inspection of Boiler Bank Tubes –

In case of Boiler Bank Tubes, only the Front Column of Tubes are available for Inspection for Thickness Testing externally there is no other way to check the rest of the Tubes. Thus we have only 5 % of the Tubes available while we miss out on an inspection of the rest of the 95% of the tubes.

RFET was done previously to ascertain the condition of the rest of the 95% of the tubes but due to the limitation of the technique it is also not a right testing method. Then we tried IRIS on these tubes and got amazing results.

HOW IT WORKS –

Because IRIS is an ultrasonic technique, it requires a couplant. In this case, water. Tubes under test must therefore first be flooded to use this technique. IRIS relies on a transducer to generate an ultrasonic pulse parallel to the axis of the tube under test. It also relies on a rotating mirror that directs the ultrasonic wave into the tube wall.

The mirror is driven by a small turbine powered by the pressure of water pumped into the tube. Part of the ultrasonic wave is reflected by the inner diameter (ID) wall, while the rest is reflected by the outer diameter (OD) wall of the tube. Because the ultrasonic velocity of the tube’s material is known, it is possible to assess the thickness of the wall by calculating the difference in times of flight between the two diameters. As the probe is pulled, the spinning motion of the mirror results in a helical scan path.

A critical aspect of IRIS is ensuring that the mirror is at the centre of the tube. An off-centre ultrasonic pulse yields a distorted scan image because of the different ID and OD wall sound paths. That’s why our IRIS kits are equipped with centring devices helping operators keep the system centred.

About IRC Engineering Pvt. Ltd.

IRC is one of the fastest growing Testing and Inspection company in India. We at IRC provide Liquid Penetrant Testing, Non-Destructive Testing, Destructive Testing, Advanced NDT, Third Party Inspection, Condenser Testing, Electrical Testing, Residual Life Assessment of Power Plant, O&M Services, Fitness For Service, Civil Testing and Training services.

How Mild Steel Corrodes In Boiler Water?

by ircengg on June 4, 2015

Waterwall tubes in most of the fossil-fired boiler are made of carbon steel. In pure DM water, or in very dilute acid or alkaline solutions at boiler temperature, it normally corrodes very slowly to form the black iron oxide known as magnetite (Fe3O4). The overall reaction is:

3 Fe + 4 H2O ——-> Fe3O4 + 4 H2

The corrosion rate is dependent by the rate at which the reactant (water) can reach the metal surface and the reaction product can leave the surface. In nearly neutral solutions, magnetite is very slightly soluble and it deposits as coherent and tenacious surface film, which greatly impedes this two-way chemical traffic. The transport processes are dominated by slow state diffusion through the oxide layer and the corrosion rate is virtually independent of solution composition. The corrosion rate is also diminishes with time as the oxide thickness grows. Even after years of exposure, the layer is no more than a few microns thick.

In more alkaline or more acid solutions, magnetite becomes increasingly soluble and precipitates in a different physical form. Instead of yielding a strongly coherent film, it has a more porous structure. Soluble species can now diffuse relatively rapidly through the film and the corrosion rate is much faster, although it still falls off the time as the oxide accumulated.

In many countries there are mainly three types of Medical Laboratories as per the types of investigations carried out. 1. Clinical Pathology 2. Clinical Microbiology & 3. Clinical Biochemistry laboratories. 1. Clinical Pathology: Haematology, Histopathology, Cytology, Routine Pathology2. Clinical Microbiology: Bacteriology, Mycobacteriology, Virology, Mycology, Parasitology, Immunology, Serology. The deposition of salts and corrosion products observed in different waterwall tubes has shown in the photograph no.1-4.

Photograph no.1&2 showing waterwall tubes of 60 MW boilers having a very thick deposit ranging thickness 1.0-1.5 mm.

Photo 1

Photo 2

Photograph no.3 & 4 showing a waterwall tube of 200 & 110 MW boilers having a thick iron oxide as well as hardness salt deposition which is non uniform in nature.

Photo 3

Photo 4

IRC is regularly carrying out such studies to find out the reason of the failures and are also giving remedial actions to prevent it, if required we can also carry out chemical cleaning of boiler.

IRC is a service provider having expertise in ndt, residual life assessment, fitness for service, advanced ndt, failure investigation, chemical cleaning, certification of storage tanks as per chief controller of explosives guidelines , consultancy for boiler water chemistry and training

How Caustic Corrosion And Acidic Corrosion Are Formed In Case Of Boilers?

by ircengg on June 4, 2015

Caustic Corrosion

If the salt concentrated under the deposit is having high pH due to concentration of caustic from TSP dosing, it start dissolution of protective magnetite (Fe3O4) layer on the evaporator tube wall inner surface and form sodium ferrite (NaFeO2) and sodium ferroate (Na2FeO2) as shown in the equation.

Fe3O4 + 4 NaOH ——-> NaFeO2 + Na2FeO2 + 2 H2O

ACIDIC CORROSION

Solution of low pH is generated in high pressure boilers in two different ways:

pH of the entire boiler water is reduced when contaminants which are acidic or becomes acidic when heated in to the boiler.
The bulk boiler water remains alkaline but acidic solutions are generated within corrosion pits by the action of dissolved oxygen and chloride. The most common acid forming contaminant is sea water or a river water which is low in carbonate and sulphate. In the boiler, the acidity is increased locally to corrosive concentrations by boiling.

In the acidic or highly alkaline conditions, iron reacts and hydrogen is liberated.

Fe + 2 NaOH = Na2FeO2 + H2

Fe + 2 HCl = FeCl2 + H2

If the hydrogen is liberated in an atomic form, it is capable of diffusing into the steel. Some of this diffused, atomic hydrogen will combine at metal grain boundaries or inclusions to produce molecular hydrogen, or it will react with iron carbides in the metal to produce methane.

Fe3C + 4 H = CH4 + 3 Fe

Because neither molecular hydrogen nor methane is capable of diffusing through the steel, these gases accumulate, primarily at grain boundaries. Eventually, the gas pressure created will cause separation of the metal at its grain boundaries, forming discontinuous, intergranular micro cracks as shown in the micrograph.

As these micro cracks accumulate, tube strength diminishes until stresses imposed by the internal pressure exceed the tensile strength of the remaining, intact metal. At this point a thick-walled, longitudinal burst may occur depending on the extent of hydrogen damage as shown in photographs below:

Photographs of 210 and 110 MW boilers, waterwall tubes show the failure due to hydrogen damage due to localised acidic condition.

IRC is regularly carrying out such studies to find out the reason of the failures and are also giving remedial actions to prevent it, if required we can also carry out chemical cleaning of boiler.
IRC is a service provider having expertise in ndt, residual life assessment, fitness for service, advanced ndt, failure investigation, chemical cleaning, certification of storage tanks as per chief controller of explosives guidelines , consultancy for boiler water chemistry and training

Boilers

Inspection of Boiler Bank Tubes | Internal Rotary Inspection System (IRIS)

Are you unable to carry out the Inspection of Boiler Bank tubes?

Are you regularly carrying out Capital Investment by purchasing tubes rather than finding only the defective Bank Tubes?

Here is the solution for inspection of Boiler Bank Tubes –

HOW IT WORKS –

How Mild Steel Corrodes In Boiler Water?

How Caustic Corrosion And Acidic Corrosion Are Formed In Case Of Boilers?

Caustic Corrosion

ACIDIC CORROSION

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