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Liquid Penetrant Testing – Advantages, Disadvantages & Alternatives

by Ishita Kapoor on January 5, 2019

Out of all the non-destructive testing techniques, only a few are suited for every type of application. For instance, on most composite materials, electromagnetic testing techniques don’t work, while computed tomography and X-rays are ideal. Likewise, ultrasounds are adaptable, but require appropriate coupling and direct contact with the surface under test, which is not possible in all cases.

What’s Liquid Penetrant Testing? 

Liquid penetrant testing, also known as dye penetrant inspection (DPI) or liquid penetrant inspection (LPI) is one of the most common and affordable solution and one of the oldest, if compared to non-destructive testing challenges. 

The method leverages capillary action, i.e, the ability of a liquid to flow into narrow spaces without help, even in opposition to, external forces such as gravity—to detect surface-breaking defects.

The excess is removed and a developer is applied after applying the penetrant and letting it dwell for a certain period. From surface-breaking defects, the developer draws the penetrant where it’s seeped, revealing their presence.

Advantages of LPI

Liquid penetrant testing has the following advantages:

  • Works on complicated geometric shapes
  • LPI materials are compact
  • Sensitive to small surface interruptions
  • Few material limitations such as—works on non-metallic, metallic, non-magnetic, magnetic, non-conductive and conductive materials
  • Liquid penetrant testing materials are individually very cost-effective
  • Visual, real-world results

Disadvantages of LPI

Liquid penetrant testing has the following disadvantages:

  • Extensive, time-taking pre-cleaning critical—surface contaminants can mask defects
  • Sensitive to surface-breaking defects only
  • Direct connection to the surface under test necessary
  • Works on relatively non-porous surface materials only
  • No depth sizing
  • Multi-process testing procedure
  • Time-taking; post-cleaning also necessary
  • No recordable data handy for progress monitoring
  • User dependent
  • Environmental concerns—may require disposing of chemicals and expensive handling

The biggest disadvantage is that despite lower costs and over time (cheaper materials, less training), Liquid penetrant testing is more than a screening tool; one can measure their length and locate defects, but using this method, it’s impossible to monitor the advancement of defects or determine the severity of its depth. It relegates the method to a pass/fail evaluation, that leads to discarding healthy parts and retaining unhealthy parts—which can both prove expensive.

Hence, in totality, despite the instant captivation of this cost-effective solution, it possesses various downsides that must be looked at before dismissing more progressive and more expensive inspection solutions, whether you contract inspections or perform them on your own.

Alternatives to Liquid Penetrant Testing

Eddy Current Array (ECA)

This offshoot of ECT enhances on the technology using multiplexed arrays of coils that are displayed in rows (instead of one or two coils), allowing to cover a larger area in a single scan pass. Below are the advantages of ECA:

  • Eddy current array probes offer better data than manual raster scans; the larger ECA probes lowers operator dependence
  • Making defect progress monitoring possible, data can be recorded.
  • Wider coverage significantly results in faster scans
  • The simpler ECA scan patterns makes analysis easier, quicker and accurate
  • ECA offers superior detection abilities, and correct defect positioning as the inspection data can be encoded, and—perhaps—sizing.

About IRC Engineering Pvt. Ltd.

IRC is one of the fastest growing Testing and Inspection company in India. We at IRC provide Liquid Penetrant TestingNon-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.

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Ishita KapoorLiquid Penetrant Testing – Advantages, Disadvantages & Alternatives

All You Need to Know About Corrosion Under Insulation (CUI) Plague

by Ishita Kapoor on October 22, 2018

Corrosion under insulation is one of the major challenges faced today by the process industries and it costs huge to the industry.

Corrosion under insulation (CUI) is a type of corrosion that occurs because of moisture buildup on the insulated equipment’s external surface. The buildup can be caused by one of multiple factors mentioned below. The corrosion itself is most commonly chloride, galvanic, alkaline or acidic corrosion. The results of CUI can result into the shutdown of a process unit or an entire facility, if undetected. Also, in rare cases it may result into a process safety incident.

CUI is one of the most tough corrosion processes to prevent. No matter what precautions are taken, water seeps into the insulation every time and thus leads to  process leakage.

Temperatures Leading to Corrosion Under Insulation

It is believed that low-alloy steels and carbon operating between –4ºC (25ºF) and 149ºC (300ºF) are at danger from CUI, but aggressive CUI has also been noticed in the 149ºC+ (300ºF+) range. From that outlook, preventing CUI is a matter of ensuring that there’s no water in insulation systems below approximately 177ºC (350ºF)  and the intermittent boiling and flashing that occurs above a metal temperature of 100ºC (212ºF) produces a fairly aggressive CUI environment.

Environmental Conditions Leading to Corrosion Under Insulation

Foreseeing CUI rates is difficult—they can be highly localized or somewhat general in nature. Listed below are some of the environmental conditions that lead to higher CUI rates:

  • Hot or humid environments
  • Marine environments
  • Steam tracing leaks
  • Climates with higher rainfall
  • Contaminants from the insulation or the atmosphere (such as chlorides and sulfides) dissolving in water
  • Systems that operate below typical atmospheric dew point
  • Intermittent wet-dry conditions
  • Insulation systems that don’t allow moisture drainage
  • •Insulating materials that hold moisture

Preventing Corrosion Under Insulation

Keeping water and electrolytes away from coming into contact with the unprotected metal surface is the most adequate way of preventing CUI. However, it’s nearly impossible to assure that the coating or insulation will not be breached. 

Weather barriers and effective protective coatings can minimize the potential for CUI, but effective maintenance practices will also prevent corrosion damage before becoming a severe problem. But remember that maintenance isn’t an effective solution alone. It requires a well thought out inspection strategy; none of the mitigation practices above ensure the complete prevention of CUI.

Various Inspection Methodologies for Corrosion Under Insulation

Determining the presence of Corrosion under insulation without removing the insulation is possible with very few inspection methods. All the Inspection Methodologies are generally having certain Limitations to detect the CUI and mostly are screening tools

Non-destructive testing if Corrosion is Undetected in Visual Inspection:

Pulsed Eddycurrent Testing: This testing can be used to detect CUI and can carry out inspection over GI, SS, Al cladding. The thickness of the Insulation can be upto 300mm and the Metal Thickness can be upto 100 mm. Further Advancements have been made in this system such as Pulsed Eddycurrent Array, which is very fast in screening the Piping. Generally the limitation of this technique is on accuracy of the reading which can have a variation of 10% but the most important benefit is it can be performed during the running condition of the piping.

Long Range Ultrasonic Testing: This Testing can be done from 2” Pipe Diameter. It requires a certain portion of Insulation to be removed for the collar to attach to the Pipeline. The ultrasonic Waves can detect Corrosion in the piping from length of 5 meters from the collar to may be 200 meters which depends on factors like coating, corrosion, buried etc.. This system can detect corrosion above 3% of the cross sectional area.

Computed Radiography: This Testing can be performed on the bends of the Piping to check for corrosion, erosion on the Bends. This is an accurate system but takes a lot of time for testing of each Bend due to the use of Radiography. Also with larger diameters Pipes would require Cobalt source thus making use of  this technique in running plant not possible for larger diameters.

Infrared Thermography: It is of great help to find moisture under insulation, which may help find CUI. False calls—wet insulation, no CUI; dry insulation that’s very wet, CUI. 

About IRC Engineering Pvt. Ltd.

IRC is one of the fastest growing Testing and Inspection company in India. We at IRC provide 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.

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Ishita KapoorAll You Need to Know About Corrosion Under Insulation (CUI) Plague

LONG RANGE ULTRASONIC TESTING OF PIPING

by Ishita Kapoor on September 4, 2018

High quality engineering is made possible only by constantly testing the quality of the equipment. In piping, it is made sure that the integrity of the component has not been compromised. For the purpose of inspecting pipes and evaluating distribution of corrosion around the circumference, long range ultrasonic testing is one of the most preferred testing methods, also known as LRUT. This method is frequently used to inspect long pieces of pipes and proves especially valuable where the location is inaccessible such as culvert or those at high elevation, making it an economical way to test the pipes without spending on excavation, insulation removal and scaffolding.

IRC Engineering Services provides in-depth services that employ LRUT for accurate assessment of the condition of piping. Our inspectors are thoroughly qualified to assess piping through long range ultrasonic testing helping to maintain the best condition through early detection of any possible corrosion or ruptures.

Method

LRUT is performed through a process utilising low frequency flaw detector, pulse receiver, transducer rings and a monitoring apparatus such as a computer or laptop. The transducer rings are placed around the pipe and a low frequency set of waves is then generated, which traverses along the length of the pipe. When placed at equal distance from each other, these transducers emit waves that move symmetrically through the length of the pipe. Therefore, once these waves meet the corroded portions of the pipe, they are reflected back to the transducer. 

Since this method does not destroy the piping in any way and reduces the need to excavate the entire pipe to check for flaws, it is a cost effective method of testing that is preferred by industries all over the world. Our equipment Teletest FOCUS uses optimum level of ultrasound that is required to prime this equipment to be most sensitive to even the most minimal damage to piping. One can therefore rest assured that piping can be checked for all damages.

Benefits 

  • In-service inspection prevents production losses or downtime.
  • Entire length of the pipe can be assessed for integrity at a one time.
  • Examine 180 meters from a single test location. (90m on each side of transducer)
  • Examine 100% of the pipe circumferential wall from a single test location.
  • Reduction in maintenance cost as there is no requirement of removal of surface coating or insulation except the location at which the transducer has to be fixed.
  • C-scan imaging gives a pictorial view of the scanned area which helps in locating the defects.
  • GPS system installed in the equipment helps to locate inspected area.
  • Secondary focusing technique is a unique feature to focus the concerned area.
  • No couplant is required for the transmission of ultrasound.

Applications

LRUT has several field applications in various industries. The most popular use of LRUT is in the detection of corrosion. Full set of applications for long range ultrasonic testing is mentioned below:

  • Assessing pipes that are located deep below the surface of the ground.
  • Assessing pipes that are deep within walls or are encased.
  • Assessing pipes that are otherwise inaccessible such as overhead pipes, etc.
  • Assessing insulated pipes, as this method can detect the sound waves even through surface coating or padding.
  • Can be used to detect corrosion, areas of concern, weld root erosion for piping in refineries, chemical plants, power stations, underwater, in farms, sewers, etc.

IRC Engineering Services recommends that LRUT be performed in conjunction with other testing methods such as phased array ultrasonic testing, which will additionally test the pipe to check for the precise thickness of the pipe wall. Pulsed Eddy Current testing can also be done to check for the average thickness of the pipe wall, in case only a general figure is required.

Through this method of nondestructive testing, industries can detect any concerns with their piping to prevent or correct adverse situations such as pipe corrosion. With the help of our highly qualified engineers, you will receive a full assessment of the condition of your pipes through our precision methodology. 

Limitations

While long range ultrasonic testing of piping is an effective method to gauge any damage in piping, this method does have certain limitations. It is important to be aware of these limitations so that the correct tests can be performed on the piping to do a thorough check for corrosion, leakage etc.

  • Gives only an approximate measurement of wall thickness: Long range ultrasonic testing can detect variations in thickness of pipe walls, but does not provide an accurate estimation of the wall thickness. For this, a combination of tests can be done, included PAUT, PECT together with LRUT.
  • Complexity in assessment of pipes in waterlogged conditions: In case the pipe is lying beneath the surface in wet ground, travelling of sound waves can be made more difficult. This can hamper the testing process, leading to inaccurate results. It is therefore prescribed for use when the pipe is lying in dry conditions.
  • Cannot be used in very narrow or short pipes: Due to the passage of sound waves, the pipe has to have a minimum diameter of 1.5 inches in order for proper testing to be done. The pipe also needs to be of a suitable length to accurately assess. The recommended minimum pipe length is 5 meters.
  • Does not differentiate between type and location of corrosion: Long range ultrasonic testing is an effective method to detect the presence of corrosion, but does not allow the assessment of corrosion that is taking place actively or passively. It also does not allow the assessor to determine if the corrosion is taking place within the pipe or on its surface. Further testing using other methods needs to be done in this case.

Long range ultrasonic testing of pipes is meant to be used primarily as a screening method for pipe damage and should not be used to determine specifics about the type of corrosion, exact wall thickness etc. 

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Ishita KapoorLONG RANGE ULTRASONIC TESTING OF PIPING