NON-DESTRUCTIVE TESTING IN THE OFFSHORE INDUSTRY

Lecture 6

04 April 2015

1.Introduction in Nondestructive testing.

2.Discontinuities , origin and classification

3.Visual testing.

4.Liquid penetrant inspection method.

5.Process control and interpretation of liquid penetrant inspection

6.Magnetic particle testing theory.

7.Magnetic particle inspection applications and interpretations

8.Ultrasonic inspection method.

9.Ultrasonic inspection equipments and materials.

10.Ultrasonic inspections process control and safety.

11.Eddy current inspection method.

12.Application on eddy current inspection

13.Acoustic emission testing.

14.Thermal infrared testing.

Topic

5. Process control and

interpretation

of liquid penetrant inspection

TECHNIQUES AND VARIABLES

It should be apparent by now that there are a number of PT techniques that

can be used with the different materials described.

A summary of these techniques is listed in table.

Technique I, Process A (I-A)

Technique I Process A uses a fluorescent water-removable penetrant that can be used with either dry, aqueous, or non-aqueous developers. This technique is generally used for the following applications:

1. When a large number of parts or large surface areas are to be examined.

2. When discontinuities that are not broad or shallow are anticipated.

3. When parts to be examined have complex configurations such as threads, keyways, or other geometric variation.

4. When the parts to be examined have surfaces that are rough, such as with sand castings or as-welded conditions

Advantages: 1. Higher sensitivity 2. Excess penetrant is easily removed with a coarse spray 3. Easily adaptable for large surfaces and large quantities of small parts 4. The cost is relatively low Limitations: 1. A darkened area is required for evaluation 2. Under- or over removal of penetrant material is possible 3. Water contamination can degrade the effectiveness of the penetrant 4. Not effective for broad or shallow discontinuities 5. Dryers are required (usually) when using developers 6. This technique is usually not portable

Technique I Process B (Lipophilic) and

Process D (Hydrophilic)

Technique I, Processes B and D use a fluorescent postemulsifiable penetrant, a lipophilic (L) or hydrophilic (H) emulsifier, and dry, aqueous, or nonaqueous developers.

The materials used are very similar to those described for Technique I Process A, except that these penetrants are not water-removable without emulsification.

A lipophilic or hydrophilic emulsifier must be used after the dwell time has expired.

This technique is generally used in the following situations:

1. When a large quantity of parts must be examined

2. When discontinuities that are broad and shallow are anticipated

3. For the detection of stress cracks or intergranular corrosion

4. For the detection of small discontinuities such as grinding cracks

5. Applications requiring higher-sensitivity techniques

Advantages:

1. High sensitivity for the detection of smaller discontinuities

2. For broad or shallow discontinuities (when they are expected)

3. Adaptable for high-quantity testing

4. Not easily affected by acids

5. Less susceptible to over removal than Technique I-A

Limitations:

1. This technique has an additional step, which requires an emulsifier. Therefore, more time and material is necessary.

2. It is not as effective for parts with complex shapes (e.g., threads) or rough surfaces, as is Technique I-A.

3. The emulsification time must be closely controlled.

4. As with Technique I-A, it requires drying prior to the application of dry or non-aqueous developers.

5. It is usually not portable.

Technique I, Process C (I-C)

Technique I Process C uses a fluorescent penetrant, which is solvent-removable, a solvent cleaner/remover, and a non-aqueous developer.

The excess surface penetrant is first removed with a dry cloth, followed by cleaning with a cloth dampened with a solvent remover.

This process is generally used when removal with water is not desirable due to part size, weight, surface condition, water availability, or when a heat source is not readily available for drying.

Advantages: 1. Can be used for spot examinations on large parts 2. Effective when water removal is not feasible Limitations: 1. The use of solvent for removal limits this technique to smaller areas 2. A black light and darkened area are required 3. The sensitivity can be reduced if excessive remover is applied 4. A “background” may occur with this technique, which could affect the contrast ratio, especially with rougher surfaces

Technique II Process A (II-A)

Technique II Process A uses a visible color-contrast, water-removable penetrant and an aqueous or non-aqueous developer. Dry developer is not usually used with Technique II

penetrants. Some specifications, in fact, do not permit the use of dry developers with Technique II penetrants.

The penetrant contains an emulsifier, making it water-removable. This technique is generally used for the following applications:

1. Examinations of a large quantity of parts or large surface areas

2. For discontinuities that are generally tight

3. For the examination of parts with threads, keyways, and other complex geometries

4. For parts with generally rough surfaces

Advantages: 1. No black light or darkened area is required for evaluation. 2. It is relatively quick and inexpensive. 3. The excess penetrant is easily removed with a coarse water spray. 4. It is effective for the examinations of a large quantity of parts. 5. It can be used for rough surfaces, keyways, threads, and other complex geometries. Limitations: 1. Its sensitivity is inferior to Technique I-A. 2. Penetrant can be over removed. 3. Water contamination can degrade the effectiveness of the penetrant. 4. It is not usually effective for the detection of broad or shallow discontinuities.

Technique II Process B (II-B)

Technique II Process B uses a visible color-contrast, postemulsifiable penetrant, an emulsifier, and an aqueous or non-aqueous developer. The materials used (except for the penetrant) are very similar to those described for Technique I Process B. An emulsifier (usually lipophilic) is applied to the surface penetrant after the dwell time to make it water-removable.

Technique II Process B is generally used for the following applications:

1. When a large quantity of parts must be examined

2. Whenever lower sensitivity than that achieved with Technique I is acceptable

3. When broad and shallow discontinuities are anticipated

Advantages:

1. No black light or darkened area for evaluation is required.

2. Broad or shallow discontinuities may be detected.

3. Useful when there are large quantities of parts to be examined.

4. This technique is not as susceptible to over removal, as are the Process A penetrants.

Limitations:

1. The additional step of an emulsifier requires more time and additional material.

2. It is not as effective for parts with a complex geometry (e.g., threads), as is Process A.

3. The emulsification time is very critical and must be closely controlled.

4. Drying is required if non-aqueous developers are used.

Technique II, Process C (II-C)

Technique II Process C uses a visible, color-contrast, solvent-removable penetrant, a solvent cleaner/remover, and an aqueous or non-aqueous developer.

The excess penetrant is not water-removable and must be removed with a solvent remover.

This technique is widely used for field applications when water removal is not feasible, or when examinations are to be conducted in a remote location.

Advantages: 1. This technique is very portable and can be used virtually anywhere. 2. It can be used when water removal is not possible. 3. Black lights or darkened evaluation areas are not required. Evaluation is done in visible light. 4. It is very adaptable for a wide range of applications. Limitations: 1. The use of solvent to remove excess surface penetrant limits the examinations to smaller areas and parts without a complex geometry. 2. Sensitivity is reduced when an excessive amount of remover is used during the removal step. 3. Excess penetrant removal is difficult on rough surfaces, such as sand casting and as welded surfaces, and usually results in a “background.” 4. This technique has a lower level of sensitivity compared to Technique I penetrants. 5. It is more “operator-dependent” due to the variables involved in the removal step.

EVALUATION AND DISPOSITION

After the penetrant process has been completed and the indications are observed and recorded, the final step will be to establish whether or not these conditions are acceptable or rejectable.

The size of the indication can usually be related to the amount of penetrant entrapped in the discontinuity. The larger the discontinuity volume, the greater the amount of penetrant that will be entrapped and, therefore, the larger the bleed-out after development.

The shape of the indication is important because it relates to the type or nature of the discontinuity; e.g., a crack or lack of fusion will show up as a linear bleed-out rather than a rounded one.

A linear indication, by most codes and specifications, is defined as a bleed-out whose length is three times or greater than its width. The intensity of the bleed-out gives some evidence as to how tight the discontinuity is.

A broad shallow type discontinuity will tend to be diluted, to an extent, by the remover liquid, and not be as brilliant as a bleed-out from a very tight discontinuity.

It is essential that corrective action be taken to remove or repair the discontinuity if it is deemed to be rejectable.

In most cases, a crack or other serious discontinuity will be cause for the rejection or scrapping of the part. Repairs to discontinuities will often be accomplished by grinding.

A recommended technique to assure complete removal of an indication after grinding is to merely reapply the developer.

This usually verifies whether the discontinuity has been removed, since the bleed out will reappear if it has not.

This process should be repeated, i.e., grinding, reapplication of developer, and then grinding again until no further bleedout occurs. At that point, the area that has been ground out must be reexamined, following the penetrant procedure from the beginning, to assure that in fact, the discontinuity has been totally removed. It is further recommended that the grinding be performed in the same direction as the longest dimension of the discontinuity. This is to minimize the possibility of smearing the material over the discontinuity. After the repair is completed, the repaired surface must be reexamined. During the evaluation process, it is necessary that a suitable light source be used. For visible penetrants, a light source of 100 foot-candles is common, although codes and specifications may require different light intensities. A black light is used for evaluation of fluorescent penetrants. An intensity of between 800 to 1200 W per square centimeter is typically required.

Penetrant indications must be recorded.

For recording purposes, a number of satisfactory techniques can be used, including photographs, hand sketches, and the transparent tape lift-off technique. Photographic techniques employed for recording visible penetrant indications are quite standard. When photographing indications under black light conditions, specialized exposures and filters may be necessary when using photographic film.

Digital cameras are usually quite exceptional for recording both visible and fluorescent penetrant indications.

Hand drawings, when used with test reports, should be prepared with as much accuracy and detail as possible.

PENETRANT TESTING APPLICATIONS Penetrant testing is extremely versatile and has many applications. It is used in virtually every major industry and for a wide variety of product forms. Industries that widely use penetrant testing techniques include:

– Power generation, both fossil- and nuclear-fueled – Petrochemical – Marine/Shipbuilding – Metalworking, including foundries and forging shops – Aerospace – Virtually all of the various welding processes and metals-joining industries

Another unique application of penetrant testing is for the detection of through-wall leaks. With this application, penetrant is applied to one surface, for example, of a tank, and developer applied to the opposite surface. If there is an unimpeded through leak, the penetrant will follow that path and be exposed on the opposite developed surface as an indication. Some leak tests, such as those for complex components and piping systems that must be evaluated to determine the source and location of a leak path, are conducted using fluorescent tracers.

LIQUID PENETRANT PROCESS CONTROL

This section provides basic, operating and advanced level information on the procedures necessary to assure a high quality performance from the penetrant inspection system.

We present the reasons for process and materials control.

QUALITY CONTROL CONSIDERATIONS In order to control the many variables associated with penetrant testing, there are a number of quality control (QC) checks that should be made periodically. The applicable requirements will specify which ones must be made and how often. There are three major areas that include the various QC issues, and once these are identified as essential or required, they should be added to the procedure. The three major categories are:

1. Material checks » New » In-use

2. System checks 3. Equipment checks

Materials and process deficiencies are not always obvious. It is not easily determined if a penetrant has lost its ability to penetrate into a given flaw. Penetrant inspection, as well as, all other nondestructive inspection processes, is not a perfect process. Flaws can be present and not be indicated for a number of reasons. The two main reasons for discrepancies in inspection results are: • Substandard inspection materials due to either receipt of bad material from the manufacturer or degradation in storage or service. • Process deviations in equipment, procedures, or operating conditions

Material Checks—New

All incoming or new penetrant materials should be verified to assure compliance with specifications. Some codes and specifications contain unique requirements regarding the presence of contaminants in the PT materials such as chlorine, sulfur, and halogens.

Some codes and standards include analytical tests to determine the amount of these contaminants but most of the penetrant manufacturers will provide material certifications that specify the amount in that particular batch of materials.

Material Checks—In-Use It is good practice, and in some cases a requirement, that penetrant materials be checked periodically to assure that they have not degraded and are still performing satisfactorily. These checks may include but are not limited to the following: 1. Contamination of the penetrant 2. Water content in the penetrant (for water-removable penetrants) 3. Water content in lipophilic emulsifier 4. The condition of the dry developer (fluorescent penetrant carry-over, etc.) 5. Wet developer contamination 6. Aqueous developer concentration (using a hydrometer) 7. Hydrophilic emulsifier concentration The results of these checks should be entered in a logbook to permit periodic reviews of the condition and the trends of the materials as they are being used.

Causes of Material Degradation. Materials Contamination. Materials contamination is a primary source of penetrant system performance degradation. There are a number of contaminating materials and their effect on performance depends upon the contaminant type. Some of the common contaminants frequently encountered are: • Water- Probably the most common type of contaminant. This can occur by careless or improper rinsing or carry over from other parts. • Organic materials - Paint, lubricants, oils, greases, and sealant are other sources of contamination. These materials, if not removed from parts during precleaning, can dissolve in the penetrant and react with or dilute it, so it loses some or all of its ability to function. • Organic solvents - Degreaser fluid, cleaning solvent, gasoline, and antifreeze solution are common types of solvent contaminants. These materials dissolve in the penetrant and reduce its effectiveness in proportion to the amount present.

• Dirt, soil, other insoluble solids - Soil/solid contamination can be carried into the penetrant, emulsifier, and developer as a result of improper pre-cleaning and carry-over from other parts. Another common source of soil contamination occurs when the dwell stations are used to store parts. Most dwell stations have drain pans, which return the effluent back to the immersion tanks. Any soil falling from unclean parts into the drain pan will be washed into the tank with the drain effluent. • Acid and alkaline materials - Acid and alkaline contamination is extremely serious. They react with the penetrant to destroy fluorescence brightness even when present in fairly small quantities. They are usually residues from etching; plating or the cleaning processes. • Penetrant - Penetrant is a normal contaminant of emulsifier in the postemulsifiable process. It can be carried in on penetrant covered parts during the penetrant dwell step. As the penetrant builds up in volume, it will gradually slow the emulsifying action, and if the level becomes high enough, the emulsification process will stop.

Evaporation Losses. Penetrant materials used in open tanks are continuously undergoing evaporation. The rate of evaporation is increased with warmer temperatures and large tank surfaces. Evaporation losses of penetrant result in an increase in viscosity, thus slowing penetration and emulsification. Evaporation of water washable penetrant may slow or speed washability, depending on the penetrant formula. Evaporation losses in developer solutions increase the concentration, which produces a heavier coating that may mask smaller indications. Since evaporation losses take place very gradually, performance change may become significant before it is noticed. Process Degradation. Not only do materials degrade, but equipment and procedures (other elements of the process) can deteriorate as well. Black-light bulbs age, degrade, and also become dirty, reducing their output. Drying oven thermostats can be improperly set or may malfunction, resulting in excessive temperatures causing critical procedures to be performed incorrectly. Materials, equipment, and procedures SHALL be periodically audited during their service life to assure satisfactory process performance.

Heat Degradation. Penetrants, especially fluorescent penetrants, are sensitive to elevated temperatures. Exposing penetrants to temperatures over 60°C can reduce the fluorescence; and temperatures over 121°C may destroy the penetrant completely. High temperatures also speed evaporation of the volatile components of penetrants, causing undesired performance changes. High temperatures exposure of penetrants can occur from the following: Immersion of heated or hot parts.

Inspection of hot surfaces resulting from exposure to the sun, such as flight-line aircraft.

Improper storage of penetrant materials (such as in direct sunlight) before being placed in use.

Excessive exposure to heat in drying ovens.

System checks

Probably the most effective overall checks of the performance of penetrant systems involve the periodic use of panels or samples containing known discontinuities There are other systems checks that may be waived if the system performance check with the panels is satisfactory.

They include tests for: 1. Penetrant brightness

2. Penetrant removability

3. Penetrant sensitivity

4. Emulsifier removability

Equipment Checks There are routine checks that should be made on the equipment that is used in the PT process. They include, but are not limited to, the following: 1. Black lights—for intensity, filter condition, bulb, and reflector. 2. Light meters, black and white—for functionality and calibration status. Some codes require that these meters be periodically calibrated; this is usually done by the meter manufacturer. 3. The temperature of the drying oven—to assure that it is within the specified range. 4. The water used for removal—temperature and pressure. 5. Pressure gauges, when compressed air is used for application of the penetrant, emulsifiers, and developers

Acceptance criteria for visual testing (VT),

magnetic particle testing MT) and

penetrant testing (PT)

Extract from

NORSOK standard M-601

Application of Liquid Penetrant like a chemical tracers to find leakage

Flaws which extend completely through thin-wall metal containers — tanks, tubing, and vessels — are readily detected by the dye penetrant process. The procedure differs from the standard process: dye penetrant is applied to one side while developer is applied to the opposite side; there is no dye penetrant removal step. Dye penetrant leak test procedures are often used for inspection of tank walls. During leak testing, the inside of the tank is coated with dye penetrant. The other side is covered with developer. The dye penetrant migrates through the flaw, and when it reaches the opposite side, reveals the flaw as a red mark on a white background.

Flaw passages must be free of contaminants — water, solvents, or oils. Moisture from air pressure tests conceivably can interfere with through penetration. The dye penetrant leak test should be used prior to other tests, such as hydrostatic and ultrasonic, to minimize the possibility of flaw passage contamination. An extended dye penetrant dwell time is usually necessary. Dye penetrant leak testing has limitations and, typically, the process is restricted to wall thicknesses of 1/4 inch or less. The rate of through penetration depends in part on the shape of the capillary passage with a narrow tube providing the best passage. Porosity retards dye penetrant movement. If wall thickness is near maximum and if poor capillary action is anticipated, the dwell time should be extended. Thirty minutes is suggested. A second application of dye penetrant during the 30 minute dwell may also prove advantageous.

Another application is to incorporate a small amount of fluorescent penetrant inside a pressurized system, taking care to ensure that the penetrant is compatible with the system fluid, then to examine the accessible side with an ultraviolet light.

This is an extremely sensitive technique for finding pinholes in welds.

Example : examining a heat exchanger wall for leakage.

Another application is to detect and locate leak points in an automobile radiator. The method is inexpensive yet accurate.

END