Volume V: ASME Pressure Vessels

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  • Volume V: ASME Pressure Vessels

ASME PRESSURE VESSELS

The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.

NONDESTRUCTIVE EXAMINATION METHODS

NDE is a general term used to identify all methods that permit evaluation of welds and adjacent areas without destroying their usefulness. Here the following basic NDE methods will be discussed:

  1.  Visual
  2.  Penetrant
  3.  Magnetic particle
  4.  Radiographic
  5.  Ultrasonic

It should be noted that NDE does not eliminate the need for destructive testing but rather complements it. The general knowledge presented here should be of valuable assistance to the reader as it provides an overview of the examination methods without unnecessary details.

Visual Examination (VT)

The integrity of most welds is verified principally by visual examination. Even for weldments with joint specified for inspection throughout by other NDE methods, VT still constitutes an important part of practical quality control. The most extensively used of any method of NDE, VT is easy to apply, quick, and often requires no special equipment other than good eyesight and some relative simple and inexpensive tools.

Despite the many advantages of VT, a major disadvantage is the need for an inspector who has considerable experience and knowledge in many different areas which encompass visual welding examination. The inspector must be familiar with materials, drawings, codes, specifications, weld procedures, performance qualification, procedure qualification requirements, and workmanship standards, and all aspects of good shop practice. Some codes and specifications require that the welding inspector be qualified and certified by examination.

Certain tools are sometimes necessary for some aspects of VT. Various measuring scales and gauges are used for checking the dimensions of the welds. There are many different types of fillet weld gauges used to determine the size of fillet welds. Other gauges can be used to verify root opening, weld reinforcement, and weld bevel angle. Measuring devices are used to check root openings, clearance dimensions of materials, backing materials, and alignment and fit-up of the work pieces. Temperature indicators verify preheat and interpass temperatures. Borescopes, video scopes, flashlights and mirrors are used in areas of limited accessibility. The development of flexible fiber optic inspection systems enables the inspector to visually inspect areas inaccessible to other devices.

Liquid Penetrant Examination (PT)

PT is a sensitive method of detecting and locating discontinuities, provided the discontinuities are clear and open to the surface. The method employs a penetrating liquid dye which is applied to the properly cleaned surface to be examined and which enters the discontinuity. After a suitable swell time, the excess penetrant is removed from the surface and the part is dried. A developer is then applied which acts as a blotter, drawing the penetrant out of the discontinuity.  The penetrant drawn from an opening on the surface indicates the presence and location of a discontinuity.

There are two basic classification of the penetrant method, both using a similar principle. One uses a visible dye and the other uses a fluorescent dye which is only visible with exposure to UV light. Visible penetrant is usually red in color to provide a contrast against the white developer background. Normal white light is usually sufficient to view the discontinuities.

Fluorescent penetrants provide a greenish yellow indication against a dark background when viewed in darkened area under a black (ultraviolet) light source. The fluorescent method is inherently more sensitive due to the fact that human eye can more easily discern a fluorescent indication.

PT is widely applicable on magnetic and non-magnetic materials, but it is particularly useful on nonmagnetic materials such as aluminum, magnesium, and austenitic stainless steels where MT examination cannot be used. It is also useful for locating cracks or other discontinuities which may cause leaks in containers and pipes.

There are two common methods of recording a PT indication for evaluation. A photo may be taken of the discontinuities exposed by the examination. Another method involves the application of clear plastic tape over the indication.  When the tape is lifted off the test surface, the indication will adhere to the tape and may be transferred to the inspection report for future reference. These techniques also apply to MT method.

PT method is relatively inexpensive. The process is simple and the operators find little difficulty in learning to apply it properly. The success of this method, like most other examination methods, depends on the visual acuity of the inspector. It should be pointed out that some substances in penetrants can have deleterious effect on either welds or base metals and can affect the service life of the weld or application of the product. Penetrants are difficult to remove completely from discontinuities, and if corrosive to the material, or otherwise not compatible with the product application, they should be avoided.

Magnetic Particle Examination (MT)

This method is used for locating surface or near surface discontinuities in ferromagnetic materials. MT is based on the principles that magnetic lines of force will be distorted by a change in material continuity, i.e. discontinuity creating a magnetic field or flux leakage.

A weldment can be magnetized by passing an electric current through the weld area (direct magnetization) or by placing the weldment in a magnetic field (indirect magnetization). When the magnetic field has been established within the workpiece, magnetic particles (medium) are applied to the surface to be examined. The magnetic particles can be dry or suspended in a liquid. Discontinuities can be further enhanced using fluorescent magnetic particles and observing them under black light. After removal of excess particles, the remaining particles trapped in the leakage field of a discontinuity reveal the location, shape, and size of a detectable discontinuity. These indications usually are distinguished by their appearance as sharp, well defined lines of medium against the background of the weld or heat-affected zone surface.

MT can be very beneficial as an in-process evaluation technique. Assurance of a sound weld before the weld is completed may prevent costly repair of the final product. In-process MT has become more of a common practice due to the portability of modern lightweight equipment. This advantage aids in reducing production time.

The cost of MT is considerably less expensive than radiography (RT) and ultrasonic (UT) – both in terms of the equipment cost and the cost of training the personnel. Using MT, the inspector obtains an instant visible indication of the size and orientation of the discontinuity, and allows the inspector to judge if the discontinuity is acceptable or rejectable. Compared to PT, this method has the advantage of revealing discontinuities that are not open to the surface, and therefore not detected by PT. MT is generally faster, requires less surface preparation, and is therefore usually more economical than PT (neglecting equipment costs).

The MT method is limited to ferromagnetic materials. Welded joints made between metals of dissimilar magnetic characteristics may create irrelevant magnetic particle indications even though welds themselves are sound. Most weld surfaces are acceptable for MT after the removal of slag, spatter or other extraneous material which may mechanically hold the test medium.

Radiographic Examination (RT)

RT is a method of NDE that utilizes radiation to penetrate a weld and reveal information about its internal condition. When a weld is exposed to penetrating radiation, some radiation will be absorbed, some scattered, and some transmitted through the weld onto recording device (See Figure 13). Most conventional RT techniques used today involve exposures that record a permanent image on a photographic film, although other image recording methods are also used.

The basic process of radiographic examination involves two general steps:

  1.  The making of radiograph, and
  2.  Interpretation of the radiograph

The essential elements needed to carry out these two operations are:

  1.  A source of radiation
  2.  Weld to be radiographed
  3.  Weld identification markers, station markers and image quality indicators
  4.  An X-ray film enclosed in a light tight film holder
  5.  A skilled person capable of producing an exposed film
  6.  A means of chemically processing the exposed film
  7.  A skilled person capable of interpreting and evaluating the radiographic images

Two types of radiation sources commonly used in weld inspection are X-ray machines and radioactive isotopes. X-radiation is produced by machines which range from portable, low energy units capable of radiographing relatively thin objects, to mammoth linear accelerators and betatrons capable of radiographing thick steel welds up to 20 in. of steel. Gamma radiation is emitted by radioisotopes, the two most common being Cobalt 60 which will penetrate to approximately 5 in. of steel, and Iridium 192 which is limited to a steel thickness of approximately 3 in.

The radiographic process is dependent upon varying amount of radiation being absorbed by different areas of the weld. The differences in the absorption occurring during the exposure process account for the dark and light regions on the radiograph. The interpretation of a radiograph involves identifying the images resulting from various light and dark regions on the film. The dark regions represent the easily penetrated parts of the weld (i.e., thin sections and most discontinuities) while the lighter areas represent the more difficult areas to penetrate (i.e. thick sections).

A significant limitation of radiographs is that discontinuities must be favorably aligned with the radiation beam to be reliably detected. This is usually not a problem for discontinuities such as porosity or slag since they are usually round in cross section and align with a beam from any direction. This is not the case with planar discontinuities such as cracks, incomplete fusion, and laminations. There are several other limitations associated with radiography:

  1.  It presents a potential radiation hazard to both personnel and the public
  2.  The cost of radiographic equipment, facilities, safety programs and related licensing is relatively high
  3.  There is usually a relatively long time, when compared to MT, PT and VT, between the exposure process and the availability of results
  4.  Accessibility to both sides of the workpiece is required to set up the apparatus.

However, there are some advantages as well:

  1.  It is generally not restricted by type of material
  2.  Both surface and subsurface discontinuities may be detected
  3.  Radiographic images aid in characterization (identification) of discontinuities
  4.  It provides a permanent record for future review
  5. The radiograph may be used to make a map to locate the exact defect orientation and assist in the removal of the unacceptable condition in the weldment

Ultrasonic Examination (UT)

UT is becoming one of the most widely used methods of NDE. Its primary application is the detection and characterization of internal discontinuities. It is also used to detect surface discontinuities, to define bond characters, and to measure thickness. In this method, high frequency sound waves are introduced into the material to detect surface and subsurface discontinuities. The sound waves travel through the material with some loss of energy (attenuation) and are reflected at interfaces. The reflected sound beam is detected and analyzed to define the presence and location of discontinuities.

UT is usually performed with either longitudinal waves (straight beam) or shear waves (angle beam). In longitudinal beam testing (commonly used to examine plate material), sound in the form of ultrasonic vibrations is introduced into part perpendicular to the entry surface by straight beam search unit. When the entry surface and the back surface are parallel, a back reflection will appear on the display screen. A discontinuity lying between the front and back surfaces will also be displayed on the display screen. By measuring the height of the reflection on the display screen, from a real or artificial discontinuity of a known size, a reference level can be established such that reflections from discontinuities of unknown sizes may be evaluated.

The angle beam technique is used for the examination of welds. Ideally, only discontinuities should appear on the display screen during the angle beam inspection. This is not always the case, however, since the geometrical boundaries of the part often reflect sound in same manner as a discontinuity. Therefore care must be taken during UT of joints with complex geometries (such as welds with backing bars) to assure that the indications are the result of the presence of discontinuities and not simply due to the configuration of the joint.

It is generally desirable to have the sound beam intercept the plane of the discontinuity at or near 90o so that the maximum amount of sound is reflected to the transducer. However, cracks that are not oriented perpendicular to the UT beam can be detected because their surfaces are not smooth and sound is reflected from the facets that are approximately perpendicular to the beam. Selection of test surface for scanning with the search unit depends upon accessibility. Scanning surface selection is also based on the weld shape and structure. Since it is important to intercept the discontinuity at or near 90o, it is common for more than one angle unit to be used to examine a particular weld.

The principal advantages of UT over other NDE methods for metal parts are:

  1. Allows the detection of discontinuities deep in the part
  2.  High sensitivity permits the detection of very small discontinuities
  3. Greater accuracy in determining the position of internal discontinuities, estimating size and characterizing the orientation, shape and nature
  4. Only one surface need be accessible
  5. Provides almost instantaneous indication of discontinuities. This makes the method suitable for immediate interpretation, automation, rapid scanning, production line monitoring, and process control. With some systems, a permanent record of inspection results can be made for future reference
  6. Scanning ability enables examination of a volume of a metal extending from front to back surface of a weld

Some disadvantages of UT include:

  1. Manual operation require careful attention by experienced technicians
  2. Extensive technical knowledge is required for the development of examination procedures and the interpretations of indications
  3. Parts that are rough, irregular in shape, very small or thin or in-homogenous are difficult to examine
  4. Discontinuities that are present in a shallow layer immediately beneath the examination surface may not be detectable
  5. Couplants are needed to provide effective transfer of UT wave energy between search units and the part being examined
  6. Reference standards which duplicate the exact examination conditions are needed for calibrating the equipment

Interrelationships among Welding Processes, Discontinuities, and Examination Methods

Table 2 relates the examination methods to various types of discontinuities. Table 3 relates the joint types to the applicable NDE methods.

Table 2: Common Weld Inspection Methods vs. Discontinuities Inspection Methods Discontinuities 

Inspection Methods
Discontinuities RT UT PT MT VT ET LT
Porosity A O A O A O A
Slag inclusions A O A O A O O
Incomplete fusion O A U O O O U
Incomplete joint penetration A A U O O O U
Undercut A O A O A O U
Overlap U O A A O O U
Cracks O A A A A A A
Laminations U A A A A U U

Notes:

  1. Surface
  2. Surface and slightly subsurface
  3. Weld preparation or edge of base metal
  4. Magnetic Particle Examination is applicable only to ferromagnetic materials
  5. Leak Testing is applicable only to enclosed structures which may be sealed and pressurized during testing

Legend:

  • RT – Radiographic testing
  • UT – Ultrasonic testing
  • PT – Penetrant testing, including both DPT (dye penetrant testing) and FPT (fluorescent penetrant testing)
  • MT – Magnetic particle testing
  • VT – Visual testing
  • ET – Eddy current testing
  • Lt – Leak testing
  • A – Applicable method
  • O – Marginal applicability (depending on other factors such as material thickness, discontinuity size, orientation and location)
  • U – Usually not used

Table 3: Applicable Examination Methods vs. Five Weld Joint Types

Inspection Methods
Joints RT UT PT MT VT ET LT
Butt A A A A A A A
Corner O A A A A O A
Tee O A A A A O A
Lap O O A A A O A
Edge O O A A A O A

Legend:

  • RT – Radiographic testing
  • UT – Ultrasonic testing
  • PT – Penetrant testing, including both DPT (dye penetrant testing) and FPT (fluorescent penetrant testing)
  • MT – Magnetic particle testing
  • VT – Visual testing
  • ET – Eddy current testing
  • Lt – Leak testing
  • A – Applicable method
  • O – Marginal applicability (depending on other factors such as material thickness, discontinuity size, orientation and location)

Sources:

  • AWS B1.10 – Guide for the Nondestructive Examination of Welds
  • ASME Section V – Nondestructive Examination

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Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers. With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers.

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  •  Stacks, Scrubbers
  •  Thermal Oxidizers
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  •  Large Diameter Piping

The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.

BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.

Please contact: John W. Smith, P.E. Engineering Manager jsmith@boardmaninc.com 405-601-3367

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