Radiographic Testing with High-Intensity Panoramic Technique & Safety Consideration for Spherical Tank

Radiographic Testing with High-Intensity Panoramic Technique & Safety Consideration for Spherical Tank

#RT #Sphericaltank # Radiographictesting  #RTpanoramic  #HighActivitySource  #WeldInspection #byLAW #DOEB

Author: Sunsook Unsereephap / Puripong Klamdith , Plant Inspection, GCME

Key Point:

“Panoramic radiographic testing with high-activity sources enables efficient and comprehensive weld inspection of spherical tank, while maintaining quality and safety compliance under code & standard, regulatory and site safety practice requirement.”

1. Introduction:

              The inspection of weld integrity in Spherical tanks is a critical step in ensuring operational safety, particularly for spherical tanks that operate under high internal pressure. This five-year periodic inspection is mandated by the Department of Energy Business (DOEB) to ensure that such equipment remains structurally sound and safe for continued use.

              The technique employed in this inspection is panoramic radiography using a high-activity radiation source, which is particularly suitable for circumferential weld inspection on spherical tanks. This method provides continuous imaging coverage, allowing the entire weld area to be captured in a single exposure cycle. As a result, it reduces the time required for shot setup and enhances overall operational efficiency.              

              All operations must be conducted in accordance with ASME Section V – Nondestructive Examination, and strict adherence to radiation safety protocols is required to prevent adverse effects on personnel and the environment.

2. Radiographic Testing (RT):

              Radiographic Testing (RT) is a non-destructive testing (NDT) method that utilizes penetrating radiation to examine internal discontinuities within materials, such as cracks, porosity, or foreign material inclusions in welds. The resulting image—captured either on film or through a digital system—reflects variations in radiation absorption by the material, allowing for the detection and evaluation of internal flaws.

2.1 Operating Principle

·      The type of radiation used in this application is gamma radiation emitted from an Iridium-192 (Ir-192) source.

·      As the radiation passes through an object, such as a weld in a pressure vessel, it is absorbed to varying degrees depending on the material density at each location.

·      Defective areas, such as cracks, absorb less radiation and thus appear as darker spots on the film.

2.2 Properties of Iridium-192

·      Iridium-192 is a portable gamma radiation source widely used in industrial applications.

·      Radiation energy: approximately 300–600 keV.

·      Suitable for inspecting materials with thicknesses ranging from 10 to 60 mm.

·      It has a small source size, which contributes to high image resolution.

2.3 Components of the RT System

·      Source Projector: A device used to contain and deploy the radiation source.

·      Guide Tube and Collimator: Direct and shape the radiation beam while minimizing scatter.

·      Radiographic Film or Digital Detector: Records the radiographic image.

·      Densitometer: Measures the optical density of the image.

·      Survey Meter and Dosimeter: Monitor radiation levels to ensure operational safety.

2.4 Selection of Imaging Technique

·      The positioning of the film and radiation source must be appropriate for the geometry of the object being inspected.

·      For spherical tanks, the panoramic radiographic testing (Panoramic RT) technique is used. In this method, the radiation source is placed at the center of the tank while the film is arranged around the outer circumference to capture a 360-degree image.

3.Code & Standard:

              Radiographic Testing (RT) for weld inspection must be performed in accordance with the ASME Boiler and Pressure Vessel Code, Section V: Nondestructive Examination, specifically Article 2 – Radiographic Examination, which governs film-based radiographic testing.

3.1 General Requirements

·      The technique used must clearly reveal internal discontinuities.

·      The radiation source and film must be suitable for the material type and thickness of the test object.

·      Film quality must be controlled through parameters such as density, sensitivity, and Image Quality Indicator (IQI).

3.2 IQI (Image Quality Indicator) Placement

·      IQIs should typically be placed on the surface closest to the radiation source. However, if access is not possible, they may be placed on the film side, provided they are in direct contact with the test object.

·      For spherical tanks using panoramic radiography, multiple IQIs must be placed around the tank to ensure image quality consistency throughout the circumference.

·      The size and material of the IQIs must comply with the requirements specified in Article 2.

3.3 Material Thickness and Radiation Selection

·      Iridium-192 (Ir-192) is suitable for inspecting materials with thicknesses up to approximately 60 mm.

·      The selected radiographic technique must provide sufficient contrast based on differences in radiation absorption.

·      When using the panoramic RT technique, key variables such as film density, radiation dosage, exposure area, and radiation scatter must be verified to remain within acceptable limits.

3.4 Image Quality Requirements

·      Film density must fall within the range of 2.0 to 4.0, depending on the film type and IQI position.

·      Details of the wire or pinhole IQI must be clearly visible.

·      The film must be free from defects such as creases, chemical stains, or light leaks.

3.5 Documentation and Certification

·      A radiographic technique sheet must be prepared, detailing radiation settings, film type, source-to-film distance, exposure time, and other relevant parameters.

·      A radiographic report, including film interpretation results, must be maintained and signed by qualified personnel.

·      Film interpreters must be certified in accordance with SNT-TC-1A, CP-189, or other applicable job-specific requirements.

4. Technique:

              Panoramic radiography is a specialized technique used to inspect the welds of cylindrical objects such as pressure vessels, gas tanks, or other pressurized containers. It is particularly suitable for components with circumferential welds.

4.1 Principle of the Panoramic Technique

·      The radiation source is positioned at the center of the vessel or along the axis of the circumferential weld.

·      Radiographic film and Image Quality Indicators (IQIs) are arranged around the outer surface of the tank, surrounding the weld line.

·      When the radiation is emitted, it disperses in a 360-degree pattern, capturing the entire weld circumference in a single exposure.

4.2 Radiographic Image Quality

              IQI stands for Image Quality Indicator, a standardized device used in radiographic testing to verify the clarity and detectability of flaws in radiographic images. It ensures that the film provides adequate sharpness and sensitivity for defect detection. Common types include wire-type and hole-type IQIs.
              IQI help assess whether the image sensitivity is within acceptable limits. If the required wire or hole cannot be clearly seen in the film, the radiographic technique must be improved—such as by using a higher activity source, adjusting the Source-to-Film Distance (SFD), or increasing the exposure time.

Figure 1 : IQI Wire type 

Figure 2 : IQI Hole type

Figure 3 : Layout of RT Source, Film, and IQI for Spherical or Cylindrical Vessel Weld Inspection According to ASME

Figure 4 : The source is installed at the internal central position of the tank

Figure 5 : Films is installed at the external tank shell of the tank

5. Radiation Intensity and Exposure Time:

5.1 Radiation Intensity

              Radiographic inspections conducted within controlled areas of industrial facilities typically utilize radiation sources with an activity not exceeding 30 curies (Ci). However, when inspecting large cylindrical tanks using panoramic radiographic techniques, it is often necessary to employ sources with activities exceeding 300 Ci. This requirement arises due to the following factors:

·      The tanks possess large diameters and substantial wall thicknesses, necessitating higher radiation energy to achieve adequate penetration.

·      To ensure uniform exposure across the entire circumference of the film, sufficient radiation intensity is required. Insufficient radiation strength can lead to underexposed areas (film density < 2.0), rendering the radiographs unacceptable.

·      Reducing exposure time is critical, particularly in field operations where access to controlled areas is time-restricted to minimize disruptions to other operational teams.

Figure 6 : Good film quality

Figure 7: Underexposed Film

5.2 Exposure Time calculation

              Exposure time refers to the calculated duration for which a radiation source (e.g., Ir-192) must be activated to deliver sufficient radiation intensity, ensuring the radiographic film achieves the desired image quality (appropriate density). The calculation is based on the following formula;

* The exposure factor is highly specific and depends on various parameters, including the type of film, film processing method, desired film density, material type and thickness, as well as the radiation energy.

6. Calculation of Safety Distance:

              In field operations, it is imperative to establish controlled areas to ensure that radiation exposure does not exceed prescribed limits. For radiation workers, the annual effective dose must not exceed 20 millisieverts (mSv), or 400 microsieverts (µSv) per week. For the general public, the annual limit is 1 mSv, or 20 µSv per week. Furthermore, the radiation level at the boundary of the controlled area must not exceed 25 µSv per hour. The calculation of the safety distance involves the following equation;

Example Calculation

              Ir-192 source with an activity of 330 Ci and a steel shell thickness of 57.5 millimeters, determine the safe distance for controlled area boundary.

Given;

·      Characteristic intensity of Ir-192: 4,800 µSv/h per Ci at 1 meter.

·      Permissible exposure limit for the general public in controlled areas: 7.5 µSv/h.

Formula;

Safety Plot plan

Figure 8 : Safety Distance and Road Closures during X-ray Photography.

7. Safety Measures

7.1 ALARA Principle (As Low As Reasonably Achievable)

              The ALARA principle is a fundamental concept in radiation protection, aiming to minimize radiation exposure to levels as low as reasonably achievable, even if the doses are within legal limits. This principle is implemented through three key factors:

·      Time: Minimize the duration of exposure to radiation sources.

·      Distance: Maximize the distance from radiation sources, as radiation intensity decreases proportionally to the square of the distance (inverse square law).

·      Shielding: Utilize appropriate shielding materials such as lead, concrete, or tungsten to attenuate radiation. 

7.2 Authorization and Qualifications of Personnel

·      Personnel must:

ü Receive training in radiation safety.

ü Undergo health examinations and possess a valid radiation worker license.

·      A Radiation Safety Officer (RSO) must be appointed to oversee and ensure compliance with radiation safety regulations throughout operations. 

7.3 Safety Controls During Radiographic Operations in Controlled Areas

·      Obtain necessary work permits and have radiation sources inspected by authorized personnel before commencing operations.

·      Clearly demarcate the area using warning signs and barrier tapes indicating radiation work in progress.

·      Assign personnel to monitor and prevent unauthorized access to the radiographic area.

·      Employ survey meters to verify the absence of radiation leakage beyond the designated safety perimeter. 

7.4 Emergency Response and Contingency Plans

·      Develop contingency plans for incidents such as loss, damage, or misplacement of radiation sources.

·      Establish evacuation procedures for personnel from hazardous areas.

·      Notify relevant authorities (e.g., Office of Atoms for Peace) in the event of an incident.

·      Maintain a Radiation Emergency Plan that is reviewed and practiced annually to ensure preparedness.

8. Summary and Recommendations

      Summary

·      Panoramic radiographic imaging is an effective technique for inspecting circumferential welds on cylindrical tanks, reducing the number of film placements and inspection time.

·      Utilizing high-activity Ir-192 sources enables radiographic imaging of thick-walled tanks within a short duration; however, it also increases radiation exposure risks.

·      Implementing the ALARA principle and determining safety distances based on actual dose rates are essential and must not be neglected.

·      Understanding the properties of Ir-192, such as radiation energy, exposure constant, and the inverse square law, is crucial for both imaging quality and safety.

      Recommendations

·      Exercise caution when employing the panoramic technique, especially in confined spaces; plan safety distances and area demarcations clearly.

·      Provide regular training and refreshers on radiation safety for the team, including conducting emergency drills at least once a year.

·      Maintain strict records of personnel's cumulative radiation exposure and evaluate the data to improve control measures for future operations.

·      The Radiation Safety Officer (RSO) must closely oversee the design of operations and the authorization process for radiographic work.

·      Promote a safety culture that prioritizes employee well-being over operational urgency.

Panoramic radiographic testing (RT) is highly efficient for weld inspections but carries inherent radiation risks that are not visible to the naked eye.

"Safety must always come first."

Adhering to standardized procedures and appropriate radiation control ensures high-quality and safe operations for all involved.