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Failure to perform quality control inspections introduces the potential for inherent discontinuities to become defects. Nondestructive testing (NDT) is a process that detects the presence of defects or flaws in a part or component without damaging or destroying the material being examined. Here, Vikas Panchal offers a basic NDT for flaw detection in industrial pumps and valves.
Industrial pumps and valves consist of assemblies with various rotating and stationary components, manufactured by casting, forging, rolling and welding, with subsequent heat treatment and machining operations. The American Society for Nondestructive Testing (ASNT) has defined NDT as “the process that includes the inspection, testing or evaluation of materials, components and assemblies for material’s discontinuities, properties and machine problems without further impairing or destroying the part’s serviceability”(1).
Nondestructive Evaluation (NDE) is a term often used interchangeably with nondestructive testing. However, technically, NDE is used to describe examinations that are more quantitative in nature. For example, an NDE exam would not only locate a defect, but it would also be used to measure its size, shape, and orientation, as well as its effect on the remaining life of structures and components. NDE may be used to determine material properties such as fracture toughness, formability and other physical characteristics.
NDT is a very broad and interdisciplinary field that plays a critical role in assuring structural components and systems function in a reliable, cost-effective manner. NDT utilizes various test methods to detect and locate flaws and characterize material conditions that could lead to pumps and valve failures. These tests are performed in a manner that does not affect the future use, such as serviceability of the object. Because NDT allows parts and materials to be inspected and measured without damaging them, it provides an excellent balance between quality control and cost-effectiveness.
Pump system components that are usually subjected to NDT include pressure containment castings such as casings, covers, nozzle heads, impellers, shafts, bearing housings and other critical parts. Similarly, valve bodies are subjected to NDT. NDT does not replace the need for hydrostatic testing of pressure containment parts of pumps and valves, but it supplements the pressure testing.
Discontinuity vs. defect
Differentiating discontinuity from a defect and understanding the impact on component use and serviceability is critical to the evaluation and interpretation of NDT results.
Not all discontinuities are defects, but all defects are discontinuities. A discontinuity is an interruption (disruption, interference or imperfection) in the normal structure or configuration, whereas a defect is a discontinuity that exceeds the allowable limits or tolerances of applicable codes.
The term flaw or defect means a detectable lack of continuity or a detectable imperfection in a physical or dimensional attribute of a part. The term nonconforming means only that a part is deficient in one or more specified characteristic. Although many noncomforming parts are entirely capable of functioning properly, some designs may need to be reworked in order to comply with certain specifications.
Every certified NDT technician must have a thorough knowledge of a part’s material, manufacturing processes, service use, quality requirements, typical discontinuity types, established evaluation criteria and any supplemental criteria for the specific component. It is advisable to contact a metallurgist for expert guidance on possible metallurgical impacts and to fully evaluate the situation prior to making any decisions.
Common discontinuities
During the various stages of material processing, certain discontinuities are expected. The origin of discontinuities in a material can take place during manufacturing, or during assembly, installation, commissioning or in-service use. For the ease of understanding, discontinuities are classified into three major groups: inherent, processing and in-service discontinuities.
Inherent discontinuities: Discontinuities that appear during the basic formation of an object by means of the primary processing are known as inherent discontinuities. Examples include: ingot pipe, ingot cracks, non-metallic inclusions, blow holes, cold shuts, and gas and shrink porosity. Many of these are removed by cropping, but a number of them can remain in the ingot. On further processing with rolling and forging, these discontinuities get elongated as stringers or laps and might affect the serviceability of the product manufactured from these metal ingots.
Pump shafts are expected to have process discontinuities developed from inherent discontinuities. Forged valve bodies might have the above imperfections if processed from the raw materials containing inherent discontinuities, while cast valve bodies are expected to have inherent discontinuities such as inclusions, blow holes, porosity and cold shuts.
Processing discontinuities: Discontinuities that originate during further processing, such as hot or cold forming, are called processing discontinuities. Inherent discontinuities mentioned above, may propagate and become detrimental by further processing of wrought product by forging, rolling and drawing. Common primary processing discontinuities include seams, laminations, stringers, cooling cracks, laps, cupping, bursts, hydrogen flakes, welding discontinuities, grinding cracks, heat treating cracks, pickling cracks and plating cracks.
In-service discontinuities: Service life of engineered components depends on their design compatibility, operating condition, service environment and quality of maintenance. In-service stresses may initiate or result in cracking and/or corrosion discontinuities. Fatigue and creep cracking, stress corrosion cracking and hydrogen cracking are a few of the most commonly occurring discontinuities in service environments. Pumps and valves are used within various end-use environments and are susceptible to inherent, processing and in-service discontinuities that might lead to unplanned failures. It is important to consult a metallurgy expert to ensure specific material and quality specifications are being upheld.
Basic tenants of NDT for flaw detection
Fracture mechanics and NDT
Fracture mechanics involves the applied mechanics of crack growth or propagation in a material. Defects present in materials lead to failure by growing to a critical, self-propagating size. Fracture mechanics concepts allow one to calculate the critical sizes of defects as a function of depth, length and active stress. By knowing the dimensions of defects present in a component, it is possible to estimate both remaining life and extent of degradation using fracture mechanics concepts, where NDT plays a premium role in detecting physical or dimensional imperfections.
Acceptance and rejection
The parameters for acceptance and rejection of a component vary widely among industries. General criteria for the acceptance and rejection of parts include the material, configuration, expected usage, known stress factors, location and number of indications, and failure history. The acceptance criteria state the size and the type of discontinuities acceptable in a specified area.
NDT methods for pumps & valves
The following are the major six (or basic) NDT methods that provide adequate and effective means to detect the discontinuities and are widely used in routine services in the pumps and valves industries. • Visual Testing (VT)• Magnetic Particle Testing (MT)• Liquid Penetrant Testing (PT)• Radiographic Testing (RT)• Ultrasonic Testing (UT)• Eddy Current Testing (ET)
Acoustic emission, thermography and leak testing are also useful NDT techniques. The above NDT methods are covered in a separate section with referenced ASME codes and ASTM standards.
Visual Testing (VT)
Visual testing (VT) is the oldest and most commonly used NDT method. VT provides a means of detecting and examining a variety of surface flaws, such as corrosion, contamination, surface finish and surface discontinuities on joints (for example, welds, seals, solder connections and adhesive bonds). Visual inspection is also the most widely used method for detecting and examining surface cracks, which are particularly important because of their relationship to structural failure mechanisms.
Nondestructive testing begins with visual testing, which can be defined as “the process of examination and evaluation of systems and components by use of human sensory systems aided only by such mechanical enhancements to sensory input as magnifiers, dental picks, stethoscopes, etc.”
The methods of visual inspection involve a wide variety of equipment, ranging from examination with the naked eye to the use of interference microscopes for measuring the depth of scratches in the finish of finely polished or lapped surfaces. The process relies on human visual acuity and can be aided by light sources, magnifying devices and sensors such as microscopes, borescopes and video crawlers. Successful use of the technique requires good lighting and vision for optimal sensitivity. Training and experience are vital for accurate interpretation of features.
VT is usually classified in two groups of processes including:
• Direct visual inspection: uses magnifying glass, stereoscopes, microscopes, borescopes and real-time video. • Indirect visual inspection: involves examination of photographs, radiographs or videotapes.
Advantages: • Minimum part preparation required• Inexpensive and can be performed rapidly• Complex sizes & shapes can be inspected
Limitations:• Useful only for surface discontinuities• Surface finish, roughness and cleanliness can interfere with inspection• Some equipment is expensive
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