A lot of flange literature and standards refer to the type and quantity of welds that a flange should have. It is beyond the scope of this course to discuss welding in detail as it is a complex topic, as is post-weld treatment. However, it is worth knowing that all welds have their own advantages and disadvantages, particularly concerning ease of application, suitability for a given application, and integrity (the ‘quality’ and thus reliability of the weld). In addition, the option to prove a weld using various non-destructive testing (NDT) techniques varies depending upon the weld joint type. Depending upon a flange’s service conditions, and the standards used, it may be a requirement that the weld be proven.
The above image shows various weld joints that are used within the piping industry. Each of the common flange types will be discussed in this section, with additional information provided concerning their associated weld joint type and quantity. When referring to flanges, the most common weld joints are the butt weld and fillet weld, so try to remember the appearance of these weld joint types.
Non-Destructive Testing (NDT)
Non-destructive testing (NDT) allows personnel to evaluate the condition of a material without damaging it; the opposite of non-destructive testing is destructive testing, where a material is tested until it fails. NDT is also known as non-destructive examination (NDE) or non-destructive inspection (NDI).
As with the topic of welding, the topic of NDT is vast and beyond the confines of this material. However, it is important to understand the basics and gain a rough overview of what NDT is, how it is used, why, and its limitations. The most common NDT techniques within the piping related industries are:
- Visual Inspection (VI) – a visual inspection of the test material by qualified personnel. The simplest and easiest NDT technique.
- Ultrasonic (UT) – high frequency sound waves are transmitted through the test material to identify any imperfections; this technique is predominantly used for surface inspections.
- Radiography (RT) – gamma radiation passes through the test material and is gathered on the opposite side with a receiver. The receiver indicates imperfections in the material as well as material density.
- Eddy Current (ET) – an electrical current is passed through the test material which results in electric fields being created around all conductive parts, and induced currents occurring in neighbouring conductive parts. Examination of the resultant eddy currents created during this electromagnetic test reveal any defects in the material.
- Magnetic Particle Testing (MT) – the test material is initially magnetised, then magnetic iron particles are spread over the surface of the material. Visual examination of the patterns formed by the iron particles enables the viewer to identify material defects.
- Acoustic Emission (AE) – the test material’s state is changed via loading, temperature, or pressure, and measured for acoustic emissions. Acoustic emissions occur due to the production of stress waves within the material; these waves are released when the material’s state is changed. The resultant acoustic waves are measured and analysed to determine the material’s condition.
- Liquid Penetrant (PT) – an NDT technique used to find surface imperfections. A dye is spread over the surface of the test material, allowed to ‘soak’, then wiped from the surface. A developer is then applied to the surface which draws-out any dye that has leaked into cracks or imperfections within the material’s surface. Surface imperfections are shown by the contrasting colour of the dye that has been drawn-out by the developing agent.
- Hydrostatic Testing (HT) – a sealed system or sealed vessel is pressurised to a given pressure, the pressure within the system is measured, then a waiting period ensues e.g. 30 minutes, to see if the pressure reduces over time. A reduction in pressure indicates leakage; the rate of reduction indicates the severity/size of the leak. Typical hydrostatic testing occurs at 1.5 times the design pressure or more, although this varies depending upon what is required in order to conform to relevant legislation and standards etc.
Note: Hydrostatic pressure tests can pose a serious danger to personnel if performed incorrectly. Trained personnel and approved procedures should always be used when performing any hydrostatic pressure test.