Flanges are piping elements commonly used in the Oil and Gas industry to form mechanical joints. The main purpose of flanges is to interconnect pipes, to connect piping to flanged fittings of equipment, and to isolate piping sections. A flange to flange connection always consists of bolts, nuts, and a sealing material between the two flange faces i.e. a gasket. Flange to pipe connections, or flange to fitting connections, are mostly done via welding; flanges are the second most common joining method.
There are several flange types widely use today, these include the Welding Neck Flange, Slip on Flange, Socket Weld Flange, Lap Joint Flange, Threaded Flange and Blind Flange types. The different flange types, their ratings, and their dimensions etc. are often taken from standards produced by the American Society of Mechanical Engineers (ASME), although other organisations exist.
ASME standards provide guidance and minimum recommended requirements for flange installations and inspections. For example, depending upon the necessary strength of a flanged joint, the type of pumped media (pumpage), and the requirements for the joint assembly/disassembly, certain flange types may or may not be applicable/suitable.
There are many different types of piping flange due to the different service conditions in which they operate. Flanges are chosen based on their corrosion resistance properties, erosion resistance properties, pressure temperature rating (Class), number of welds required for attachment, physical size (some flange designs may not fit into the space allowed) and cost. The most common flange types are:
- Welding Neck Flange
- Slip-on Flange
- Socket Weld Flange
- Lap Joint Flange
- Threaded Flange
- Blind Flange
The other types you may encounter are the male and female, and tongue and groove types. Less common designs include orifice, expander, reducing and long welding neck flanges.
Flange ends may be screwed, welded, or lapped (metal to metal) to their associated piping.
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 article to discuss welding in detail as it is a complex topic, as is post-weld treatment the welded material. 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.
Although the topic of welding will not be covered in further detail, the above image can be used as a reference when looking at different flange types. The image shows various weld joints that are used within the piping industry. In the saVRee flanges video course, each of the common flange types will be discussed in detail along with their associated weld joint type and quantity.
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) and non-destructive inspection (NDI).
As with the topic of welding, the topic of NDT is vast and is beyond the confines of this article. 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, with relation to piping systems. 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 (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 a 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.
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.
Flange Face Types
The most common flange faces are:
- Flat face.
- Raised face.
- Ring type join (RTJ).
See our flange faces article for further information.