PIPING
FLANGE
FUNDAMENTALS
Second Edition, 2024
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Piping Flange Fundamentals
Contents
Introduction .................................................................... 1
Flange Terminology .............................................................. 1
Bolted, Threaded and Welded Joints ................................. 2
Expansion Joints ................................................................... 3
Flange Construction ............................................................. 3
How Flanges Work ................................................................ 4
Basic Flange Math ................................................................ 5
Flange Defining Factors ................................................. 7
The Need for Standardisation ....................................... 8
Regulatory Organisations .................................................... 8
Insurance ............................................................................... 8
Codes, Standards and Specifications ................................. 9
Pipes and Piping ............................................................ 11
Piping Systems .................................................................... 11
Pipe Types ........................................................................... 12
Pipe Ends ............................................................................. 13
Pipe Sizing History .............................................................. 15
Nominal Pipe Size (NPS) .................................................... 15
Diameter Nominal (DN) ..................................................... 16
Larger Pipe Sizes ................................................................ 16
Pipe Wall Thickness ............................................................ 17
Schedule (SCH) .................................................................... 18
Flange Schedule .................................................................. 19
Defining A Flange .......................................................... 20
Flange Pressure Class ........................................................ 20
Flange Pressure Class Examples ...................................... 20
Determining Pressure Class .............................................. 21
Pressure Class Tables ........................................................ 22
Pressure Class Table Examples ........................................ 23
Flange Pressure Class Effects ............................................ 25
Flange Dimensions ............................................................. 25
Flange Material Selection .................................................. 26
Flange Materials ................................................................. 27
Flange Faces ................................................................... 28
Plain Face/Flat Face (FF) .................................................... 28
Raised Face (RF) ................................................................. 29
Ring-Type Joint (RTJ) ........................................................... 29
Flange Face Summary ....................................................... 30
Flange Face Surfaces .................................................... 31
Surface Finish ..................................................................... 31
Smooth Flange Surface Finish .......................................... 31
Serrated Flange Surfaces .................................................. 32
Concentric Circular Groove Surfaces .............................. 33
Spiral Groove Surfaces ...................................................... 33
Roughness and Surface Finish .................................... 34
Roughness Average (Ra) ................................................... 34
Gaskets and Sealing Agents ........................................ 35
Gaskets ............................................................................... 35
Soft Gaskets........................................................................ 35
Hard Gaskets ...................................................................... 36
Composite Gaskets ............................................................ 37
Screw Thread Sealing Agents ........................................... 37
Nuts and Bolts............................................................... 39
Bolting Criteria ................................................................... 39
Bolts .................................................................................... 39
Nuts ..................................................................................... 40
Washers .............................................................................. 41
Bolting Procedure .............................................................. 41
Alignment ........................................................................... 42
Flange Types .................................................................. 43
Welds ................................................................................... 43
Non-Destructive Testing (NDT) ........................................ 44
Welding Neck Flange ......................................................... 45
Slip-on Flange ..................................................................... 46
Socket Weld Flange............................................................ 47
Lap Joint Flange (LJF) .......................................................... 48
Threaded Flange ................................................................ 50
Blind Flange ........................................................................ 51
Flange Types Summary ..................................................... 52
Special Flange Types ......................................................... 52
Final Thoughts............................................................... 53
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Piping Flange Fundamentals
Discover the Essentials of Piping Flanges with Our
Piping Flange Fundamentals
Online Video Course!
You'll learn about:
•
Flange Terminology and Nomenclature
•
Flange Types: Welding neck, slip-on, lap joint, and more
•
Piping Codes and Standards: ASME, ASTM, ANSI, ISO, EN, API
•
Pipe Types and Ends: Seamless, welded, bevel, threaded
•
Nominal Pipe Size (NPS) and Schedule
•
Pressure Classes: 150#, 300#, etc.
•
Flange Materials, Faces, and Surfaces
•
Gasket and Fastener Types
•
Non-Destructive Testing Methods
Click HERE to see the full course.
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Piping Flange Fundamentals
Introduction
Flanges offer a mechanical means of joining pipes, fittings (elbows, tees etc.), and valves. Compared to welds, flanges
are a non-permanent type of joint that can be easily assembled and disassembled (ideal for systems that require
maintenance). Flanges are installed via welding, screwing, or lapping, and they are the second most popular joining
method after welding.
A flange assembly consists of:
•
Flange (blade, hub).
•
Gasket (metallic, composite, or non-metallic).
•
Fasteners (nuts, bolts, or studs).
Flange Assembly
Flange Terminology
Flange terminology and nomenclature can be confusing due to the similar terms, definitions, and phrases that are used.
To make the learning process easier, readers should clearly understand the following terms:
•
Flange types – refers to the flange design. Examples of flange types include the welding neck (weld neck), slip-
on, socket weld, threaded, blind flange and lap joint type flanges. Flange types are selected based on the
temperature and pressure requirements, and are identifiable by their geometry.
•
Flange faces – refers to the area used for sealing of the flange; a gasket is usually installed between the two
opposing flange faces. Examples of flange faces include the flat, raised, ring-type joint (RTJ), lap joint, tongue
and groove, and male and female designs.
•
Flange surfaces – refers to the condition of the flange face sealing surface. A flange face surface may be smooth,
or serrated1. The smoothness of a flange face surface is defined by its Roughness Average (Ra) or Arithmetic
Average Roughness Height (AARH).
All of the afore mentioned topics will be further discussed. It is important to realise that there are many aspects that
influence not only what flange type is chosen for a particular application, but also what face and what surface. For example:
•
Certain systems may require welded joints that can be easily inspected (this is not always possible with certain
flange types).
•
Certain flange faces may not be suitable for higher pressure systems because the maximum sealing pressure is
too low (flat face designs).
•
Certain materials will tend to have poor finishes that yield a correspondingly rough sealing surface; these rough
surfaces require a gasket if a leak tight seal is to be achieved e.g. cast-iron flanges.
1 ‘Serrations’ are machined grooves cut into the surface of a flange’s face. Gasket material flows into the grooves, which results in a more reliable seal being obtained;
the grooves also help hold the gasket stationary.
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Piping Flange Fundamentals
When selecting a flange, the material is chosen to meet process requirements first, whilst the temperature and pressure
requirements are then met based on the material chosen.
Bolted, Threaded and Welded Joints
Flanges are a type of bolted joint. Other common types of joint include threaded joints and welded joints.
•
A bolted joint requires a flange and fasteners (nuts, bolts, or studs).
•
A threaded joint requires a male and female screw thread, the male thread screws into the female thread.
•
A welded joint is made using a weld (the process of melting/fusing metal by applying heat).
Bolted Joint
The type of joint used depends on many factors, including pressure, temperature, type of process fluid, operating
characteristics of the system, and the surrounding environment. A bolted joint may be used if:
•
Other types of joint are not suitable e.g. welding may not be possible within areas that pose a fire or explosion
risk (Ex areas); this is mostly a concern for an already operational piping system, not one that is under construction.
•
A machinery item must be disconnected from the service line in order that maintenance or replacement of the
machine can occur.
•
Quick field assembly is required using only basic hand tools.
•
The item (e.g. tank, pipe, machine) to which the flange is connected must be frequently maintained; it is quick and
easy to disassemble and assemble a flange, but not a weld.
Some of the main disadvantages associated with a bolted joint include:
•
Insulating a bolted joint (thermal insulation) costs more than insulating a threaded or welded joint.
•
Bolted joints require more physical space than threaded or welded joints.
•
Each bolted joint represents an additional leakage point (even if assembled correctly).
As a general rule, threaded joints are suitable for lower pressure and temperature applications only, whilst bolted and
welded joints are suitable for higher pressure and higher temperature applications. If a threaded joint must be leak tight,
and leakage cannot be tolerated, it can be seal welded. The seal welding technique is only used for higher service pressure
conditions and is not an ideal solution because it creates a stress concentration point which will be prone to fatigue failure.
The advantage with welded joints is that the weld can be proved using non-destructive testing (NDT) techniques e.g.
penetrant testing, ultrasonic testing, magnetic particle testing, hydrostatic pressure testing etc.; proving a flange -and flange
gasket- is more difficult.
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Piping Flange Fundamentals
Expansion Joints
A less common type of joint is the expansion joint. Expansion joints cater for thermal expansion of a piping system as its
temperature increases. Although expansion joints are usually considered only to cater for expansion, it is important to
realise that they need to cater for the entire temperature range the piping system operates at, during both contraction
(lower temperatures) and expansion (higher temperatures). To do this, they should not be overloaded during
compression (‘squeezing’) or tensile (‘stretching’) loading. There are four main types of expansion joint:
•
Rubber bellow
•
Metal bellow
•
Slip
•
Ball
Metal Bellow Expansion Joint
Expansion joints are used if the installation of expansion loops (piping laid in a semi-circular shape) is not practical.
Pipe Expansion Loop
If thermal expansion of the piping system is not catered for, loads will be transferred to stationary supports and equipment,
which may lead them to failing. The problems arising from thermal expansion must be dealt with at the design stage.
Systems that have wide ranging temperature changes e.g. power plant steam systems, are particularly susceptible to
damage arising due to thermal expansion.
Tip - within the piping industry, expansion joints are being phased-out for new piping system designs and many
companies now prohibit their usage. The reason for this phase-out is because environmental factors often cause
expansion joints to malfunction, and they require constant inspection and maintenance (at additional financial cost).
Flange Construction
Flanges are split into two main areas, the ‘blade’, and the ‘hub’.
•
The flange blade encompasses the area where the bolts penetrate through the flange and the flange face.
•
The flange hub is the area that accommodates the pipe which attaches to the flange.
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Piping Flange Fundamentals
To ensure no leaking between the mating2 flanges occurs, gaskets are used. It is possible to mate two metal flanges
together without the use of gaskets, but sealing is difficult and can only be achieved with specially designed flanges.
The end connection specifies how the flange is connected to its accompanying pipe (threaded connection or welded).
Flange Design
How Flanges Work
A flange is created when two opposing surfaces are intentionally pressed together in order to create a leak tight seal. To
obtain a seal, force must be applied and maintained to each of the opposing flange faces. As many flange faces have
manufacturing imperfections (scratches, dents, pits etc.), it is necessary to put a softer material between the two mating
sealing surfaces to obtain the seal; this softer material is the gasket.
Flange Assembly
2 Mating – refers to the pressing together of two opposing flange face sealing surfaces.
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Piping Flange Fundamentals
Basic Flange Math
To understand how flanges work, we must first understand the concept of pressure. Pressure is defined as:
Pressure = Force / Area
P = F / A
Flanges seal because pressure is applied to the mating sealing surfaces; this pressure is known as the ‘gasket compression’
or ‘sealing pressure’. The applied pressure causes the two faces to either:
•
Crush a gasket between the two mating faces.
•
Press the two mating faces against each other.
In the gasket example, the gasket is deformed due to the pressure applied; this deformation causes the gasket to ‘flow’
into any surface imperfections that may be present on either sealing face. Because the surface imperfections have been
filled by the gasket material, leakage is no longer possible.
The second example assumes no gasket is present and that two flange faces are pressed together. It is hard to create a
leak tight seal using this method, although it is possible if the surfaces are well machined and very clean. The sealing
pressure applied will often need to be significant, as the flange surface may be manufactured from metal, which does not
easily deform under pressure (material and flange class dependent). Metal to metal flange face sealing is expensive and
thus not common.
To create the necessary sealing pressure, the variables of force and area can be adjusted.
•
Force refers to the tightening torque (bolting load) applied to the mating flange faces when the nuts on a flange
assembly are tightened. Force (F) depends upon the torque (T) applied, torque friction (K) and nominal bolt
diameter (D). The force described is classed as ‘bolt pretension’ or ‘bolt preload’, or ‘bolt prestress’, and is
represented by the equation F = T/(KD)
•
Area refers to the size of the sealing face area.
The amount of pressure on the flange sealing faces corresponds to the amount of force applied when tightening the flange
assembly. Thus it is possible to regulate the pressure by adjusting the amount of effort that is exerted when tightening the
bolts during flange assembly.
The sealing area of a flange cannot be as easily adjusted as the force used during assembly. A larger sealing face requires
more force to obtain a certain amount of pressure, compared to when using a smaller sealing face. The below example
highlights this point, but without the use of units.
Example
A given flange assembly requires a pressure of 10 to seal. This can be achieved by applying a lot of force onto a small sealing
face:
Pressure = Force / Area
10 = 40 / 4
Or, it is possible to decrease the size of the sealing face (area) and thus reduce the amount of force required to create the
same amount of pressure3:
3 Standards such as ASME B16.5 and B16.47 dictate the size of the sealing face required.
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Piping Flange Fundamentals
10 = 20 / 2
The relationship between pressure, force, and area, can be briefly summarised:
Decreasing the sealing face area leads to a decrease in the force required to create a given amount of pressure.
Increasing the sealing face area leads to an increase in the force required to create a given amount of pressure.
The amount of force that can be applied to a flange assembly is limited because of problems relating to physical strength
(nuts are often hand-tightened), gasket blow-out4, and stripping5 of the flange bolt threads; but these problems can be
overcome if the size of the sealing face is reduced. The type and size of the sealing face used will be dictated by relevant
piping standards once the temperature and pressure rating of the flange is known.
Based on what has been discussed in this section, it can be determined that flanges required to seal at higher pressures,
have smaller sealing faces. It is possible for a viewer to guess the pressure at which a system operates by visually inspecting
the flange sealing faces e.g. large flange sealing faces indicate low pressure systems.
4 Refers to the expulsion of the gasket from the sealing face due to pressure; this usually occurs due to overtightening of the flange during assembly.
5 Refers to the removing of the threads from the stud or bolt; the result is a spherical piece with no screw threads.
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Piping Flange Fundamentals
Flange Defining Factors
Flanges are categorised based upon certain criteria, and these categories are usually defined by relevant piping standards
and specifications (discussed later). A flange is defined by:
•
Type – the geometry of the flange as a whole. Welding neck, slip-on, and socket weld, are examples of different
flange types.
•
Face – the sealing area of the flange. Flat face, raised face, and ring type joint, are examples of different flange
faces.
•
Standards and Specification – flanges are manufactured to comply with given standards and specifications.
Standards and specifications dictate the dimensions, geometry, schedule, and material, of a given flange (to
name a few factors).
•
Dimensions – the dimensions of a flange’s hub, face, blade etc. Dimensions depend upon nominal pipe size (NPS)
and the pressure class required for a given application.
•
Nominal Pipe Size (NPS) – a dimensionless unit of measurement defining the size of the item (pipe, fitting etc.)
that connects to the flange.
•
Pressure Class – the pressure-temperature rating of the flange for a given material. Despite the name ‘pressure
class’, this factor is material and temperature dependent.
•
Material – the material from which the flange is manufactured e.g. cast iron, carbon steel, stainless steel etc.
•
Schedule (SCH) – a pipe’s thickness/schedule. The schedule of a pipe is relevant only for welding neck and lap-
joint flanges because the schedule of these flanges must match the associated pipe schedule to which they are
connected. The other flange types either slide partly into, screw into, or penetrate through, their associated flange,
thus the flange schedule does not need to match the pipe schedule. The schedule is relevant for swivel-ring
flanges, but these have limited application and will not be discussed further.
All of the aforementioned bullet points will be discussed in a logical order in the coming sections. For now, it’s important to
realise that flanges are not unique items. Flanges are manufactured for a specific purpose, with many design factors already
considered. Should a flange ever fail, the exact same flange can -theoretically- be ordered to replace its predecessor6; this
has significant real-world benefits, which will be discussed later in the Standardisation section.
6 A root cause analysis (RCA) should be conducted on any flange that has failed unexpectedly. If the cause of failure is not determined, the same failure may occur
again even with a new flange.