Pipework can appear straightforward until a flange, pipe or threaded fitting has to be specified, identified or replaced. Nominal sizes do not necessarily correspond to dimensions measured with a rule or caliper, pressure designations need to be interpreted correctly, and components with apparently similar descriptions are not automatically interchangeable.
This guide explains how to approach the selection and identification of flanges, pipe sizes and pipe threads before using the detailed RoyMech dimensional tables.
Before looking for a flange or pipe dimension, establish what the connection has to do. For an existing installation this may mean identifying components already fitted. For a new design it means establishing the service requirements before selecting the components.
Important questions include:
These questions are related. Selecting a flange solely because its nominal diameter appears to match the pipe is not sufficient.
DN is the nominal size designation used for pipes, flanges, valves and associated piping components. It provides a common size designation for compatible components within a specified system.
DN should not be interpreted as an exact measurement of either the pipe bore, pipe outside diameter or flange diameter.
For example, when identifying an existing pipe, measuring its outside diameter and comparing that measurement directly with the DN number can give an apparently unexpected result. The correct relationship must be obtained from the applicable pipe dimensional standard or table.
PN is the nominal pressure designation used for components in PN-designated piping systems. RoyMech includes dimensional reference pages for PN16, PN25 and PN40 flanges.
A higher PN designation generally identifies a flange arrangement intended for a higher pressure rating within the applicable standard, but PN should not be treated simply as the maximum permissible working pressure expressed in bar.
The allowable pressure for a particular flange depends on the applicable standard and factors including:
The required flange pressure designation should follow from the design conditions and the applicable piping or equipment specification. It should not be selected simply by choosing the lowest PN value above the normal operating pressure.
The design should consider the specified design pressure and temperature, material, possible transient conditions and any requirements imposed by the piping system, equipment or governing design standard.
Once the required PN designation has been established, the corresponding RoyMech dimensional page can be used to obtain the principal flange dimensions, drilling and bolting information.
| Flange reference | RoyMech dimensional page |
|---|---|
| BS EN 1092-1 PN16 | PN16 Flange Dimensions |
| BS EN 1092-1 PN25 | PN25 Flange Dimensions |
| BS EN 1092-1 PN40 | PN40 Flange Dimensions |
A flange designation contains more information than nominal size and pressure designation. The method by which the flange connects to the pipe or component is also important.
Depending on the applicable standard and system, flange arrangements can include weld-neck, hubbed slip-on, plate, threaded, loose and other flange types. Different arrangements suit different methods of fabrication, loading, assembly and maintenance.
When selecting or identifying a flange, establish:
BS EN 1092-1 specifies flange types and facings as well as dimensions, tolerances, bolting, materials, marking and pressure/temperature ratings.
The flange facing forms part of the joint and should not be considered independently of the gasket and mating flange.
Two flanges may have the same nominal size and pressure designation while still requiring further checks before they can be joined. The facing geometry, surface finish, gasket arrangement and bolting all contribute to the completed joint.
For an existing installation, inspect the mating flange and identify the existing facing rather than assuming that a replacement flange with matching DN and PN will necessarily be suitable.
Not necessarily.
Matching DN and PN designations are useful starting points, but they do not by themselves prove that two flanges can be assembled correctly or that the resulting joint is suitable for service.
Check the:
For the steel flange information covered by this RoyMech section, BS EN 1092-1 is the current reference for new work. The RoyMech BS 4504 pages are retained because BS 4504 flange dimensions continue to be encountered on existing plant, older drawings and replacement components.
For a new design, the applicable current standard and project specification should normally determine the flange requirement.
For maintenance or replacement work, however, the problem is often different: the engineer first has to establish what was originally fitted.
| Situation | Starting point |
|---|---|
| New steel flange design | Establish the applicable current standard and project specification; BS EN 1092-1 is the principal current steel flange reference used by the RoyMech tables. |
| Existing BS EN 1092-1 installation | Identify DN, PN, flange type, facing, material and dimensional details before selecting the replacement. |
| Older plant or drawing specifying BS 4504 | Use the legacy BS 4504 data to identify or check the existing component and its mating flange. |
| Standard unknown | Measure and identify the existing component rather than assuming a standard from DN or PN markings alone. |
RoyMech retains the following BS 4504 reference tables for engineers working with existing machinery, pipework and historic drawings:
Pipe size and pipe wall thickness are related but separate selections. A nominal pipe size identifies the general size of the pipe, while the schedule identifies a wall-thickness series.
For a given nominal pipe size, different schedules can therefore have the same nominal outside diameter while having different wall thicknesses and consequently different inside diameters.
As wall thickness increases, the available internal diameter normally decreases for the same nominal outside diameter.
RoyMech provides dimensional data for commonly encountered pipe schedules here:
Because different wall thicknesses can be used with the same nominal pipe size, the actual bore depends on the selected pipe dimensions.
This matters when bore affects:
When the internal diameter is important, use the actual pipe dimensions rather than assuming it from the nominal size.
Older drawings and installations may specify pipe dimensions using standards that are no longer the normal choice for new designs.
RoyMech retains dimensional information from BS 3600 for this reason:
Legacy BS 3600 Pipe Dimensions
BS 3600:1997 has been withdrawn. For current dimensional information for seamless and welded steel tubes, BS EN 10220 is an applicable current reference, subject to the requirements of the particular pipe product and design standard.
One of the most important distinctions between pipe-thread systems is whether the thread itself forms part of the pressure-tight joint.
This distinction separates the principal thread systems covered by the RoyMech pipe-thread reference.
| Thread system | General purpose | Important point |
|---|---|---|
| BS EN ISO 228 | Pipe threads where pressure-tight joints are not made on the threads. | The threads are parallel. Where a pressure-tight assembly is required, sealing is provided outside the thread by the appropriate sealing arrangement. |
| BS EN 10226 | Pipe threads where pressure-tight joints are made on the threads. | Taper external threads are used with the corresponding parallel or taper internal thread arrangements specified by the applicable part of the standard. |
This is more important than simply deciding that a component needs a "BSP thread". The thread designation and sealing method need to be understood together.
The letter preceding a pipe-thread size identifies the thread system and form. The RoyMech pipe-thread page includes the commonly encountered G, R, Rp and Rc designations.
| Designation | General meaning | Connection principle |
|---|---|---|
| G | Parallel pipe thread to BS EN ISO 228 | Pressure tightness, where required, is provided by a sealing arrangement outside the threads rather than by thread interference. |
| R | Taper external pipe thread to BS EN 10226 | Used as part of a pressure-tight threaded joint with the appropriate mating internal thread and sealing arrangement. |
| Rp | Parallel internal pipe thread to BS EN 10226 | Used with the appropriate taper external R thread for a pressure-tight threaded joint. |
| Rc | Taper internal pipe thread to BS EN 10226 | Used with the appropriate taper external R thread for a pressure-tight threaded joint. |
For dimensions, pitch, major diameter and other thread data, see:
Pipe Thread Dimensions and Designations
Pipe-thread sizes are nominal designations associated historically with pipe sizes. The stated size is therefore not a direct measurement of the thread outside diameter.
This is particularly important when identifying an unknown threaded fitting. Measuring the outside diameter of the thread and selecting the nearest nominal fractional size can lead to incorrect identification.
Instead, check the measured thread diameter and pitch against the appropriate thread table and determine whether the thread is parallel or taper.
When working with an unknown fitting, a useful identification sequence is:
Visual similarity alone is not sufficient evidence that two pipe threads are compatible.
Replacement work often requires more care than new design because the original standard, material or component designation may not be immediately available.
Where possible, start with the drawing, equipment documentation or markings on the existing component. If these are unavailable, record enough dimensions and features to identify the connection rather than relying on a single measurement.
For a flange, useful checks include:
Several measurements agreeing with a standard are much stronger evidence of identification than one matching dimension.
| If you need to... | Start here |
|---|---|
| Find current PN16 steel flange dimensions | BS EN 1092-1 PN16 |
| Find current PN25 steel flange dimensions | BS EN 1092-1 PN25 |
| Find current PN40 steel flange dimensions | BS EN 1092-1 PN40 |
| Identify an older BS 4504 PN16 flange | BS 4504 PN16 |
| Identify an older BS 4504 PN25 flange | BS 4504 PN25 |
| Identify an older BS 4504 PN40 flange | BS 4504 PN40 |
| Find pipe outside diameter and schedule dimensions | Pipe Schedule Dimensions |
| Work with an older BS 3600 pipe specification | BS 3600 Pipe Data |
| Identify a G, R, Rp or Rc pipe thread | Pipe Thread Dimensions |
The principal standards associated with the subjects covered by this guide include:
The final question should not simply be "Does this flange or fitting have the right nominal size?" It should be: "Have I established that the complete connection is compatible and suitable for the required service?"
Nominal designations help engineers identify components. They do not replace checking the dimensions, materials, mating components, sealing arrangement and service conditions that determine whether the completed joint is appropriate.