How to Select the Right Dismantling Joint Size and Pressure Rating

A dismantling joint may look like a relatively simple pipeline fitting, but selecting the wrong size or pressure rating can lead to serious installation and operating problems. The joint may not match the valve flange, may provide insufficient axial adjustment, or may fail to maintain a reliable seal when the pipeline experiences pressure surges.

Correct selection therefore involves more than checking the nominal pipe diameter. Engineers, contractors, and purchasing teams should evaluate the flange standard, available installation space, operating pressure, surge pressure, test pressure, pipeline medium, temperature, coating, gasket, and fastener materials.

This guide explains how to select the right dismantling joint size and pressure rating for municipal water supply, wastewater treatment, irrigation networks, pumping stations, industrial pipelines, and other flanged piping systems.

What Is a Dismantling Joint?

A dismantling joint is an adjustable flanged fitting installed next to a valve, pump, flow meter, check valve, strainer, or another pipeline component.

Its telescopic structure provides controlled axial adjustment during installation. This makes it easier to position equipment between two fixed pipeline flanges. It also creates the clearance required to remove the connected equipment during future inspection, repair, or replacement.

Once the equipment and dismantling joint are correctly positioned, the flange bolts, gland bolts, and tie rods are tightened to form a restrained connection. The tie rods help transfer axial forces through the assembly and prevent uncontrolled separation during operation.

Solid Joints supplies ductile iron dismantling joints for above-ground pipelines and chamber installations. The standard selection range covers DN50 to DN2000, with PN10, PN16, and PN25 pressure classes. Available options include ductile iron bodies, EPDM or NBR rubber seals, fusion-bonded epoxy coatings, and galvanized, coated, or stainless-steel fasteners. Flange drilling can be manufactured according to ISO, EN, BS, DIN, ANSI, or project-specific drawings.

1. Confirm the Nominal Diameter

The first selection parameter is the nominal diameter, normally expressed as DN. For example, a DN300 dismantling joint is generally intended for a DN300 pipeline, valve, pump, or other flanged component.

However, DN is only a nominal designation. It does not necessarily equal the actual pipe outside diameter, flange outside diameter, or internal bore.

Two components marked with the same DN may still have different flange dimensions when they are manufactured according to different standards.

Before ordering, confirm the following dimensions:

• ·   Pipeline nominal diameter

• ·   Valve, pump, or meter flange size

• ·   Flange outside diameter

• ·   Bolt circle diameter

• ·   Number of bolt holes

• ·   Bolt-hole diameter

• ·   Flange thickness

• ·   Available flange-to-flange distance

For a new pipeline project, these values can usually be obtained from approved drawings and equipment data sheets. For a replacement or maintenance project, field measurements are strongly recommended.

Do not depend only on the nameplate of an old valve or an approximate pipe diameter.

2. Match the Flange Standard and Drilling

One of the most common purchasing mistakes is ordering the correct DN but the wrong flange drilling.

For example, a DN200 PN16 flange manufactured according to EN 1092-2 is not automatically interchangeable with every DN200 flange manufactured according to another standard.

The flange standard determines several important dimensions:

• ·   Flange outside diameter

• ·   Pitch circle diameter

• ·   Number of bolt holes

• ·   Bolt-hole diameter

• ·   Flange thickness

• ·   Raised-face or flat-face dimensions

• ·   Gasket contact area

ISO 7005-2 specifies requirements for circular grey iron, malleable iron, and ductile iron flanges. This is why the flange standard must be confirmed together with the nominal diameter and pressure class.

Common flange references include:

• ·   EN 1092-2

• ·   ISO 7005-2

• ·   BS 4504

• ·   DIN flange dimensions

• ·   ASME or ANSI flange patterns

• ·   AWWA requirements

• ·   Project-specific flange drawings

A complete purchasing description should read:

DN300 PN16 dismantling joint, flange drilling according to EN 1092-2

This is much more precise than simply requesting a “300 mm dismantling joint.”

When the existing flange standard is unknown, send a dimensioned drawing or provide the measured flange outside diameter, bolt circle, number of holes, and hole diameter to the manufacturer.

3. Measure the Required Installation Length

Dismantling joints are selected not only by nominal diameter but also by their face-to-face length and axial adjustment range.

The joint must fit between the fixed pipeline flange and the connected equipment flange. At the same time, it should retain sufficient adjustment for installation and future maintenance.

Before ordering, confirm:

• ·   Actual flange-to-flange gap

• ·   Minimum joint length

• ·   Maximum joint length

• ·   Nominal face-to-face length

• ·   Permitted axial adjustment

• ·   Required dismantling clearance

The equipment should first be positioned correctly. The distance between the equipment flange and the fixed pipeline flange can then be measured.

Avoid installing the dismantling joint in a completely compressed or completely extended position unless this is specifically approved by the manufacturer. Keeping the joint within its recommended adjustment range provides more reliable gasket compression and leaves useful movement for future removal.

A dismantling joint is primarily designed to compensate for installation tolerances and provide equipment removal clearance. It should not automatically be used as a substitute for a rubber expansion joint or metal expansion joint when the pipeline must continuously absorb vibration, thermal expansion, or large axial movement.

4. Identify the Maximum System Pressure

The pressure rating of the dismantling joint must be equal to or higher than the maximum pressure that the pipeline assembly will experience.

Do not select the pressure class using only the normal reading shown on the pressure gauge.

The pressure evaluation should include:

• ·   Normal operating pressure

• ·   Maximum pump discharge pressure

• ·   Pump shut-off pressure

• ·   Static head pressure

• ·   Water hammer

• ·   Transient surge pressure

• ·   Pressure during rapid valve closure

• ·   Hydrostatic test pressure

• ·   Temporary abnormal operating conditions

PN10, PN16, and PN25 are commonly associated with nominal pressure classes of approximately 10, 16, and 25 bar under specified reference conditions.

However, PN should not be treated as a universal guarantee that the fitting can operate at exactly that pressure under every possible temperature, material, medium, and installation condition.

The final allowable working pressure can also be influenced by:

• ·   Flange standard

• ·   Body material

• ·   Gasket material

• ·   Bolt grade

• ·   Operating temperature

• ·   Pipeline medium

• ·   Manufacturer’s design

• ·   Applicable project standard

The safest approach is to select the pressure class according to the highest credible system pressure rather than the average operating pressure.

5. Consider Water Hammer and Surge Pressure

Water hammer occurs when fluid velocity changes rapidly. Typical causes include:

• ·   Rapid valve closure

• ·   Pump start-up

• ·   Sudden pump shutdown

• ·   Power failure

• ·   Check valve closure

• ·   Sudden interruption of flow

The resulting transient pressure can be significantly higher than the steady operating pressure.

For example, a pipeline that normally operates below 10 bar may still require a PN16 dismantling joint when hydraulic calculations show that surge pressure could exceed the permitted PN10 pressure.

Surge analysis is particularly important for:

• ·   High-head pumping stations

• ·   Long water transmission pipelines

• ·   Pipelines with quick-closing valves

• ·   Systems with frequent pump starts and stops

• ·   Firewater networks

• ·   Industrial process pipelines

• ·   Pipelines with large changes in elevation

When accurate surge information is not available, consult the pipeline designer or pumping-system engineer. Do not simply add an arbitrary pressure margin without considering the complete system design.

A higher pressure class may change the joint’s flange dimensions, bolt requirements, wall thickness, weight, and cost. The selected pressure class should therefore be technically justified.

6. Check the Ratings of Connected Components

A dismantling joint is only one component in the flanged assembly.

The following components must also be suitable for the selected pressure class:

• ·   Valve

• ·   Pump nozzle

• ·   Flow meter

• ·   Pipe flange

• ·   Gasket

• ·   Bolts and nuts

• ·   Check valve

• ·   Strainer

• ·   Adjacent fittings

Installing a PN25 dismantling joint next to a PN10 valve does not make the complete assembly suitable for PN25 service.

The allowable system pressure is normally limited by the lowest-rated component.

Pressure classes should also not be mixed without checking flange compatibility. Bolt patterns and flange thicknesses may vary between PN10, PN16, and PN25, particularly at larger nominal diameters.

Even when two flanges appear to use similar drilling, their mechanical pressure ratings may not be identical.

7. Select the Correct Gasket Material

Pressure class alone does not guarantee reliable sealing. The gasket must be compatible with the pipeline medium and operating temperature.

EPDM Gaskets

EPDM is commonly selected for:

• ·   Potable water

• ·   Municipal water supply

• ·   Wastewater

• ·   Cooling water

• ·   Irrigation systems

• ·   General water-service pipelines

For drinking-water projects, the gasket should comply with the potable-water approval required by the destination country or project specification.

NBR Gaskets

NBR may be selected for applications involving:

• ·   Oil-contaminated water

• ·   Fuel

• ·   Certain hydrocarbons

• ·   Industrial fluids requiring improved oil resistance

NBR should not be selected automatically for every gas or chemical application. Chemical compatibility must be verified using the actual medium, concentration, and operating temperature.

Before requesting a quotation, provide the supplier with:

• ·   Transported medium

• ·   Maximum operating temperature

• ·   Minimum operating temperature

• ·   Chemical concentration

• ·   Potable-water approval requirements

• ·   Presence of oil, fuel, or solvents

Special rubber compounds may be required for aggressive chemicals, high temperatures, or unusual industrial fluids.

8. Choose the Body, Coating, and Fastener Materials

Ductile iron is widely used for dismantling joint bodies because it offers the strength and durability required for water and infrastructure projects.

Fusion-bonded epoxy coating protects the internal and external surfaces against corrosion. Different coating thicknesses, colors, and specifications may be required by local water authorities or project consultants.

Fastener selection is also important.

Galvanized carbon-steel bolts may be suitable for many normal water-system installations. Stainless-steel bolts and tie rods may be considered for:

• ·   Coastal environments

• ·   Wastewater treatment plants

• ·   Desalination facilities

• ·   Humid valve chambers

• ·   Chemically aggressive areas

• ·   Locations with frequent condensation

• ·   Projects requiring extended corrosion resistance

Material selection should consider the actual installation environment rather than focusing only on the pipeline medium.

Quick Pressure Rating Selection Guide

This table should only be used as an initial reference. Final selection must follow the approved pipeline design, relevant standards, and the manufacturer’s technical confirmation.

Common Dismantling Joint Selection Mistakes

Ordering by DN Only

The nominal diameter may be correct while the flange drilling, face-to-face length, or pressure class is incorrect.

Using Normal Pressure Instead of Maximum Pressure

Normal gauge pressure does not include pump shut-off pressure, surge pressure, or hydrostatic testing conditions.

Ignoring the Installation Gap

A correctly sized dismantling joint may still be impossible to install when its minimum or maximum length does not match the available space.

Selecting the Wrong Gasket

An incompatible rubber seal may swell, harden, crack, or gradually lose its sealing performance.

Mixing Pressure Classes

A pipeline assembly is limited by its lowest-rated component. Installing a higher-rated dismantling joint does not increase the rating of a lower-rated valve or flange.

Treating It as an Unrestrained Expansion Joint

A dismantling joint provides installation adjustment and maintenance clearance. Pipeline thrust, continuous thermal movement, and vibration must be controlled through the correct joint type, anchors, supports, and restraint design.

Final Selection Checklist

Before approving a dismantling joint order, verify that:

• ·   The DN matches the pipeline and connected equipment

• ·   The flange standard matches both sides

• ·   The bolt pattern is correct

• ·   The face-to-face length fits the available space

• ·   The axial adjustment is sufficient

• ·   The pressure class covers maximum and surge pressure

• ·   The test pressure is acceptable

• ·   The gasket is compatible with the medium

• ·   The gasket is suitable for the operating temperature

• ·   The coating matches the installation environment

• ·   The bolts and tie rods meet strength and corrosion requirements

• ·   All connected components have compatible pressure ratings

Conclusion

Selecting the right dismantling joint size and pressure rating requires a complete review of pipeline dimensions, flange standards, installation space, operating pressure, transient pressure, medium, temperature, and material compatibility.

DN and PN are important starting points, but they are not sufficient by themselves.

For most water and infrastructure projects, the most reliable purchasing approach is to provide the manufacturer with the nominal diameter, flange drilling, installation gap, maximum system pressure, test pressure, medium, temperature, and material requirements.

Solid Joints offers standard and customized ductile iron dismantling joints for valves, pumps, meters, check valves, and other flanged pipeline equipment. By submitting complete project drawings and operating parameters, buyers can confirm the appropriate size, pressure class, gasket, coating, and fastener configuration before production.