Expansion loop or expansion joint – which way of absorbing expansion suits the pipe?

With natural expansion compensation, pipe bends and sufficiently long pipe legs absorb the thermal movements by elastic deformation. Besides a purpose-built U-shaped expansion loop, existing L- or Z-shaped pipe runs can therefore also be suitable. The prerequisite is that the resulting stresses and connection loads are kept under control.

For design this means: before an expansion joint is specified, it is worth examining the pipe routing. Not every long or hot line automatically needs a bellows expansion joint.

In a pipe that is continuously connected so as to transmit tension, the pressure force path through the pipe wall is maintained. An expansion loop does not interrupt this force path in the way an unrestrained axial expansion joint does. The additional bellows pressure force, which would otherwise have to be resisted by appropriately designed anchors, therefore does not arise. That does not mean, however, that the pipe or its supports are free of forces altogether.

On pipe racks in particular this difference matters: the pressure thrust of an axial expansion joint acts along its axis – with a horizontal arrangement, therefore, horizontally. With large effective bellows cross-sections and high pressures this can result in a governing load on the structural steelwork.

An expansion loop avoids this particular load from an unrestrained bellows. Thermal spring forces, weight loads and the other actions must nevertheless be taken into account.

For modifications to existing plant, the investigation should therefore not begin with the question “Where can another expansion joint be fitted?” but with “Which possibilities for movement does the existing plant offer?”

Our design approach at ENTRACON is to examine alternative pipe routes and support concepts together. This includes, for example, making use of existing changes of direction, sufficiently long connecting legs or a different arrangement of guides and anchors. A pipe stress analysis serves as the basis for comparing the variants.

The aim is not to avoid an expansion joint as a matter of principle. The aim is a solution in which movements, pipe stresses, support loads and nozzle loads fit together.

A newly arranged expansion loop needs space for the pipe run and its movement. Additional pipe length and bends also mean more material, welded joints, insulation and possibly further supporting structures. Added to this is additional flow resistance. These consequences have to be assessed as part of the comparison of variants.

The process side must not be neglected either: on steam lines, unfavourably arranged loops can encourage condensate to collect. Slope and drainage must therefore suit the chosen geometry.

An expansion loop is thus not a better solution across the board – but a variant that should be examined before an additional flexible component is used.

In an unrestrained axial expansion joint, the internal pressure tries to pull the bellows apart. The pressure thrust results from the pressure difference between inside and outside and the effective bellows area.

What governs is the effective bellows area from the manufacturer’s data, not simply the nominal size or the free pipe cross-section. For the anchors, the spring forces of the bellows and the friction forces of the pipe supports must additionally be taken into account.

A worked example for the order of magnitude: with an assumed effective bellows diameter of 500 millimetres and 10 bar gauge pressure, the pressure thrust is around 196 kilonewtons. With an effective diameter of 1,000 millimetres, the same pressure already produces around 785 kilonewtons. Doubling the diameter quadruples the force here.

These example values do not replace the design of the component. They do illustrate, however, why a supposedly small intervention in the pipe can have considerable consequences for anchors and structural steelwork.

It is important to distinguish between movement of the pipe and movement at the expansion joint: the longitudinal expansion of a main line can, with a suitable change of direction, be absorbed as lateral movement at a lateral expansion joint. The expansion joint therefore does not necessarily have to work axially just because the connected line grows longer.

A fixed tie-rod restraint can transmit the pressure thrust across the expansion joint. This, however, also prevents its free axial movement. An expansion joint restrained in this way can therefore not be planned at the same time as a freely moving axial expansion joint for absorbing the longitudinal expansion.

In addition, tie rods, limit rods and shipping restraints fulfil different functions. A limit device with free travel does not replace a permanently effective pressure thrust restraint. Likewise, factory-set tie rods must not be adjusted without authorisation, and structural restraints must not be confused with shipping restraints that are to be removed.

The statement “axial movement and pressure balancing are mutually exclusive” would nevertheless be too sweeping: specially designed pressure-balanced axial expansion joints permit both. This function, however, results from the design as a whole, not from merely attaching threaded rods.

Correctly selected expansion joints can considerably reduce the connection forces and moments arising from restrained movements. This can be a significant advantage particularly at sensitive pump, turbine or equipment connections.

The entire force path must be examined, however. Depending on the connection concept, pressure forces can be carried by the pipe, by a restraint or by the fixing of the machine. A lower load at one point therefore does not necessarily mean that the system as a whole is relieved.

For the design, nozzle loads, machine fixing and pipe supports should therefore be assessed together. An expansion joint is no substitute for a coordinated connection and support concept.

An enquiry stating “DN 300, PN 16 and 50 millimetres of expansion” does not yet describe the task completely. For a suitable selection, the associated pressure and temperature conditions, the fluid, the directions of movement and possible additional loads are decisive in particular. Corrosion, erosion, deposits and pressure pulsations must also be taken into account where necessary.

The allowable movement must, moreover, be matched with the intended number of load cycles and the actual service conditions. Axial, lateral and angular movements must not, without verification, each be combined up to their separately stated maximum values. Additional components of movement can impair the service life.

The bellows must not be treated as an arbitrarily deformable make-up piece for installation errors. The installed length, any pre-setting that may be required and the installation temperature must match. Torsion and damage from weld spatter or unsuitable handling must be avoided. The insulation must not block the mobility of the bellows convolutions.

Anchors and guides must be functional before the pressure test. The assessment must take account of the actual test pressure; it can govern the pressure thrust forces that occur.

Nor must the expansion joint take over the task of a missing pipe support. Pipe weight and additional loads are to be carried by suitable supports.

Witzenmann and BOA, for example, describe their metal expansion joints, when used accordingly, as maintenance-free. It does not follow from this, however, that their condition can be disregarded throughout the entire service life. They expressly recommend periodic checks of the piping system and set the frequency depending on application and loading.

Useful points to check are, for example, corrosion, leaks, conspicuous deformations, loosened components and blocked or damaged movement elements. Changes at anchors and guides can also be relevant. A generally applicable annual interval cannot be derived from this; scope and frequency should suit the specific system and the manufacturer’s specifications.

ENTRACON additionally recommends including them in the operator’s risk assessment and defining periodic inspections. Accessibility for checks and a possible later replacement should therefore already be considered at the design stage.

An expansion joint selected for thermal movements is not automatically suitable for any kind of vibration or pressure surge. Particularly near quick-closing valves, pressure-reducing stations or other sources of turbulence, additional measures may be required. Depending on the application, a suitable internal sleeve protects the bellows from flow-induced loading.

This is an important difference between expansion compensation, vibration isolation and the control of pressure surges: these tasks can be related to one another but must each be assessed separately.

For ENTRACON, the focus is therefore not on selecting an individual component that is as cheap as possible. We consider the variants including pipe routing, additional structural steelwork, supports, installation, accessibility and later operation.

As part of the pipe stress analysis we compare the resulting movements, stresses, support loads and connection loads. This makes it possible to decide transparently whether existing flexibility is sufficient, an expansion loop makes sense or a deliberately selected expansion joint is the better solution.