Axial expansion joints usually work along their own axial direction only. They can support either compression or traction expansions, but they are generally used between pipelines sections or to connect various equipments provided that their operating performance take place along an absolute rectilinear section only. Their operating performances are necessary in the following circumstances: 1st) in heat exchanger equipments in order to absorb differential expansions between outer shell and tube bundle; 2nd) as "demounting joints" in order to facilitate the assembling and disassembling of valves, gate valves and so on. In fact thanks to their elasticity, during unscrewing of the bolts they leaves a little clearance between flange and counterflange large enough to pull the valve out. They are also used in intake and exhaust pipes in order to attenuate moderate axial or lateral frequency vibrations.

Because of strengths generated by inside pressure, axial expansion joints have been planned to lengthen and to buckle sidewards. Slides must therefore assure an extreme axial freely movement to pipeline and at the same time must prevent any traverse and upwards movement since owing to the presence of the peak load pipeline even might rise in consequence of an eventual slight starting eccentricity too. Therefore, swing and catenary suspensions must be strictly avoided and strictly avoided when they have no lateral restraints too. To eliminate risk of not alignment it's fundamental to foresee in every rectilinear section already setted, anchor points strong enough and some restraining guide towards all lateral directions, placed in suitable space as per instructions listed on the following table.

In order to take a full advantage of bellow power compensation it's necessary that the same work either in traction or in compression at the same time. Axial expansion joints "AS" must be therefore placed already pretensioned. Should pipeline be placed at ambient temperature only or, anyhow, when working temperature is low enough, while quoting assembling joint expansion length it's important to take into strictly account installation room temperature.
As above for the reason that during insertion of expansion joints, pipeline may be already partially or totally expanded as to maximum thermical ranges involved. In other words it may happen that a pipeline for cold fluids be already wholly expanded during installation of expansion joints. Should pipeline has to be exposed to sunlight during summer season, then instead of proceeding to a pretension, we shall be obliged to submit expansion joints to a precompression. We are able however to prevent this disadvantage, by supplying you with our "APS" model in suitable requested lengths, already pretensioned and ready to be placed in pipeline. When expansion joint have to perform like a vibration damper it must be installed without any prebukling just in its own real length.

Anchor points have the task to anchor the pipeline in order to pre-establish the direction of expansions. Usually anchor points are stressed by the following forces:

TOTAL AMOUNT OF FRICTIONAL FORCES. Slides frictional forces are depending either on respective own coefficients and or on the pipeline weight.

BELLOWS FLEXIBILITY. It is nothing but the strength that bellows opposes against its own extension or shrinkage. In technical tables, bellows elastic strength is calculated for every 1 millimetre (plus or less) of expansion. Should the expansion joint not to be pretensioned, its own strength is valued as product of bellow flexibility for 1mm per elongation. Should, on the contrary, the expansion joint be pretensioned by 30%, its bellow flexibility is given by last product per a 0.6 ratio.

REACTION DUE TO WORKING PRESSURE. Since axial expansion joint has been structured in order to resist external strains and planned to buckle in elastic way, along its own axial direction, under the action of internal pressure a thrust will arise in it. Owing to the latest strength, pipeline comes to be subjected to a peak load and the value of these strains depends on either maximum working pressure or effective cross sectional area. It will be product of effective cross sectional area per working pressure.

On the basis of previous considerations, may happen that in a pipeline where has been mounted an axial expansion joint the following four extreme cases may take place:

COLD PIPELINE WITHOUT PRESSURE (pipeline stressed by traction). Thrust against anchor points is due to the resistance of the same expansion joint

COLD PIPELINE SUBMITTED TO A TEST PRESSURE (pipeline stressed by compression and by peak load). Stress arising around anchor points can be considered as a product of test pressure by the cross-sectional area. To that thrust we have to algebraically add the further share due to the strength just given by the same expansion joint in connection with the assembly cold-draw-gap eventually already done.

HOT PIPELINE WITHOUT PRESSURE (pipeline stressed by compression and by peak load). In this case, strain towards anchor points is just due to strength expansion joint only.

HOT PIPELINE UNDER WORKING PRESSURE (pipeline stressed by compression and by peak load ). In this case the anchor points are stressed by a thrust due to the operative pressure multiplied by cross-sectional area and by the resistance of the same expansion joint.