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Spiroflex d.o.o.
Ljudevita Gaja 7
35 208 Ruščica
Croatia

 

Phone: +385 35 _212420
Telefax: +385 35 _457105
e-mail: info@spiroflex.hr

 

Axial expansion joints

Axial expansion joints are designed to accept movements of extension and compression in the axis direction. The length of expansion joint in neutral position is called free length. The longest service life is achieved by exploiting the axial shift in both directions as extension (+Δax) and compression (-Δax). When it is known that movement will be in one direction, to utilize the complete all available movement we propose to install expansion joint under “prestress”. The expansion joint service life depends on correct installation.

Application of axial expansion joint

  • pipeline for central heating
  • pipeline for hot water and steam
  • petrochemical and refinery technology
  • electrical engineering (volumetric displacement)
 
  • heat exchangers
  • ship building and exhaust systems
  • vacuum technology
  • as sealings in valves

Materials

For standard applications austenitic stainless steel DIN EN 1.4541 covers a wide range of needs in many operating conditions. Material DIN EN 1.4571 is suitable for pipeline of steam, crude oil, diesel engine exhaust systems and in any case of marine services.
Heat resistant materials AISI 309 and Incoloy 800 are suitable for high temperatures up to 900 °C.
For chemically aggressive media our engineers will recommend you nickel based alloys such as Hastelloy C4, Inconel 600, Inconel 625, Monel 400 and Incoloy 825.

Design procedure of axial expansion joints systems

When you design your axial expansion joint system it is necessary to observe the following points:

  1. Planning of system and installation of axial expansion joint
  2. Planning of fix points and pipe guides
  3. Calculation of thermal expansion of pipeline
  4. Cold-pull of axial expansion joints
  5. Calculation of the forces on fix points

1Planning of system and installation
of axial expansion joint

The purpose of installation of expansion joint is to absorb the thermal expansion of pipe. Usually the temperature of the flow medium is the major source of dimensional changes, but in extreme cases ambient temperature can cause thermal movement.

Axial expansion joint at fix point in the straight pipeline

 
Axial expansion joint in the middle of straight pipeline

Axial expansion joint in the middle of straight pipeline

Axial expansion joint in straight part of "Z" pipeline

Axial expansion joint in straight part of "Z" pipeline

 
Axial expansion joint in "T" pipeline

Axial expansion joint in "T" pipeline

2Planning of fix points
and pipe guides

The purpose of fix points (anchors) is to divide a piping system into individual expanding sections. Furthermore, the function of pipe anchors is to limit and control the amount of movement between fix points.
Pipe anchors must be designed to accept all of the forces acting upon them.
Pipe guides insure proper alignment of movement to the expansion joint and prevent buckling of the line. The expansion joint flexibility in combination with internal pressure loading can also cause buckling. General recommendation is to put axial expansion joint near to anchor. First guide should be located at maximum of 4 pipe diameters away from expansion joint. The distance between first and second guide is 14 pipe diameters. The maximum recommended spacing of other guides is shown in diagram below.

Proper location of expansion joint, anchors and guides

Proper location of expansion joint, anchors and guides

 
Influence of guides on stability

Influence of guides on stability

Maximum spacing of guides

3Calculation of thermal
expansion of pipeline

An increase of temperature of the pipeline from ambient to working temperature will cause thermal expansion as change in the length of pipe.

 

∆L=L•∆t•α [mm]

ΔL – expansion of the pipe [mm]
L – length of the pipe [m]
Δt – difference between ambient and operating temperature [°C]
α – expansion coefficient [mm/m°C x 10-3]

Koeficijent produljenja / Mean thermal expansion coefficient α [mm/m°C x 10-3]
Prema / In acc. to: EN 14917:2009

Materials Temperature ranges from 20 to 500 °C
100 °C 200 °C 300 °C 400 °C 500 °C
Carbon steels 12,5 13,0 13,6 14,1 14,5
Austenitic steels 16,0 16,5 17,0 17,5 18,0
Copper 15,5 16,0 16,5 17,0 17,5
Aluminium alloy (AlMg3) 23,7 24,5 25,3 26,3 27,2

Example

L = 50 m length of carbon steel pipe for transport of hot water 90 °C.

Installation temperature is 20 °C.

α in range 20 – 100 °C = 12,5 x 10-3 mm/m°C

 

ΔL = L • Δt • α
ΔL = 50 • 70 • 12,5 x 10-3 = 43,75 mm

4Cold-pull of axial
expansion joints

The axial movement of axial expansion joints is generally expressed as compression (-Δax/2) and extension (+Δax/2) by one half of movement each. The longest service life is achieved by exploiting the axial shift in both directions. In order to utilize maximum capacity of bellows the expansion joint should be installed under cold pull.

Cold-pull:

 

Hp=

 

      

ƥ(tinst Рtmin)

 

[mm]

2

tmax – tmin

Install length:

L=L0+Hp [mm]

 

 

Δ – total expansion of pipe [mm]
L0 – free length of expansion joint [mm]
L – installed length of expansion joint [mm]
tmax – maximum operating temperature [°C]
tmin – minimum operating temperature [°C]
tinst – installation temperature [°C]

Axial expansion joint must always be cold pulled opposite to its working movement direction and installed in this cold condition. The amount of cold pull depends on installation temperature.

Amount of cold pull

Amount of cold pull

 
Example of axial cold pull

Example of axial cold pull

5Calculation of the forces
on fix points

1. Pressure thrust force

Pressure thrust force is the axial force due to the effect of pressure (see Fig.). The amount of the pressure thrust force is determined by the equation:

Ft=p•A [N]

 

A=

 

d2m •π

 

[mm2]

4

 

 

p  –   pressure [N/mm2]
A  –  effective area of bellows [mm2] (given in the dimensional tables)

 

When calculating pressure thrust force the pressure value should be equal to maximum anticipated pressure. For this reason, pressure test must be considered.

Pressure thrust

Pressure thrust

 
Lack of fixed points

Lack of fixed points

2. Spring force

The axial bellows spring force is the force required to extend or compress an expansion joint.

Fs=kaks•∆aks

 

 

kaks  –   axial spring rate (given in dimensional table)
Δaks  –  axial movement