Axial expansion joints

Application of axial expansion joint

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.

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.

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.

 

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 (-Δ_ax/2) and extension (+Δ_ax/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.

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.

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:

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

2. Spring force

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