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Thick Wall Multi-ply Bellows - The Right Choice

Engineering Appliances has been supplying expansion joints for Mechanical Building Services, Power Generation and Industrial Process Systems for 50 years. Our experience and knowledge of customer requirements has led us to choose thick wall spirally wrapped multi-ply bellows membranes, being the most technically advanced available anywhere. Such membranes have advantages over all conventional single and multi-ply designs where only 2 or 3 ply technology is used. Thick wall multi-ply membranes are typically 2 to 3mm thick and 6-12 ply resulting in the final product being robust, immensely strong yet highly flexible. The membranes have thick walls and yet due to the many layers or plies the bellows remain very flexible, therefore the overall length is short and the thermal cycle life is high. The spiral wrapping gives the bellows an in-built early warning which indicates when the inner ply has been damaged. The visual sign being a slow weep from a hole in the outer layer only, drilled for this purpose. A catastrophic failure is therefore virtually impossible.

Quality is Assured

EA has a Lloyds Register accredited Quality Management System to ISO 9001. BOA as major suppliers are also accredited to ISO 9001. EA expansion joints are designed to the American EJMA standard, which incorporates large safety factors. All products are manufactured and finished to the highest standard using approved weld procedures to EN288. Material certificates to EN 10204 and pressure test certificates are available on request.

Advice Service

We have many years experience in analysing pipe work expansion compensation. We stake our reputation on only including expansion joints where there is no alternative. Our engineers can often reduce the cost of a system providing they are involved at an early design stage, as the most economical solution should not just consider the cost of the expansion joint but also take into account the cost of anchoring, guiding and installation. Frequently expansion joints are not required and can be designed out by using the natural flexibility of the pipe work. Our Area Engineers are always pleased to advise you.

Unrestrained Expansion Joints

Axial Expansion Joints are the simplest way of absorbing expansion in pipe work systems. They are designed to be installed in a straight run of pipe where they absorb axial (or longitudinal) expansion by being compressed. The fact that they can only compensate for expansion in one plane limits their application and offers little opportunity for engineering initiative (Fig. 1 ).

Fig. 1

                               

14D4D4D14D

When applying an axial expansion joint it is crucial to understand the concept of pressure thrust. Under pressure a convoluted bellows acts like a piston (Fig. 2) and will extend unless it is restrained. Axial expansion joints are unrestrained and therefore the pipe work needs to be securely anchored to resist this tendency to extend. The forces exerted by an unrestrained expansion joint can be very significant and is calculated as follows :-

Fig. 2

Pressure thrust = Effective area of bellows x pressure Fpt = CSA (Cm2) x P (bar) kgf

The effective cross sectional area of the bellows is given in the data sheets. For anchor load calculations always use the highest pressure, usually the test pressure, and take into consideration the total thrust from multiple pipes.

Example: A pipe system which has a 150mm n.b. flow and 100mm n.b. return, with an axial bellows installed in both at a test pressure of 15 bar. The total thrust exerted by both bellows is:-

(267.1cm2 + 129.3cm2) x 15.

= 58.8 kN)

If it is not possible to build anchors of sufficient strength, a restrained expansion joint must be used. It is usually either not possible or uneconomical to use unrestrained units at high level. Axial bellows also introduce an instability into the pipe run and therefore the rules on guiding detailed in BS 6129 must be strictly adhered to. (Also see EA’s Guide to the use of Expansion Joints). The first guide should be spaced 4 pipe diameters from the expansion joint, and the second 14 pipe diameter from the first guide, Fig. 1. As this may not always be possible for small diameter pipe runs, EA have introduced a range of bellows where the above rule can be stretched (see data sheet for EA15, 16, 04). For further information, please consult the EA Guide to the use of Expansion Joints or contact one of our engineers.

Restrained Expansion Joints

Restrained Expansion Joints are designed not to extend when an internal pressure is applied. The pressure thrust reaction force is restrained by means of hinges or tie bars.

Fig. 1.

Restrained Expansion Joints are versatile and can be used in many different configurations and combinations. The loads imposed on anchors is generally much lower than for unrestrained types. Movement in angular expansion joints and lateral expansion joints is achieved by controlled angulation of two bellows in opposite directions. They can only absorb movement at 90° to their axis. Angular expansion joints have to be used in pairs (2 pin system) or in threes (3 pin system).

Fig. 2.

Lateral expansion joints included in this manual incorporate two bellows which are restrained using tie bars. They can move in two planes which makes them extremely versatile, Fig. 3. They are sometimes called articulated expansion joints.

Fig. 3

Movement

The anchor load imposed by all restrained units is the sum of their spring rate plus restraint friction losses. However, generally the frictional loads caused by the pipe supports are by far the largest component of anchor load. No special guiding is required and they are therefore suitable for mounting at high level or wherever guiding and anchoring may prove difficult or expensive. Restrained expansion joints are more complex than axial units and are therefore more expensive. However, to offset the higher cost, savings can result from a reduced need for guiding anchoring and number of expansion Joints.