PUMPS, VALVES & ACTUATORS LEARNING FROM EXPERIENCE
Adam Fox, director at vibration control specialist Mason UK, outlines how getting the engineering right, when it comes to the design of rubber expansion joints, can limit the risk of failure
allow for thermal expansion. This reduces pipe stress and allows for movement but an often- overlooked benefit is that if correctly designed they can reduce the risk of noise and future failure of pipes through vibration. In addition, if a joint or connector were to fail or burst, the cost in damages could be astronomical. When fluid is pumped through a system it
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typically happens as a series of pulses rather than a continuous flow. This can have undesirable effects, such as generating noise or vibration elsewhere in the building. Over time, this can cause pipes and joints to fail through fatigue cracking, causing wider damage through flooding or in the worst case, severe injury. Rubber expansion joints are a type of
flexible pipe connector that offer a solution to this problem. The rubber body of the joint provides the necessary movement in all directions, but if designed with a spherical cross-section it can expand and absorb the energy as pulses pass through. The right design greatly reduces the risk of pipe noise or potential failure further down
lexible pipe connectors are normally used to allow movement, due to connection to moving equipment, or to
the line. These connectors are therefore commonly found on the inlet and outlet of pumps, as well as where thermal expansion of pipes occurs. The consequences of flooding could be
severe, so the cost of failure is high. Buildings have been written off and injury can easily result, especially with steam pipes.
Choosing the right material With regard to vibration control, we learn a lot from the technical literature. True expertise, however, comes from combining this with decades of testing applications and research. For example, the introduction of our SAFEFLEX rubber expansion joints in 1996 was the culmination of twenty years of research. A key improvement made by our engineers
was the switch of elastomer from Neoprene to Ethylene Propylene Diene Monomer (EPDM). The latter is superior to Neoprene in minimising water swell and resistance to oxygen, so suits applications involving water pipes. Other benefits include superior ozone ageing and high temperature tolerance. We had assumed, like everyone else, that
Natural Rubber encapsulated between two Neoprene or EPDM layers was safe at higher temperatures. Practice taught us otherwise:
exposure to high temperature over time led to a hardening of the inner carcass. The solution was to produce a tire or body
that was made from the same material throughout. The first key feature of SAFEFLEX was the switch to EPDM throughout. Yet this step, although crucial in our goal of reducing heat failures, was only half the story. We discovered that EPDM’s ability to tolerate
high temperatures was partly dependent on the curing system – the chemicals that stabilise or vulcanise EPDM during curing. Sulphur based curing systems are commonly used because they are cheaper and quicker than the alternatives.
Research and experience Our research and experience demonstrated peroxide was a superior curing agent. It costs more and the production takes longer, but this method truly reduces the risk of heat failures. In SAFEFLEX, the EPDM cover, liner and frictioning materials are all peroxide cured. A further discovery was the effectiveness of
Kevlar. We learned that Polyester (Du Pont Dacron) was much more resistant to higher temperatures, while very close to the Nylon tensile strength. We also realised that Kevlar had a higher elongation modulus, meaning
26 NOVEMBER 2021 | PROCESS & CONTROL
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