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Externally Pressured Expansion Joints

When applying internally pressurized expansion joints, the designer has to be concerned with extension, compression, angular and torsional motion. Like automobile tires, rubber expansion joints are thick skinned forgiving creatures that tolerate abuse. Stainless steel is very reliable too, but only if expertly designed, properly anchored and guided. All of us have taken a strip of metal and bent it back and forth until it cracked and snapped. Multiple corrugations designed to low stresses eliminate the problem. An anchor close to an expansion joint stabilizes one side. Then two guides, one 4 diameters from the joint and the other a minimum of 14 diameters from the first, lead the piping straight in and out. If the anchors are both up and down-stream, four guides are required, two on each side. Improper anchoring or guiding leads to failure. If major movements are required, it may become necessary to increase the outside diameter to prevent buckling. This increases both thrust and cost. There is always the worry of personal injury from hot liquid or steam, even though the evolution from steel to galvanized steel, copper, bronze and finally stainless steel, has increased service life and operating pressures. According to the standards established by The Stainless Steel Expansion Joint Institute, an expansion joint is considered safe when this collapse occurs at 2.25 times the working pressure. That means the safety factor is only 2.25. Most manufacturers do not publish their collapse ratios, but a 2.25 safety factor seemed very low. Braided hoses have safety factors between 3 and 4. Why is 2.25 acceptable in an expansion joint? Flexible products are always riskier than solid pipe, so it seemed only right that our housed expansion joints and expansion compensators should have a safety factor between 3.5 and 4 as published and what we have worked to in all designs.

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