Attached you'll find some more images. Imagine a situation your "fabric expansion joint" is more akin to a flexible rubber hose. And suppose it inflates like a rubber balloon. We could likely all agree that when it inflates, the ends could expand outwards, and thus would exert a force on whatever is on the ends of it.
Let's not concern ourselves just yet to commiserate what the forces might be on the ends.
Let's instead take that same flexible hose/expansion joint and attach a sleeve to it. Our sleeve consists of braided fibers, which surround the piece in a helical fashion. My image shows 2 such fibers that are parallel, but let's say we have many more fibers, and half run perpendicular to this helix, i.e. in the opposite direction. Unlike our flexible rubber material, let's assume the fibers are inextensible. As the "balloon" inflates, it causes the fibers to go into tension, and they in turn cause the entire unit to shorten - not elongate.
Are you going to say that the net resultant force on your piping elements is outwards from the center of the expansion joint, or inwards?
There are few things here.
Yes.Fabric expansion joint bulges outwards limited by how much there is free fabric. More it bulges larger the effective area and therefore higher pressure thrust. Metal expansion joint works similar. It is just more rigid and doesn't bulge so much especially round. If you have rectangular or square the bulging can be a lot if the design is not correct.
Flex hose with braiding is restraint design. Braiding holds the axial force and therefore works like solid pipe. Rubber expansion joints have internal fibers. Steel, or some other material. They increase the pressure capacity and axial stiffness a bit. Pressure thrust issue remains exactly the same as on metal element.
Force balance in the element is shown in the PDF file in the previous post. Stresses and therefore forces in the element material are key design issue for expansion joint designer. Attached is an explanation how the formulas work. Paper is for EJMA 6. Latest is EJMA 10. Basics are still the same. Details of the formula have changed. Author is in a way "father" of EJMA standard. Element stresses are limited to allowable stress of the material with some correction factors - pls see EJMA or ASME VIII Div 1 App 26.
Where the expansion joint is standard design and the pressure inside is higher than outside the pressure thrust is trying to push the unit longer. If the pressure is higher outside the force direction changes and unit wants to become shorter. If the expansion joint is special externally pressurized design the element gets shorter but the assembly extends if the pressure inside the pipe is higher than outside.
Be free to do some experiments using CAESAR II. It is not expansion joint design program (like the software I have developed and supply) but by putting anchors and looking loads on them or without anchors look into the end movements. COADE consulted long time ago Robert (Bob) K. Broyles and as a result if the program is used correctly the results are good.
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