Expansion Joints, Bellows and Metal Hoses: Back to Basics (Part 1)

Metal expansion joints are primarily needed for the thermal expansion of piping. If straight pipe sections between pipe anchors expanded without added flexibility, the load on the anchors would likely be too much for the system to withstand.

If there are no anchors and the pipe heats up due to thermal expansion, the pipe will grow between the fixed anchors and bend – which could cripple, buckle or crack the pipe. This damage could lead to the buildup of an immense thermal load on anchors, causing them to fail. If expansion joints have been added between the anchors and the pipe were to heat up and expand, the joints would compress to compensate for the movement, with minimal thermal loads. Adding metal joints is therefore a possible way to handle thermal expansion of pipes.

By Gobind Khiani, M.Eng, P.Eng, Fellow – Piping/Pipelines, SME

Metal Joints

Metal joints usually have stainless steel bellows which help to accommodate loads with multiple corrugations. They also typically achieve long lives, even in demanding systems such as ones with steam. Flanges are usually welded afterwards to give term/shape as expansion joints.

There are three movements a joint could make: Axial compression or extension, Angular movement and Lateral offset. Which action a joint can make is ultimately decided by the manufacturer, based on the client’s process requirement specifications. The parameters used are ply thickness, number of piles, pitch, number of corrugations and corrugation height; these parameters are used to ensure the required movement is achieved per provided design conditions. The bellows cannot be made too long; if they are too long, they will fail in squirm mode. The bellows tie rods are used to limit the extension and sometimes the compression of the joint. The nuts on the rods prevent them going through the holes.

Bellows

Bellows are usually stainless steel and have piping made of carbon steel. Liners may be considered in the manufacturing process, and if used, are slipped in to provide erosion protection and to protect/prevent flutter if the application has high flow rates. Covers also protect the thin bellows from potential damage where that may be a concern. They are manufactured by a mechanical forming method or, preferably, through hydro forming.

Although the single bellow joints are most common, they do not move very far in lateral offset. For large, lateral movements, a universal configuration with two bellows is needed. The longer the center pipe spool, the more lateral movement possible. Longer axial compression from thermal expansion is often beneficial and can be supplied by externally pressurized metal joints (such as flexicraft model EP). As the housing allows the system’s fluid to pressurize the bellows from outside, it is more stable and can be made longer, which allows for considerably more compression.

Thrust Loads

What exactly is pressure thrust load? When looking at a bendable straw, as an example, the bending section represents the expansion joint. If one end of the straw were plugged and someone were to blow into the open end of the straw, the pressure from the air being passed through the straw, but unable to exit, would cause the bend in the straw to become stretched out. The pressure would not affect the stiff portions of the straw. The pressure force that is acting on the plug is therefore the thrust load and is equal to the pressure multiplied by the cross sectional area.

When an expansion joint is added to a pressurized piping system, a new pressure thrust load is being introduced to the piping anchors. This new force on the anchors is the main reason why an expansion joint hole should not just be added by cutting into an existing pipeline, even though the large thermal anchor loads from the pipe growth would be minimized. A new anchor support load could therefore be difficult to understand at first, but it is critical to both the choices between products and also how they are applied to the system in question.

Pipe System Models

If looking at a pressurized pipe system model that is plugged and does not have anchors, there is no pipe movement, as the pipe wall is too stiff to be moved by the thrust load. If a hole is cut into the same pipe, and an expansion joint is added, the joint in the pressurized pipe will stretch out to failure, as seen with the straw example. When anchors are added to the system that has expansion joints and the pipe is pressurized, the thrust load is not able to stretch the joint. There is still a large risk of joint and pipe failure if the anchors are not designed to withstand the thrust load they are subjected to; if the anchors fail due to thrust load, the joint will stretch out and will likely fail as well.

Tie rods are sometimes added as an attempt to absorb the thrust load so that the anchors do not have to. The tie rods limit the extension, and sometimes the compression of the joints. The rods loosely fit through holes and lugs that are welded through the flanges or weld ends. The nuts on the rods prevent them from going through the lugs.

The issue with using a tie rod in a straight pipe is that if the joint compresses the nuts attached to the rod, the rod will separate from the lugs. When the two pieces separate they no longer shoulder the load. Using a tie rod to handle the thrust load therefore only works when axial movement is needed.

References
– Numerous Bellow Expansion Joint Manufacturers worldwide.
– Expansion joint manufacturers association (www.EJMA.org)
– Fluid Sealing Association (www.FSA.org)
– Tytec LP, Edmonton, Alberta, Canada
– Knowledge share during meetings with various industry colleagues.