Metal expansion joints are primarily needed for the thermal expansion of piping. When straight sections of pipe, between the pipe anchors, expand without added flexibility, the load on the anchors can be too much for the system to withstand. If there are no anchors and pipe experiences thermal expansion, it will grow and bend, which could cripple, buckle, or crack the pipe. This damage could lead to the buildup of an immense thermal load on the anchors, causing them to fail. The addition of expansion joint to a piping system can therefore mitigate the risk of damaging the pipes, as the joints would compress to compensate for the movement, with minimal thermal loads.
By Gobind Khiani, Consulting Fellow-Piping/Pipelines
Properties of Expansion Joints
Almost all metal expansion joints are manufactured by a mechanical forming method or, preferably, through hydro forming, and have bellows which are designed to help accommodate loads with multiple corrugations. Bellows are typically made of 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. The flanges, found on either side of the bellows, are generally welded on after, to give term/shape as expansion joints.
Expansion joints cannot be made too long; if they are too long, they will fail in squirm mode. While 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 therefore needed. The longer the center pipe spool, the more lateral movement is possible. Longer axial compression from thermal expansion is often beneficial and can be supplied by externally pressurized metal joints.
There are three movements a joint can 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 to determine these actions include: 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. Despite their use in demanding systems, such as steam applications, expansion joints often achieve long lives.
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 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. While a new anchor support load can be difficult to understand, an operator’s comprehension is critical to both the product selection process as well as how they are applied to the system in question.
Potential Pipe Issues
As a pipe wall is too stiff to be moved by a thrust load, a pressurized pipe system that is plugged and does not have anchors will have no pipe movement. 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 a system that has expansion joints and the pipe is pressurized, the thrust load is not able to stretch the joint. There is 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 therefore 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 the compression of the joints, which can help prevent failure. 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.
An anchored piping system with an expansion joint will generally be pressurized prior to heating up. At this stage, if tie rods are in place, they will work to absorb the thrust load so that the anchors are not required to. In some cases, the tie rods will disengage from the lugs when the system starts to heat up, and the thrust load will be transferred to the anchors. If this scenario takes place, the anchors could also fail. To ensure that the system does not fail, the following solution can be implemented.
Ensure the anchors are large enough to withstand the thrust load. If the correct requirements are established, the rise in pressure and temperature will not negatively impact the system. The anchors must therefore be designed for the forces of the thrust load; the force is equal to the effective cross sectional area of the bellows, multiplied by the pressure. In addition to the thrust load, a second load – the spring load – is added to the anchors when the joint moves; this is equal to the joint spring rate multiplied by the movement. The spring load is usually considerably smaller than the thrust load. The effective area and spring rate parameters of the joint are listed by the manufacturer in the joint’s technical manuals. These rates normally act as the input parameters required for computer piping design models.
Additional Application Details
There are special considerations that can be taken into account to eliminate the thrust load for thermal growth applications. For example, a configuration, namely Zee band (Z), can use a universal joint with tie rods. As described earlier, a universal expansion joint uses two bellows separated by a spool pipe section to allow long lateral movements. If the joint is placed where it will only move laterally but not axially, the added tie rods will never be forced to disengage. This can be set up for piping thermal expansion by placing the joint perpendicular to the main pipe run (the anchors will be far apart). This configuration can be quite helpful, especially in larger diameter piping systems.
The universal joint configuration design for lateral movement can be contrasted with another double bellows configuration, by using an anchor base. Sometimes it is advantageous to obtain twice the amount of axial movement by placing a double expansion joint in the middle of a pipe run. The anchor base essentially splits the length of pipe between main anchors in half with a single joint acting on each half.
This configuration can also be used with externally pressurized joints. In this case the thrust and spring loads from each bellows acts on the anchor base in opposite directions which cancels them out. Without these loads this anchor is referred to as an intermediate anchor, as opposed to a main anchor. Main anchors are normally positioned at major changes in the pipe direction and must be designed to consider the thrust and spring loads from the expansion joints.
In addition to special anchor applications, pipe guides are also often used with metal expansion joints. Pipe guides allow movement in the axial direction only. This limitation prevents the possibility of buckling instability in a pressurized pipe run with an expansion joint. With no guides, it is possible that the pipe run with the joint may buckle as it expands.
Buckling guides are added to stabilize a system that moves as expected. The spider type guides are the most popular and include an x-shaped spider that attaches to and moves with the pipe. These are not designed to take any loads from the main pipe beyond keeping it stable. In a horizontal pipe run it is possible to use slide guides that can take a downward load. The fixed bottom plate is most commonly Teflon lined to allow sliding and has rating for the load. The pipe is normally welded to the sliding portion in the field; it can also be made with clamps if preferred.
Braided Expansion Joints
These are composed of short braided flexible hose sections that consists of a stainless-steel corrugated piece/sheet metal in a stainless-steel braid. These are referred to only as flexible connectors because they are not technically an expansion joint. They are not meant for axial compression or extension because of the braid. As the braided expansion joint can withstand the thrust load because of its axial stiffness and extension, it essentially acts like a tie rod.
Braided expansion joints are therefore most often used on pumps and other equipment as flexible connectors to eliminate the thrust load on the equipment nozzles, which act as anchors. Although they cannot absorb the thermal growth compression the joints can move for lateral offset and absorb vibrations well.
Flexible Pipe Connectors
Flexible pipe connectors are usually either braided metal or spherical rubber joints with tie rods.
Like straight metal expansion joints, flexible loops are made with corrugated braided metal hose but with two independent legs. They can move in all three directions, i.e., axial, lateral and angular (including axial compression and extension), exceptionally well. Their ability to move, combined with the fact that the braid absorbs the thrust load, makes the loops a key alternative in situations where thrust load may be difficult for anchors to handle. The lack of thrust load combined with larger movements in all three directions also makes them key for absorbing seismic movements. The advantages of the flexible loops in thermal expansion application are the lack of thrust load and low spring load on the anchors. The reduction of space and extra insulation work, when compared to a hard pipe loop, are also significant.
By understanding the different properties, uses, and additional components that can be used with a metal expansion joint, operators can mitigate the risk of piping systems failures caused by thermal expansion. The correct use of these applications on the system will not only prevent failures but help to reduce unnecessary downtime.
About the Author
Gobind N Khiani, a UCalgary alumnus of Masters in Mechanical Engineering is a seasoned changemaker. He has a proven track record in technical and value engineering and holds a Fellowship in Engineering and an MBA. He is the Chairman of the End User Group at API and Vice Chairman of the Standards Council of Canada. He has done peer review on Emissions Management regulatory documents for ECCA and participated in research and development initiatives. Further, his experience is in the energy sector in the improvement of standards, technical compliance, strategy, governance, digital innovation, engineering management, technology, sustainable development, and operations. He is also skilled in Asset Integrity and Maintenance Management. As a volunteer, he is involved in technical standards (energy, tech, public safety) and has been a mentor/judge at First Robotics Canada. He is also the past chair of the CBEC of APEGA.