The displacement of metal expansion joint, these four words seem simple, but many people who select the model reported the number incorrectly in the first step. Two days ago, I met a customer who made pipelines, and he said, "Our expansion joint here needs 40mm displacement". I asked-how much is the axial direction? How much horizontally? How much in the angular direction? As a result, the other party was stunned for a long time and said, "Let's go back and look at the drawings again."
First, the displacement is not a single number: axial, transverse and angular. First, distinguish where the pipeline moves
Axial, transverse, angular. The axial direction is the expansion and contraction along the center line of the pipeline, the transverse direction is the horizontal misalignment perpendicular to the center line of the pipeline, and the angular direction is the relative rotation of flanges at both ends. How the pipe moves is determined by the arrangement of the pipe system and the direction of thermal expansion. A long straight steam pipe is elongated after heating, and the expansion joint is compressed, which is axial displacement; But if there is an elbow in the middle of the pipe, thermal expansion will push this point diagonally upward, and then it will come out axially and transversely at the same time.
Is that the truth? If you only report a maximum displacement value, the manufacturer can't judge how many waves the bellows should make and whether or not the hinge structure is needed. The correct method is to disassemble the displacement vectors on the drawing, mark the axial, transverse and angular directions respectively, and then match their respective allowable errors.
2. The pull of displacement and fatigue life: how big the single wave displacement is, directly determines how many years the bellows can last
Let's talk about the pulling of displacement and fatigue life. Bellows belong to low-cycle fatigue elements. Every time they expand and contract, the peaks and valleys undergo plastic deformation. The larger the single-wave displacement is given, the lower the number of cycles allowed. In the industry, it is often checked according to the design life of 1000 times, but if the actual working conditions start and stop several times a day, the design goal will have to be thousands or even tens of thousands of times.
For example, a bellows with a wave height of 40mm may have a life span of tens of thousands of times when the single wave displacement is 2mm. If you put the single wave displacement to 4mm, the life will directly drop by one or two orders of magnitude. This curve is very steep. Don't expect to get past it by "overdoing it". Therefore, the displacement of metal expansion joint must be mentioned together with the expected fatigue life when selecting the type, and cannot be discussed separately.
3. The way of "digesting" the displacement by the expansion joints of different structures: what is the use of general type, compound hinge and straight pipe pressure balance type
Let's talk about how the expansion joints of different structures "digest" the displacement. The general corrugated expansion joint is the most common axial type, which absorbs the displacement by the axial expansion and contraction of the corrugated pipe, and also allows a little angular direction, but the lateral ability is very weak. External pressure single axial type expansion joint puts the bellows on the outside, the medium passes directly, and does not contact the node. It is suitable for clean medium or the occasion where liquid drainage is needed to plug the leak, but it also has only axial displacement.
The lateral expansion joint of compound hinge is different. It connects two corrugated joints with hinges, and the internal pressure thrust is carried by hinges and pull rods. The node is responsible for rotation, and the lateral displacement is "screwed" out by the rotating shaft of the hinge. A balance bellows or a balance ring is added inside the straight pressure balance expansion joint to offset the blind plate force, and there is no need for heavy main fixing brackets at both ends of the high-pressure straight pipe section. Curved pipe pressure balance expansion joint is suitable for pipe systems with limited space and need to absorb complicated displacement at the same time. What structure to choose depends not on the price, but on the displacement pattern and pipeline stress.
4. How to "hide" the displacement into the pipeline when installing pre-stretching/pre-compression, and the restriction of the guide tube on the displacement condition
Another thing that is easy to miss is installing pre-stretch/pre-compression. To put it bluntly, this trick is to "hide" the displacement into the pipeline. For example, when the pipeline heats up after operation, the expansion joint is pre-pulled for a section when installed, and the compression amount just fills this displacement when it is hot, so that the bellows is always in a reasonable range within its life. Conversely, if it is a cold-shrunk pipe, it is pre-compressed. Many projects leaked this step, and as a result, the bellows was pressed to the limit as soon as it was started, and its life was frighteningly short.
But pre-stretching/pre-compression is not about pulling as much as you want. The existence of the guide tube will limit the displacement condition. The guide tube is installed in the inner layer of the node to protect the bellows from being washed by the medium and reduce the flow resistance. However, if the medium contains particles or the flow rate is high, a sealing gap must be maintained between the conduit and the conduit, which determines the allowable amount of axial displacement. Lateral displacement is the short board of the guide tube. Deadlifting will break the tube crooked and jam the bellows, and the compensator will basically be wasted.
So, how to determine the displacement of the metal expansion joint? Four things: distinguish the axial and transverse angles, think about the fatigue life, choose the right structure, and calculate the pre-installation and pre-tension. The order should not be messy. If any step is missing, there is a possibility of rollover later. Before the next selection, take the as-built drawing and operating conditions of the pipe system, and check these numbers cleanly first, which is more useful than anything else.