How to determine the length of non-metallic expansion joint? Don't just stare at the pipe diameter
Two days ago, I met a customer and asked directly on the phone, "The diameter of my pipe is 800mm. Can you make the length of your non-metallic expansion joint 800mm?" As soon as I heard it, I knew something was going to happen. If the length is chosen wrong, no matter how good the non-metallic expansion joint is, it will be useless, ranging from early air leakage to direct tearing of fabric fibers. Today, I'll talk to everyone clearly-how to determine this length.
Why is length so critical?
Non-metallic expansion joints (also called fabric fiber expansion joints) rely on the deformation of flexible bands to absorb the thermal displacement of the pipe. You make it short, the compensation amount is not enough, hard pull and hard tug, the stress at the root of the loop band is concentrated, and it will crack after a few heat cycles. Not to mention that the cost has gone up for a long time, but it is also easy to become unstable, sag and block dust. The length is not determined by patting the head, it directly determines that the expansion energy saving can not operate safely during the life cycle. According to JB/T 12235-2015 standard, the length design must be verified by calculation, and the diameter cannot be copied without trouble.
The three core factors that determine the length: compensation amount, wave number and fatigue life
Let's start with the amount of compensation-this is fundamental. How big the thermal expansion and contraction of the pipeline, how much the vibration amplitude of the equipment, and how much the welding deviation are, all have to be counted. For example, when the temperature of the flue gas pipeline rises by 100℃, the linear expansion coefficient of steel is about 0.012mm/m·℃, and the expansion amount of a 10-meter-long pipeline is 12mm. Non-metallic expansion joints can absorb limited displacement in a single wave (single turn). Usually, one wave can absorb 10~20mm axial displacement, depending on the structure and the number of fabric layers.
How to determine the wave number? Total compensation ÷ single wave compensation, and then a safety factor (generally 1.2~1.5). But with more wave numbers, the length will naturally be longer. Here we also have to consider the fatigue life-the same amount of compensation, the more the wave number, the smaller the burden of each wave, the longer the life. In turn, you insist on shortening the length and reducing the wave number, and the single wave deforms too much, and the fatigue life falls off a cliff. For example, the preheater outlet pipeline of a cement plant requires a compensation amount of 30mm. We designed it according to 3 waves, with a total length of 450mm. In fact, it took 5 years without any problems. Later, they were greedy for cheap and changed to a short 2-wave one, which cracked in less than half a year.
Is the length algorithm the same for rectangles and circles?
Not even close. The circular non-metallic expansion joint mainly does axial compensation, and its length is basically equal to wave number × wave pitch + flange thickness at both ends. Rectangular non-metallic expansion joints (such as ourRectangular non-metallic expansion jointProduct) In addition to axial, consider lateral and angular displacements. The rectangular section has a large aspect ratio, and the stiffness of the side wall is low. When the length is not enough, the side wall is easy to bulge. Moreover, the rectangle generally has a guide tube (such as an expansion joint guide tube), and the guide tube itself also occupies the internal space. Therefore, the length of rectangular pieces is often 10% ~15% longer than that of circular pieces with the same diameter. Don't take the round set and put it directly on the rectangle.
Limited on-site installation space? Don't push it
What about that? Customers often say, "I only have this little place on the spot, you shove it in for me!" It is easy to shove it in, but the consequences caused by insufficient length are more troublesome. Stress concentration will make the fabric fiber expansion joint tear prematurely at the root of the flange, or "squeeze" the loop belt into wrinkles, affecting the circulation area and accumulating dust and scale. We have dealt with a case: the kiln tail pipeline of a cement plant, the site space is only enough to install 350mm long non-metallic expansion joint, but according to the calculation, it needs 500mm. The customer hardly installed it, and after two months, the side of the ring belt was torn, and the ash leakage was serious, so he was forced to stop the kiln.
How to deal with it? We customized a model for our customersNon-metallic expansion joint (fabric fiber expansion joint)Reduce the number of fabric layers from 4 layers to 3 layers but use higher strength aramid cloth, at the same time, make the wave pitch smaller, the wave number remains unchanged, the final length is controlled at 420mm, the compensation amount reaches the standard, and the fatigue life test passes. The key still has to be counted, not forced.
A practical case: tearing of fabric fiber expansion joint in cement plant
Last year, a cement factory came to us and said that their non-metallic expansion joint was torn after less than half a year, and it was still broken after two replacements. When we went to the scene, the problem lies in the length of selection. The diameter of the pipe is 1200mm, and the design compensation amount is 25mm, but the installation space left at the site is only 350mm. In order to stuff it in, the supplier made a 2-wave design, and a single wave should absorb 12.5mm, far exceeding the recommended value (generally, a single wave does not exceed 8mm). As a result, the band was repeatedly bent at the trough, and the fibers were broken. Our plan: discuss with the owner, change the pipeline slightly, make 50mm space, make 3-wave design, single-wave compensation of 8.3mm, plus reinforcing ring and wear-resistant layer. It runs for one year after the modification and is in perfect condition. Therefore, if the length is not enough, don't make it hard. Changing the pipeline or changing the structure is the right solution.
The 3 easiest pits to step on when selecting a model
Pit 1: Think of the diameter as the length.The pipe diameter is DN and the non-metallic expansion joint length is L, two different things. Many people say "DN800 length 800" when reporting the size, which is probably wrong. The length should be calculated according to the wave number determines the length, and it has no direct relationship with the diameter. The diameter only affects the flange size and the inner diameter of the guide tube.
Pit 2: Ignore the influence of the guide tube.The function of the guide tube is to protect the ring belt from erosion by high-speed medium (see the specific function of the expansion joint guide tube for details). However, the guide tube itself has a thickness, the inner diameter is one circle smaller than that of the pipe, and it should be fixed on the inside of the flange, which will occupy part of the effective length. If you don't include the installation size of the deflector, the actual band length will be shorter, and the compensation ability will be discounted. We have encountered customers placing orders according to the total length of the loop belt, but as a result, the guide tube couldn't fit in and was reworked. Correct approach: Total length = loop length + flange thickness at both ends + guide tube extension length.
Pit 3: Forgot to leave an installation allowance.When installing non-metallic expansion joints, it is often necessary to pre-stretch or pre-compress, especially when installing pipes in cold state and running in hot state. For example, for steam pipelines, the ambient temperature is 20℃ during installation, 200℃ during operation, and the expansion amount is 50mm. Then you have to pre-compress the expansion joint by 20~30mm during installation, leaving space for thermal expansion. However, many people don't give an allowance when placing an order, and directly do it according to the theoretical length. When they arrive at the site, they can't install it, so they can only cut flanges or add gaskets to destroy the seal.
Anyone who fell into one of these three pits would be in trouble. So next time to set the length of non-metallic expansion joint, ask first: How much is the compensation amount? How to calculate wave numbers? Is there enough space on site? Are deflectors, flanges, and allowances all considered? Find out before placing an order, don't just stare at the pipe diameter.