Most Asked Question by Customers: My Pipes Expand and Contract Thermally, How Long Expand Amount Is Exactly Needed?
Two days ago, I met a steam pipeline purchaser. When I came up, I said, "How long can your metal expansion joint stretch? Give me one that can stretch 30 centimeters." I laughed when I heard it – brother, are you asking for expansion joints or expansion joints? The expansion joint is not a rubber band, so the stretching amount is really not determined by patting your head.
Many engineers come to ask "how much does the metal expansion joint stretch", which actually simplifies the problem. From the cold state to the hot state, the elongation of the pipeline can not be solved only by the maximum stroke of the expansion joint itself, but depends on the working conditions, structure and type selection. Today, this article will break it up and talk about it. After reading it, you can step on at least three pits less.
The core of the amount of stretching: the effective area and wave number of the bellows
Let's talk about the principle first. The amount of tension of a metal expansion joint (also called a corrugated compensator) is essentially the single wave displacement of the bellows multiplied by the wave number. How much each wave can be pulled depends on the geometry of the bellows-wave height, wave pitch, wall thickness, and, most critically, effective area.
The word effective area sounds mysterious. To put it bluntly, it is the equivalent force area of the axial force generated by the bellows when it is subjected to internal pressure. The larger this area, the greater the thrust generated under the same pressure, and the higher the requirements for the pipe support. However, the amount of stretching itself is mainly stacked out by the wave number. For example, if a wave can pull 10mm, and you design 10 waves, the theoretical stretching amount is 100mm-provided that the fatigue limit of the material is not exceeded.
And guess what? Many manufacturers mark the single wave displacement quite high, but that is the ideal data in the laboratory, and it has to be discounted in actual use. Because you also have to consider temperature, pressure, number of cycles. If you have to ask "how much does the metal expansion joint stretch", then I can only say: first calculate the thermal elongation of the pipeline, and then find the product with the corresponding wave number and fatigue life.
The amount of stretching varies greatly under different working conditions: from a few millimeters to tens of centimeters
Let's just say that for a water pipe at room temperature, the temperature difference is only a few tens of degrees, and maybe a few millimeters of compensation is enough. However, if it is the main steam pipeline of the power station, the temperature soars to five or six hundred degrees, and the thermal extension of the pipeline of tens of meters can be hundreds of millimeters casually. At this time, you take a general-purpose corrugated expansion joint to make up, which is looking for death.
This site'sUniversal corrugated expansion jointThe axial compensation amount is generally at the level of tens of millimeters, which is suitable for ordinary thermal pipelines; WhileHigh temperature axial expansion jointSpecially aimed at the high-temperature working conditions of power stations and cement industries, the wave number can be done a lot, and the stretching amount can reach 200mm or even higher. However, note that the allowable stress of bellows material will decrease at high temperature, and the actual available displacement should be discounted. Don't expect the nominal value to be full.
And there are more extreme- -things likeDouble hinge expansion joint for air-cooled island vacuum pipelineThis is absorbing the lateral displacement, and the axial stretch amount is instead a small head. So don't expect a general-purpose product to overcome the world. Different working conditions are too different.
Pits that are easy to step on during model selection: pre-stretching, tie rod nut, guide tube
There are three most common mistakes to make when selecting models, and I'll talk about them one by one.
1. Install the pre-stretch in the reverse direction
Many expansion joints leave the factory in cold length and need to be pre-stretched or pre-compressed for installation, with the aim of keeping the compensator in the best working position when hot. For example, the pipeline will extend after heating up, so the expansion joint should be partially compressed during installation to reserve an extension space. However, some people tried to save trouble and install it directly. As a result, as soon as the temperature rose, the bellows was pulled over and scrapped directly.
So how do you know how much to pre-stretch? Look at the cold tightness value given by the design. General axial type expansion joints (e.g.External pressure single axial expansion joint) will be clearly written in the installation instructions, so don't guess it yourself.
2. Improper adjustment of tie rod nut
The tie rod nut of the expansion joint (see Q&A "How to adjust the tie rod nut of the expansion joint") is used to limit excessive displacement or adjust the installation length. Some people screw the tie rod nut to death after installation, which is equivalent to turning the expansion joint into rigid, so why do you need it? The correct method is: after pulling to the designated position, loosen the locking nut to the design gap to ensure that the bellows can expand and contract freely. You screw it to death. When the pipe is hot, the stress is all held in the bellows. It's weird that it doesn't crack.
3. Ignore the influence of the guide tube
Many people think that the deflector tube is optional. Wrong! Guide tube (refer to the question and answer of this site "Specific function of expansion joint guide tube") First, it guides the flow direction of the medium and reduces the turbulent erosion of the bellows; The second is to prevent solid particles from accumulating. In dusty gas or steam pipes, there are no guide tubes, and the bellows are quickly worn out. More importantly, the guide tube itself will occupy a part of the axial space, which will affect the effective stretching stroke of the expansion joint. If the length of the guide tube is neglected during the selection, the calculated stretching amount may be less than the actual one.
How to verify whether the stretching amount is qualified in actual operation?
OK, assuming that you have selected the type and the product arrives at the site, how can you confirm how much it can pull? Two ways.
First, look at the nameplate. Regular manufacturers (such as the products of this site) will mark the design displacement, working pressure and temperature range on the expansion joint. Check against the procurement contract.
Second, do the cold displacement test. Before installation, fix the expansion joint on the stand, slowly stretch it to the design value with a hydraulic jack or hand hoist, measure the length change of the bellows, and check whether there is instability and bulging phenomenon.Universal corrugated expansion jointAndThe maximum axial displacement is included in the technical parameter table of high-temperature axial expansion joint, which can be used as a reference. However, note that the test should not exceed 1.5 times the design displacement, otherwise the bellows may produce plastic deformation and be directly scrapped.
Plus, if the plumbing system is vibrating or starts and stops frequently, you also have to consider the fatigue life. At this time, the amount of single stretch is not the only indicator, and the number of cycles is equally critical. This siteStraight pipe pressure balanced expansion jointThe balance of pressure and thrust is considered in the design of such products, which is suitable for high-frequency working conditions.
Summary: Three elements of tensile quantity calculation-temperature difference, pipe length and compensator type
To say a thousand words, return to the core question "How much does the metal expansion joint stretch?" The answer is three factors:
- Temperature difference: The difference between the operating temperature of the pipeline and the installation temperature determines the total amount of thermal elongation. Formula: Δ L = α × L × Δ T, α is the coefficient of linear expansion, L is the length of the pipe, and Δ T is the temperature difference.
- Pipe length: The length of the straight segment that needs to be compensated between the two fixed brackets. Pipes with many turns can be compensated by nature, not necessarily all by expansion joints.
- Compensator Type: axial type (e.g.Universal corrugated expansion joint、High temperature axial expansion joint) mainly absorbs axial displacement; Lateral type (egCompound hinge transverse expansion joint) absorbing lateral displacement; Angular type is used to absorb angular displacement. Choose the wrong type, no matter how much stretch is, it doesn't work.
Don't just stare at the stretching amount. If any of the pressure level, guide tube, fatigue life and bracket stress leaks, something will happen. If you have specific working conditions at hand, directly refer to the technical parameter table under each product classification of this site, or flip throughMetal expansion joint weight tableAndModel and size of expansion jointQuestions and answers are basically correct.