Two days ago, I met a customer and asked, "What is the standard length of your metal expansion joint? Give me a quote." I was stunned and asked him, "What are your working conditions? What is the diameter? What is the displacement? What about the pressure and temperature?" He was also stunned. "Just according to the standard length." Alas, the word "standard length" really tricked many people.
Find out first: Is there a "standard length" for metal expansion joints?
The word "standard" in the name refers to the design and manufacturing standards, such as GB/T 12777 and EJMA, which does not tell you that the length has a fixed value. The same is the general-purpose corrugated expansion joint of DN200. Some factories make 500mm long, while others make 800mm long. Why? Different wave numbers, different takeover lengths and different structural forms naturally differ a lot in length. If you ask the standard length when you come up, you can't answer it directly.
Core factors affecting length: diameter, pressure, displacement, fatigue life
To put it bluntly, length is forced out by "demand". The larger the amount of thermal expansion of the pipe, the more wavenumber the bellows needs, and the length is naturally elongated. Under the working conditions of high pressure and high temperature, the wall thickness of the bellows must be thickened, the single wave compensation amount will change, and the wave number must be recalculated. There is also fatigue life. You require the design life to be mentioned from 1,000 to 10,000 times, and the wavenumber has to be added up. Therefore, it is not unusual that the length of the same diameter is two or three times different. When you ask "what is the standard length", it is like asking "what is the standard wheelbase of a car"-can cars and trucks be the same?
Differences in length between structural types are beyond imagination
The influence of structure type on length is often greater than that of diameter. Take a practical example: the external pressure single axial expansion joint, the bellows is on the outside, and the shell and end tube that bear the internal pressure occupy a lot of dimensions, which is longer than the ordinary single expansion joint under the same diameter. And straight pipe pressure balance type expansion joint, with balance bellows and connecting pipe in the middle, the length is increased exponentially. What about the double hinge transverse expansion joint? It wants to absorb the lateral displacement, and an intermediate tube has to be added between the two sections of bellows. Can the length not be long? Therefore, when selecting the type, you have to determine the structure first, and then talk about the length. The structure is uncertain, and asking the length is empty talk.
How to determine the length in engineering? Design processes and common misconceptions
Calculate the thermal displacement of the pipeline first, then deduce the required wave number according to the allowable fatigue life, and then add the dimensions of the end pipe, guide tube and connector pipe, and finally get the total length. Note that the thickness and length of the guide tube must also be counted, especially for high-temperature dusty media, which is easy to blow through if the guide tube is short. Here are two common myths. First, some people think that the longer the length, the stronger the compensation ability, and they can't wait to make a super long one. However, there is a stability problem of the bellows. If the length is too long, the critical pressure of column instability will drop. Before it is in place, the bellows will bulge first. Second, someone directly sets the "conventional length" on the sample, and the one on the sample is only a reference. Your actual working conditions, pipeline layout and fixed bracket position are different from the sample, and something will happen if you set it hard.
There is no fixed length, so what exactly does the standard care about?
The standard tube is not the length, but the "rules" of design method, material, fatigue life and test requirements. For example, GB/T 12777 stipulates the design formula, stress assessment, fatigue life prediction method of corrugated pipe, and how to carry out the pressure test and airtightness test before the product leaves the factory. EJMA is an international universal design standard for expansion joints, and many manufacturers make calculation books according to it. There is JB/T 12235-2015 for non-metallic expansion joints, and there are corresponding standards for metal hoses. But no standard will say "the length of the expansion joint of DN100 must be 500mm". Therefore, next time you ask about the standard length of metal expansion joint, it is better to report the diameter, displacement, pressure, temperature and medium directly to the engineer of the manufacturer. People can calculate the reasonable length for you in a few minutes, which is much more reliable than guessing the "standard" yourself.
Don't be too troublesome, the selection data is filled in, the manufacturer can save you a lot of trouble, and it can also avoid being unable to install or use on site. Metal expansion joints are not standard pieces, they are custom pieces that are "figured out". If you buy it as a standard part, you will suffer.