As the master of dry pipe installation knows, the word "cold stretching" is often marked on metal expansion joints. Some drawings will also draw an arrow with "Pre-stretched 50%" written next to it. You look at that bellows and mutter to yourself: Isn't this thing used to absorb thermal expansion? How to pull it first when installing it? Why do you pull it?
Cold stretching is not messing around: the pipeline expands and contracts with heat, and the expansion joint takes some effort in advance
The pipe thermally expands, and the expansion joint is compressed, right? Half wrong. When the metal expansion joint operates in a hot state, the bellows is indeed in a compressed state, but the amount of compression does not come out of thin air. If the bellows is in the middle position when installing, as soon as the temperature goes up, all the expansion of the pipe is pressed on the bellows, and the bellows is compressed to the limit all the way. It's okay in the short term, but what about the long term? The bellows is repeatedly deformed near the extreme compression position, and the fatigue life is directly discounted.
In cold installation, artificially stretching the bellows for a certain distance is equivalent to making the bellows "take some effort in advance". When the temperature of the pipeline rises and begins to expand, the expansion amount is not compressed from zero, but first rebounds from the stretching amount to the middle position, and then slowly walks to the compression end. In this way, the bellows moves within the allowable displacement range of the design throughout the entire operation cycle, rather than holding it unilaterally.
To put it bluntly, cold stretching is to advance the compressive displacement in the hot state to the cold state to bear it. A walk on both sides of the bellows will naturally last longer. This principle is the same as people. Always let one leg be stressed, and sooner or later it will lame.
How to determine the amount of cold stretching: Don't pat your head, calculate according to temperature, tube length and stiffness
The amount of cold stretch is not randomly determined. A numerical value is usually given on design institute drawings, such as "the amount of pre-stretch is 50% of the design displacement". How did this 50% come from? It is calculated based on the amount of pipe thermal expansion Δ L. Δ L = α × L × Δ T, α is the coefficient of linear expansion, L is the length of the pipe between the two fixed points, and Δ T is the difference between the maximum operating temperature and the installation temperature. Calculate the total thermal expansion, and then determine the cold stretching amount according to the displacement stiffness and allowable displacement range of the expansion joint.
The conventional practice is to take 50% of the design compensation amount as the cold draw amount. But it's not an iron rule. If the operating temperature of the pipeline fluctuates greatly, or the stiffness of the expansion joint is relatively large, 50% is not necessarily optimal. Under some working conditions, the amount of cold drawing should be corrected according to the actual installation temperature. For example, if the design installation temperature is 20℃, and the actual installation ambient temperature is already 30℃, the amount of cold stretching will be reduced accordingly-because the pipe itself has already expanded partly.
In another case, the pipeline has a plurality of expansion joints arranged in series, and the displacement of each expansion joint is different, so the cold stretching amount should be calculated separately. At this time, we can't just look at a single expansion joint, but also calculate the stress distribution of the whole pipe system. Otherwise, it is easy to have an embarrassing situation that this expansion joint is overstretched and that expansion joint is not pulled in place at all.
Hands-on: Cold stretching steps, tooling, and those easy-to-miss details
On-site cold stretching operation looks simple, but there are many actual pits. The conventional steps are: firstly, the pipes at both ends of the expansion joint are fixed to ensure that the expansion joint is in a free state; Then install stretching tooling-typically tie rods and nuts, or jacks plus channel steel frames; Slowly apply tension to stretch the bellows to the design value; Finally, lock it with temporary support or nut, and then fix it with pipe spot welding.
There are four key details.
First, before stretching, be sure to confirm that there is no debris inside the expansion joint and that the guide tube is not stuck. There was a project before, when the bellows was stretched, I heard a "click" sound. When I disassembled it, the welding slag left by the guide tube was stuck in the corrugated gap, which forcibly deformed the bellows.
Second, the amount of stretching should be carried out in stages, not in place at once. Especially for the expansion joint with large displacement, it is easy to lead to uneven force of the bellows and excessive local corrugation deformation when pulled into place at one time. Stretch in 2-3 times, 10-15 minutes apart, to allow the bellows to fully release stress.
Third, the strength of the tooling should be enough. Don't underestimate this tensile force. Although the rigidity of the bellows is not large, the compressive displacement stiffness of the expansion joint with a large diameter can multiply the displacement amount, and the tensile force can reach several tons. Just find a few steel bars and weld a shelf? That's a joke about safety.
Fourth, and most easily overlooked – once stretching is complete, the temporary locking device should be removed or loosened as required by design. Some workers try to save trouble, and directly leave the bolts on the tie rod after the cold stretching is completed. As a result, once the pipe is heated, the bellows has no time to move at all, and the tie rod holds up the displacement, and the expansion joint is useless. Do you think this wave is a loss or not?
Cold stretching and pre-compression, a word difference, a big thing will happen in the reverse direction
The reverse operation of cold stretching is called pre-compression. The direction is completely different, and the application scenario is completely different.
Cold drawing is to stretch the bellows, which is suitable for the situation where the thermal expansion of the pipe is greater than the cold shrinkage-most steam pipes and hot water pipes belong to this category. Pre-compression is to shorten the bellows, which is suitable for occasions where the cold shrinkage of pipelines is greater than the thermal expansion, such as low-temperature pipelines and liquefied natural gas pipelines. There are also some special working conditions, when the pipeline is shut down, the temperature is lower than when it is installed, so it is necessary to pre-compress it at this time.
What happens if the direction is reversed? Think about it, the pipe is heated to expand, but the expansion joint is pre-elongated, which means that the expansion amount has not yet come, and the bellows is already near the tensile limit. Once the temperature rises, the bellows is directly pushed beyond the allowable displacement. From plastic deformation of bellows to rupture of bellows, pipeline leakage and shutdown for maintenance, it's all money.
Therefore, be sure to read the marks on the design drawings before installation. Some expansion joints will draw arrows at the ends, and the arrows point to represent the flow direction of the medium, but the amount of cold stretching is often represented by "S" plus a number, such as "S =50mm". Seeing this logo, first confirm whether it is pre-stretched or pre-compressed before doing it.
Which expansion energy saving for cold drawing: how to deal with common products such as general-purpose type and straight tube pressure balance type
Not all expansion joints are suitable for cold stretching. Whether you can do cold stretching depends on the structural form of the expansion joint.
The universal corrugated expansion joint is most commonly done cold stretching, which has a simple structure, straight connection between the two ends of the corrugated pipe, convenient stretching operation and the most obvious effect. Straight pipe pressure balanced expansion joint can also be done, because it comes with its own balanced bellows and end plate. When cold stretching, pay attention to the synchronous displacement of the main bellows and the balanced bellows, and don't just pull one side.
External pressure single type axial type expansion joint is also suitable for cold drawing. Its bellows is compressed when working, and cold drawing can offset the initial compression to a certain extent and improve the working state. Compound hinge transverse expansion joint and compound straight pipe bypass pressure balance expansion joint can also realize transverse pre-offset through the tie rod system, but the calculation is more complicated, and the pre-adjustment data is generally provided by the manufacturer.
Rectangular non-metallic expansion joints are different from fabric fiber expansion joints. The flexible part of non-metallic expansion joint is fiber fabric, which has low elastic modulus and fast stress relaxation after stretching, so cold stretching is of little significance. The metal rectangular expansion joint part can be done, but the stress concentration of the four corners should be considered, and the stretching amount should be controlled more conservatively. Because the sleeve pipe expansion joint slides by the sleeve clearance, there is no bellows deformation, so there is no concept of cold stretching.
Some expansion joints have been pre-stretched when they leave the factory, so they don't need to be operated on site. This product will be marked "pre-stretched" or "factory pre-strained" on the nameplate. Just install it directly during installation. Don't pull it again, otherwise the bellows will exceed the elastic limit and be scrapped directly.
In the end, cold stretching is essentially a "pre-installation service" installation program, not just doing a simple welding job in the workshop. It involves calculation, tooling, sequence, and direction. Which step is lazy, the back pipeline system will "repay" you with vibration and leakage. After more than ten years in this business, I have seen the scene of cold stretching rollover. Nine times out of ten, it is not that I don't understand the principle, but I don't take cold stretching seriously. So next time you see the words "cold stretching" written on the expansion joint on the construction site, don't rush to pull the gourd. Find out how much to pull, where to pull, and how to fix it after pulling. That pull well, the pipeline system runs safely and steadily for ten years; If you pull it wrong, you will have to rework it in half a month. This account was still clearly calculated.