Cold draw amount of metal expansion joint? Don't laugh, this is not a question that everyone can answer. When the master of dry pipeline installation sees "cold drawn 50mm" marked on the drawing, most of them will scratch their heads: What does this have to do with the compensation amount? Which way? What happens if you pull too much?
Let's get this straight first.
What is cold drawing amount: Understand the difference between it and compensation amount and pre-displacement
The compensation amount is the total maximum thermal displacement of the pipe from cold to hot, which is the design value and is the part of the deformation that the expansion joint is going to "eat". And what about the amount of cold pull? It is an artificial pre-tension (or pre-pressing) of the expansion joint in the opposite direction of thermal displacement during installation. To put it bluntly, it is to "advance" part of the thermal deformation in advance.
Pre-displacement is the broader term and includes cold drawing and cold pressing. Cold drawing is only the most common type of pre-displacement-for thermal expansion displacement of pipes. Don't be confused by these names: the amount of compensation is "to happen" and the amount of cold pull is "you did it ahead of time". One is the formula in the textbook, and the other is the wrench at the installation site.
Why Cold Draw: What is the true displacement of thermal expansion and contraction?
Some people think that the expansion joint is installed and it's done? Why do you pull it cold?
Settle the account for you. A section of 30-meter-long steam pipeline, the working temperature is 200℃, the ambient temperature during installation is 20℃, and the linear expansion coefficient of carbon steel is calculated according to 12×10⁻⁶/℃:
Δ L =12×10⁻⁶ ×30000mm × (200-20) =64.8mm
More than 63mm. If you install the bellows directly from its natural length, it will have to carry a compression of 65mm when it is hot. The wave pitch is pressed to the limit, and the fatigue life is directly cut in half. What about the cold pull half? 32mm pre-compression +32mm hot compression, the cycle amplitude is small, and the fatigue life is doubled up.
How to determine the amount of cold drawing: calculate step by step according to the temperature, length of pipe section and position of fixed bracket
Δ L = α × L × (TWork-TEnvironment)
α is the linear expansion coefficient (about 12×10⁻⁶/℃ for carbon steel and 16×10⁻⁶/℃ for stainless steel), L is the length of the pipe section between the two fixed brackets, and T is the temperature difference. After calculating the total thermal displacement Δ L, 50% to 70% of Δ L is taken as the cold drawing amount, which is a commonly used "cold drawing ratio".
- Steam pipeline, temperature difference> 150℃, cold pull ratio 50% ~65%
- Hot water pipeline, temperature difference 80~150℃, cold pull ratio 60% ~70%
- Low temperature or temperature difference
Of course, this is only an estimation method. The project also depends on the position of the fixed bracket, the direction of the pipeline and the number of elbows. If there are several bends in the same pipe section, the thermal displacement is no longer pure axial, and the cold drawing amount has to be corrected according to the results of stress analysis software. You are marked with the amount of cold drawing on the drawing, and you are done with it, but you know how it came from, and you have a bottom in your heart when checking the calculation.
How to operate on site: pre-compression, tie rod, jack, measurement record
The operation process is not difficult to say, but the details are the worst.
In the first step, remove the expansion joint from the package and check that the set nut on the transport protection tie rod is loose. In the second step, the cold drawing direction is determined according to the drawing. When the pipe heats up, it extends, so the bellows is pre-compressed during installation; Pipes are pre-stretched if they are cold shrunk, such as cryogenic pipes.
The third step is practical execution. If the diameter is small and the cold drawing amount is less than 30mm, just screw the tie rod nut directly. If the diameter is large or the cold drawing amount exceeds 50mm, don't screw it hard. Use a jack and an auxiliary frame to push it. Two people on the side will tighten the flange bolts diagonally at the same time, so that the bellows will deform evenly, otherwise the wave pitch will leak later.
Step 4, record. Don't be too troublesome-ambient temperature, pipe wall temperature, length of corrugated pipe before cold drawing, length after cold drawing, actual cold drawing amount and number of adjustment turns of tie rod are all written into the construction record. Which link is wrong, these data are clues to find the cause.
Consequences of mistaken cold drawing amount: bellows instability, bracket exceeding limit, nozzle leakage
Two years ago, there was a project of thermal pipe network, the general-purpose corrugated expansion joint of DN600. During the installation, the workers didn't draw it cold, so they welded it directly. Three months into operation, the bellows cracked at the trough, and the steam blew out far away. When I removed it, I saw that the wave pitch had been pressed together, and the bellows was completely unstable. When the manufacturer sent an engineer to the scene, the first thing he asked was: Has the cold drawing been done?
Even if the amount of cold pull is too large, it won't work. The pre-compression is excessive, and the bellows is close to the yield limit in the cold state. Before it is put into production, the fatigue life has been burned by a large part. The axial force bearing by the fixed bracket will also exceed the design value, and there have been cases where the guide bracket has fallen off.
Bellows instability, bracket force exceeding limit, nozzle leakage-any one of these three things is a shutdown accident.
Cold Drawing Quantity and Matching of Different Types of Expansion Joints
Different structures of expansion joints have different concerns in cold drawing operation.
Universal corrugated expansion jointMost commonly, the structure is simple, just adjust the tie rod nut in place directly when cold drawing. Be careful not to install the deflector in the opposite direction, and the medium should flow in the direction of the arrow.
External pressure single axial expansion jointThe bellows is on the outside of the guide tube and has a protective sleeve on the outside. When cold drawing, you can't see the state of the bellows from the outside, only the scale mark on the outer cylinder. The advantage of this structure is that the bellows is not directly washed by the medium, but you must look at the marking line when pulling it in cold, and don't forget to lock the nut after pulling it in place.
Straight pipe pressure balanced expansion jointThe most particular. It has a balancing bellows and a working bellows inside, which relies on the self-balancing of internal pressure and thrust. If the cold drawing amount is only calculated according to the thermal displacement, and the force on the balance end is not considered, the actual displacement of the bellows after installation will not match the design value. It is suggested that when this model is cold drawn, the bellows on both sides should be pre-displaced at the same time, and only 20% ~30% of the cold drawing amount should be adjusted each time, and the operation should be alternated so that the pressure balancing system should always be in a symmetrical state.
AndCompound hinge transverse expansion jointIt absorbs lateral displacement by the change of hinge angle. There is no such thing as axial cold pulling, but the rotation direction of hinge should be found according to the drawings when installing, and the reason is similar.
Okay, that's it. The formula of this set of cold-pulled things is not complicated, but the difficulty lies in that someone has to be serious in every link. Next time you go to the construction site and see that the bellows is crooked before installation, don't be surprised, that may be the right cold drawing site.