Anyone who does pipeline compensation knows that the metal welding expansion joint looks like an iron knot. In fact, if it is not selected or installed correctly, it will be full of trouble. Two days ago, a friend from a power plant complained to me, saying that one of their steam pipelines was equipped with a general-purpose corrugated expansion joint. As a result, the corrugated pipe cracked after less than half a year of running. It was found that the medium temperature exceeded the design value, and the high-temperature creep was not considered at all during the selection. Alas, I've heard too much of this kind of thing.
So today, I simply broke the metal welding expansion joint from the principle to the selection to the installation pit, and crumpled it into pieces at once to make it clear. You can use it as a reference book or a guide to avoiding pits. Anyway, it will be worth it if it can save you tens of thousands of rework fees.
1. What is a metal welding expansion joint? What is the difference between it and a ripple compensator?
To put it bluntly, the metal welding expansion joint is to assemble bellows, end pipes, tie rods and guide tubes into a flexible element that can absorb the thermal displacement of the pipeline through welding technology. Its core is the bellows, which utilizes the elastic deformation of the corrugations to compensate for axial, transverse and even angular displacement.
As for the difference with the ripple compensator? It is often mixed in the industry, but strictly speaking- -expansion jointIs a functional description,compensatorIt's the device name, and both refer to the same thing. You searched on my siteProduct ListIt can be seen that there are both "universal corrugated expansion joints" and "straight pipe pressure balance expansion joints", which are all metal welded expansion joints in essence, but their structural forms are different. Don't worry about the name anymore, focus on the model parameters.
2. Four common types: general-purpose type, high-temperature axial type, pressure balance type and direct burial type
Universal corrugated expansion joint
The most common and cheapest. Single or multi-layer bellows, with flanges or connections at both ends, absorb displacement by corrugation expansion and contraction. The applicable temperature is generally not more than 350℃, and the pressure is within 1.6MPa. If you use room-temperature water and low-pressure steam in the pipe gallery, this is the most cost-effective. But don't expect it to carry a large displacement. The single-wave compensation amount is usually 20-40mm. If it is not enough, use a double one.
High temperature axial expansion joint
This one is a little particular. The medium temperature of the main steam pipeline of the power plant and the flue gas pipeline of the cement kiln head is always five or six hundred degrees, and the material of ordinary corrugated pipe is directly softened. The high temperature axial type is generally made of Inconel 625 or 321 stainless steel, and is lined with high temperature resistant thermal insulation to prevent heat radiation from burning through the corrugations. Note when selecting the model:The permissible fatigue life is reduced by at least half for every 100℃ increase in temperatureSo don't just look at the amount of compensation, the number of fatigue times is real money.
Pressure balanced expansion joint
When the internal pressure of the pipeline is high and the large displacement needs to be compensated, the common expansion joint will produce a huge blind plate force, which will push the fixed bracket flying. The pressure balance type (such as straight pipe pressure balance type and curved pipe pressure balance type) eats the blind plate force by itself by setting the balance bellows or tie rod structure, and the thrust on the bracket is almost zero. Suitable for high pressure steam, chemical pipe system. The disadvantage is that it is expensive and large, so you have to leave enough installation space in advance.
Direct buried (fully buried) type expansion joint
Don't think that you don't need compensation if you bury the pipeline directly. The temperature difference of the underground heat pipeline is also large, but it can't be repaired. The directly buried expansion joint is covered with an anti-corrosion layer and an insulation layer, and the bellows has an external pressure structure, which is not afraid of soil corrosion. Weld directly to the pipeline during installation, just backfill. But there is a hard indicator:Must be strictly pre-stretchedIf you don't stretch it, the bellows will be pressed into twists as soon as the pipe heats up.
3. You can't pat your head when you select a model: how to weigh the medium, temperature, pressure and displacement?
Many designers took the selection table and put it on, and as a result, there were problems on the spot. I summarize a native method, and you judge it in this order:
- Step 1: Look at the media。 Corrosive media (acid, alkali, sulfur-containing flue gas) must be made of corrosion-resistant materials, such as PTFE-lined hoses or high-nickel alloys. 304 is enough for ordinary water vapor, don't waste money.
- Step 2: Look at the temperature and pressure。 These two parameters directly determine the number of layers and wall thickness of the bellows. Multi-layer thin wall can be used for high temperature and low pressure, but single layer thick wall should be used for low temperature and high pressure. Remember a mantra:High temperature layering, high pressure thickening。
- Step 3: Calculate the displacement amount。 The axial displacement is calculated according to the pipe thermal elongation formula Δ L = α · L · Δ T. Lateral displacement often comes from pipe deflection or equipment interface misalignment. Angular displacement generally occurs at the elbow. When three kinds of displacements exist at the same time, use compound hinge type or universal hinge type, and don't force a general-purpose type.
What about the conflict between the three? For example, if the temperature is high and the displacement is large, don't use the axial type alone, but try to connect a pressure balance type in series. Don't think of an expansion joint package to conquer the world. When designing a pipeline, set up a compensator group.
4. The five easiest pits to step on in the installation process
The equipment was chosen correctly, and I have seen at least double-digit cases of installing rollovers. The following five points must be checked before you install them.
① Install the guide tube in reverse direction
The function of the guide tube is to guide the medium to the inner wall of the bellows to prevent high-speed fluid from impacting the bellows. The direction of the arrow must point to the direction of the medium flow. If it is reversed, the guide tube will press against the bellows, and it is only a matter of time before it wears out. As specifically mentioned in the FAQ of my station, the arrow on the expansion joint is the flow direction of the medium, so don't twist it.
② Improper adjustment of tie rod
Many tie rod expansion joints are locked when they leave the factory, which is used to prevent the bellows from deforming during transportation. After installation in place, the tie rod nut must be loosened to the specified length according to the design drawings, otherwise the expansion joint is equal to a rigid connection, and there is no compensation effect. How exactly to adjust? Look at what we sent earlierQ&A, with detailed steps.
③ Pre-stretching is not done or reversed
The directly buried expansion joint and the high-temperature axial expansion joint have a large temperature difference between the ambient temperature and the operating temperature during installation, so they must be pre-stretched. The amount of pre-stretch is equal to half the amount of thermal elongation. Note: The direction of stretching is opposite to the direction of displacement. If you pre-compress, the system will directly fold the bellows as soon as it heats up.
④ The fixed bracket becomes a decoration
Although the pressure balanced expansion joint can be self-balanced, it does not mean that the bracket can not be fixed. The main fixing bracket must be set between the two compensators or at the end of the pipe, otherwise the pipe will still run. I've seen people mount the fixing bracket on the expansion joint body, so why not just weld it to death? The thrust of the fixed bracket should be calculated according to the type of expansion joint, and the blind plate force should not be estimated by feeling.
⑤ Welding quality is not up to standard
Metal welding expansion joints, welding is the life gate. In particular, the circumferential welds of end pipes and bellows must be permeation tested or radiographic tested. The construction site often rushes to the construction deadline, and the welder just clicks twice and finishes it, resulting in leakage. When accepting, you must read the welding process evaluation report, the grade of welding rod should be correct, and preheating and post-heating should not be saved.
5. From power stations to cement, how to choose between different industries?
Working conditions vary greatly from industry to industry. Take power stations and cement —
Power station industry: Main steam, reheating steam, water supply pipeline, high temperature, high pressure, large diameter. High-temperature axial expansion joint, double-hinged transverse expansion joint or double-hinged expansion joint for air-cooled island vacuum pipeline are recommended. We have a specialCorrugated expansion joint for power station industrySeries, the material is heat-resistant steel, and the fatigue life is designed according to 1000 times (there are only so many cycles in the actual power plant life cycle, so there is no need to invest 3000 times, which is a waste of money).
Cement industry: The flue gas pipelines at the head and tail of the kiln have large temperature fluctuations (from normal temperature to 400℃), large dust content, and may be condensed and corroded. At this time, the metal corrugated expansion joint of cement industry is used, lined with wear-resistant lining, and the guide tube is thickened. The pressure is not high, generally 0.1-0.5MPa, so the wall thickness does not need to be too thick, but the bellows chooses multi-layer stainless steel plus outer anti-corrosion coating. In addition, cement plants are compact in space and often require rectangular expansion joints or non-metallic expansion joints. We have rectangular expansion joints, too, lookMetal rectangular expansion joint。
Others, like chemical industry, metallurgy and heating, have their own ways. But the core logic remains unchanged: the medium determines the material, the temperature and pressure determine the structure, the displacement determines the type, and the installation determines the life. If you understand these four dimensions thoroughly, the metal welding expansion joint selection will not overturn.
Don't always think about saving money and buying a general-purpose model. Calculate the downtime loss caused by improper selection. The expensive price difference is nothing at all. If you happen to have an item that you are unsure of, you can basically find the answer by flipping through the product catalogue and FAQ on my site.