Metal expansion joint bellows compensator, this thing looks inconspicuous in the pipeline system, but it is chosen wrongly, ranging from rework and rewelding to pipeline tearing, media leakage and even production shutdown accidents. Two days ago, I met a customer, and a universal corrugated expansion joint was installed on the steam pipeline. As a result, the displacement was not calculated correctly, and it took three months for the corrugated pipe to bulge and scrap directly. Alas, in fact, these problems can be avoided by understanding the following five core problems.
1. Are metal expansion joints and bellows compensators the same thing?
Let's make this basic concept clear first. You see "metal expansion joint" on the drawing, the contract says "bellows compensator", and the supplier quote becomes "compensator" again – is it the same thing? Yes. Expansion joints and compensators are different names for the same type of equipment in industrial pipeline systems, and their functions are exactly the same: absorbing heat displacement, reducing vibration and noise, and protecting pipelines. So don't worry about the name, the key depends on the parameters and working conditions.
However, note that although the name is common, the structure is divided into two main categories: metallic and non-metallic. Metal expansion joint bellows compensator mainly relies on the elastic deformation of bellows to compensate for displacement, while non-metallic expansion joints (such as fabric fiber expansion joints) use flexible materials. If you buy metal, the product catalogue corresponds to general-purpose corrugated expansion joints, high-temperature axial expansion joints, and directly buried (fully buried) expansion joints.
2. From general-purpose to rotary compensator, which of the dozen varieties should I pick?
General-purpose corrugated expansion joint, rotary compensator, high-temperature axial type, direct burial type, external pressure single axial type, double hinge transverse type, straight pipe pressure balance type, curved pipe pressure balance type... There are more than 20 kinds of light metals. How to choose? Take your seat according to the working conditions, and don't be blind.
-If you are a long straight steam pipe with large axial displacement but limited space, use External pressure single axial expansion jointIt can concentrate the displacement on the bellows and has a compact structure.
-If the pipe is displaced in multiple directions, such as horizontal plus vertical, use Compound hinge transverse type expansion joint orCurved tube pressure balance expansion jointAnd can absorb angular displacement and lateral displacement.
-Buried pipes? Don't hesitate, come on Direct buried (fully buried) type expansion joint, the shell is anti-corrosion wrapped, and it is directly buried in the soil without making a well.
-Requires rotation compensation? Rotary compensatorIt is compensated by rotation, which is suitable for high temperature and large displacement scenes.
For flue gas pipelineNon-metallic expansion jointOrHigh temperature axial expansion jointFor corrosion-resistant applicationsPTFE-lined hoseOrPTFE compensator。 The core of type selection is one: find out the temperature, pressure, medium and displacement direction of the pipeline system.
3. Two parameters that are most afraid to be ignored in model selection: pressure and displacement
Many engineers only look at the caliber and length when selecting models, and feel that they are "almost the same". And the result? If the pressure is selected small, the bellows will burst; If the displacement is selected small, the bellows is stretched to the limit, and the life is directly returned to zero. Neither of these two parameters can be less.
The calculation of pressure not only looks at the working pressure, but also considers the test pressure and impact pressure. For example, the design pressure of steam pipeline is 1.0MPa, but the pressure test should be 1.5 times. If you choose the expansion joint of 1.0MPa, it will be finished during the pressure test. Correct practice: Select the nominal pressure level according to the design pressure × safety factor (generally 1.25~1.5).
The displacement is even bigger. When you calculate the thermal expansion of a pipe, you have to consider the temperature difference between cold and hot, the length of the pipe, and the number of compensators. Each compensator can absorb a limited amount of displacement, and it will fail if it exceeds the design value. There is a common misunderstanding here: some people think that installing a few more compensators can share the displacement, but if you install a general-purpose type, it can only absorb axial displacement, and lateral displacement will damage the bellows. So be sure to clarify the displacement type – axial, lateral or angular?
By the way, the stiffness of bellows directly affects the displacement compensation performance. There is a special article on our station about "stiffness and calculation formula of bellows". If you are interested, you can turn it over. Simple understanding: the smaller the stiffness, the easier it is to deform, but the pressure resistance will decrease, which is a balance.
4. Can the tie rod nut be removed before installation? Which way does the deflector face?
The two details that are easiest to overturn during on-site installation may save half of the after-sales trouble.
Let's start with the tie rod nut. Many expansion joints come out of the factory with tie rods to prevent accidental damage to the bellows during transportation and installation. Once installed in place, the tie rod nut must be loosened-note that it is not removed, but loosened to the specified length to allow the expansion energy saving to expand and contract freely. Some customers forgot to loosen it. As a result, as soon as the pipe ran, the tie rod jammed the displacement and the bellows tore directly. Refer to our question and answer of "How to adjust the tie rod nut of the expansion joint". Conventional operation: first tighten the nut and fix it. After the pipeline connection is completed, screw the nut out counterclockwise to the marked position to ensure the displacement margin.
Look at the arrows. The expansion joint is usually marked with an arrow pointing to the direction of the media flow. When installed, the guide tube becomes a throttle, which not only increases the resistance, but also will be eroded. Especially for flue gas pipelines with high temperature and high flow rate, if the direction is wrong, the guide tube will be worn out in half a year.
5. Short life and fast leakage? It's probably because you don't understand how to calculate fatigue life
The most afraid thing about the metal expansion joint bellows compensator is fatigue failure-no corrosion, no overpressure, that is, it starts to leak after several years of use. Why? Every time the bellows expands or contracts, the material fatigues. The design life is usually determined by the number of cycles, such as 1000, 5000, 10000. You didn't calculate how many times you started and stopped every day, so you chose one with a life span of 1,000 times. As a result, the steam pipe started and stopped once a day, and it was over in three years.
The steam pipeline of a chemical plant has a temperature of 300℃, a length of 50 meters and an expansion of about 80mm. They chose a high-temperature axial expansion joint, which allows an axial displacement of 100mm. Looks like enough, right? But they didn't count the number of cycles-the boiler starts and stops once a day, and with the frequent expansion and contraction caused by load fluctuations, the actual number of cycles per month exceeds 30. The design life was only given 1000 times, and it was leaked in two and a half years. Later, it was changed to a model with the same specification but a fatigue life of 5,000 times. Although it was 30% more expensive, it took 8 years without any problems.
How to calculate fatigue life? The core is to look at the stress level and material fatigue curve of the bellows. When selecting, the supplier should provide the number of design cycles. You can estimate the number of annual cycles according to the actual working conditions, and then multiply it by the safety factor (generally 1.5~2.0) to deduce the required design life. If you are unsure, choose a higher level. For example, if you use it 1,000 times routinely, you will choose 3,000 times. The cost is 15% ~20% more, but it is worry-free.
In the end, selection is a balancing game: pressure, displacement, temperature, medium, and fatigue life are all indispensable. Next time, I will choose the metal expansion joint bellows compensator. After going through these 5 questions, I will basically not step on pits.