Eight out of ten people who do pipeline designs have been tossed by the performance calculation of metal expansion joints. Obviously, I chose a model according to the sample, and when I installed it at the scene, either the displacement was not enough, or the life span was frighteningly short. What's the problem? The high probability is not that the manufacturer's parameters are falsified, but that the calculation ideas and actual working conditions are twisted. Today, break up these five links and explain them clearly. Next time, you will make a selection, and you will know where to put your strength into it.
1. Before performance calculation, find out these three displacements: how to take the values in axial, transverse and angular directions
When you get the working condition, the first step is not to turn over the sample, but to understand the displacement first. The axial displacement is easy to understand, and the thermal elongation of the pipe is multiplied by L × α × Δ T. But when you look at the pipeline on the spot, how can any of them only extend forward honestly? Once the fixed bracket is set and there are more elbows, the pipe system will have lateral displacement in both horizontal and vertical directions-at this time, it is far from enough to calculate the axial direction alone.
Angular displacement is more troublesome. It is not a number that can be measured directly, but is determined by the angle at which the pipe bends and rotates. Many designers directly input the displacement of the pipe system to the manufacturer for the manufacturer to calculate. This is not impossible, but you at least have to know how the angular displacement comes from. For example: a 6-meter pipe is fixed at both ends and thermally expanded by 30mm in the middle. If the pipe system is arranged asymmetrically, the displacement of 30mm will be converted into a certain angular displacement and applied to the expansion joint. If you ignore this, no matter how beautiful the numbers in the calculation book are, the expansion joint on the spot will still strike.
To put it bluntly, the key to the value of displacement lies in: we can't just focus on the elongation in a single direction, but we should take into account the spatial displacement of the whole pipe system. The pipeline layout drawing is flat, which direction is constrained and which direction can be freely expanded and contracted, pushed down one by one, and finally taken a combined displacement condition and put it into calculation. Only when this step is done solidly can the subsequent calculations make sense, otherwise it will be a castle in the air.
2. Stiffness, effective area and pressure thrust: Three numbers determine the basic disk of the bellows
Stiffness, effective area, pressure thrust. Don't be boring, these three figures directly determine that expansion energy saving can't stay in this pipe system.
The concept of stiffness, to put it bluntly, is the ability of bellows to resist deformation. The unit is N/mm, meaning how much force you need to compress it by 1mm. The greater the stiffness, the greater the elastic reaction force, and the greater the load on the fixed bracket. Choosing an expansion joint with too much stiffness is equivalent to plugging a spring into the pipeline system. As soon as the temperature changes, the bracket will suffer.
The word effective area sounds mysterious, but it is actually the equivalent force area when the bellows bears internal pressure. Calculation formula: A = π/4× (Dm) ², Dm is the average diameter of the bellows. Once this number is calculated, multiply it by the working pressure and you get the pressure thrust. That's a real effect on the blind plate force of the pipeline. For example, the expansion joint of DN500 has a working pressure of 1.0MPa, an effective area of about 0.2 square meters, and a pressure thrust of more than 200 thousand N-you calculate, which is equivalent to more than 20 tons of weight walking against the pipeline.
The pressure thrust must be factored into the load of the fixed bracket. If you don't count, the consequences are serious. We have seen a power plant project in which a straight pipe pressure balance expansion joint was installed on the main steam pipeline. The fixed bracket given by the design institute was designed according to the thrust-free working condition. As a result, the bracket was directly deformed when it was pressed on the spot. Later, the manufacturer redesigned it, installed a double hinge transverse expansion joint, and locked the thrust inside the pipe system, which solved the problem.
3. The calculation of fatigue life cannot only count wave numbers: the influence of temperature correction, pressure correction and pre-deformation
Fatigue life is the most likely to roll over. Many people think that the wavenumber of bellows will have a long life. That's a wrong idea. More wave numbers can indeed increase the compensation amount, but in the fatigue life calculation formula, wave number is a denominator term-the more wave numbers, the smaller the displacement of each wave, and the life will indeed become longer. But if you just increase the wave number without considering the temperature correction and pressure correction, it will still roll over.
The fatigue life calculation is based on the formula in the EJMA standard. The temperature correction coefficient ft and the pressure correction coefficient fp must be considered in the conversion between the design fatigue life Nc and the actual number of allowable cycles [N]. How did the temperature correction factor come from? The fatigue performance of corrugated pipe material will attenuate at different temperatures. The allowable stress of 304 stainless steel at 400°C may be only about 60% of that at room temperature. If you take the fatigue curve at room temperature to calculate the working condition of 400°C, the calculated life will double directly, which is going to happen.
The influence of pre-deformation is also often overlooked. The so-called pre-deformation is to pre-stretch or pre-compress the bellows during installation, so that it can be in an ideal stress state at the working temperature. The method of taking the pre-deformation amount is particular, and you have to calculate it comprehensively according to the installation temperature, working temperature and pipeline cold tightness. Many construction teams don't understand this. During installation, they directly pull the bellows to half of the nominal displacement, regardless of whether the cold tightening direction is correct or not. As a result, the bellows cracked within two months after the expansion joint was installed. You said that it was a product quality problem, rather than saying that it was pre-deformation.
4. Does the calculation result not match the actual selection? Common misunderstandings lie in the guide tube, pull rod and installation method
Sometimes you choose a model according to the calculation results, and when you look at the quotation sheet, you find that the size, weight and price don't match the model. Don't be in a hurry to scold the manufacturer, look back first to see if you stepped on these pits.
The guide tube occupies the displacement space. The role of the guide tube is to protect the inner wall of the bellows from high-speed media scour, but it has a length itself. For high-temperature axial expansion joints, one end of the guide tube is fixed and the other end is movable, and the movable end must have enough clearance for the bellows to expand and contract. When calculating the displacement, if you ignore the restriction of the guide tube on the effective compensation amount, choose the type according to the full displacement. Once the site is run, the guide tube will hold up first, and the bellows will not reach the designed displacement at all.
Tie rods and hinges change the way constraints are restrained. Many customers think that this thing can absorb the axial displacement of 50mm when they see "axial compensation ± 50mm" written on the sample. But you should know that expansion joints with tie rods, such as compound hinge transverse expansion joints, are mainly used to absorb transverse displacement, and axial displacement can be realized mainly within the clearance range of tie rods. You have to use it to absorb a lot of axial displacement, and the ends of the tie rod are quickly pulled and bent. On the other hand, you use the universal corrugated expansion joint to make lateral compensation, and you have to check the load-bearing capacity of the hinge or tie rod, so you can't overload the tie rod.
The third pit is also concealed: the installation direction is reversed. The expansion joint is marked with an arrow for the flow of media, which many people think is decorative. In fact, for an expansion joint with a guide tube inside, the fixed end of the guide tube must face the flow direction of the medium. Install backwards, the medium directly flushes the bellows, and it wears out in a few months. So the first thing when you get the goods, look at the flow mark, and don't wait for it to be loaded before reworking it.
5. Before the calculation sheet is delivered to the site, there is one step left: the verification of media, corrosion allowance and product standards
Printed out the calculation book, all the work? It's still early. The last check is life-saving.
In terms of media, many people think "isn't it steam" or "isn't it smoke", but the details are all in it. The medium in the flue gas desulfurization system has high sulfur content and the temperature is below the acid dew point, so condensed acid will precipitate. If you choose a 304 stainless steel bellows, it will corrode like briquettes in a few months. At this time you have to use 316L, or even higher grade corrosion resistant alloy. The performance calculation of metal expansion joint is not only computing power, but also material. Wrong choice of materials, everything is useless.
How to consider the corrosion allowance? There is a concept of corrosion allowance in pressure vessel design, and expansion joints are also applicable. The wall thickness of bellows is very thin, generally 0.5mm to 2mm. Adding a corrosion margin may account for 30% or 40% of the wall thickness. Many industry standards have rigid regulations on the minimum wall thickness of bellows, but in actual selection, the medium working conditions reported by customers are vague, and manufacturers are not good to take the initiative to thicken, because thickening will affect the flexibility, thus affecting the life and compensation amount. This requires Party A's designers to think clearly in advance and clearly write the corrosion allowance in the inquiry document.
Not to mention product standards. Expansion joint products have national standard GB/T 12777, non-metallic products have JB/T 12235, and special standards for electric power and petrochemical industries. You have designed a parameter according to the national standard, but the site requires the implementation of ASME standard. The proportion of weld non-destructive testing and hydraulic test pressure are different. Not to mention those export projects, you have to consider PED certification and classification society certification. This is not just a matter of adding a page to the calculation book, but the whole design input conditions have to be reorganized.
Then again, is it difficult to calculate the performance of metal expansion joints? There are only a few formulas that are turned over and over. The key is to think clearly at each step: Whose position did you make this calculation? Is it selected according to samples or customized according to working conditions? The thinking of the two is completely different. The former is to take the existing products to make up the working conditions, while the latter is to take the working conditions to push back the structure. The truly reliable selection must be the latter. Go through the above five links, and you will find that the expansion joint is not a general part that is "bought back and installed", but a functional unit that is deeply integrated with the pipeline system. Calculate each parameter well and check in place, so that the expansion joint can stay on the pipeline quietly and realize its design life.