Over the years of designing and calculating metal expansion joints, I have seen too many engineers who turn over samples and set formulas as soon as they come up. And the result? The calculation book looks pretty, but it leaked within a few months of being installed in the pipeline. What's the problem? The parameters of the working condition were not thoroughly understood. Today, let's break it apart and talk about it clearly.
The first step of design calculation: Get accurate displacement, pressure and temperature, and don't rush to set formulas
Many people ask, "How much displacement does this expansion energy saving make?" In fact, we should ask first: How much displacement does this pipeline system need to absorb? Thermal expansion, equipment end point displacement and installation deviation, do you distinguish these three things clearly? For example, for a 50-meter-long steam pipeline, when the temperature rises from 20℃ to 300℃, the linear expansion coefficient of carbon steel is calculated as 0.012mm/ (m·℃), and the total elongation is 50×0.012×280=168mm. This number is not difficult to calculate. The difficulty is whether you know how many natural elbows in the direction of the pipe can absorb some of the displacement. After deducting the natural compensation part, it is the job that the expansion joint should do. The same is true for pressure. Don't just look at the working pressure on the nameplate, but also consider the hydraulic test pressure and transient impact pressure. The temperature is even more terrible. The allowable stress of bellows material drops with the temperature. The wave thickness calculated by normal temperature may not be enough to 500℃.
How to calculate bellows stiffness and fatigue life: How to choose data from single wave displacement to overall compensation amount
Stiffness determines thrust, and thrust affects the design of fixed bracket. The stiffness of the bellows is the series result of each wave of stiffness. The larger the single wave displacement, the more the wave number, and the lower the overall stiffness. But stiffness and fatigue life are contradictory. If you take the single wave displacement larger, the wave number will be less, the stiffness will decrease, and the fatigue life will drop visibly to the naked eye. How to choose? According to the formula in EJMA standard, the key to calculate the number of cycles is to calculate the equivalent axial displacement accurately: when there is lateral displacement, don't forget to convert it into equivalent axial displacement, and don't take the synthetic displacement directly. I met a customer two days ago, saying that his double hinge transverse expansion joint cracked after three months. I asked him to measure the actual transverse displacement at the site. Good guy, the design gave it 50mm, but the actual deviation was 80mm, so it was weird that it didn't crack. Don't pat your head with data, but leave a margin, but the bigger the margin, the better-if you leave more, the wavenumber will increase, the stiffness will become smaller, and the system stability will go wrong. Generally, the fatigue life is multiplied by the safety factor according to the actual number of cycles, which is more than 15 years.
Doorways in structure selection: axial type, double hinge type and pressure balance type are used under what working conditions respectively
When it comes to structural selection, I am most afraid of the "universal mentality". The universal corrugated expansion joint can only absorb axial displacement. If you force it to eat lateral displacement, the bellows will have to twist and break fast. There is transverse displacement on the pipeline, and the compound hinge transverse expansion joint is preferred-it converts the transverse displacement into angular displacement through the linkage mechanism between the two hinges, and the bellows is uniformly stressed. However, note that the double hinge type has blind plate force, which has to be borne by hinges and pull rods, so the fixed bracket should be calculated clearly. If the pressure is high and the diameter is large, the blind plate force can easily be tens of tons. At this time, straight pipe pressure balanced expansion joint or curved pipe pressure balanced expansion joint have to be used. They cancel out the internal pressure thrust by balancing the bellows, and pipelines and equipment hardly bear the blind plate force. How to choose? Look at the direction of the pipeline: the straight line section uses straight pipe pressure balance type, and the corner uses curved pipe pressure balance type. For the working conditions of high medium temperature and large dust, such as flue gas and high-temperature air duct, don't use ordinary bellows, choose high-temperature axial expansion joints, or simply match guide tubes. Selection is not to pick expensive, but to pick "match".
Guide tube, tie rod and connector thickness: Three details that are most easily overlooked in design calculation
These three places are the most easily overlooked "small roles" in design calculation, but they are precisely the high-incidence areas of problems.
The function of the flow guide tube may be more than just flow guidance. It is necessary to block the high-speed fluid from directly washing the inner wall of the bellows, so the length, wall thickness and gap of the guide tube have to be calculated. When the medium flow rate is high or contains particles, wear-resistant surfacing should be added to the front end of the guide tube, otherwise it will be worn out in a few months. Also, leave enough clearance between the guide tube and the bellows, otherwise the bellows will get stuck as soon as it deforms.
Many people think that the pull rod is a safety rope. Wrong. For the transverse expansion joint of compound hinge, the tie rod is the main load-bearing part, which bears the blind plate force and the moment generated by transverse displacement during normal operation. The diameter of the tie rod, the strength of the thread and the length of the weld of the connecting ear plate should be checked according to the maximum stress condition. Two days ago, a customer asked, "How to adjust the tie rod nut of the expansion joint?" I asked him: Do you know whether the tie rod nut is adjusted to a stressed state or loose when it leaves the factory? If you don't know, don't screw it randomly. If you screw the wrong bellows, it will become unstable directly.
Take over thickness not to mention. Sometimes, in order to save cost, the wall thickness of the connecting pipe is thinned, and as a result, the connecting pipe is not strong enough after being corroded and thinned by the medium, and it is broken from the connecting pipe position. The thickness of the joint should be calculated separately according to the design pressure, temperature and corrosion margin of the pipeline, and the thickness of the bellows cannot be directly sleeved.
After calculation, check the effects of installation constraints, cold tightness and field deviation on the calculation results
Everything will be fine when the calculation book is done? It's far away. On-site installation will in turn affect the calculation results.
Have you thought about installing constraints? If rigid branch pipes are connected to both ends of the expansion joint, the actual displacement distribution will be completely different from what you assume. For example, in the design, it is assumed that 50% of the displacement is absorbed in each direction. As a result, the fixed bracket on one side is not welded firmly, and the displacement all goes to the other side, and the bellows exceeds the limit. Therefore, when calculating, you should put forward requirements for the stiffness of the fixed bracket. Don't just draw a △ Symbols.
Cold tight amount is another pit. To take full advantage of the displacement capacity of the bellows, half of the thermal elongation can be pre-stretched (cold tightened) during installation. But cold tightness is a double-edged sword-if you calculate the cold tightness wrong, it will make the bellows in a pre-compressed state in the cold state, and exceed the limit after the hot state is superimposed. How to take the cold tight amount? Based on the actual temperature curve, don't slap your head.
The on-site deviation can't be avoided. The pipe is a few millimeters away from each other. You think it's okay, so you pull it over and weld it. But on the expansion joint, these millimeters are the real initial displacement. Therefore, when designing and calculating, at least the displacement margin of installation deviation should be reserved, which is generally 10% ~20% of the total displacement. If the deviation on the spot is too large, it is recommended to use a metal hose for transition, and don't let the corrugated expansion joint carry this pot.
In the final analysis, the design and calculation of metal expansion joint is a systematic project: calculate the displacement, stiffness, fatigue, blind plate force, check the guide tube and tie rod, and finally consider the installation deviation. Every step is supported by data, and every step can stand up to on-site inspection. This is the truly reliable design.