1. What exactly is the metal expansion joint coefficient? Don't be fooled by the name
Anyone who has done pipeline design knows that every time they encounter the words "metal expansion joint coefficient", some people always think that this is a slang used by manufacturers to fool people. Two days ago, a friend who made steam pipelines asked me, "Is your coefficient set by slapping your forehead?" Tsk, not really. This coefficient thing is, to put it bluntly, a set of numbers-telling you how hard this expansion energy saving can carry, how many times it can hold up, and how many displacements it can swallow. Don't think of it as calculus, it's just a selection manual.
The coefficient of metal expansion joint essentially contains three levels:stiffness coefficient(whether the bellows is hard or not),Fatigue life coefficient(How long will it break after use),Displacement coefficient(How much can be stretched and compressed). Plus an easily overlookedPressure thrust coefficient— — These four parameters determine whether the product can really bear it on the spot. And guess what? 80% of selection accidents are because these coefficients are not understood.
Second, stiffness, fatigue life and displacement-three core coefficients determine whether the product can bear it
Let's startstiffness。 The rigidity value of the bellows directly determines how much reaction force the expansion joint produces on the pipe. The stiffness is too large, and the expansion joint cannot be pushed when the tube expands and contracts thermally-it is equal to white installation; The stiffness is too small, and the bellows will bulge as soon as the internal pressure comes up. Remember one thing: the stiffness coefficient is not a fixed value, it varies with wave pitch, wave height, wall thickness. For example, this site'sUniversal corrugated expansion joint, the stiffness of the multi-layer corrugated design is 30% ~50% lower than that of the single layer, which is suitable for large displacement but low pressure scenarios.
Next lookFatigue life coefficient。 This stuff is the easiest to overlook. "The customer said it would take 10 years, so I designed it according to 10 years"-Wrong. Fatigue life is not simply calculated by time, it is directly linked to the number of displacement cycles you actually run. Like aHigh temperature axial expansion jointIf it starts and stops twice a day, more than 700 times a year, the fatigue life design should be calculated as more than 10,000 times. However, if the site is running continuously and the shutdown is very occasional, 2000 times is enough. So don't open your mouth and have a "high life factor". Who will bear the extra cost?
And finallyDisplacement coefficient。 This includes three directions: axial, transverse and angular. Many design drawings only write "axial compensation amount ±50mm". As a result, the on-site pipeline installation deviation plus thermal displacement forcibly ate 20mm in the lateral direction-the bellows was directly twisted into a twist. When selecting the model, be sure to count the installation deviation into the displacement amount, leaving at least 10% margin.
3. How to adjust the coefficient under different working conditions? Take the high temperature axial type and the straight pipe pressure balance type as examples
Two days ago, a power station project required to useHigh temperature axial expansion jointThe medium temperature is 650 °C and the pressure is 0.5 MPa. Under this working condition, the stiffness coefficient will decrease-because the elastic modulus of the material will drop at high temperature, and the trough will easily become unstable. We adjusted the wave height and layer number for the customer, and finally the stiffness coefficient decreased from the original 120N/mm to 85N/mm, and the fatigue life was raised from 3,000 times to 5,000 times. Why? Because the bellows is softer and the local stress is less. Is that counter-intuitive?
In the desulfurization flue, the wind pressure is high and low, and the pipeline is also thrust by the equipment. At this timeStraight pipe pressure balanced expansion jointIt's more suitable than the normal axial type. Its pressure thrust coefficient is digested by itself through the balance ring and is not transmitted to the fixed bracket. If you calculate, if you choose a conventional expansion joint, the fixed bracket will have to bear tens of tons of thrust, and the civil construction cost will more than double. However, the coefficient of the pressure balance type is also particularly adjustable-the area difference of the balanced bellows must be accurate, and the difference of one millimeter will break the thrust balance.
Fourth, the easiest pit to step on when selecting the model: the coefficient is large ≠ safe, and the guide tube and tie rod have to be counted
Many people think that "it's always right to keep the coefficient larger", but I have seen too many cases of rollover. If the stiffness coefficient is too large, the pipeline cannot be pushed, and the thermal stress will be transmitted to the equipment end, which will crack the flange of the pump; The fatigue coefficient is too high, the bellows is made thick and hard, and the actual working conditions can't reach so many cycles at all, which is a waste of money. To put it bluntly, "exceeding the standard" coefficient is sometimes more dangerous than not being sufficient-because you will have the illusion of "definitely OK" and ignore other hidden dangers.
guide tubeAndtie rodThe match of. LikeSpecific Function of Expansion Joint Guide TubeIt is clear in the FAQ-it protects the inner wall of the bellows from being washed by the medium, but the guide tube itself has thermal expansion. If your displacement coefficient does not account for the axial expansion and contraction of the guide tube, the guide tube may hold up against the flange at high temperatures, causing the seal to fail. In the same way,Function of expansion joint tie rodIs to limit excessive displacement, but how to adjust the tie rod nut? If the locking torque is not set according to the coefficient, it is useless if the tie rod is loose, and it will eat all the displacement if it is tight. These two things are not included in the selection coefficient table, you are laying a mine for yourself.
5. Inverse design from coefficients: 30% cost can be saved by changing another expansion joint scheme for the same project
At the beginning of this year, there was a cement plant project with a smoke exhaust pipe diameter of 2.2m, a temperature of 350℃ and a design displacement of ±60mm in the axial direction. The original scheme used 3Metal Corrugated Expansion Joints in Cement Industry, each with guide tube and tie rod, the total quotation is more than 100,000. I recalculated the coefficient for them: the actual thermal expansion of the pipe is only 35mm, and the stiffness of the fixing brackets on both sides is sufficient. So changed the scheme toExternal pressure single axial expansion joint(Absorb axial displacement) plusA duplex hinged transverse-type expansion joint (absorbs minor lateral offsets in the pipe path). Two pieces in total, with a total price of tens of thousands. Saved 33%.
Why can it be saved? Because the stiffness coefficient of the external pressure single axial type is lower than that of the ordinary axial type, and there is no need for a liner guide tube (low medium flow rate, no scouring). The transverse type of compound hinge uses hinge structure instead of tie rod, which is less costly and can compensate for angular displacement. This is the idea of inverting design from coefficients-find out the actual demand behind each coefficient, and don't stack parameters.
So stop thinking that the metal expansion joint coefficient is metaphysics. It is a tool. If it is used well, it can save hundreds of thousands. If it is used, it may cause a safety accident. Next time you select, spread these four parameters on the table and ask one by one: Is the stiffness soft enough? Is fatigue enough? The displacement package does not contain deviations? Who bears the pressure thrust?