Find out one thing first: Does big thermal expansion test the bellows or the frame?
When many people talk about expansion joints, their eyes are fixed on the bellows-how good the material is, how much the wavenumber is, and how much the temperature resistance is. But have you ever thought that the bellows are cracked and crushed. In many cases, it is not the bellows itself, but the frame can't hold it first. If the frame is not hard enough, the bellows will be completely messed up under the force, and no matter how good the stainless steel bellows is, it will be in vain.
To put it bluntly, in the face of great heat expansion, the frame is the one who carries the matter. The bellows is responsible for deformation and absorption of displacement, and the frame is responsible for restraining the messy forces such as blind plate force, internal pressure thrust and thermal stress. The frame broke down, and the bellows were the scapegoat.
Three mainstream choices of profile frames: carbon steel, stainless steel and non-metal composite frames, each with its own destiny
Carbon steel, stainless steel, non-metal composite. Carbon steel is cheap and high in strength, but its temperature resistance is limited. If it exceeds 350℃, alloy steel or lining with heat insulation layer must be applied. Stainless steel is good in temperature resistance and corrosion resistance, but it is expensive, and the stiffness will drop at high temperature, depending on the specific grade. What about non-metallic composite frames? Light weight, strong corrosion resistance, but weak pressure bearing capacity, generally used in low pressure and large cross-section smoke duct occasions.
Choosing a frame is not the more expensive the better, but to see who can hold the bottom line at what temperature and load. When you take the carbon steel frame to carry the smoke at 800℃, it will be oxidized and peeled in two days, the stiffness will plummet, and the bellows will suffer.
Take the power station and cement industry to talk about things: How to choose the frame of high-temperature axial expansion joint and metal rectangular expansion joint
The flue at the tail of power station boilers and cement kilns can expand by tens or even hundreds of millimeters. This kind of place is commonly usedHigh temperature axial expansion joint, the bellows are Inconel or high nickel, but what about the frame? Many design institutes only give a Q235B channel steel, as long as it is strong enough. And the result? After half a year of operation, the frame was deformed, the bellows was forcibly twisted and pulled, and the weld cracked.
Metallic rectangular expansion joints in the cement industry are more typical. The stiffness of the rectangular section itself is poor. If the frame is not reinforced by box girders or stiffened plates, the out-of-plane deformation will directly press on the bellows as soon as the internal pressure comes up. Two days ago, I met the owner of a cement factory, saying that their flue expansion joint had changed the bellows three times a year, and then it was replaced with an integral stainless steel frame with reinforcing ribs, and there was no accident.
What happens if the frame is not stiff enough? Measured data tell you the common reasons why bellows are cracked
The axial expansion joint of DN500 has a design pressure of 0.25MPa and a temperature of 600℃. The frame is made of No. 14 channel steel and the bellows stiffness is 120N/mm. During operation, the thermal displacement is 60mm, and the lateral deformation of the frame reaches 8mm. As a result, the actual bending stress of the bellows is 40% higher than the design value-the fatigue life is directly cut by half. There are a lot of cases in the air duct and desulfurization pipeline of power stations.
The bellows were pulled and cracked, and eight out of ten were caused by frame deformation. You think that the quality of the bellows is not good, but in fact, the stiffness of your frame is not enough. During the design, only the strength was checked, but not the stiffness. When the frame is bent in the hot state, the bellows is not pure axial compression, but the composite force of tension + bending + torsion. Who can bear it?
Don't just look at temperature resistance, but also look at the restraint mode-tie rods, hinges, pressure balance, and the frame structure changes accordingly
What material to choose for the frame is one thing, the structural form is another. Tie rod expansion joint, the frame to bear the reaction force from the tie rod, then you have to calculate the buckling of the tie rod and the strength of the connecting ear plate. Hinged type, hinge seat must be installed on the frame, the force points are more concentrated, and the local stiffness requirements are higher.
The pressure balance type is more complex, such asStraight pipe pressure balanced expansion jointOrCurved tube pressure balance expansion jointThe forces of the middle bellows and the balanced bellows should be self-balanced through the frame. If the frame stiffness of this structure is insufficient, the bellows are involved with each other, and it is easy to become unstable. Do you think the frame is nothing more than a few steel plates welded together? When it was actually designed, it was full of doorways.
Type selection suggestions: What working conditions use carbon steel frame and what working conditions must use stainless steel or non-metal frame
The flue gas temperature is lower than 350℃, the pressure is not high, the medium is not corrosive, and the carbon steel frame is enough, but remember to add coating or temperature-resistant paint. If the temperature exceeds 400℃, or there is sulfide corrosion (such as the flue behind the desulfurization flue gas baffle door), don't hesitate to use a stainless steel frame, at least 304L, and 316L or higher for the high-temperature section.
For large-diameter rectangular flue, such as cement kiln tail and power station induced draft fan outlet,Rectangular non-metallic expansion jointIt was more appropriate at this time. The non-metallic expansion joint itself absorbs the displacement by fabric fibers. The frame is made of non-metallic composite materials or carbon steel frames lined with stainless steel, which is light in weight and corrosion resistant. The key is that the cost is much lower than that of all stainless steel.
Let's talk moreMetal rectangular expansion jointThis kind of dust collector used in the import and export of power plants, the frame must be made of reinforcing ribs, and it is best to weld hot-rolled steel into an integral frame, instead of welded thin-walled steel plates. Don't ask me how I know – I've seen too many cases of rectangular corrugated plates tearing due to insufficient frame rigidity.
So, back to the question at the beginning: What profile frame can eat big heat expansion? The answer is: a frame with sufficient rigidity, right material and smart structure. Don't just focus on the bellows. If you choose the right frame, the expansion joint can be really worry-free. Next time a manufacturer blows to you that "our bellows use imported materials", you should ask first: What materials do you use for your frame? How do you check the stiffness? If he can't answer it, you know it.