Why do you have to do pre-displacement? This is not only to save trouble, it is to save your life
Two days ago, I met a customer. After the flue expansion joint was installed and put into test run, I heard a "rattle" in less than two hours, the tie rod broke directly, and the bellows was twisted into a twist. Only after asking did I know that there was no pre-displacement at all during installation, and it was welded to death directly according to the factory length. When the hot state runs, the elongation of the pipe is fully pressed on the corrugated joint, which is strange if it is not bad.
Pre-displacement of flue expansion joint, to put it bluntly, means that when cold installation, the expansion joint is deliberately let in a pre-compressed or stretched state to offset the displacement caused by thermal expansion of pipeline. This thing is not the icing on the cake, it is a means to save your life. Think about it, the flue temperature of steel mills and power plants always moves by three to four hundred degrees. The elongation of carbon steel pipes per meter is calculated as 0.012mm/℃, and the temperature of a 50-meter-long flue rises by 300℃, which is 180mm. Without pre-displacement, the expansion joint will either be crushed to death or cracked, the weld cracking and media leakage will be light, and the whole flue may collapse.
So, don't think this is unnecessary. If the pre-displacement is done correctly, the expansion energy saving can steadily absorb the displacement within the design life; If you do it wrong, it will cost more to rework it on site than to buy new equipment.
How exactly is pre-displacement calculated? Don't be afraid, experience works better than formulas
Δ L = α × L × Δ T, and then the pre-displacement magnitude takes Δ L/2. But here's the question-the alpha value is affected by the material. Is L the actual calculated length or the distance between two fixed points? Is Δ T the design temperature or the operating temperature? Everyone who engages in engineering knows that the theoretical value is often different from the actual working condition.
Don't die on the formula, first find out the owner's flue gas temperature and pipe layout. For example, the actual operating temperature of the desulfurization flue of a power plant is often 20-30℃ lower than the design value, and the pre-displacement calculated by you according to the design temperature may be too large, resulting in excessive compression of the expansion joint in the cold state and laborious installation. On the other hand, the flue temperature of cement kiln head fluctuates greatly, and the peak value may exceed the design value, so the pre-displacement amount has to be left with a margin. The general practice is to take 40%-60% of the design elongation, and then fine-tune it according to the direction of the pipe system and the type of expansion joint. Remember a phrase: It's better to owe a little than to live too much. Excessive pre-displacement will make the bellows in a high stress state in the cold state, and the fatigue life will be directly discounted.
You have to find out before you start: What kind of expansion joint are you using?
Different structures of expansion joints have completely different pre-displacement techniques. Don't use the universal corrugated expansion joint method to set the double hinge transverse expansion joint, that will cause big trouble.
- Axial type (such as general corrugated expansion joint, high temperature axial expansion joint): The pre-displacement is adjusted by the tie rod nut to shorten or elongate the bellows length. Note that only axial compression or stretching, radial deflection is strictly prohibited.
- Transverse type (e.g. double hinge transverse type expansion joint): Pre-displacement is achieved by adjusting the hinge angle, the purpose is to allow the intermediate connection to generate a lateral displacement in advance to offset the lateral thermal expansion of the pipe. This thing requires the highest installation accuracy. The angle difference is one degree, and the actual displacement deviation is several millimeters.
- Pressure balance type (such as straight pipe pressure balance type expansion joint, curved pipe pressure balance type expansion joint): There is a bellows group inside, and the pre-displacement should not only adjust the external tie rod, but also pay attention to the balance of internal pressure and thrust. When leaving the factory, the manufacturer will give the pre-displacement value according to the rigidity of the bellows, so don't adjust it randomly.
In addition, rectangular non-metallic expansion joints and fabric fiber expansion joints are flexible and usually do not need mechanical pre-displacement. However, the expansion margin should be reserved during installation, fixed by a limit frame, and the limit should be removed after the hot state is stable. Many novices ignore this point, resulting in the expansion joint being elongated in the cold state, but unable to retract in the hot state.
The key actions during installation, screwing the nut, adjusting the pull rod, looking at the arrow
At the practical level, I disassemble the steps according to the most commonly used axial expansion joints (such as general corrugated expansion joints):
- Check Arrow Direction— — There are generally arrows on the expansion joint cylinder pointing to the medium flow direction, and some are also marked with "installation length" and "pre-displacement position". First, make sure that the direction of the arrow is consistent with the flow direction of the flue. If you install it backwards, it will be a disaster.
- Pre-adjustment tie rod nut-The tie rod nut is usually in a free state when it leaves the factory (i.e. the natural length of the expansion joint). According to the calculated pre-displacement, first tighten the nut at one end and then the other end. Note: The nuts on both sides should be twisted alternately, 1/4 turn each time to prevent the bellows from deflecting. After screwing to the target scale, measure the total length of the bellows with a caliper, and confirm that the error from the design value is within ±1mm.
- Welding and fixing— — After the pre-displacement is adjusted in place, fix the tie rod and bellows with a temporary fixture to prevent the welding deformation from affecting the pre-displacement value. Symmetric segment welding is used during welding to avoid stress concentration in bellows caused by local overheating. Release the fixture after the weld has cooled.
- Inspection confirmation-Check the clearance of the flange sealing surface with a gauge, and check the levelness with a level ruler. If there is a guide tube, also make sure that the direction of the guide tube is consistent with the direction of the medium flow.
These four steps look simple, but the common mistakes on the spot are: after the nut is screwed to the head, it is found that the pre-displacement amount is exceeded, and it is loosened back, resulting in the thread slipping; Or forget to remove the temporary fixture after welding, and the expansion joint cannot expand and contract freely in the hot state. Taste yourselves, isn't that the truth?
How to verify it after installation? Run the hot state and you will know if you have stepped on a pit
Finished doesn't mean finished. The most reliable verification method is hot tracking. During the heating process, record the displacement of the expansion joint every half hour (mark the outer wall of the bellows and measure with a vernier caliper). At the same time, observe whether the tie rod nut is loose and whether the bellows is deflected.
- If the actual compression amount of the expansion joint is less than the design value, it means that the pre-displacement amount is too large, and the cold state is pressed too hard. Relax the nut appropriately when stopping the car next time.
- If the actual compression amount is much greater than the design value, it means that the pre-displacement amount is not enough, the bellows is over-stretched in the hot state, and it needs to be shut down for adjustment (unless you can be sure that there is still margin in the pipe).
- If the bellows is found to have bulge or local wrinkles, it is mostly caused by deflection or welding stress during installation, and must be replaced.
Also, don't forget to check the brackets and guides adjacent to the expansion joint. After pre-displacement, the force distribution of the pipe system will change, and the fixed bracket may bear extra thrust. I have seen more than one accident in which the bracket deforms and cracks when it runs to the hot state.
The pre-displacement details of expansion joints with different working conditions and structures (such as corrugated expansion joints in power station industry, metal corrugated expansion joints in cement industry, expansion joints matched by desulfurization flue gas baffle doors, etc.) vary greatly. It is best to discuss with the manufacturer's technology before installation, and get the recommended value of pre-displacement provided by them. After all, manufacturers know their products best, and you may lose the warranty if you play around.