What the hell are the limit devices caring about? — — Start with Function
Many people think that the limit of metal expansion joint is to tighten a few tie rods and a few nuts. Two days ago, I met a customer, who took photos on the spot and asked, "Why did the bellows bulge when I installed this expansion joint?" I looked at it, good guy, the tie rod nut was tightly screwed, and the bellows couldn't move at all. If the limit becomes fixed, is it still called an expansion joint?
Within the allowable displacement range, allow the bellows to expand and contract freely; When it exceeds the design range, hold the thrust of the pipe hard to protect the bellows from being pulled or crushed。 To put it bluntly, the limit is to draw a "safe zone" for the bellows.
First, prevent the bellows from exceeding the design displacement due to installation error or pipe system thrust; The second is to bear the blind plate force generated by the medium pressure of the pipeline, especially the large-diameter pipeline or high-pressure system; Third, it is used as a temporary fixation during installation to facilitate butt welding joints. There is no limit, and the expansion joint is a "soft persimmon", and it will be wasted as soon as the pipe system exerts force.
Common limit structures: pull rod, nut, screw, limit ring, how to work each
Take the general-purpose corrugated expansion joint in our station for example, the most common one isTie rod + nutStructure. The tie rod runs through the flanges at both ends of the bellows, and the two ends are equipped with double nuts. Its working principle is simple: when the bellows is stretched outward, the tie rod is pulled; When compressed inward, the nut presses against the flange. The tie rod is only subjected to pulling force, not pressure.
But many people can't telltie rodAndscrewThe difference between. The tie rod is a full-length thread with nuts at both ends; The screw is typically a middle polished rod, threaded at both ends, or welded at both ends to the flange. The adjustment distance of the pull rod is realized by screwing in and out of the nut; The screw has to rely on the welded stop or limit ring. Don't get confused when selecting, otherwise the limit amount will not match.
Againlimiting ringThis thing is mostly used for external pressure single axial expansion joint or directly buried expansion joint. The limit ring is sleeved on the outside of the bellows, and both ends are welded on the connecting pipe. When the bellows is elongated to the design limit, the end plate hits the limit ring and rigid cutoff. The advantages are compact structure and not afraid of loosening; The disadvantage is that once welded, it can't be adjusted, which requires high installation accuracy.
And there's anotherLarge tie rodStructure, commonly found in double hinge transverse type expansion joint and air-cooled island vacuum pipe double hinge expansion joint. This kind of tie rod is thick and exclusively bears the shear force caused by lateral displacement, and is not used to adjust the axial direction. Don't replace it with the tie rod of the axial expansion joint.
Installation and Adjustment: When is the tie rod nut removed? When to screw to death?
This is the most problematic part of the scene. Many construction teams got the equipment and saw the nut on the tie rod, thinking it was a fixture and welding it directly to the pipeline-and then the bellows was scrapped.
- Tie rod nuts for transport fixing must be removed after installation.The angle steel and bolt connectors that are temporarily welded to prevent deformation when leaving the factory shall be removed. Not dismantling is equivalent to welding the expansion joint to dead, and when the pipe system is hot, the bellows will be directly cracked.
- As a tie rod nut for limiting position, it cannot be removed, but a gap should be left.How to stay? Look at the amount of pre-stretch/pre-compression on the design file. For example, if the steam pipe needs to be pre-stretched, then screw the nut to the designated position to leave a thermal expansion allowance. If there is no pre-displacement in the design, the nut is screwed and retreated by 1-2 buckles to ensure that the bellows has room for movement.
Two days ago, there was a question and answer "How to adjust the tie rod nut of the expansion joint", which made it very clear: when adjusting, first loosen the locking nut, twist the adjusting nut to make the bellows reach the designed length, and then lock it. The operation is simple, but only if you know what the design length is. Don't be blind if you don't know, call the manufacturer.
As forDoes the screw need to be removed?The screw is a limiting part, not a transport fixing part, and does not need to be disassembled. However, it is necessary to ensure that there is a gap between the screw and the flange hole, and it cannot be stuck.
Consequences of Improper Limit: Lessons from a Real Case
There was a case of a cement factory last year that is still fresh in my memory. Their high-temperature fan outlet pipeline uses metal corrugated expansion joints in cement industry. Parameters: working temperature 350℃, pressure 0.15MPa, axial compensation ±40mm. During the on-site installation, the construction team screwed the limit tie rod nut to death, leaving no gap at all. After the pipe system heats up, the bellows can't extend axially, and the end flange pulls the pipe hard. As a result, the pipe support and hanger fall off, the wave pitch of the bellows is crushed to death, and the weld is cracked. It was shut down for two days and lost hundreds of thousands.
First, the design drawings indicated pre-stretching 30mm, but the construction team didn't understand it; Second, they feel that "the limit is fixed" and screw it to death on their own initiative. Alas, this is actually quite common.
In turn, the limit nut can't be screwed too loosely. In the high-temperature axial expansion joint of a petrochemical plant, the limit nut retreats too far, and the blind plate force compresses the bellows to the bottom during the pressure test of the pipe system, which causes the bellows to become unstable and bulge directly. SoThe limit is not simply "loose" or "tight", but precisely controls the displacement range。
Selection suggestion: How to select metal expansion joint with limit for different working conditions
The key to the selection is the type of displacement and pressure level of the pipe system.
Mainly axial displacement: Universal corrugated expansion joint or external pressure single axial expansion joint is preferred, with tie rod nut limit. Pay attention to the thickening of the tie rod when the pressure is high, otherwise the blind plate force will pull and bend the tie rod. For high-temperature working conditions (such as corrugated expansion joints used in power station industry), the limiting structure should consider the thermal expansion gap, and the tie rod material should be heat-resistant steel.
Predominantly lateral or angular displacement: Double hinge transverse expansion joint or double hinge expansion joint of air-cooled island vacuum pipeline, with large tie rod or limit ring for limit. Under this working condition, the tie rod should not only bear the limit position, but also bear the shear force, and the cross-section size should be checked according to the strength.
Large diameter or high pressure: For example, large-diameter thick-walled expansion joints and straight-pipe pressure-balanced expansion joints, the limit structure often adopts built-in limit rings and external tie rods for double protection. One tie rod can't bear tens of tons of blind plate force, so multiple ones have to be evenly distributed.
Direct or fully buried pipeline: The limit of the directly buried (fully buried) expansion joint must be discarded from the nut adjustment mode because it cannot be maintained. Generally, the limit tie rod or limit sleeve with a preset length from the factory is welded to the pipeline. The compensation amount must be checked before installation, and it will be wasted if the weld is wrong.
Don't just look at the parameters on the product sample,The design fatigue life of the limiting device should also be matched。 Our station has a piece of information called "Service Life of Expansion Joint", which mentions that the fatigue point of the limiting structure is often the root of the thread or the weld. When selecting the type, write the fatigue check working condition clearly, so as not to break the limit before the equipment reaches the design life.