Expansion joints without pre-deformation when they leave the factory will be scrap iron at the site
The factory state of the metal expansion joint directly determines whether it can work after being installed. Many people receive the goods and see that the bellows are shiny and the paint is intact, and they feel that everything is okay-wrong. If the expansion joint is not pre-deformed when it leaves the factory, it is basically a decoration when transported to the site, and may even become a potential safety hazard of the pipeline system.
Why do you say that? Because the displacement compensation ability of bellows is achieved by the elastic deformation of corrugations. If the bellows is at a completely free length when leaving the factory, once the pipe heats up after installation, the bellows will be forcibly compressed (or stretched) from the free state to absorb the heat displacement. The problem is that the fatigue life of bellows is calculated according to the number of cycles, and it is installed directly in the free state, which is equivalent to overdrawing a part of the displacement quota that should be left for cold state to hot state in advance. For example, if a general-purpose corrugated expansion joint with a design compensation of 100mm is pre-stretched by 50mm before leaving the factory, it will be in a semi-compressed state in the cold state, and then compressed by 50mm in the hot state. If it is not pulled when it leaves the factory, it will be installed directly at the site. When it is hot, it will be compressed by 100mm, the stress of the bellows will be directly doubled, and the life will be reduced by a cliff.
So, what is the factory status of metal expansion joints? It is not "what you receive", but "what the manufacturer should be according to your working conditions". This point is confused, and there are pits behind it.
Several common conditions at the time of leaving the factory, which one do you have
Expansion joints with different working conditions have completely different factory conditions. There are generally four types.
The first, the free state.This is most commonly found in small diameter, low pressure, and little temperature change. For example, some non-metallic expansion joints (fabric fiber expansion joints) used in ventilation ducts have low stiffness, and their absorption displacement mainly depends on structural rotation rather than corrugation stretching, so they don't need pre-deformation when they leave the factory. However, note that the expansion joints of metal bellows, especially the high-temperature axial expansion joints used in power stations and cement industries, will not leave the factory in a free state.
The second is pre-stretched or pre-compressed.This is the most particular category. Straight pipe pressure balance expansion joint, external pressure single axial expansion joint, curved pipe pressure balance expansion joint, etc. Before leaving the factory, the manufacturer will pre-stretch or compress the bellows for a certain distance according to the cold tightness (also called cold drawing) of the pipeline. Pre-stretching is used to compensate for thermal expansion (pipes become longer when heated), and pre-compression is used to compensate for thermal shrinkage (pipes become shorter when cooled, such as cryogenic lines). How much to pull and how much to press are written on the drawings and nameplates. Just work according to the drawings during installation.
The third type, with a limiting device.Many tie rod and hinge expansion joints will be equipped with temporary limit screws or limit blocks when they leave the factory to prevent the bellows from being accidentally stretched, compressed or twisted during transportation and hoisting. For example, compound hinge transverse expansion joint and compound straight pipe bypass pressure balance expansion joint have large transverse displacement. Once the bellows is twisted during transportation, the corrugations will produce plastic deformation, and the whole process will be wasted. The limit device is used to lock the displacement, and then remove it after installation and alignment at the site.
The fourth type, with protective support.Large-diameter thick-walled expansion joints, directly buried (fully buried) expansion joints, such as large self-weight, thick bellows wall and large wave height, will be wrapped with a protective sleeve or support frame on the outer ring of the bellows when leaving the factory to prevent bumps, extrusions and scratches on the corrugated surface during transportation. Because any pit or scratch in the bellows is a potential stress concentration point.
The factory position of the tie rod and nut, whether to dismantle or not
This question has been asked countless times in the background. Every time I rhetorically ask: Have you seen the nameplate?
There are two kinds of tie rods on the expansion joint, one isStructural tie rod(Permanent tie rod), such as the tie rod on the single hinge expansion joint and the large tie rod expansion joint. This set of rods participates in the force and is used to bear the blind plate force generated by the internal pressure of the pipeline. The nut on this tie rod is in a locked state, which is set after leaving the factory. It must not be loosened or disassembled during installation. You remove it, and the pipe is pressed, and the bellows bulge out like a balloon.
The other istransport tie rod(temporary tie rod), which only serves the transportation and hoisting stages. It is usually made of thinner screws or angle steel, with one end welded to the end pipe and the other end locked with a nut to the flange or tube sheet. The role of this tie rod is to maintain the factory length of the bellows while locking in the amount of pre-tension/pre-compression. After installation and alignment, the tie rod must be removed or loosened to the extent that it is completely unstressed. If it is not disassembled, the expansion joint becomes a rigid short joint, which can't absorb any displacement. All the thermal stress of the pipeline is transferred to the equipment and bracket, and the root of the bellows will crack after a few heat cycles.
To judge what kind of tie rod it is, look at two points: first, thickness, structural tie rods are generally thick, and high-strength bolts are used; The second is to see if there is a mark for adjusting the spacing of nuts. The transport tie rod is usually marked with the words "remove after installation" with paint. Really unsure, turn out the factory information, it is clearly written on it.
What records are left behind by factory inspection to be considered qualified
The factory status of the metal expansion joint is not only the physical posture, but also the inspection record. When a qualified expansion joint leaves the factory, these things must be attached with the box: hydraulic test record, air tightness test record (when gas medium is required), stiffness test data and displacement data.
Hydraulic pressure test, generally according to the design pressure of 1.25 times or 1.5 times the water pressure, hold the pressure for 30 minutes without leakage, this is a hard index. Air tightness test is used for pipelines transporting flammable, explosive or toxic media, and the requirements are stricter. Usually, low-pressure air tightness of 0.05~0.1MPa or helium mass spectrometry leak detection is directly performed. The most critical stiffness data are axial stiffness, transverse stiffness and bending stiffness. These three values are directly calculated into pipeline stress analysis. Whether the data is correct or not, and whether the values deviate greatly from the design must be verified on the spot. The displacement data refers to the maximum compensation amount allowed by this expansion joint in all directions, axial, transverse and angular directions. Comparing it with the drawings, you can know whether the manufacturer has cut corners.
After you get this information, don't rush to put it away, check it with the product nameplate. The nominal diameter, design pressure, design temperature, compensation amount and factory number on the nameplate should be aligned with the certificate. If you don't, call back to the manufacturer.
Shipping and storage, which state parameters can be quietly destroyed
There is a saying that "it is good when you leave the factory, but it is bad when you go to the construction site". During the period from the factory to the installation of metal expansion joints, there are three state parameters that are most prone to problems: pre-stretching amount, limiting device and surface protection.
What does the amount of pre-stretch maintain? By the limit rod. However, the practice of many construction sites is that the equipment is thrown in the yard directly in the open air, exposed to the sun and soaked in rain. If the limit tie rod is exposed to humid environment for a long time, the thread will rust, and it can't be removed even if it wants to be removed during installation. Rust is not the worst thing. The worst thing is that someone tries to save trouble by cutting the tie rod directly with gas cutting-that's how the expansion joint is treated as scrap iron.
For surface protection, there is generally stainless steel or spray protective layer on the outside of the bellows, but if the sling is bound at the position of transportation and hoisting without a rubber pad, the wire rope will be directly corrugated, and a strangulation mark will be a crack source. In a dusty environment such as cement plants and power plants, dust accumulates in the trough of corrugated pipes. If it absorbs moisture and chloride ions are enriched, stainless steel corrugated pipes can't withstand pitting corrosion.
Pad sleepers below, above 30cm from the ground, cover with rainproof cloth on top, oil the limit rod regularly to prevent rust, and don't pile any heavy objects on the bellows. There are not many projects that can be done with such a small thing.
On-site acceptance three-minute self-inspection checklist
No nonsense, just give you a set of moves.
- Touching the nameplate:Is there a nameplate? Are the model, pressure, temperature, compensation amount and factory number on the nameplate clear and unaltered? Expansion joint without nameplate, refuse to sign for it.
- Second, look at the ripples:Are there any pits, scratches, welding slag splashes and arc damage on the corrugated surface? Is the corrugation spacing even? If the wave distance is wide or narrow, it means that plastic deformation has occurred during transportation, and the product will be returned.
- Three pairs of limits:Pre-tensioned/pre-compressed expansion joint, is the limit rod intact and the locking nut loose? Is the paint mark on the tie rod clear? If the mark is gone, confirm with the manufacturer what the pre-deformation amount is.
- Quad core records:Are there any factory hydraulic test reports, stiffness data and displacement data? Are the data consistent with the nameplate? Is the stamp on the document complete?
- 5. Check the attachments:Is the deflector installed? Is the direction of the guide tube consistent with the direction of the medium flow (along the direction of the medium flow)? Is the arrow on the nameplate in the same direction as the deflector?
These five steps can be completed in a little more than two minutes. If there is a problem, take photos on the spot, write records, notify the manufacturer, and don't drag it out until the day of installation before unpacking.
What is the factory status of metal expansion joints? It is the pre-deformation position calculated by the manufacturer according to your working conditions, and it is the reference point to ensure the safe operation of the bellows during the life cycle. The first thing you do after receiving the goods is not to rush to hoist them, but to confirm that these "states" are still there.