The expansion joint is a "small part" in the pipeline system, but once it strikes, the whole line has to stop. How to judge how old it is to the extent that it should be replaced? Don't wait for it to leak before panicking. Everyone in this industry should know the following methods of judgment.
1. Why do expansion joints age? Understand the aging mechanism first
Expansion joint aging is not metaphysics,Physical and chemical factorsEach and a half. Metals-such as general-purpose corrugated expansion joints and high-temperature axial expansion joints-mainly carry fatigue and corrosion. Repeated action of alternating stress, coupled with chloride ions and sulfide in the medium, cracks will occur in the bellows over time. Non-metallic ones are more troublesome, such as non-metallic expansion joints (fabric fiber expansion joints) and rubber compensators. Ultraviolet rays, ozone and temperature will make rubber hard and fabric fibers delaminate. To put it bluntly, high temperature, high pressure and corrosive media are three "killers". Only by accelerating aging and figuring out these can the judgment have a basis.
The desulfurization flue gas baffle door used in power plant desulfurization system, the non-metallic expansion joint next to it, the medium temperature is high and the acid condensate is also present, and the life time is often less than half of the design value. This is the result of the acceleration of dielectric corrosion superposition temperature.
2. Aging signals visible to the naked eye
No matter how professional the testing is, the first step is always to look. Rust pits on the surface of the metal expansion joint and fine cracks at the trough of the bellows are typical signals. Non-metallic expansion joints are more pronounced: cracked, sticky, bulging surfaces, or layered blistering of fabric layers. In addition, pay attention to whether there is any abnormal displacement in the installation position, such as the deformation of the guide tube and the loosening of the tie rod nut (how to adjust the specific question and answer on how to adjust the tie rod nut of the expansion joint), which may mean that the internal structure can no longer hold up.
Two days ago, a customer reported that the non-metallic expansion joint next to the flue gas baffle door had a large area of delamination in three months, and when it was disassembled, it was seen that the fibers were carbonized. Did you say it was miserable? This kind of thing can be detected by the naked eye, and there is no need to wait for the detection data.
For directly buried (fully buried) expansion joints, the surface cannot be seen, but it can be checked whether there is abnormal uplift or settlement on the ground, and indirectly judge whether the interior fails.
3. Performance data will not lie: measure stiffness and displacement
If your eyes can't see correctly, talk to the data. Once the stiffness of the expansion joint (refer to the article on the stiffness and calculation formula of the bellows) changes by more than 20%, it will basically fail. How to measure? Measure axial stiffness with a simple loading device or directly compare factory reports. The displacement compensation ability is also a hard indicator-if the expansion joint cannot be restored to its original position in actual operation, it means that the bellows has undergone plastic deformation.
The corrugated expansion joints and compound hinge transverse expansion joints used in the power station industry should be regularly tested for displacement rebound. Don't be too troublesome, you can screen out 80% of the hidden dangers with one test. And guess what? Some factories are too troublesome, and after three years of accident, the bellows got stuck, the stress of the whole pipeline exceeded the standard, and the brackets were crooked.
There is also the system stress exceeding the standard caused by stiffness attenuation, which can be known by measuring the stress of the pipe support with a strain gauge. Don't wait for the bracket to deform before checking, then the cost will double.
4. Key points of aging judgment of different types of expansion joints
Different materials and structures have different "dead points" of aging. Don't look for non-metals with the one you use for metals.
- Metal expansion joint(Including direct burial type, straight pipe pressure balance type, curved pipe pressure balance type, etc.): Focus on the thinning of bellows trough thickness,Ultrasonic thickness gauge is the most accurate。 Large diameter thick wall expansion joints also have to check the weld parts, especially the tie rod joints.
- Rubber compensator and rubber PTFE compensator: The hardness tester brushes and brushes, and it is basically wasted if the hardness increases by more than 20 degrees; Press again to see if there is any elastic restoring force. After the rubber is aged, it becomes hard and can't compensate for the displacement, so the hard brace will crack.
- Non-metallic expansion joints (fabric fiber expansion joints, rectangular non-metallic expansion joints): Pull a piece of fiber to see the strength, or use water pressure to check the sealing performance. The non-metallic expansion joint matched by the flue baffle door is even worse. It runs in high-temperature flue for one year, and the aging rate is three times faster than that indoors.
- Special scenario: Desulfurization flue gas baffle door, air-cooled island vacuum pipeline double hinge expansion joint, these should consider medium corrosion and vacuum fatigue, and it is easy to miss judgment by appearance alone. The fine cracks caused by vacuum fatigue can't be seen clearly with the naked eye, so they have to be tested by penetration.
In addition, the rotary compensator, sleeve-type pipe expansion joint and other poor maintainability, once aging, the repair cost is higher than replacement. So judge more decisively.
5. Break when you break: When should you change instead of repair?
If you find aging, don't always think about making do. Several hard indicators are well held:
- The crack depth of the metal expansion joint exceeds the wall thickness by 10%;
- The leakage test of the non-metallic expansion joint is unqualified, or the damaged area of the fabric layer is more than 10%;
- The stiffness attenuation causes the system stress to exceed the standard (it will be known by measuring the force of the pipeline support with a strain gauge);
- The surface crack depth of the rubber component exceeds 2mm.
At this time, don't worry about "how long it can be used", and go directly to the manufacturer to replace it. For example, rotary compensator, sleeve-type pipe expansion joint, maintenance requires dismantling the pipeline, and the labor cost is more expensive than new parts. Remember, the life of expansion joints is usually only 60%-80% of the design life (refer to the Q&A on the service life of expansion joints), so making spare parts plans in advance is the way to save money. Don't wait until the middle of the night to make a call in a hurry. At that time, doubling the spot price will delay the construction period.
The core of the aging judgment method of expansion joint is "eye diligence, hand diligence and data diligence". Regular inspections and records are better than anything else.