Inspection Basis: What exactly does JB/T 12235-2015 say?
When many old iron get non-metallic compensators, their first reaction is to peel open the fabric layer to see if there are any holes and feel whether the sealing surface is flat. Appearance is important, of course, but if you only do this, the device will have to show you color sooner or later. The core that really determines the life of the compensator is the national standardJB/T 12235-2015That complete inspection system. This standard not only stipulates the requirements of materials, design and manufacturing, but more importantly, it gives the "ghost gate" that must be passed before leaving the factory-airtightness, compressive strength and displacement compensation ability, all of which must be missing. You didn't test according to the standard? Then don't shout if something happens.
JB/T 12235-2015 divides non-metallic expansion joints (that is, we often call non-metallic compensators) into two categories: rectangular and circular, each of which has corresponding test methods. For example, the air tightness test clearly requires that 1.5 times the design pressure be kept for 5 minutes, and the pressure drop does not exceed 5% before it is qualified. This is not a number set by slapping the forehead, but behind it is a summary of a large number of failure cases. Therefore, when you accept, don't just look at the certificate given by the manufacturer, you have to sample it yourself, or ask a third party to report.
Appearance inspection: fabric layers, sealing layers, metal frames, which pits are most common?
Appearance inspection is a basic skill, but many people take a look. Let's take it apart and say:
- Fabric layer (body of non-metallic expansion joint): The most common defects are localized wrinkles, scratches, or loose fibers. Think about it, once there is a crack in the fabric layer, as soon as the high-temperature smoke rushes, the internal thermal insulation cotton will immediately carbonize, and the sealing layer will be finished. Therefore, when inspecting, you have to use a flashlight to shine from the inside out, especially at the corner of the weld-where stress is concentrated and it is easiest to fluff.
- Sealing layer (PTFE or rubber PTFE composite layer): If rubber PTFE compensator or PTFE compensator is used, focus on whether there are bubbles, cracks and delamination. Once the sealing layer leaks, the whole compensator is a chimney, and the air tightness test will definitely hang later. In addition, note that there should be no hollow drum between the sealing layer and the fabric layer, which can be felt by pressing it with your hand.
- Metal frames (flanges, plates, ribs): The frame is generally made of Q235B or stainless steel, common problems include welding splashes, unpolished burrs, and eccentric bolt holes. Especially flange flatness, the national standard requirements should not exceed 2mm/m. You take the ruler and lean against it. If the gap is large, it will be returned immediately. Otherwise, when the bolts are tightened after installation, the frame will be deformed and the fabric layer will be twisted.
Two days ago, a customer bought a batch of rectangular non-metallic expansion joints for use in desulfurization flue. They looked beautiful in appearance, but they leaked after three months of installation. After checking, it turned out that the sealing layer had micropores before leaving the factory, but it was not found at that time. Therefore, the appearance inspection can't only look at "whether there are holes", but also use penetrant or phosphor to assist in detection, which many masters ignore.
Air tightness and withstand pressure test: leak or not, let the data speak for itself
To put it bluntly, no matter how sharp your eyes are, you can't see micron-scale cracks. Air tightness and pressure resistance tests are the hard indicators. How to measure? According to JB/T 12235-2015:
First do the pressure test-install the compensator on the test bench, slowly pressurize it to 1.5 times of the design pressure, hold it for 5 minutes, and observe whether there is obvious deformation, abnormal noise or weld cracking. Note that the pressure should rise slowly, don't go up at once. The rigidity of non-metallic compensator is not as strong as that of metal corrugated expansion joint, and the impact pressure may crack the fabric layer.
Then there's the airtightness test. Reduce pressure to design pressure and apply soapy water or leak detector to all possible leakage points-flange sealing surfaces, fabric layer and frame joints, around bolt holes. If continuous bubbles appear, it is directly judged as unqualified. If conditions permit, it is recommended to use helium for leak detection, which is more sensitive. Many power plants are interested inDesulfurization flue gas baffle doorThis is how the compensator next to it checks. After all, there is corrosive gas in the smoke, and it can't stand the leakage at all.
The holding time of airtightness test is 5 minutes in the standard, but in actual engineering, some large-size rectangular expansion joints (such as rectangular non-metallic expansion joints of 3m ×4m) have great structural flexibility, and the holding time can be appropriately extended to 10 minutes, so that the stress of the whole system can be relaxed and stabilized before reading. This is not over the standard, but practical experience.
Verification of displacement compensation ability: axial, angular and transverse, how to measure it reliably?
The core value of non-metallic compensators is to absorb displacement. But when you bought it, the manufacturer's nominal "50mm in axial direction and 5° in angular direction" can actually be achieved? Must be verified before leaving the factory. The test method is not complicated: use a special displacement test bench, fix the compensator first, and then apply axial compression/tension, angular deflection and transverse misalignment respectively. Note that each direction should be measured separately, and don't think about completing the test at once, which will interfere with each other.
Axial displacement: Apply slowly with a jack, measure the actual displacement, and observe whether the fabric layer has obvious wrinkles or tension. If the fabric layer "bulges" after the displacement is in place, it means that the inner skeleton or insulation cotton is not filled evenly, which is a manufacturing defect.
Angular displacement: Fix one end of the compensator, rotate the other end with a hinge mechanism, and record it with an angle meter. The actual angle that can be turned is often 10%-20% smaller than the theoretical value. Why? Because when the fabric layer is deformed in the angular direction, the force on the edge is uneven, which is easy to produce local threshold effect. Therefore, it must be measured when inspecting the goods, and don't believe the theoretical value on the sample.
Lateral displacement: This one is the easiest to fake. Some small factories push it directly by hand, and it feels almost over. The correct thing to do is to adjust precisely with a screw, a dial indicator reading, and at the same time check the metal frame for scratching the fabric layer-the platen edge may cut the cloth like a knife when displaced laterally.
After the displacement test, the airtightness test has to be done again, because the original seal state may be destroyed after deformation. Think you, is that the truth?
Review and daily inspection after installation, don't wait for something to remember
The compensator is piped, not once and for all. Three things must be reviewed as soon as the installation is complete:
- Installation direction: Non-metallic compensators have arrows to indicate the flow direction. Installation backwards will lead to the failure of the internal baffle or guide tube, and high-temperature gas directly washes the fabric layer. In particular, the corrugated expansion joints used in the power station industry-no, non-metallic expansion joints-also have directional requirements, and they will be perforated within a few months after being installed backwards.
- Bolt tightening torque: The tighter the flange bolt is not the better. If the moment is too large, the pressure plate will be deformed, resulting in the stress concentration at the edge of the fabric layer; If the torque is too small, the sealing layer will leak. It is recommended to use a torque wrench, and check the standard torque table according to the bolt specifications, such as the M16 bolt, grade 8.8, with a torque of about 200N·m.
- Cold tight pre-displacement: Many pipes are installed at room temperature, but they expand at high temperatures during operation. Therefore, when designing, the compensator will be required to preload or preload a certain amount in the installed state. Is this the right amount? You have to measure it with a ruler, not by your feeling.
What about daily inspections? At least once a month. Focus on whether there is ash, oil, or condensation on the surface of the fabric layer-if so, it means that the sealing layer has failed and the interior may have leaked. Plus, listen to the sound. If you hear a "hissing" sound during running, it probably leaks. Use the back of your hand close to the weld and sealing surface to feel if there is any airflow. In windy weather, you have to check whether the metal frame bolts are loose. Don't be too troublesome. There was a cement factory two years ago. Because of the sloppy inspection, the compensator had a hole, the smoke flowed backwards, and the whole production line stopped for three days.
Replacement and scrapping criteria, don't save that little money
When should I change? It's not that you change it when it doesn't look good to the eye, or that you change it when it leaks. According to industry experience, the following situations are directly judged to be scrapped:
- Penetrating fracture or ablation depth exceeding 1/3 of the thickness of the fabric layer。 Don't think about local repairs. Once the temperature resistant layer is destroyed, the patch won't last long.
- The sealing layer is aged and cracked, or a bulge with a diameter greater than 5mm appears。 The bulge indicates that the internal medium pressure has debonded the sealing layer and may explode at any time.
- Metal frame deformation exceeds 10% of mounting clearance, or cracks appear in welds。 Once the frame is deformed, the displacement capacity of the compensator will be messed up, and the adjacent pipeline may be broken.
- Air tightness test failed。 As I said just now, if the pressure drop exceeds 5%, change it directly. Don't make do.
In addition, if the compensator is used in a corrosive environment (such as desulfurization system and the back section of the flue gas baffle door), it is recommended to change a batch every two years, so as not to save thousands of dollars. The cost of your one replacement can't afford the loss of one day of discontinuation. How is it? As brutal as it sounds, that's the value for money.
Inspection requirements for non-metallic compensatorsIn this matter, the standard is there, and the key is implementation. Don't wait for an accident to turn over the national standard. That's called making up for the situation. It is not a dream to make the inspection solid at ordinary times and run the equipment for five more years.