Typical manifestations and hazards of non-metallic expansion joint leakage-what signals are telling you it can't hold up?
Non-metallic expansion joints (also called fabric fiber expansion joints in the industry) are widely used in power plants, cement plants and chemical pipelines, especially when high-temperature corrosive media such as flue gas pipelines, air ducts and desulfurization systems flow through. If this thing leaks, the most typical signals are three:Running ashes, smoke, dripping water。 When you inspect, you see dust floating out of the flange connection, or smoke running out along the edge of the compensator, or a pool of acid water on the ground-directly indicating that the sealing layer has broken.
A more serious situation is when the device vibrates suddenly. The non-metallic expansion joint has the function of vibration isolation. If it produces abnormal jitter during operation, there is a high probability that the skin is torn or the internal insulation cotton is blown away, resulting in a sudden change in stiffness. At this time, don't think of "lasting for a few more days". Once the leakage expands, the high-temperature flue gas (such as the sulfur-containing flue gas above 120℃ at the desulfurization inlet of the power plant) directly washes the metal frame, which will burn through the whole guide tube and end structure. And guess what? I have seen a rectangular non-metallic expansion joint at the kiln tail of a cement plant. Because it was not dealt with in time, the leakage caused the surrounding cable trays to burn, and it was shut down for three full days.
The harm goes beyond discontinuation. If the leaked medium is acidic (such as the raw flue gas in front of the desulfurization tower), it will corrode the metal hose and pipe support next to it, and even threaten the safety of operators. So don't wait for the naked eye to see the big hole before you start. Those inconspicuous "small spray ash" are early warning signals.
Five major causes of leakage: material aging, installation deviation, medium corrosion, overtemperature and overpressure and structural design defects
The reasons for the leakage of non-metallic expansion joints are nothing more than these five categories, but each category has its own way.
1. Material aging-the unavoidable life cycle
The skin of non-metallic expansion joint is usually laminated by fluororubber, silicone rubber, polytetrafluoroethylene, etc., and reinforced by glass fiber cloth and ceramic fiber. These polymer materials will slowly become brittle and crack under ultraviolet light, temperature cycle and humidity. Generally, the design life is 3~5 years, but in fact, many factories have not changed it after 6 years-can it not leak? Especially if it is installed outdoors, wind and sun exposure, the appearance of turtle cracks on the skin surface is the ironclad evidence of aging.
2. Installation deviation-many leaks are actually man-made
During installation, the pipe is not well aligned, and the expansion joint is forcibly screwed on, resulting in the skin in a twisted or stretched state. Or forget to remove the transport fixing screw (this problem occurs repeatedly), resulting in the expansion joint failing to expand and contract properly, and the stress concentration is torn somewhere. There is also a common mistake: the deflector is installed in the opposite direction. The function of the expansion joint guide tube is to protect the internal thermal insulation cotton from being washed by the airflow. If the direction of the arrow is wrong, the medium will blow directly on the inner wall of the skin, and it will be worn out in a few months.
3. Media corrosion-the nightmare of acids, bases and salts
The non-metallic expansion joint behind the desulfurization flue gas baffle door has been exposed to wet flue gas containing sulfuric acid and sulfurous acid for a long time. If the skin is not made of corrosion-resistant polytetrafluoroethylene compensator material or is not lined with PTFE layer, the result will be chemical corrosion leading to powdering and perforation of the sealing layer. Similarly, alkaline dust at the end of the kiln in a cement plant can also corrode the fiberglass layer.
4. Overtemperature and overpressure-scrapped in an instant
Non-metallic expansion joints have definite temperature grades (for example, the conventional type can withstand 200℃, and the high-temperature type can reach 600℃ for ceramic fibers). However, under accident conditions (such as the sudden rise of wind pressure and deflagration of furnace caused by the failure of fan frequency converter), the instantaneous temperature or pressure exceeds the design value, and the skin bursts directly. This is irreversible damage and must be replaced in its entirety.
5. Structural design defects-not considered in the selection
Some equipment selection only considers axial compensation and ignores lateral or angular displacement. Or circular non-metallic expansion joints are used on rectangular pipes, resulting in local stress concentration. There is also insufficient frame stiffness, excessive deformation during operation, and the skin is stretched and torn. To put it bluntly, the design itself planted the seeds of leakage without finding the right matching rectangular non-metallic expansion joint or high-temperature axial expansion joint.
Three-step method for leak diagnosis: Application of visual inspection, pressure testing and infrared thermography
How to confirm after finding signs of leakage? Don't dismantle it as soon as it comes up-follow these three steps first.
The first step, visual inspection.The easiest and most effective. Use a flashlight to see if there are cracks, bulging and discoloration on the surface of the skin. Touch the edge of the flange with your hand (pay attention to anti-scalding) to see if there are wet stains or dust. This inspection is done at least once a week, especially before and after shutdown for overhaul. But there are blind spots in sight: for example, the leakage in the mezzanine can't be seen outside.
The second step is the pressure test (airtightness test).If hidden leakage is suspected, low-pressure compressed air (such as 0.05~0.1MPa) can be introduced into the pipeline, and then soapy water can be sprayed on the surface of the expansion joint. Where there are bubbles is the leak point. Note: For running pipelines, this method is only suitable for out-of-operation, not under pressure.
The third step, infrared thermal imaging.This technology is becoming more and more popular now. When the non-metallic expansion joint leaks, if there is a difference between the medium temperature and the ambient temperature (for example, the leakage is high-temperature flue gas or low-temperature condensed water), the infrared thermal imaging camera can directly display the abnormal temperature zone. Two days ago, I met a user of a thermal power plant, and I found a tiny perforation in the skin by thermal imaging-it was completely invisible, but the bright spot in the temperature field was too obvious. Save the hassle of disassembly and downtime.
These three steps are not all used every time. Look at the scene: visual inspection, shutdown maintenance for airtightness, and thermal imaging of difficult and complicated diseases.
Treatment schemes for different leakage scenarios: local repair, overall replacement or upgrade selection?
What to do after discovering a leak? Don't be one size fits all, it depends on the degree of damage and working conditions.
Scenario 1: Small cracks and slight trachoma on the skin surface.Such as perforations less than 5 mm in diameter, or cracks no more than 1/3 thick in depth. This situation can be temporarily sealed with high-temperature silicone rubber sealant, or partially repaired with a patch of the same material (with special adhesive). But remember, this is only an emergency measure, which can last at most one maintenance cycle. It must be recorded and replaced as a whole the next time it stops.
Scenario 2: Large-scale skin aging, multi-layer tearing, and loss of thermal insulation cotton.Needless to say, just replace the entire non-metal expansion joint. Note when selecting: To check the original design parameters, it is best to use the same brand or the same grade products, otherwise the stiffness and compensation amount of the expansion joint will not match, and it will still leak after installation. Special reminder: When dismantling the old and replacing the new, be sure to check whether the metal frame is corroded or deformed-if the frame is rusty, it is useless to change the skin, and you have to change it together with the frame.
Scenario 3: Repeated leakage, and the reason points to improper selection.For example, in the case of high-temperature acid flue gas, the ordinary silicone rubber skin is changed three times a year. Don't fix it at this time, upgrade and select the model. Use PTFE compensator liner instead, and add multi-layer ceramic fiber + stainless steel wire mesh reinforcement layer. Or replace the original circular expansion joint with a rectangular non-metallic expansion joint, the one with a guide tube and drain hole. It is a little more expensive, but it can quadruple its lifespan. You do the math – which saves money, changing it every three years vs changing it three times a year?
Scenario 4: Burst caused by overtemperature and overpressure.Such accidents often damage adjacent metal bellows or smoke baffle doors at the same time. The treatment plan is: first cut off the medium, repair the pipeline system, then replace all damaged components uniformly, and add overpressure protection devices (such as burst discs) upstream. Don't just change the expansion joint. If the root cause is not solved, it will explode next time.
Preventing Leakage from Source: Selection and Matching, Installation Specification and Scientific Setting of Inspection Cycle
Instead of waiting for the leak to be repaired, avoid the pit from the beginning. Prevention is in place, and non-metallic expansion joints can be used for five or six years without any problem.
Selection matching is the first step.First, find out the pipeline media composition (temperature, pressure, corrosiveness, particle content), compensation amount (axial, transverse, angular) and pipeline layout. For example, the dusty hot air duct at the outlet of a coal mill in a power plant is preferably a wear-resistant PTFE-lined hose or a non-metallic expansion joint with a wear-resistant guide tube. The outlet of cement grate cooler is high temperature and contains alkali, so it is necessary to use high temperature axial expansion joint or ceramic fiber temperature-resistant type. Don't buy standard parts cheaply, customization is the right way.
Installation specifications must be strictly grasped.Is the transport screw disassembled or not? Is the guide tube in the right direction? Is the tightening torque of flange bolts according to the requirements of the manufacturer (usually with a torque wrench, symmetrically twisted turn by turn)? Is the amount of pre-compression in cold drawing of pipeline sufficient? A technical briefing meeting is held before these details are installed, and the manufacturer's instructions are printed out and posted on the site. Many times leakage is less than two turns of bolts or forgetting to remove the screw. Do you say it is wrong?
The inspection cycle should be scientific.It doesn't mean "watch it once a week" is over. It is recommended to be divided into three levels: daily inspection (listening, watching and smelling every day in the operation post), professional inspection (the equipment staff scans it once a week with thermal imaging), and shutdown for maintenance (performing air tightness test for each overhaul and recording the remaining life of the expansion joint). By accumulating the data, the aging curve of the same expansion joint can be drawn, and it is clear when to change it.
In addition, if there is a metal hose or corrugated expansion joint next to the non-metallic expansion joint, it should also be checked simultaneously. They are an integral part of the entire compensation system, and whichever breaks down first will implicate the entire pipe system.
80% of the leakage of non-metallic expansion joints can be avoided through early selection and daily management. Don't wait for the smoke to remember to replace it-the downtime losses are more than ten times more expensive than the equipment itself.