What exactly do expansion joint metal pads do? Don't think of it as a normal flange spacer
When you choose the expansion joint, do you pay all attention to the material, wave number and compensation amount of the corrugated pipe? Metal mat, do you think "it's just a gasket, just choose a temperature-resistant one"?
I can't do it. The metal gasket of the expansion joint is completely different from the ordinary gasket between the pipe flange. Ordinary flange gasket only needs to fill between two flange surfaces without leakage; The metal pad of the expansion joint has to do a harder job-it has to withstand the shear and extrusion caused by high temperature, high pressure, and repeated displacement at the connection between the bellows and the end pipe, or with the flange. To put it bluntly, it is the most underrated link in the seal chain. Once it fails, the medium runs directly into the corrugated root of the bellows, and a set of corrosion, fatigue and fracture is taken away, and the pipe will be scrapped for you.
Two months ago, a buddy from the power plant complained to me that one of their main steam linesHigh temperature axial expansion jointIt leaked after less than half a year of use. When I removed it, the root of the bellows was corroded like a honeycomb. At the end of the investigation, the crux is that the metal pad material can't match the working temperature, the graphite layer is carbonized in advance, and the seal is broken. You see, gaskets don't do the trick, and expensive expansion joints become consumables.
Common metal pad material combinations: stainless steel, graphite, PTFE... How to build it?
Now on the marketExpansion joint metal padThe typical structure is "metal skeleton + flexible overlay". The skeleton gives strength, and the overlay is responsible for sealing. There are several common combinations:
- Stainless steel corrugated metal pad + flexible graphite: This is the main force of high temperature working conditions. 304 or 316L stainless steel corrugated tooth matrix, covered with a layer of high purity graphite. The temperature resistance can reach 650°C (graphite will be lost in oxidation atmosphere, but it can be held in a short time), and the rebound rate is high, which is suitable for steam, hot oil and flue gas pipelines.
- Stainless steel metal winding pad + graphite/PTFE: Winding pads are more used in the flange connection of general-purpose corrugated expansion joints. V-shaped or W-shaped stainless steel tape and fill tape are alternately wound for stable compression rebound performance. However, it should be noted that the winding pad requires high flange surface finish, and it is easy to leak if there is a slight scratch.
- Stainless steel metal toothed pad + PTFE (PTFE): Standard for anti-corrosion scenarios. Such as PTFE-lined pipes orPTFE compensatorWhen matching, the gasket body is made of stainless steel, and the tooth surface is covered with a layer of modified PTFE, which is resistant to chemical corrosion, but the temperature resistance can only reach about 260°C. PTFE will soften and creep at high temperature.
- Nickel-based alloy (Inconel 625/600) + mica: Extreme high temperatures (> 800°C) or corrosive environments containing chloride ions, such as flue gas pipelines for waste incineration power generation. Mica layer has high temperature resistance, but is prone to brittleness and cracking, which requires a metal skeleton to provide adequate elastic compensation.
There is no universal solution to material matching. You have to choose according to your own medium (oxidizing? Reducing? Sulfur?), temperature peak, and pressure fluctuation amplitude. Don't listen to the manufacturer saying "this is general" to place an order.
Selection and rollover site: How to weigh hardness and compression ratio? What should I do if the gasket is deformed under high temperature and pressure?
chooseExpansion joint metal padThe two most troublesome parameters: hardness (expressed by Brinell or Rockwell) and compressibility (the percentage of thickness reduction of the gasket under preload).
The hardness is too high, such as an all-solid metal pad (such as a pure stainless steel flat pad), and it can't be pressed. The flange must provide great bolt preloading force to fill the microscopic unevenness. However, when the preloading force is large, the flange may be deformed, and the expansion joint end tube may also twist. The hardness is too low, such as pure graphite pad, and the creep relaxation is too severe. After a few days of running, the preloading force drops out and begins to leak.
It is reasonable to choose a "flexible metal pad"-the metal skeleton has appropriate tooth or wave shapes and the surface is covered with a soft seal layer. In this way, the compression ratio is controlled between 15% and 25%, which can be compacted with reasonable bolt torque and will not collapse under high temperature.
What about gasket deformation under high temperature and high pressure? Here are two data references:
- When the temperature exceeds 400°C, the yield strength of 304 stainless steel decreases by more than 30%. If the design thickness of the gasket skeleton is not enough, the wave teeth will be flattened and lose the rebound ability.
- When the pressure exceeds 2.5MPa, it is recommended to choose a metal wound pad with an inner ring or an outer ring. The inner ring can prevent the gasket from bursting inward, and the outer ring limits excessive compression.
Don't just look at the material grade, get what the manufacturer providesCompression-rebound curveAndHigh temperature relaxation rateData. If you can't give these, just pass.
Install that thing: the pre-tightening force is not enough will leak, the gasket is waste when screwed too hard, and the pull rod adjustment should be paid attention to
loadExpansion joint metal padIt's a craft job. Two days ago, a master in a chemical factory asked me, "Is it enough to hit the torque wrench to 550Nm?" I asked him back, "What size is your flange bolt? What is the outer diameter of the gasket?" He couldn't answer.
The preload force is not enough, and the stress of the gasket relaxes after a few hours of operation and begins to leak. Twist too hard? The metal skeleton is pressed to plastic deformation, the corrugated teeth fail directly, and the sealing layer is extruded-the gasket is completely wasted. These two extremes meet every year.
According to the gasket manufacturerInstallation Stress RangeCalculate the torque and use a torque wrench to tighten diagonally in three times. Measure the bolt elongation after each screwing (with ultrasonic or caliper) to ensure that each bolt is evenly stressed.
There is another point that is often overlooked —expansion joint tie rod。 Many expansion joints, such asCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint, the tie rod nut is locked when it leaves the factory to prevent the bellows from being stretched during transportation. When installing the metal pad, remember to loosen the tie rod first to allow the bellows to be in a free state before installing the gasket. Otherwise, when you tighten the flange bolts, the bellows will be dragged by the pull rod, and the gasket will be unevenly stressed. It will be strange that it will not leak while crushing to death.
How to adjust the tie rod? Refer to the article "How to Adjust the Tie Rod Nut of Expansion Joint" we wrote before. The core principle is that the tie rod nut cannot be locked after adjustment, and the thermal displacement margin should be left.
Real case: Gasket failure scrapped the high-temperature axial expansion joint. Where is the root cause?
Last year, a blast furnace hot air duct (temperature about 950°C, pressure 0.6MPa) was used in a steel plant in the south of ChinaHigh temperature axial expansion joint, suddenly a lot of smoke after a year of normal operation. After stopping the furnace for inspection, it was found that the graphite layer of the metal pad at the connection between the bellows and the end pipe had been burned out, and the metal skeleton was covered with oxide scale, losing its sealing ability. The high-temperature flue gas directly washes the root of the bellows, and the stainless steel is corroded out of the deep pit by sulfur, which finally leads to the thinning of the wall thickness of the bellows to less than 1mm, and the whole expansion joint is scrapped.
When selecting the gasket, the technician chose a stainless steel + graphite combination pad with a nominal temperature resistance of 600°C, thinking that "although the hot air temperature is high, the gasket is inside the flange, and the actual temperature will be lower". However, in fact, the external insulation of the hot air duct was not done well, the temperature at the gasket exceeded 700°C, and the graphite ablated rapidly under the oxidation atmosphere. What's more fatal is that the gasket is a flat gasket structure, with no wavy teeth and no secondary sealing ability at all.
What's the lesson?Expansion joint metal padA margin of at least 100°C must be left for the temperature resistance rating of the gasket, and a heat insulating liner must be provided on the outer ring of the gasket. In addition, for ultra-high temperature working conditions, don't save that cost, and go directly to the configuration of nickel-based alloy + mica.
When communicating with manufacturers, what three questions should you ask about metal pads
The next time you call a supplier for a quote, don't come up and ask "how much". Asking these three questions clearly can filter out 80% of unreliable manufacturers:
First question: What is the compression rate data of your metal pads? Is it measured at high temperature or room temperature?
Many manufacturers fool people with the normal temperature curve. At high temperatures, the sealing layer will soften and the compression rate will drift. You must look at the measured data under high temperature conditions.
Second question: Do you have experimental data on the mode and life expectancy of gasket failure?
Good manufacturers will do the aging test of the leakage rate-bolt stress-temperature cycle. Without this data, that's a pat on the head.
Third question: If my expansion joint is with a tie rod, what should be done with the tie rod when the metal pad is installed?
If you can answer clearly "loosen first, install then adjust", it means that they know the scene.
The metal pad may seem unremarkable, but it is the last line of defense for the expansion joint seal. Choose the right one, and the pipeline will run safely and steadily for ten years; Choose wrong, and the downtime loss on one line is enough for you to buy a hundred sets of gaskets. Tell me, should this money be spent clearly?