1. Why insulate the metal expansion joint? Not just to save energy
Many people think that the insulation layer for the pipe is just to save some steam money. However, in the metal expansion joint, the significance of heat preservation is far more than energy saving. If you think about it, the core function of the expansion joint is to absorb the thermal displacement of the pipe-the bellows has to expand and contract freely to work. If the temperature is too high, the creep strength of the bellows material (such as 304 or 316L stainless steel) will drop dramatically. For example, when 304 stainless steel is at 550℃, the allowable stress is only about 1/3 of the normal temperature. At this time, the heat insulation is not in place, and the bellows may be fatigued and cracked in advance before the design life.
A more insidious problem is condensation corrosion. If the insulation layer of steam pipe is damaged, the local temperature drops below the dew point, and the water vapor condenses into acidic water, which directly corrodes the bellows wall. I have seen a corrugated expansion joint used in a power station industry in a power plant. Because the outer iron sheet of thermal insulation was not sealed well, the rain seeped in and stuffed, and it rusted out in two years. Therefore, insulation is not only to save coal and gas, but also to protect the expansion joint from heat damage and chemical corrosion. To put it bluntly, if the insulation is not done well, it is only a matter of time before the expansion joint is scrapped.
Second, how to choose insulation materials? Don't let the heat "burn through" your protective layer
When choosing insulation materials, first look at the temperature resistance grade. Don't listen to the manufacturer's bragging that "composite silicate felt can withstand 1000℃", you have to leave an allowance in actual working conditions. For steam pipes (usually ≤350℃), rock wool or aluminum silicate fiber wool is enough, with a density of more than 120kg/m³. However, if it is flue gas and high-temperature gas, such as the kiln tail flue gas connected by the metal corrugated expansion joint in the cement industry, the temperature can reach 600~900℃. At this time, ceramic fiber blanket or aerogel felt must be used. Aerogels have low thermal conductivity and are three times more insulating than rock wool at the same thickness, but they are expensive-you have to calculate the full life cycle cost.
In addition, it should be noted that the insulation material cannot contain chloride ions that are harmful to stainless steel. The chlorine content of some inferior rock wool exceeds the standard, and chloride will precipitate when exposed to water, which will lead to stress corrosion cracking of bellows. I recommend that the chloride ion content test report be required at the time of purchase (standard general requirement ≤25ppm). Let's talk about the actual details: the thicker the thickness of the insulation layer, the better. After the thickness exceeds the critical point, the outer layer dissipates heat and increases the cost. Generally, 100~150mm for steam pipeline and 200~250mm for high-temperature flue gas. When you are not sure, you can find the manufacturer to calculate the heat loss. Don't slap your head.
Third, the easiest pit to step on during construction: the wave section, guide tube and tie rod can't be wrapped
This is the core problem of metal expansion joint insulation. Many people directly wrap the thermal insulation cotton on the bellows, and then pack it with iron sheet-it's done, and the expansion joint is directly wasted. Why? Because the axial expansion and contraction and lateral displacement of the bellows require space. If the insulation layer is stuck between the peaks and valleys, once the pipeline expands thermally, the bellows will be hardly supported by the insulation material, which will reduce the compensation amount at the light and squeeze the corrugated shell into deformation at the worst.
Correct practice: There must be an air gap between the insulation layer and the bellows.How to do it specifically? First, make an independent layer of detachable thermal insulation box (also called thermal insulation cover) outside the expansion joint, make the box body with stainless steel sheet, fill the inside with thermal insulation cotton, and keep a gap of 50~80mm between the box body and the outer wall of the bellows. This insulates without restricting movement. Don't forget the deflector-it is installed inside the expansion joint and is responsible for deflecting the medium and protecting the bellows. The guide tube itself does not need heat insulation, but during construction, do not stuff the heat insulation material into the annular gap between the guide tube and the bellows, otherwise the guide tube will deform, the medium vortex will intensify, and even cause vibration.
And tie rods and nuts. The tie rod on the general corrugated expansion joint and the large tie rod expansion joint is used to bear the internal pressure thrust and cannot be wrapped in the insulation layer. Because the tie rod needs to be checked regularly for looseness (to see if the nut is displaced), if the package is dead, the inspector can't see it at all. The correct way is to expose the pull rod and adjustment nut, and the insulation box only covers the main section of the bellows. Similarly, the reinforcing ribs on the rectangular expansion joint must also leave a heat dissipation gap.
4. What should I do if the insulation layer fails? Look at these three signals for daily inspection
The insulation layer will not break suddenly, and it will wear out slowly. Keep an eye on three points in daily inspection:
- Abnormal increase in surface temperature:Sweep the outer wall of the insulation box with an infrared temperature measuring gun. If the temperature somewhere is more than 20℃ higher than the adjacent temperature, it means that the insulation cotton has fallen off or water has entered. Especially the corrugated section of the high-temperature axial expansion joint is most prone to hot spots.
- Deformation or bulging of outer sheet:If the stainless steel plate used in the insulation box bulges, water may accumulate in it. The expansion of water vapor under heat will push the iron sheet up and aggravate corrosion at the same time. Seeing this situation, quickly take it apart to check whether the insulation cotton is wet.
- Insulation box fixing loose:The expansion joint vibrates when it runs, and the hoops and bolts will slowly loosen. During the inspection, push the insulation box with your hand. If the shaking is obvious, tighten it again.
Expansion joints on steam pipes, recommended once a month; High-temperature flue gas (such as near the desulfurization flue gas baffle door) is best seen every Monday because of the high risk of corrosion. If the insulation layer is found to be damaged, don't drag it, and replace it immediately when the machine is shut down for maintenance. Otherwise, the little labor cost you save won't be enough to pay for an expansion joint.
5. Comparison of insulation schemes under different working conditions: steam, flue gas and high-temperature gas are different
The insulation strategies of different media vary greatly. Let's go through them one by one:
Steam pipeline:The medium temperature is generally 250~400 ℃, and the pressure is medium and low. Insulation scheme recommended: rock wool + stainless steel insulation box, thickness 100~150mm. Note that the steam pipeline has hydrophobic requirements, and a drainage hole (a small hole of Φ 6mm is enough) should be left at the bottom of the insulation box to avoid the accumulation of condensed water. If the external pressure single axial expansion joint is used, the external pressure cylinder also needs to be insulated, but the guide sleeve should be ensured to slide freely.
Flue gas pipeline:Flue gas baffle doors and expansion joints commonly found in power stations and cement industries. The flue gas temperature fluctuates greatly (200~700℃), and contains sulfur and dust. The insulation material must be acid-resistant ceramic fiber with a thickness of more than 200mm. It is recommended to use a double-layer structure during construction: the inner layer is high-temperature resistant ceramic fiber blanket, and the outer layer is waterproof aerogel felt. Non-metallic expansion joints (fabric fiber expansion joints) that are in direct contact with smoke usually do not need external insulation, but rely on their own multilayer insulation structure. However, the metal rectangular expansion joint must be wrapped, and a rain cover should be added outside the insulation box-because the smoke will condense due to the decrease in temperature and corrode the bellows.
High temperature gas (e.g. hot air, inert gas):Temperatures may exceed 900°C. In this case, it is recommended to use aerogel composite insulation structure, or simply make internal insulation-spray high-temperature insulation coating on the inner wall of the expansion joint. However, the internal insulation will affect the installation of the guide tube, so it needs to be specially designed. For example, the double hinge expansion joint of air-cooled island vacuum pipeline, the medium is a mixture of high-temperature steam and air, and anti-freezing should be considered outside, and the thickness of insulation layer may exceed 300mm.
Finally, there is no universal "universal insulation scheme". When selecting, be sure to send the medium, temperature, pressure and environmental conditions of your pipeline to the expansion joint manufacturer, and ask them to help calculate the insulation thickness and structure-don't copy other people's schemes by your own feeling. After all, how to insulate metal expansion joints is ultimately an engineering problem, not a mathematical problem.