Metal expansion joint insulation cotton: wrong choice, bad package, high temperature pipeline problems sooner or later

Why do metal expansion joints have to be equipped with thermal insulation cotton? — Not just to save some heat

Many people think that the metal expansion joint is packed with thermal insulation cotton, just to save energy and heat. Actually, this is only half true. In high-temperature pipeline systems, such as power stations, cement plants and chemical plants, the temperature is always three to four hundred degrees, or even higher. If the bellows is not protected by insulation layer and is directly exposed to the environment, it will lead to two fatal problems: First, the temperature difference between the inside and outside of the pipe wall is too large, which generates thermal stress and accelerates fatigue; Second, after the medium heat is lost, the downstream process parameters fluctuate, which may affect the stability of the whole system.

There is also a more insidious risk – condensation corrosion. When the temperature of the pipeline conveying medium is lower than the environmental dew point, or the cold and hot alternate after shutdown, the outer wall of the bellows will condense water droplets. The water droplets are mixed with the sulfide in the smoke, and they become dilute sulfuric acid. Within a few uses, stainless steel bellows pitting. Insulation cotton actually plays the dual role of "insulation + anti-condensation" here. So don't just think about saving money. If the insulation cotton is chosen wrong or the package is not good, the life of the metal expansion joint will be folded directly in half.

Picking insulation cotton can't rely on feeling: How to choose materials under different working conditions

Don't be in a hurry to place an order, first find out what medium is running in your pipeline, how high the temperature is, and whether there are any corrosive gases. The three most common insulation materials on the market: aluminum silicate wool, rock wool, aerogel felt. Each applicable scenario is completely different.

• Aluminum silicate wool: The highest temperature resistance, the long-term use temperature can reach more than 1000 ℃. High temperature flue gas pipeline suitable for power station and cement industry. For example, in the product list on our siteCorrugated expansion joint for power station industryAndCement industry metal corrugated expansion joints, the matching insulation cotton is almost all aluminum silicate. Its fibers are long, elastic and not easy to collapse, but the disadvantage is that the water absorption rate is on the high side, and it needs a waterproof layer outdoors.
• rock wool: The price is cheap and the temperature resistance generally does not exceed 600℃. It is fine to use it on steam pipes and hot water pipes. However, the rock wool fiber is short, and it is easy to settle over a long time, resulting in a cavity above the bellows and losing the heat insulation effect. Moreover, the heat insulation performance of rock wool drops linearly after exposure to water, so it is not recommended to be used in flue gas system or occasions where condensation is possible.
• Aerogel felt: The thermal insulation performance is 3-5 times that of traditional materials, and the thickness can be reduced by half. It is especially suitable for occasions with limited space, such as steam turbine bypass and high-temperature air duct. But the price is expensive, and it is generally only used in key parts. If you are usingHigh temperature axial expansion jointOrDirect buried (fully buried) type expansion jointThe space is inherently limited, and the comprehensive cost of using aerogel felt is lower.

Look at the temperature, the medium, and the environment. Don't put tens of thousands of expansion joints in it just to save hundreds of dollars.

Conflict between insulation cotton and expansion joint structure: guide tube, tie rod, bellows, which position is most afraid of being crushed to death?

I met a customer two days ago who boughtCompound hinge transverse expansion jointDuring the installation, the workers stuffed the thermal insulation cotton to the fullest, and as a result, the bellows bulged after a heat cycle. Why? Because the thermal insulation cotton blocks the moving space of the expansion joint.

The core of metal expansion joint is to absorb displacement by the deformation of bellows. Sufficient clearance must be left around the bellows to not be crushed to death. Especially in several key positions:

• guide tube: There is typically an annular gap of about 10-20 mm between the guide tube and the bellows. If the insulation cotton is plugged too tightly to block the gap, the bellows will push against the guide tube when it expands thermally, resulting in stress concentration. The guide tube itself guides the flow direction of the medium and protects the inner wall of the bellows. It is not rigid connection with the bellows, so space must be left. Refer to the question and answer in our station "The Specific Function of Expansion Joint Guide Tube", which many people ignore.
• Tie rod and nut: The role of the tie rod is to limit the excessive elongation or compression of the expansion joint (see Q&A "The role of the tie rod for the expansion joint"). If the insulation cotton is wrapped too thick, it will wrap the drawbar inside, making it impossible to adjust the nut. Once the system changes and you want to adjust the stroke and find that you can't reach the nut at all, you can only remove it and redo it.
• Bellows trough: Insulation cotton cannot be stuffed directly into the trough. The trough is the area with the highest stress, and the hard cotton block will form a local support point. When the bellows moves, it will bend repeatedly at this point, and the fatigue life will drop sharply. The correct way is to use soft insulation felt and gently cover it so that the bellows can move freely.

Insulation cotton is to insulate the pipe, not to fill the expansion joint. Leaving room for movement is much more important than stuffing a few more layers of cotton.

There are eight pits in ten installation sites: random bandaging, wrong gaps and excessive bolt compression

Let's talk about a few real cases I saw at the scene.

The first one, bandage at will. Some people try to save trouble by tying iron wire directly to the outer wall of the bellows. After the wire is tightened, it will be embedded in the bellows surface at high temperature, forming stress corrosion cracks. The formal practice is to use stainless steel belts or high-temperature resistant cable ties, and a metal sheath, such as galvanized iron or aluminum, should be wrapped around the thermal insulation cotton, which is both rainproof and mechanical damage.

Second, leave the gap wrong. The expansion seam must be left at the butt joint of insulation cotton. Someone put two pieces of cotton together tightly. Once heated, the cotton swells and has no place to run, so it directly squeezes the bellows. Correct practice: Leave a gap of 5-10mm every 1m in the axial direction and fill it with flexible sealant.

Third, the bolts are pressed too tightly. Some installers are afraid of loosening the insulation layer and screw the bolts on the outer sheath to death. The sheath itself will move with the expansion joint, and if the bolt is crushed, the sheath will restrict the bellows from deforming. EspeciallyLarge tie rod expansion jointThe tie rod itself has a preloading force, and if the sheath is fixed too dead, the actual displacement will not reach the design value, and the system stress cannot be released.

Even more outrageous, thermal insulation cotton is directly installed on the inside of the guide tube, thinking that it can protect the bellows. As a result, the high-temperature smoke burned the cotton into ash, and the ash was blown into the downstream valve, causing stuck. Alas, these pits are lessons from real money.

Insulation cotton aging is not equal to changing cotton only: how much does it really affect the life of expansion joints?

Thermal insulation cotton also has a lifespan. Organic ingredients (such as binders) slowly decompose at high temperatures, and the fibers become brittle and collapse in thickness. Generally, aluminum silicate wool starts to be obviously powdered in 3-5 years in the environment above 800 ℃. At this time, many people think that when the cotton is old, isn't it enough to change it with new cotton?

The problem is that the expansion joints have taken additional damage during cotton aging. After cotton collapse, the exposed area of bellows increases, the local temperature rises, and the metal creep accelerates. After the cotton absorbs water, the acidic substance directly contacts the outer surface of the bellows, and intergranular corrosion occurs. These damages will not be automatically repaired just by replacing them with new cotton. Therefore, there is a saying in the industry: "The replacement cycle of thermal insulation cotton actually determines the maintenance window of the expansion joint." Generally, it is recommended to conduct non-destructive testing (such as penetration detection and thickness measurement) on the bellows simultaneously when replacing the insulation cotton. If cracks or thinning exceeds 10%, you have to consider replacing the expansion joint.

In addition, the aging of insulation cotton will also cause the outer sheath to loosen and rain water to seep in. Water vapor forms water vapor at high temperature, and its volume expands, which may burst the sheath screws and even blow the insulation layer away. These chain reactions are ultimately counted on the expansion joint head.

So don't underestimate that layer of cotton. If it can't carry it, the expansion joint can't. Choose the right materials, leave enough space, standardize installation, and check cotton regularly-these four points are achieved, and your high-temperature pipeline system can run safely for ten years.