Why do metal expansion joints need thermal insulation shells? — — Not just heat preservation, but also life and safety
The medium temperature of the main steam pipeline of the power station is 560℃. If the bellows of the expansion joint is directly exposed to this temperature, the metal material will creep and oxidize, and the fatigue life will drop like a cliff-it may not last for a maintenance cycle. And guess what? In many on-site fault cases, the root cause is precisely the local overtemperature of the expansion joint caused by the design or construction defects of the insulation layer. To put it bluntly, the function of the insulation shell is to artificially reduce the wall temperature of the bellows and make the material work in a safe temperature range. In high-temperature pipelines, such as hot air pipelines in the cement industry, the medium temperature is always five or six hundred degrees Celsius. If there is no thermal insulation shell, the bellows is equivalent to being carried hard in the "furnace", and it is strange that the life span can be good.
But don't think that thermal shells are just insulation. It also undertakes multiple tasks such as protecting the bellows from external mechanical damage, preventing rain erosion, and reducing heat loss. To put it bluntly, the thermal insulation shell is the "protective armor" of the expansion joint. If it is not selected or installed properly, no matter how good the expansion joint is, it will be useless.
Material and structure of insulation shell: rock wool, aerogel or ceramic fiber?
This is not a single-choice question, it depends on the working conditions. There are several commonly used materials for insulation layer:
- rock wool: Low cost, but high water absorption. It is no problem to use it indoors or in a dry environment. If it is an open-air pipeline, the thermal conductivity of rock wool will soar after absorbing water, and the thermal insulation effect will be discounted, which may also accelerate the corrosion of the shell.
- Aluminum silicate fiber (ceramic fiber): Temperature resistance 1000℃ +, suitable for high temperature working conditions. For example, this site'sHigh Temperature Axial Expansion Joint (Class 8)With ceramic fiber insulation layer is a routine operation. The disadvantage is that it is relatively brittle and dusty during construction, so it has to be protected.
- Aerogel felt: Light and thin, top heat insulation performance, the heat insulation effect is 3-5 times that of rock wool under the same thickness. But it is expensive. A set may be more expensive than the expansion joint itself. Suitable for space constraints or extremely stringent heat loss requirements.
What about the outer metal shell? Commonly used stainless steel or carbon steel galvanized, thickness 0.5-1.5mm. There are two types of structures: detachable (convenient for maintenance of expansion joints) and integral welding (good sealing, suitable for occasions where air leakage is not allowed). Here is a key point: if the expansion joint has a guide tube (refer to Article 7 of Q&A on this site), the design of the thermal insulation shell must avoid the fixed position of the guide tube, otherwise it will affect the thermal displacement compensation-the function of the guide tube is to guide the flow direction of the medium and protect the bellows. If the thermal insulation shell is welded to the support point of the guide tube, the displacement of the expansion joint will be stuck, and the consequences can be imagined.
Key Points of Type Selection under Different Working Conditions: Differentiation of Power Station vs Cement vs Chemical Industry
Power station industry: MultipurposeCorrugated Expansion Joints for Power Station Industry (Class 5)The temperature and pressure of the main steam and reheating pipelines are high, and the thermal insulation shell must meet the Technical Specifications for Thermal Insulation of Thermal Power Plants, and there are strict limits on heat dissipation losses. Usually, aluminum silicate fiber + stainless steel shell is used, and the thickness is calculated according to the pipe diameter and medium temperature, generally between 100-200mm. Rainproofing also has to be considered-the shell joints arranged in the open air must face down, otherwise rain will flow back into the insulation layer and wait for corrosion.
Cement industry: UsingMetal Corrugated Expansion Joints for Cement Industry (Class 6)There is much dust in the air duct. The insulation shell should be prevented from dust accumulation-dust accumulates on the surface of the shell, which not only increases the load, but may also cause local overheating. The metal shell interface should face down or be provided with a dust seal. In addition, the environmental temperature of the cement plant is high, and the outer surface temperature of the insulation shell may exceed 60℃, so a protective net must be installed to prevent burns.
Chemical industry: The trouble is the risk of leakage of corrosive media. If the medium in the tube contains sulfide, corrosion under the insulation layer (CUI) is a headache-once the insulation layer is filled with water, the sulfide dissolves to form a corrosive electrolyte, and the outer surface of the bellows quickly perforates. At this time, closed-cell foam glass (non-absorbent) or shell with anti-corrosion coating should be selected. The shell drainage hole is not optional and must be set at the lowest point and checked regularly for blockage.
Common Installation and Maintenance Misunderstandings-Don't Let Thermal Insulation Shell Become a "Death Reminder"
The insulation shell is in direct contact with the bellows. The bellows produces axial or transverse displacement when working (refer to this site"Proper Installation Method of Large Tie Rod Expansion Joint"Q&A), if the thermal insulation shell is close to the bellows, it will hinder the displacement, resulting in additional stress and fatigue cracking of the bellows in advance. Correct practice: Reserve a gap of 10-20mm, or fill it with a flexible ceramic fiber blanket, which can keep heat and allow the bellows to expand and contract freely.
Another myth is neglecting drainage. Once rainwater or condensate water penetrates the insulation layer and cannot be discharged, water vapor accumulates on the surface of the bellows, and the corrosion rate doubles in the high temperature environment. It is recommended to open a drainage hole at the lowest point of the shell, with an aperture diameter of 6-10mm, the aperture facing down and an insect-proof net. Don't bother-a drain hole can save an unplanned downtime.
Also, don't violently remove the shell during maintenance. The detachable insulation shell is usually fixed with clamps or bolts. The position should be marked when disassembling and assembling. After resetting, check whether the sealing strip is pressed tightly. If the integral welded shell needs to be cut, it must be anti-rust treated after welding, otherwise the welding joint will rust through first.
Write at the end: The thermal insulation shell is the "extension cord" of the expansion joint
Selecting and installing the heat preservation shell can makeHigh Temperature Axial Expansion Joint (Class 8)OrSleeve type pipe expansion joint (Class 29)The actual service life is increased by more than 30%. Don't save money on insulation shells-the cost of a qualified insulation shell is often less than 5% of the price of the expansion joint itself, but it determines whether the whole compensation system can run safely for a maintenance cycle. When selecting the model next time, remember to write the parameters of the insulation shell (medium temperature, ambient wind speed, insulation thickness) into the technical protocol together. After all, the bellows silently bears thermal expansion and contraction in the pipe, and the thermal insulation shell is its only "coat". You can't let it streak naked, can you?