Non-metallic Expansion Joints Material Performance Analysis & High Temperature Flue Gas Application Cases

In the flue gas and ventilation pipeline system of environmental protection,building materials and waste incineration industries,a large number of low-pressure large-section rectangular air ducts are laid.The metal expansion joints used in traditional circular pressure pipelines have obvious application limitations in rectangular flues:metal bellows will generate huge blind plate thrust under the action of flue gas internal pressure,which requires the air duct frame to bear extra heavy load,and the manufacturing cost of large-size metal compensation components increases exponentially.Non-metallic Expansion Joints,which are woven from high-temperature resistant fiber composite cloth,break through the performance bottleneck of metal compensation products.The flexible fabric main body does not produce blind plate force when bearing internal pressure,and can freely absorb axial,lateral and angular three-dimensional displacement of the air duct at the same time.It has the outstanding advantages of light weight,low noise,arbitrary forming of rectangular and special-shaped sections,and has become the mainstream compensation product for industrial flue gas air ducts at present.

The internal multi-layer composite structure determines the overall performance of non-metallic expansion joints.A complete standard non-metallic expansion joint is composed of outer protective layer,heat insulation buffer layer,air tight sealing layer and inner anti-corrosion isolation layer,and each layer of fabric has independent functional positioning.The outermost protective layer is mostly made of silicone coated glass fiber cloth.Silicone coating can resist outdoor rain erosion,ultraviolet aging and flue surface high-temperature radiation,and has strong tear resistance.The middle heat insulation buffer layer is filled with high-purity ceramic fiber blanket or glass fiber cotton,which can block the outward transmission of flue heat,reduce the surface temperature of the expansion joint outer wall,and avoid scalding maintenance personnel and fire hazards caused by high temperature.The innermost sealing isolation layer is the core functional layer in contact with flue gas medium.For conventional boiler flue gas with temperature below 300℃,fluorine rubber coated fiber cloth is selected,which can resist weak acid and weak alkali corrosion produced by sulfur-containing flue gas;for waste incineration flue gas containing hydrogen chloride and high temperature above 400℃,PTFE composite fluorine cloth needs to be used as the inner lining,which can isolate strong corrosive acid gas and prevent the fiber base cloth from being quickly corroded and pulverized.Many manufacturers cut corners and reduce the number of composite layers in order to reduce costs,only retain single-layer silicone cloth without heat insulation and anti-corrosion lining,resulting in the expansion joint being burned through within 2-3 months after being put into use,and frequent flue gas leakage shutdown accidents.

According to different working condition parameters,non-metallic expansion joints are divided into three mainstream product series:ordinary ventilation type,high temperature flue gas type and strong corrosion resistant desulfurization type.The ordinary ventilation type is mainly used for building air supply and exhaust ducts and low-temperature waste gas pipelines below 180℃,with single-layer silicone glass fiber composite structure,low procurement cost,and suitable for neutral gas medium without corrosive components.The high temperature flue gas type is widely matched with cement kilns,coal-fired boiler flues and garbage incineration primary flues,with multi-layer ceramic fiber heat insulation structure,long-term stable working temperature range from 250℃to 550℃,and can resist instantaneous temperature impact of 700℃in short time.The desulfurization special anti-corrosion non-metallic expansion joint is the most widely used product in the environmental protection industry at present.The inner lining adopts full PTFE fluorine film composite cloth,which can resist the strong corrosion of sulfur dioxide,hydrogen fluoride and other acid substances in desulfurization flue gas,and the outer layer adds anti-ultraviolet aging coating,which can adapt to long-term outdoor open-air layout of desulfurization tower connecting flues.In the model selection work of environmental protection engineering designers,it is necessary to clarify three core parameters:long-term working temperature of flue gas,medium acid-base concentration and flue cross-sectional shape,so as to avoid mismatch between fabric structure and working conditions.

The production and processing technology of non-metallic expansion joints directly affects the air tightness and tear resistance of finished products.The whole processing flow includes composite cloth blanking,multi-layer cloth edge sewing,metal frame pressing,expansion joint outer protection belt winding and finished product air tightness inspection.In the blanking link,numerical control cutting equipment is required to ensure that the fabric cutting edge is flat without burrs,and the reserved expansion and contraction margin of each side of the fabric is reserved according to the pipeline displacement data.The sewing process is the key link to prevent flue gas leakage.Double-line high-temperature resistant aramid fiber thread is used for sewing,and the sewing needle distance is controlled within 3mm.After sewing,high-temperature silicone sealant is evenly coated on the sewing gap to block the tiny gap between the thread and the fabric.The metal pressing frame adopts galvanized carbon steel or stainless steel punching frame,and the composite fabric edge is clamped between the two layers of metal frames,and fixed with uniform pressure bolts to prevent the fabric from falling off from the frame under the repeated displacement of the air duct.After the finished product is produced,the factory must carry out air tightness inspection by closed inflation method,fill compressed air into the inner cavity of the expansion joint,and observe whether there is air leakage at the sewing seam and frame pressing position,and the unqualified products are re-sealed and sewn before leaving the factory.

Compared with metal expansion joints,the installation construction of non-metallic expansion joints has special anti-tearing operation specifications,which are easy to be ignored by on-site construction teams.Before hoisting,the temporary transport fixing belt reserved by the factory cannot be removed in advance.The fixing belt limits the stretching range of the flexible fabric during transportation and hoisting,and can only be cut off after the metal frames at both ends are fully fixed with the air duct flange by bolts.When tightening the frame connecting bolts,the diagonal uniform tightening method must be adopted,and the one-side single bolt cannot be tightened excessively,which will lead to local stress tearing of the composite fabric.In the layout of the air duct support,a fixed support must be set within 1m of the two sides of the non-metallic expansion joint,and the sliding support is arranged at a distance of more than 3m,so as to avoid the fabric bearing excessive unilateral pulling force when the air duct is thermally displaced.In addition,it is forbidden to weld and cut operations within 50cm around the installed expansion joint.The high-temperature welding spatter will burn through the fiber fabric,forming irreversible leakage holes.If welding construction must be carried out nearby,asbestos fireproof baffle needs to be set for isolation protection.

In the daily operation and maintenance management of flue gas pipelines,regular inspection and maintenance can greatly extend the service cycle of non-metallic expansion joints.The inspection frequency of high-temperature desulfurization flue expansion joints shall not be less than once a month,and the low-temperature ventilation type can be inspected once every six months.The inspection focus includes checking whether the surface of the outer silicone cloth has pulverization,cracking and fire burn holes;observe whether the sewing seam has flue gas black smoke leakage marks;check whether the metal pressing frame bolts are loose and rusted.When local small-area burning holes and leakage gaps are found,high-temperature silicone repair glue can be used for on-site coating repair to avoid replacing the whole set of expansion joints and reduce maintenance costs.For the expansion joints with serious aging of the inner anti-corrosion lining and large-area pulverization of the fabric,the whole set needs to be disassembled and replaced in time to prevent a large amount of corrosive flue gas from leaking into the workshop and causing equipment corrosion and environmental pollution.In winter low-temperature areas,outdoor exposed non-metallic expansion joints need to be wrapped with thermal insulation cotton and rain-proof color steel plates to prevent the silicone coating from hardening and cracking after long-term low-temperature frost.

From the perspective of engineering economic benefit comparison,non-metallic expansion joints have obvious comprehensive advantages in large-section low-pressure flue pipelines.Under the same compensation displacement parameters,the manufacturing cost of rectangular non-metallic expansion joints with a cross section of more than 2 square meters is only 1/3 of that of metal rectangular expansion joints,and the installation weight is reduced by more than 80%,which greatly reduces the load bearing pressure of the air duct support frame and the civil engineering investment cost.At the same time,the flexible fabric structure can absorb the vibration generated by the fan and boiler operation,play a good noise reduction effect,and reduce the pipeline resonance damage.The only limitation of non-metallic expansion joints is that they cannot adapt to high-pressure liquid medium pipelines,and are only suitable for low-pressure gas flue and ventilation systems.Therefore,in the early stage of pipeline design,engineers need to distinguish medium pressure and medium type to select metal or non-metallic compensation products reasonably.

In conclusion,non-metallic expansion joints are customized flexible compensation products developed aiming at the working condition characteristics of industrial flue gas air ducts.The multi-layer composite fiber structure,diverse material collocation and free forming section characteristics make it occupy an irreplaceable core position in the environmental protection flue gas treatment industry.All links from material layer selection,factory production process,standardized on-site installation to regular anti-aging maintenance will affect its service life and sealing performance.Environmental protection engineering companies,pipeline design institutes and equipment manufacturers need to fully grasp the performance limits and application standards of different types of non-metallic expansion joints,so as to match the most cost-effective compensation scheme for different flue gas working conditions,reduce the failure rate of flue gas pipelines,and ensure the stable and up-to-standard operation of waste gas treatment equipment.