What is the plastic lining of pressure vessel
What is the plastic lining of pressure vessel?
Table of Contents
Plastics are widely used as lining materials of pressure vessels due to their good corrosion resistance and light weight. Commonly used in the lining of plastics are hard PVC plastic, soft PVC plastic, polypropylene plastic, polyethylene plastic and fluoroplastics.
Method of plastic lining
The main methods of plastic lining are bonding method, hanging lining method and bolt fixing method.
- ① Hanging lining method (also known as loose lining or empty paving method). The plastic plate is processed into the shape of equipment and hung on the inner wall of steel equipment. The lining layer and steel shell are not fixed directly, which can solve the problem of expansion and contraction gap between hard plastic plate and steel shell, and ensure that the plastic lining layer will not be cracked and damaged. It is commonly used in small equipment.
- ② Bolt fixation method. Screw is set at the proper part of the inner wall of the steel shell, and the plastic plate is lined and fixed with nuts. In order to prevent creep tension and bulging defect of lining layer, flat steel strip is often used as hoop to support lining surface. All metal materials such as nuts and steel hoops shall be covered with plastic plates and welded tightly for protection.
- ③ Bonding method. Select adhesive to stick the plastic plate on the inner surface of the steel shell. The lining and the steel shell are closely bonded together, and the lining structure can improve the working pressure of the equipment. The commonly used adhesives include perchloroethylene adhesive, polyurethane adhesive and chloroprene adhesive. With the improvement of adhesive performance and the development of adhesive technology, the application of adhesive method for plastic lining is increasing.
Types of plastic lining
Rigid PVC plastic lining
Unplasticized polyvinyl chloride (PVC) plastic is made by kneading, mixing and hot pressing with PVC resin as the main component, stabilizer, lubricant, filler and other additives. It has certain mechanical strength, good machining performance and excellent chemical stability, and is widely used. When using rigid PVC plastic sheet as lining, the above three lining methods can be used, especially the bonding method.
Soft PVC plastic lining
Soft PVC is a kind of product which is made by hot kneading, plasticizing and extruding with plasticizer and stabilizer in PVC resin. Although a considerable amount of plasticizer is added, its acid, alkali and salt resistance is still excellent. It can be used as a non bearing material for lining of chemical equipment with reference to the corrosion resistance of rigid PVC. The lining methods of soft PVC plastic plate mainly include bolt fixing method and bonding method. The matrix of lining equipment is generally made of carbon steel.
PTFE plastic lining
Polytetrafluoroethylene (PTFE) plastic has excellent chemical corrosion resistance at high temperature, and can resist the corrosion of any kind and concentration of acid, alkali, salt, strong oxidant, organic solvent and other strong corrosive media (except for high-temperature molten alkali metal, element fluorine and chlorine trifluoride); Good heat resistance (- 195 ~ 250 ℃), low friction coefficient and good viscosity. Because of its low mechanical strength and stiffness, it is often used as lining material for chemical pipeline and equipment. There are two methods of PTFE plastic lining: loose lining and tight lining. Please refer to Hg 20536 “PTFE lining equipment”.
① Matsumura law. There are three ways of PTFE loose lining.
- (a) The extruded polytetrafluoroethylene pipe is put into the corresponding specification metal pipe, and then the surplus polytetrafluoroethylene pipe at both ends is cut off, and the two ends are heated for flanging, flattening and cooling. The lining pipe shall adopt flange connection structure.
- (b) Select the PTFE lined pipe whose outer diameter is slightly larger than the inner diameter of the lined metal pipe. When lining, the polytetrafluoroethylene tube is heated and stretched. At this time, the diameter of the polytetrafluoroethylene tube is reduced and the tube is elongated. After moderate cooling and shaping, the polytetrafluoroethylene tube is sent into the metal tube, and then heated to about 250 ℃. Due to the thermal stress relaxation and deformation characteristics of PTFE plastic pipe, the original size of the pipe is naturally restored, so that the PTFE lined pipe is closely attached to the inner wall of the metal pipe. Finally, both ends of the PTFE pipe head flanging flattening and cold setting.
- (c) Select the appropriate PTFE plastic pipe diameter liner, freely sleeve into the metal pipe, and then roll and stretch the metal pipe on the special machine tool. At this time, the metal pipe diameter becomes smaller, so that the PTFE pipe liner is close to the inner wall, and finally the PTFE pipe at both ends is flanged and flattened.
② Tight lining method. The tight lining method of PTFE can be divided into two kinds: bonding method and transition layer bonding method.
- (a) Adhesion. The lining surface of polytetrafluoroethylene (PTFE) plate (tube) was activated (commonly treated with sodium naphthalene tetrahydrofuran solution), and then epoxy resin adhesive was used to bond it with the metal matrix. The lap joint of Teflon plates should be welded together by welding.
- (b) Transition layer bonding. First, the PTFE plastic and glass fiber fabric are pressed into composite material under certain temperature and pressure, so that the glass fiber layer is used as the bonding transition layer, and the general bonding technology is adopted to paste it to the metal surface to complete the lining process.
- (c) Polytetrafluoroethylene plastic lining products must undergo quality inspection, usually through the appearance inspection, water pressure leakage test, high frequency electric spark detection of micro holes, etc. The quality requirements of Teflon plastic lining equipment are as follows: the lining layer is smooth, fit, the flanging is smooth, and there is no leakage at the joint of weld and cylinder; The flanging surface of the branch pipe and the mirror on the cylinder is flat, and the weld seam is free of leakage; There should be no cracks, delamination and blistering during the whole heating bending; All lining layers shall be checked by electric spark to avoid leakage.
Source: Network Arrangement – China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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