Hardcore content: A comprehensive guide to pipes and fittings

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Common Terms for Pipes and Fittings 01: Nominal Diameter (DN): The designated diameter of pipes in thermoplastic pipe systems, indicating the size of the pipe’s inner or outer diameter, or an approximation thereof. 02、Tangential bending stiffness / Circumferential stiffness: A measure of a pipe’s ability to resist tangential deformation. The value can be determined using testing methods or calculation methods, with the unit being N/m2 or KN/m2. 03、Elastic diameter deformation rate: The ratio of the vertical elastic deformation of the pipe diameter under external pressure to the diameter of the central axis of the pipe wall cross-section before loading. Expressed as a percentage, as determined by the compression test. 04、Out-of-roundness: The difference between the maximum outer diameter and the minimum outer diameter measured at the same cross-section of the pipe. 05、Standard Dimension Ratio (SDR): The ratio of the nominal outer diameter to the nominal wall thickness of the pipe. SDR=DN / en. 06. Maximum Operating Pressure (MOP): The maximum effective pressure of the fluid that can be used continuously in the piping system, expressed in MPa. 07. Nominal wall thickness en: The specified value for the pipe wall thickness, measured in mm. 08. Total usage (design) coefficient C: A total coefficient greater than 1, which takes into account the operating conditions not reflected in the lower prediction limit, as well as the properties of components such as fittings in the piping system. 09. Predicted lower limit of hydrostatic strength confidence (σLPL): The lower limit of the predicted hydrostatic strength value at temperature T and time t, at a confidence level of 97.5%; σLPL=σ(T,T,0.975), with the unit in MPa. 10. Minimum Required Strength (MRS): The stress value obtained by rounding down the value of σLPL at 20°C with a 50-year lower confidence limit to the nearest preferred number in the R10 or R20 series; unit: MPa. When σLPL is less than 10 MPa, it shall be rounded according to the R10 series; when σLPL is greater than or equal to 10 MPa, it shall be rounded according to the R20 series. 11. Design stress σs: The allowable stress under specified application conditions, obtained by dividing the MRS by the coefficient C and rounding it down to the next smaller value in the preferred R20 series; that is, σs = MRS / C. 12. Permissible deviation / Limit deviation: The deviation between the permissible limit values and the specified values. The difference between the maximum allowable value and the specified value is called the upper deviation, while the difference between the minimum allowable value and the specified value is called the lower deviation. 13. Tolerance: The allowable deviation from a specified value, expressed as the difference between the maximum allowable value and the minimum allowable value. It equals the difference between the upper and lower deviations. 14. Outer diameter at any point: The outer diameter measured across the cross-section at any point on the pipe, in mm. 15. Average diameter: The diameter of the circle corresponding to the center of the pipe wall cross-section. The average diameter is equal to the difference between the average outer diameter and the average wall thickness, or the sum of the average inner diameter and the average wall thickness, with the unit being mm. 16, 20°C, 50-year confidence lower limit (σLCL): A stress value used to evaluate material properties; it represents the confidence lower limit of the predicted long-term strength of pipes made from that material under internal water pressure at 20°C over a period of 50 years, with a confidence level of 97.5%, expressed in MPa. 17. Long-term hydrostatic strength (σLTHS): It represents the predicted average strength at temperature T and time t (the average strength refers to the lower confidence limit of the material’s strength when the confidence level is 50%). The unit is MPa. 18, 20°C, Long-term Strength at 50 Years (σLTHS): The average strength or predicted average strength of the pipe under water pressure at 20°C over 50 years, expressed in MPa. 19. Environmental stress cracking: The phenomenon of accelerated stress cracking due to the influence of environmental conditions. The environmental stress cracking resistance of materials is commonly expressed as ESCR. 20. Crosslinking degree: A physical quantity that indicates the degree of crosslinking. When cross-linking occurs between polymer chains, it is expressed as the average number of cross-linking units per polymer molecule. Also known as cross-linking point density or cross-linking index. It is usually expressed in terms of gel content. 21. Ductile failure: Failure accompanied by significant plastic deformation. 22. Fractural failure: Failure in which there is no significant plastic deformation in the fractured area. 23. Penetrative failure: A form of failure in which the pressurized fluid inside the pipe seeps through the pipe wall, resulting in visible leakage without significant cracking. 24. Circumferential stress: The stress induced in the pipe wall by internal pressure, acting in the direction of the pipe’s circumference. 25. Blasting pressure: The maximum pressure before the pipe ruptures during a hydrostatic blasting test of the pipe. 26. Fault temperature: The highest temperature reached when the piping system exceeds its control limits. 27. Structural wall pipes: A general term for pipes with composite wall structures composed of multiple layers of materials, as well as pipes with special-shaped wall structures such as open ribbed or closed ribbed (hollow) walls. 28. Special-shaped wall pipes: A general term for pipes with a smooth inner wall and an outer wall featuring a spiral, ring-shaped, or ribbed wall structure. 29. Double-wall corrugated pipe: It has a smooth and flat inner wall, while the outer wall features trapezoidal or arc-shaped corrugated ribs; it is a special-shaped tubular material with hollow spaces between the corrugations on the inner and outer walls. 30. Ringed ribbed tube: A special-shaped tubular material with a smooth inner wall and an outer wall equipped with ring-shaped reinforcing ribs. 31. Spiral ribbed tube: A type of tubular material with a smooth and flat inner wall, and reinforced ribs on the outer wall that are spiral in shape; it is a special-shaped tube wall manufactured through a spiral winding process. 32. Straight-wall pipe: A homogeneous pipe with smooth inner and outer walls. 33. Mechanically connected fittings: Fittings that are connected through mechanical means such as threaded connections or compression connections. 34. Elastomeric-sealed fittings: Fittings that achieve a sealed connection using elastomeric sealing rings. It generally cannot transmit axial loads. 35. Solvent-adhesive pipe fittings: Pipe fittings that are joined using solvent-based adhesives. 36. Electric fusion fittings: Fittings with pre-installed electric heating elements on the connection surface (bottom). 37. Hot-melt butt fittings: Fittings that have ports of the same size as those of the pipes to be connected, and are joined through hot-melt butt welding. 38. Hot-melt socket fittings: A heating tool is used to heat the inner surface of the fitting’s socket as well as the outer surface of the pipe or valve’s insertion end; thereafter, they are inserted together and fused, thereby enabling connection between the fittings. Classification of pipes: Non-metallic pipes. Non-metallic pipes include various common plastic pipes (such as rigid polyvinyl chloride pipes for water supply, ABS engineering plastic pipes, PP-R engineering plastic pipes, etc.), composite pipes (aluminum-plastic composite pipes, steel-plastic composite pipes, etc.), concrete pipes, rubber pipes, glass fiber reinforced plastic pipes, and so on. I. Concept of plastic pipes: Plastic pipes are generally made from synthetic resins, namely polyesters, to which stabilizers, lubricants, plasticizers, etc., are added; they are then produced through extrusion processes in pipe-making machines using plasticization techniques. Advantages: light weight, corrosion resistance, attractive appearance, no unpleasant odor, easy to process, and convenient for installation. Disadvantages: Lower strength and poor heat resistance. Uses: It is primarily used for piping in water supply systems for building construction, as well as for drainage, exhaust, and sewage pipes; in underground drainage systems; in rainwater pipes; and as conduit for electrical wiring installation. Classification: Based on the material used in pipe manufacturing, plastic pipes for construction mainly include polyvinyl chloride pipes, chlorinated polyvinyl chloride pipes, polyethylene pipes, cross-linked polyethylene pipes, etc. 1. Applications of rigid polyvinyl chloride pipes (PVC-U): Used for water supply (non-potable water), drainage pipes, and rainwater pipes. Advantages: Corrosion-resistant, smooth inner wall, resistant to scaling, good hydraulic properties, lightweight and easy to install, not prone to aging, low thermal strength, hard in texture, inexpensive, easy to bond, flame-retardant. Disadvantages: Not impact-resistant, poor durability, high requirements for joint bonding technology, and long curing time. Connection methods: socket bonding, plastic welding, flange connection with special fittings, threaded connection. 2. Applications of chlorinated polyvinyl chloride pipes (PVC-C): Abroad, they are commonly used as hot water pipes, waste liquid pipes, and sewage pipes; in China, they are mainly used as protective sleeves for power cables. Advantages: Best temperature resistance, excellent aging resistance, and good flame-retardant properties. Disadvantage: High price. Connection methods: socket bonding, plastic welding, flange connection using specialized fittings, and threaded connection. 3. Uses of polyethylene pipes (PE): Abroad, HDPE and MDPE pipes are widely used as urban gas pipelines and urban water supply pipelines. Currently, HDPE and MDPE pipes in China are mainly used for urban gas pipelines, with a small amount being used for urban water supply pipelines, while LDPE pipes are widely used for agricultural irrigation and drainage pipelines. Classification: Based on their density, they are divided into high-density polyethylene pipes (HDPE), medium-density polyethylene pipes (MDPE), and low-density polyethylene pipes (LDPE). Features: HDPE pipes possess high strength and stiffness ； In addition to the pressure resistance of HDPE pipes, MDPE pipes also possess good flexibility and creep resistance ； LDPE pipes have good flexibility, elongation, and impact resistance, and in particular, they exhibit excellent chemical stability and high-frequency insulation properties. Connection methods: electrothermal welding, heat fusion butt jointing, flanges, threaded connections, etc. 4. Applications of cross-linked polyethylene pipes (PE-X): Mainly used for hot and cold water supply in buildings and for floor radiant heating. Cross-linked polyethylene pipes, PEX pipes, generally come in three types: a, b, and c. PE-Xa pipes are used for hot and cold water pipelines, oil pipelines, fuel tanks, chemical pipelines, etc ； PE-Xb pipes are used for underfloor heating systems (toxic and gradually being phased out from the market) ； PE-Xc pipes are currently the most environmentally friendly heating pipes. Advantages: Good temperature resistance, good creep resistance, excellent memory effect, and easy to correct. Disadvantages: It can only be connected using metal parts, and it cannot be recycled. Connection methods: bayonet connection using specialized metal fittings, clamp connection, and threaded connection. 5. Applications of heat-resistant polyethylene pipes (PE-RT): indoor cold and hot water pipelines, especially in hot water systems. The ideal choice for radiant heating applications. Advantages: Excellent temperature resistance, suitable for use from -70°C to 90°C℃ ； Excellent thermal insulation performance, low thermal conductivity ； Long service life; can be used safely for over 50 years ； Chemical and corrosion resistance ； Good recovery of shape memory properties ； Vibration-resistant, impact-resistant ； It has excellent hydraulic properties. Disadvantage: Cannot be recycled ； The overhead span of the pipeline is small, with numerous support elements required ； Due to complex factors such as concurrent operations during construction, PE-RT pipes are prone to being punctured, and the reserved pipe ends may get flattened, resulting in numerous repairs and rework instances. Connection methods: three main categories – heat fusion connectors, electrofusion connectors, and mechanical connectors. 6. Applications of type 3 polypropylene pipes (PP-R): Mainly used for domestic hot and cold water supply and floor radiant heating. Advantages: Good temperature resistance, recyclable, an eco-friendly product, and excellent hygiene properties making it suitable for pure water transportation. Disadvantages: Higher wall thickness and flammability under the same pressure and medium temperature conditions. Connection methods: The main ones are electrofusion and butt fusion; there are also flange connections and threaded connections using special fittings. 7. Uses of polybutylene pipes (PB): Mainly used for water supply, hot water, and heating pipelines. Advantages: Good temperature resistance, excellent tensile and compressive strength, impact resistance, low creep, high flexibility; it has the thinnest wall thickness under the same pressure and medium temperature conditions, and is considered a green product. Disadvantages: Reliant on imports, expensive, flammable. Connection methods: The main ones are electrothermal fusion and heat fusion welding; sealant ring sealing welding can also be used. 8. ABS engineering plastic pipes (ABS): Uses: Abroad, they are commonly used as drain pipes for sanitary ware, gas pipelines, and industrial pipelines exposed to high levels of corrosion. In China, they are generally used for indoor hot and cold water pipes, chemical dosing pipelines in water treatment systems, and industrial pipelines subject to corrosive conditions. Advantages: High strength, impact resistance; can be threaded directly; excellent hygiene properties, making it suitable for conveying pure water. Disadvantages: Long curing time for bonding, flammable. Connection methods: The main ones include socket connection, as well as flange connection using specialized fittings, and threaded connection. II. Concept of composite pipes: These are pipes that are based on metal pipes, with non-metallic materials such as polyethylene and cross-linked polyethylene welded to their inner and outer surfaces; they thus possess the characteristics of both metal and non-metal pipes. Advantages: light weight, smooth inner wall, low resistance, and good corrosion resistance. Disadvantage: It is prone to damage when subjected to external forces, and repairing such damage inevitably results in waste. Use: Water supply and drainage pipes. Categories: aluminum-plastic composite pipes, steel-plastic composite pipes, copper-plastic composite pipes, coated composite pipes, PE pipes with steel framework, etc. 1. Uses of aluminum-plastic composite pipes: Used for cold and hot water supply in domestic applications. It is a composite pipe with cross-linked polyethylene for both the inner and outer plastic layers, suitable for hot water pipes, with an operating temperature of up to 90°C. Structure: Its basic composition should consist of five layers, namely plastic, hot melt adhesive, aluminum alloy, hot melt adhesive, and plastic, in that order from the inside out. Classification: Based on the type of polyethylene material, there are two types: cross-linked polyethylene aluminum-plastic composite pipes suitable for hot water, and high-density polyethylene aluminum-plastic composite pipes suitable for cold water. Advantages: The pipe has a certain degree of flexibility, is insulating and corrosion-resistant, impermeable with good airtightness, has a smooth inner surface, is lightweight, and is easy to install. Disadvantages: High price, limited strength, prone to damage, requires many supports for fixation, limited aesthetic appeal, and uneven wall thickness. Connection method: Aluminum alloy self-locking tight-sealing fitting. When using a plastic seal, the water temperature should not exceed 60℃ ； When an aluminum seal is used, the water temperature should not exceed 100°C. 2. Applications of steel-plastic composite pipes: They can be used not only for water supply but also for transporting corrosive liquids ； S/PP applicable temperature: -25～ 100℃ ； S/PE operating temperature: -25～ 70℃ ； Applicable temperature for S/PVC: -25～65°C. Advantages: A high-performance anti-corrosion pipe that offers excellent corrosion and wear resistance, as well as good strength and aging resistance. Disadvantages: The wall thickness is relatively large, resulting in limited pressure-bearing capacity; processing and installation require specific conditions. Connection method: threaded connection using special fittings, flange connection. 3. Applications of aluminum alloy plastic-lined composite pipes: They are specifically suitable for domestic cold and hot water pipes. Structure: The outer tube is made of aluminum alloy, while the inner tube is made of thermoplastic. The outer aluminum alloy not only serves to prevent oxygen from entering but also plays a key role in providing structural support. Advantages: attractive appearance, heat retention, corrosion resistance, impermeability, good airtightness, smooth inner surface, light weight, and easy installation. Disadvantages: The outer layer is susceptible to acid and alkali corrosion, and the fittings are external connectors, which makes hidden installation difficult. Connection methods: dedicated slip-on and flange connections. III. Concept of glass fiber reinforced plastic pipe: Glass fiber reinforced plastic pipes are made by using various resins with different properties as the inner anti-seepage layer, combined with a glass fiber reinforcement layer. Advantages: heat retention, corrosion resistance, smooth inner wall. Disadvantages: High requirements for the interface, prone to leakage, difficult to install, and expensive. Use: Suitable for large and medium-sized outdoor water supply pipelines with high geological corrosion resistance. Connection method: Ordinary sand-laid fiberglass reinforced plastic pipes are connected by socket fitting. IV. Concept of rubber hoses: Rubber products. The inner and outer rubber layer materials of ordinary hoses are typically natural rubber, styrene-butadiene rubber, or cis-butadiene rubber ； Oil-resistant hoses are made of neoprene and nitrile rubber ； Acid and alkali resistant, as well as high-temperature resistant hoses are made from materials such as EPDM, fluororubber, or silicone rubber. Advantages: corrosion resistance, vibration resistance, noise reduction, and convenient interfaces. Disadvantages: High water flow resistance and low pressure tolerance. Uses: water supply pipes, hot water pipes, steam pipes, marine pipes, food and beverage pipes, air pipes, welded pipes, gas pipes, ventilation pipes, material transport pipes, oil transport pipes, chemical pipes, high-pressure pipes, and more. Connection method: flange connection, tie-wrap connection. V. Concept of concrete pipes: These are prefabricated pipes made using cement and steel reinforcement, based on the principle of centrifugal force applied to utility poles. Uses: Large and medium-sized outdoor water supply pipelines, hydraulic engineering projects. Advantages: Low cost, and good corrosion resistance after proper treatment. Disadvantages: heavy weight, difficult to install, and accessories are prone to damage. Connection method: Concrete pipes use rubber rings and sealing tape for connection, while reinforced concrete pipes use rubber ring connections. Metal pipes 1: Ordinary steel pipes. Uses: pipes for production and fire water supply, drainage branches for sanitary fixtures, as well as non-corrosive drainage branches for production equipment; rainwater pipes in industrial buildings. Advantages: high strength, convenient interfaces, high resistance to internal pressure, smooth inner surface, and good hydraulic properties. Disadvantages: Prone to corrosion, high cost. Connection method: Welding, flange connection. 2. Galvanized steel pipes (hot-dip): Used in fire protection systems and production water supply pipelines with a pipe diameter of 150 mm or less. Advantages: High strength, convenient interfaces, capable of withstanding high internal pressures, smooth inner surface, and good hydraulic properties. Disadvantages: Prone to corrosion, high cost. Connection methods: threaded connection, flange connection. 3. Copper pipes (red copper, brass): Used for hot water pipelines. Advantages: Good corrosion resistance in freshwater, high mechanical strength, strong flexural resistance, easy to process, smooth inner surface, resistant to scale formation, and attractive in appearance. Disadvantage: The wall is thin, making it prone to damage. Connection methods: threaded connection, flange connection, welding, special fittings. 4. Uses of stainless steel pipes: transporting pure water and corrosive industrial wastewater. Advantages: corrosion-resistant, impervious, good airtightness, smooth inner wall, light weight, easy to install, high-pressure resistant, vibration-resistant. Disadvantages: Difficult to install, expensive. Connection methods: compression type, clamping type, push-type (flexible type), tapered thread type, quick-connect flange type, flange type, screw connection type, socket connection type. 5. Uses of cast iron pipes: They are commonly used in pipeline projects for water supply, drainage, and gas. Classification: Based on their manufacturing methods, they can be divided into sand-cast centrifugal socketed straight pipes, continuously cast iron straight pipes, and sand-cast iron pipes. Based on the materials used, they can be classified into gray iron pipes, ductile iron pipes, and high-silicon iron pipes. Advantages: high strength, good toughness, thin walls, light weight, impact resistance, great bending capacity, and easy installation. Disadvantages: Aging, significant thermal expansion and contraction; not suitable for prolonged exposure to sunlight; poor compressive strength; improper construction can lead to deformation. Connection types: flexible interface, flange interface, self-anchoring interface, rigid interface.