Due to the wide variety of pipe conveying gas, their performance is different, so their use is also different. The designers and constructors of gas pipelines should select the pipes correctly according to the properties of gas medium, system pressure and construction requirements, and meet the basic requirements of mechanical strength, corrosion resistance, seismic resistance and air tightness. The basic requirements of the pipe are:
- It has enough mechanical strength and tightness under the action of medium pressure and temperature;
- Good solderability;
- When the working condition changes, it has the corresponding elasticity and stability to the thermal stress and the external force;
- Durability against internal and external corrosion;
- Good anti-aging, long life;
- The inner surface roughness is small and the medium erosion is avoided;
- Low temperature deformation coefficient;
- The connection between pipes or fittings should be simple, reliable and tight;
- Transport, storage and construction should be simple;
- Adequate source of pipe, low price;
In gas engineering, metal pipe is widely used because of its high mechanical strength, thin wall, convenient transportation and easy construction.
Properties of Pipes
1.1 steel pipe
Commonly used steel pipe has ordinary seamless steel pipe and welded steel pipe, with load bearing stress, good plasticity, easy welding advantages. Compared with other pipes, the wall thickness is thinner, saving the amount of metal, but the corrosion resistance is poor, The steel pipe has high strength, good plasticity and toughness, strong ability to withstand impact and vibration load, can be constructed by welding method, easy to install; But easy corrosion, engineering maintenance costs.
1.1.1 seamless steel pipe. Ordinary seamless steel pipe with ordinary carbon steel, quality carbon steel and low alloy steel rolled from. According to the manufacturing method is divided into hot rolled and cold rolled seamless pipe. Cold-rolled seamless steel pipe has an outside diameter of 5 ~ 200mm of various specifications. The outside diameter of hot-rolled tube is 32 ~ 630mm. General seamless steel pipe for a variety of pressure levels of urban gas pipelines and gas plant process pipeline, because there is no production of high-quality large-caliber seamless steel tube equipment, seamless pipe only in DN ≤ 250mm gas pipeline.
1.1.2 welded steel pipe. Also known as welded seamless steel pipe. The main straight weld pipe (DN200 ~ DN1800) and spiral welded pipe (DN 200 ~ DN 700) two. Straight seam welded pipe is made of steel plate and welded by electric arc welding. Spiral seam welded pipe is made of steel plate spiral rolled after welding. Material to low – carbon steel (Q 235) and low – alloy steel (16 Mn) – based.
When the selection of steel pipes, when the diameter of 150mm or less, the general use of low-pressure fluid transmission welded steel pipe, It is a kind of small diameter pipe most commonly used in gas pipeline engineering, and is suitable for conveying various low pressure gas medium (such as indoor gas pipeline). According to the different thickness of the pipe wall, it can be divided into three types: thin-wall pipe, ordinary pipe and thickened pipe. Thin – wall pipe shall not be used to convey gas medium, but can be used as casing pipe. Ordinary pipe working pressure PN = 1. 0 MPa, thick-walled tube working pressure PN ≤ 1. 6MPa； Large diameter pipe mostly adopts spiral welded pipe, according to the pressure level of the medium is divided into class A pipe and class B pipe two kinds. Class A pipe is often used to transport oil, natural gas and other high-pressure media, B pipe used as low-pressure fluid transport pipe. The maximum working pressure of spiral welded steel pipe is generally not more than 2. 0 MPa, minimum diameter of 219 mm, maximum diameter of 1420 mm, length is usually 8 ~ 18 m.
Steel pipe wall thickness should be depending on the location, soil and traffic load and to be selected, not less than 3.5mm, such as in the street red line inside the not be less than 4.5mm. When the pipeline passes through the important obstacles and the soil corrosion is very strong section, the wall thickness should not be less than 8mm. Indoor pipe wall thickness is not less than 2. 75mm.
1.2 Cast iron pipe
Cast iron pipe has strong corrosion resistance. Cast iron gas pipe fittings for gas transmission and distribution pipelines are generally manufactured by casting or centrifugal casting. The tensile strength, bending resistance, impact resistance and welding performance of gray iron pipe are not as good as steel pipe, but because of its good corrosion resistance, Long service life (generally up to more than 50 years), simple production and other characteristics, in the city, the low pressure gas pipeline is still widely used. For the transmission of gas medium cast iron pipe generally need to do air tightness test.
With the development of nodular cast iron casting technology, the mechanical properties of cast iron pipes are greatly enhanced, thus improving the starting safety and reducing maintenance costs. The diameter of cast iron pipe abroad is 40 ~ 2600mm and the length is 2 ~ 9m. Various pipe diameters have been serialized. The highest gas pressure is 0.7 ~ 1.6 MPa.
Domestic common pressure continuous cast iron straight pipe, centrifugal socket straight pipe and pipe fittings, diameter 75 ~ 1500mm, wall thickness 9 ~ 30mm, length 3 ~ 6m.
In order to improve the seismic performance of cast iron pipe and reduce the difficulty of interface operation and labor intensity, the flexible mechanical interface cast iron pipes developed in China have been widely used. The diameter of the pipes is 100 ~ 500 mm and the experimental pressure of air tightness can reach 0.3 MPa. 1.3 plastic pipe
Plastic pipe has a series of advantages, such as corrosion resistance, light weight, low resistance to fluid flow, long service life, simple construction, large tensile strength and so on. Developed countries use medium density polyethylene and nylon – 11 plastic pipes in natural gas transmission and distribution system.
There are two types of plastic pipes that can be used as gas pipelines: one is thermoplastic pipes, such as polyethylene (PE) pipes, polyvinyl chloride (PVC) pipes and polybutylene (PB) pipes. At present, the most widely used is polyethylene (PE) pipe, polyethylene has high, medium and low density points, usually use medium density polyethylene as pipe material. PE pipe has the following advantages:
(1) corrosion resistance, life of up to 50 years, no electrochemical corrosion phenomenon, unlike the steel pipe to make external corrosion layer;
(2) light weight, can be rolled, low transportation and installation costs:
(3) the interface adopts electric hot melt or hot melt connection, the construction is safe, fast, low labor intensity and easy to handle the accident;
(4) The inner wall is smooth, its equivalent absolute roughness K is only 1 / 20 of steel pipe, the friction resistance is small, the gas transportation capacity is 30% higher than steel pipe;
(5) has a good flexibility, compliance, making PE pipeline system can effectively resist land subsidence, earthquake and other hazards, facilitate trenchless construction.
The maximum working pressure of PE pipe: for medium and high density PE pipes, if thickened pipe SDR = 11, PG ≤ 0.4MPa; If SDR = 17, PN ≤ 0. 25MPa。 At present, China produces medium and high density PE pipe SDR = 17. 63； Enhanced SDR = 9.33 ~ 12. The utility model can be used for medium and low pressure gas pipelines.
However, although the elongation of PE pipe is higher than steel pipe, the tensile strength is only 1 / 5 of steel pipe. The performance of bearing pressure, shearing force and bending force is lower than that of steel tube. Temperature has a greater impact on its working pressure, the ability to adapt to the temperature difference is worse than steel pipe.
The current maximum working pressure of plastic pipes used on gas pipelines is 0. 3 MPa, 0.4-0 in a few countries. 6 MPa, maximum operating temperature of 38 ° C. Because it is not as rigid as the metal pipe, so the buried construction must tamp trench bottom, in order to ensure the pipeline slope requirements. Although the plastic pipe is easy to break by violent collision, but easy to find and repair in time, The installation cost of plastic pipe is lower than that of steel pipe, such as the construction of DN100 pipe, the construction cost per meter can save 7%, DN50 pipe per meter to save about 17%.
1.4 Other tubes
Sometimes non-ferrous metal pipes, such as copper and aluminium, are also used. Because of the high price of copper tube and aluminum tube, they can not be widely used in gas transmission and distribution. In the indoor pipeline can also use aluminum soft connection pipe and aluminum-plastic, steel-plastic composite pipe, in some respects have better than the characteristics of plastic pipe.
Selection of Gas Pipes
- Pipe selection depends on gas pressure.
Polyethylene pipe, ductile iron pipe with mechanical interface, steel pipe or steel skeleton polyethylene plastic composite pipe should be adopted for medium and low pressure gas pipeline, and should conform to the current national standards. - Choose pipe according to gas properties.
The main sources of urban gas include natural gas, liquefied petroleum gas and artificial gas. In addition to different conveying pressures, the composition and physical and chemical properties of various gas sources are also very different. Therefore, it is necessary to choose pipe according to gas properties. It mainly considers the influence of gas composition on various pipe materials and the effect of gas physical and chemical properties on pipeline transportation. Generally speaking, liquefied petroleum gas pipe requirements of the most stringent, followed by artificial dry gas, wet gas, natural gas - Influence of other factors on pipe selection.
The selection of gas pipe must consider the geographical location, topography and soil properties of the pipeline. For example, for the high water table, easy settlement zone, try not to cast iron pipe, and heating pipe for the distance closer to the zone, as far as possible without plastic pipe. In addition, the choice of pipe should also pay attention to the surrounding environmental factors, such as the temperature of the PE pipe to allow the working pressure is greater, the temperature difference in large areas should not use PE pipes.