Discussion on Technology Development of CNG Natural Gas Compressor

1 The rapid development of CNG

As of 1999, there were nearly 70 CNG stations in 12 demonstration provinces and cities across the country, and the number of clean vehicles (NGV) was less than 10,000. After four years of rapid development, as of October 2003, there were 16 stations nationwide. There are 273 CNG stations in demonstration provinces and cities, 74,852 NGVs, 282 LPG stations, 89475 LPGVs.

   According to statistics from Sichuan Province at the end of 2003, there were 156 CNG stations and 45,000 NGVs. In Sichuan Province, since 1989, the China National Natural Gas Corporation has introduced Australian main engines and domestically-made equipment to build the first CNG charging station for vehicles. After ten years of research and development, it has embarked on a successful road. With the progress of the West-East Gas Pipeline, the CNG business is bound to move towards a healthier development process. The West-East Gas Pipeline plans to cover 120 cities in 2005 and 260 cities in 2010. CNG will take root and bear fruit in 331 cities along the main artery of the West-East Gas Pipeline. For the faster and more healthy development of CNG, this article discusses the technology development of natural gas compressor for CNG station, one of CNG's core equipment.

  2 The current problems and technical development of natural gas compressors for CNG

   There are many shortcomings in the natural gas compressors currently used in CNG stations. According to the current new technology development direction, there are indeed many areas for improvement.

  2.1 Insufficient understanding of oil-free purification of compressed natural gas

First, CNG station imports Australia PRESCON 3D350/6D/75R natural gas compressor; intake pressure is 0.17MPa, exhaust pressure is 24.8MPa, speed is 575r/min, displacement is 250Nm3/h, 3-stage compression, W type, adopts Splash lubrication. So far, the imported Italian SAFE Sr1 W compressor and SW1 32F2-EM are still splash lubricated. This backward technology orientation has caused some domestic CNG natural gas compressor manufacturers. Although their compressors occupy a certain market for CNG, they have not made efforts to completely oil-free gas. Some have begun to engage in oil-free structures, and after a few years, they have regressed to oil lubrication. Some advertise to users that there is no oil, the vertical arrangement is adopted, the cylinder is not filled with oil, etc., but the thin oil in the fuselage runs up the piston rod into the packing and enters the cylinder, polluting the gas. This type of compressor should be correctly defined as less oil . If it has an oil-free structure, at least an oil sling ring should be installed on the piston rod. The oil sling ring moves up and down with the piston rod. Therefore, the height of the intermediate cylinder must be greater than one reciprocating stroke S, and the oil scraping device must Scrape the thin oil of the fuselage and return it to the fuselage. It is not allowed to enter or climb up along the piston rod and then into the cylinder through the packing.

If the exposed piston rod at the junction between the middle body of the fuselage and the cylinder is blackened for a period of at least 20~30mm after a period of operation, while the rest of the part is shiny, this phenomenon must be absolutely oil-free.

In order to realize that the compressor gas is oil-free, companies such as Nuovopiginone in Italy and Nowalk in the United States have designed the machine as an integral skid-mounted, vertical oil-free structure and adopts the long single-chamber connector specified by AP1618. It is used for an oil-free lubrication structure and has enough To prevent oil from entering the cylinder, the piston rod is equipped with an oil flinger made of spark-proof material, and the ring is generally designed as a split structure. For the horizontal type, the oil-free structure must be realized, and the cylinder packing must be oil-free. Long and short double-chamber connectors should be used, and the leakage of flammable and explosive gases can be recovered. The packing seal can reach a leakage level of at most five ten thousandths. If an ordinary small-scale CNG station uses 10000Nm3/d of gas, the leakage can reach 5Nm3/d. Therefore, from the viewpoint of energy saving, the leakage should be recovered.

  The fundamental solution to the problem of CNG gas carrying oil is to use an oil-free compressor. Because no matter how well-designed the oil eliminator is, it can only remove oil mist droplets with a diameter greater than 1 μm in the gas, but it is impossible to remove all the oil mist in the gas.

   Gas with oil will contaminate the storage container, contaminate the desiccant, cause the car engine to ignite prematurely, and cause the pressure regulating valve in the fuel supply system of the car to cement. Even when the exhaust temperature is too high, carbon deposits and unsafe accidents occur.

  2.2 About adopting the concept of inverted differential piston

Some CNG compressors are designed to reduce the height. For example, when the intake pressure is 0.3MPa and the exhaust pressure is 25MPa, two rows of four-stage compression are designed into two inverted differential cylinders (refer to the diagram). This kind of design scheme is impossible to realize the cylinder packing oil-free lubrication, because according to the formula of piston rod diameter when inverted

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 Obtain d——The diameter of the piston rod mm when the difference is reversed

  Di-1-the diameter of the adjacent low-pressure stage (cover side) cylinder

  Psi——is the shaft side suction pressure MPaA

  Psi-1-the suction pressure on the cover side MPaA

The inverted differential design is aimed at the nitrogen and hydrogen compressors used in chemical fertilizer companies for ammonia synthesis, which need to use four rows of six-stage compression to appear Ⅲ-flat-Ⅴ, Ⅳ-flat-VI, low pressure gas is passed into the balance section, and high pressure grades Ⅲ, Ⅴ, Ⅳ, Ⅵ Leakage to the balance section, especially the short life of the Ⅴ and Ⅵ grade rings, makes the leakage worse, and even causes more than 10% of the gas transmission loss due to the leakage, which reduces the production of synthetic ammonia by more than 10%. The four-row compressor avoids high-pressure filling without a scheme. The inverted stage difference is actually equal to a double-acting cylinder, the length of the cylinder axis is the shortest, so the rigidity is the largest, the piston force is uniform, the working conditions of the piston ring are the best, and the leakage is less. It is also the most convenient in terms of manufacturing, installation and maintenance.

But for the inverted differential structure, the cover-side pressure ratio cannot be high. Generally, the cover-side pressure ratio is ≤2. For two inverted stages, the pressure ratios of the I and III stages on the cover side are both ≤2, then II, The pressure ratio of grade IV must be increased. A large pressure ratio means a higher exhaust temperature. The exhaust temperature of grades II and IV can reach about 160℃ when compressed natural gas, and may reach 180℃ when the air is tested. In this case, filling with PTFE will never pass the test, and oil-free will inevitably become a bubble.

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Therefore, it is better to adopt the forward differential design in the first and second stages, which is more reliable than the reversed differential structure, because the first and second stages are low-pressure stages, and the higher pressure ratio for the first stage is still set at the [PV] value (product of pressure and speed) Within the allowable value range of Ⅰ, due to the lower inlet temperature of the first stage, the temperature of the first stage exhaust is still within the oil-free permission range. The temperature at other levels is reduced accordingly. Of course, the current 3-column vertical 4-stage compression scheme not only achieves height reduction but also realizes reasonable pressure ratio distribution.

  2.3 Inadequate understanding of friction heat of oil-free gas compressors

   Insufficient understanding of the frictional heat of oil-free gas compressors is the main reason for the failure of oil-free compression, especially the inverted piston rod. The author suggests that the center of the column of piston rods is drilled with oil to cool the friction surface of the packing seal and the piston rod directly, which can conduct frictional heat in time, and the center hole piston rod structure can not only reduce the reciprocating weight of the column, but also increase the piston rod.的rigidity. Then use the stainless steel stuffing box to pass water for cooling and conduct the compression heat, which can realize the long life of the stuffing seal ring.

The cylinder mirror surface can be cooled by a water jacket. In 1965, the Italian company Simperon filled the cylinder cooling water jacket with diethylene glycol ether, and then put a cooling coil into the water. The hot gas was guided by the diethylene glycol ether. Because the diethylene glycol ether is a highly water-absorbing ether, the cylinder mirror is easy to sweat when the compression cylinder is stopped, and the diethylene glycol ether can keep the mirror dry and not rust.

  2.4 CNG compressor valve structure problem

   The latest air valve structure fungus valve or poppet valve (POPPET VALVE) used on CNG compressors is based on PEEK polyaryletherketone plus filler glass fiber and carbon fiber injection molding production process fungus valve. Victex, the world's leading industrial aromatic polyketone producer, was established in 1993 to produce PEEK since its demand has been increasing. The United States is the largest market, accounting for 48% of sales. The Grinded Company in Hartfield, Sauvinia, USA specializes in the production of compressor seals, PEK mesh valve discs, and fungus valves for Borqig, Cooper Energy, Dresserland and other companies. The metal is light, has a damping effect and can cushion the impact, so it has a long life. Generally 8000h life is guaranteed. The gas motorcycle compressor imported from my country's oil and gas fields has been used and verified before 1997. In July 1997, HOERBIGER (Shanghai) Co., Ltd. has provided users with samples of POPPET VALVE. However, the price is too high for users to bear. In contrast, the order from the United States Grinded Company is only 1/5 of the price. At present, domestic production is in small batches in Wenzhou, Zhejiang, and users are in the stage of life and reliability verification.

  2.5 Application of bellows and corrugated tube coolers

Since 1996, corrugated tubes have been used to replace old-fashioned tube heat exchangers in the ammonia synthesis process of small and medium-sized nitrogen fertilizers. Thin-walled stainless steel corrugated tube elements are used. The heat transfer coefficient is 2 to 3 times that of the straight wall method. The corrugated tubes are flexible elements. , It has automatic compensation performance in the occasion of large temperature difference and large pressure difference, small tensile and compression stress on tube sheet and cylinder, strong adaptability to changes in working conditions, safe and reliable for medium and low pressure; due to the smooth arc on the surface of the bellows Inhibit the formation of scale, and has the ability to clean itself; due to its high heat exchange capacity, it can reduce the volume and floor space of the equipment, reduce the amount of cooling water, and reduce operation and maintenance costs; independent and completely free forming process, corrugation forming process There is no forced deformation, no stress concentration, small residual stress, uniform stress distribution, and no intercrystalline defects.

   The corrugated tube type is composed of a corrugated section and a straight section intersecting. It still has the ability of self-cleaning. The tube wall is thick and can develop towards high pressure.

3 Sichuan Huaxi General Machinery Company vigorously promote CNG technological innovation

   The central government pointed out in its recommendations on the Tenth Five-Year Plan that in accelerating the technological transformation of traditional industries, it is necessary to "vigorously revitalize the equipment manufacturing industry." Sichuan Huaxi General Machinery Co., Ltd. is a professional enterprise in the design and manufacture of national petrochemical equipment and Type 1, 2, and Type 3 pressure vessels. In recent years, the company has conducted extensive investigations and technical screenings for CNG station compressors, and successively developed 2Z3.5 And 3Z3.5 vertical oil-free CNG compressor, the two models cover various working conditions such as the inlet pressure of 0.2MPa, 0.4MPa, 0.8MPa, 2.5MPa, etc., and the gas volume covers 340Nm3/h to 1500Nm3/h In the development of natural gas compressors for CNG, the 2ZW3.5-1.1/8-250 and 3Zw3.5-3/3-250 natural gas compressors were installed at the 3 CNG stations of Tongliang, Jianyang and Jiangyou respectively. Put into user assessment and make further technical improvements.