【Major Equipment in the Chemical Engineering Sector】029 – Domestic production of mechanical seals for butadiene recycle gas compressors
Thread Content
The past decade has been a period of rapid development for China’s chemical industry, marked by the emergence of many new processes, products, and equipment. There has also been significant progress in China’s chemical processing equipment, which has contributed to the swift advancement of the petrochemical sector. Here, we have compiled some of the achievements and breakthroughs made by Chinese equipment manufacturers in terms of larger-scale production and greater use of domestic equipment. We invite everyone to share their insights and engage in discussions, to cheer for Made in China and to support the development of chemical processing equipment! ***********************************
The butadiene gas compressor J-201 in Maoming Petrochemical’s No. 2 butylene unit is a screw compressor manufactured by British Hawton; its imported shaft seals are of two-stage type (the compressor consists of a low-pressure stage and a high-pressure stage), with single-face contact mechanical seals. Apart from the difference in shaft diameter, the two types of seals have basically the same structure, material selection, and parameters. When the new unit was put into operation in 2006, significant leakage occurred in the secondary inlet seal. Considering factors such as high oil consumption, high prices of imported spare parts, and long delivery times, it was decided to use domestically produced seals instead. However, after the localization was completed and up to 2012, the average service life of the sealing products was around half a year. Despite several attempts by the local manufacturers to make improvements, no significant change was achieved. Eventually, they collaborated with Zhongmi Holding Co., Ltd. to carry out further sealing modifications, which were successful; today, the maximum continuous service life of these seals has reached nearly three years.
Which machine seal is used for the renovation of Dushanzi?
It’s just that no relevant information was found – medium density? ? ?
The butadiene gas compressor J-201 in Maoming Petrochemical’s No. 2 butylene unit is a screw compressor manufactured by British Hawton; its imported shaft seals are of two-stage type (the compressor consists of a low-pressure stage and a high-pressure stage), with single-face contact mechanical seals. Apart from the difference in shaft diameter, the two types of seals have basically the same structure, material selection, and parameters. When the new unit was put into operation in 2006, significant leakage occurred in the secondary inlet seal. Considering factors such as high oil consumption, high prices of imported spare parts, and long delivery times, it was decided to use domestically produced seals instead. However, after the localization was completed and up to 2012, the average service life of the sealing products was around half a year. Despite several attempts by the local manufacturers to make improvements, no significant change was achieved. Eventually, they collaborated with Zhongmi Holding Co., Ltd. to carry out further sealing modifications, which were successful; today, the maximum continuous service life of these seals has reached nearly three years.
For the sealing renovation of this unit, two main technical options are available: non-contact dry gas seals and contact mechanical seals. The contactless dry gas seal features low operational energy consumption, minimal sealing wear (as the sealing surfaces do not come into contact, resulting in no wear over extended periods of operation), and makes it easy to achieve long-term operation. However, to carry out the dry gas seal modification, it is necessary to introduce purge nitrogen in front of the process medium gas in order to prevent the medium from approaching the seal, thus avoiding the formation of coking substances caused by the process medium and liquid solvents, which could have an adverse effect on the seal. Adding a nitrogen injection system has two major drawbacks: (1) high loss of butadiene material. To achieve optimal operating conditions, the nitrogen injection rate for the first stage is estimated to be around 16 Nm3/h, while that for the second stage is estimated to be around 24 Nm3/h; in total, approximately 40 Nm3/h of nitrogen is introduced into the process gas. The process gas mixed with nitrogen enters T-203 (the second extraction distillation column), causing the accumulation of non-condensable gases (nitrogen) within the column. To avoid affecting operations, it is necessary to continuously discharge the condensed gases from the top of the column via E-213 to the flare, which inevitably results in the emission of butadiene gas as well, leading to significant losses of butadiene. (The process estimation indicates that 40 Nm3/h of nitrogen will carry away approximately 30 Nm3/h of butadiene gas; this results in a loss of about 566 tons of butadiene gas per year. At a market price of 9,000 yuan per ton, the annual economic loss is around 5 million yuan.) (2) When nitrogen is introduced into the process gas, nitrogen itself also contains trace amounts of oxygen; these accumulate within the tower, causing polymerization of butadiene in T-203 and affecting production. Therefore, the technical approach of non-contact dry gas seal at the end face was abandoned in the modification, and the technical approach of mechanical seal with end-face contact was chosen instead.
The process medium for the J-201 gas compressor unit mainly consists of: 96% butadiene gas, 3% other gases (isobutylene, alkynes), and 1% DMF as a polymerization inhibitor. The main difficulty in shaft seal design is that butadiene gas tends to undergo self-polymerization at certain temperatures, resulting in the formation of \"carbon deposits\" which block the compensating elements of the mechanical seal, prevent the sealing surfaces from fitting properly together, and cause leakage at those surfaces ； The accumulation of carbon deposits at the sealing end face can also cause abnormal wear of that end face, exacerbate heating at it, and accelerate seal failure. The polymerization inhibitor DMF is a medium with strong permeability; once it penetrates into the end faces of graphite, the frictional heat generated there can cause the graphite material on those end faces to peel off ； The medium composed of butadiene and DMF also has a certain degree of permeation and corrosive effect on the auxiliary sealing ring, accelerating the aging and failure of the rubber material. Restricted by the inherent conditions of the unit – the space available for shaft seal installation, the layout of the shaft seals, and the auxiliary flushing systems for these seals are all fixed and cannot be changed; therefore, solutions to problems such as carbon buildup and DMF penetration can only be found within the existing constraints.
The 【Haichuan Equipment Guessing】 series is being updated continuously in 2022; all Haichuan community members are welcome to participate actively! https://bbs.hcbbs.com/thread-2079686-1-1.html (Source: Huahai Chuanliu – Haichuan Chemical Industry Forum) Summary post on the series 【Major Equipment in the Chemical Equipment Industry】 https://bbs.hcbbs.com/thread-3312079-1-1.html (Source: Huahai Chuanliu – Haichuan Chemical Industry Forum) Summary post discussing the components of cracking furnace equipment for 【Ethylene Cracking】 https://bbs.hcbbs.com/thread-3308724-1-1.html (Source: Huahai Chuanliu – Haichuan Chemical Industry Forum)