There are many kinds of chemical pumps, such as centrifugal pumps and magnetic pumps with corrosion resistance. The selection of mechanical seals for liquid hydrocarbon pumps under special conditions is very important. The liquid hydrocarbon medium is a low temperature liquefied gas with low boiling point, low viscosity and high vapor pressure. The mechanical seal used under this condition will make the sealing material appear cold brittle, the water vapor in the atmosphere will freeze on the side of the sealing device, and the liquid film at the end of the friction pair is easy to vaporize. Especially when the medium leaks slightly, the leaked liquid hydrocarbon vaporizes immediately at the atmospheric side, takes away a lot of heat, and the temperature of mechanical seal environment drops sharply. The use of general sealing materials, such as rubber or PTFE, generally becomes brittle, leading to seal failure, leakage increases and irremediable.
So it is very important to use mechanical seal when choosing liquid hydrocarbon pump. Some enterprises adopt double-end mechanical seal and set up an isolation chamber at the media and atmosphere end, during which oil is sealed to alleviate the effect of low temperature. But this kind of structure is complex and needs sealing fluid system. According to experience, it is better to use bellows mechanical seals, which are mainly manifested in replacing bellows and flexible graphite with auxiliary sealing rings. The problems of cold brittleness, elasticity loss and cushioning of sealing ring materials are solved, such as DBM type, YH604/606/609 type, etc.
1. Metal bellows are made of Ha-C, AM350 and Carpenter 20 with good low temperature resistance, plasticity and toughness.
2. Because the vaporization of the end face of the friction pair has a great influence on the performance of the mechanical seal at low temperature, besides choosing suitable materials, it is advisable to reasonably select the specific pressure of the end face (mainly the compression of the bellows, which is 15% to 30% larger than the compression that is usually given in use). The mechanical seal element is put into the air side at 25 C. About the cooling water to improve the lubrication environment of friction pairs.
3. Material of friction pair L is selected in two special cases:
For continuous operation equipment, if there are more solid particles in the medium, the "hard-to-hard" structure (YG6-YG6 is selected in practice) is preferable. Generally, the continuous operation life is more than 8000 hours.
B. For intermittent running equipment, tungsten carbide or silicon carbide are selected as friction pairs for special graphite.