Liquid hydrocarbon medium is a cryogenic liquefied gas, has a low boiling point, low viscosity, high vapor pressure and other properties. In such conditions the use of the mechanical seal, seal material will be cold brittleness, water vapor in the atmosphere freezes on the air side of the sealing device, the friction face film easy gasification. Especially when the medium is slightly leaks, leakage of liquid hydrocarbons in the atmosphere immediately gasification side, taking a lot of heat, a sharp decline in mechanical seals ambient temperature, generally sealing material such as rubber or PTFE generally brittle, leading to seal Failure leakage increases.
Some companies use double mechanical seal between the medium and the atmosphere set up a quarantine room, during pass to seal oil, to mitigate the effects of low temperatures. However, this structure mechanical seal installation complexity, and the need to seal with liquid systems, temperature is also limited, the price is relatively high.
Based on actual experience, the use of metal bellows mechanical seal is better, mainly in the metal bellows and flexible graphite secondary seal in place to address the occurrence of cold seal material brittle and lose elastic and buffering problems, such as the use DBM type, YH604 / 606/609 type. Using a bellows on the liquid hydrocarbon pump mechanical seal should pay attention:
1. The metal bellows material selection, low temperature resistance, toughness, good Hastelloy-C (Hastelloy), AM350, Carpenter20 like;
2. Rational structure designed to ensure the mechanical seal in the case of high speed without damaging the seal face and bellows assembly;
3. Rational face contact than the pressure to ensure that the mechanical seal sealing performance and prevent premature failure;
4. Reasonable friction material to ensure the normal mechanical seal friction properties and sealing properties at low temperatures, to avoid excessive wear and premature failure;
Friction material in the selection of two special cases:
① If they contain more solids, then the choice of continuous operation of the device, the media "hard on hard" structure is better, in practice selected the YG6- YG6, general life in continuous operation 800Oh above;
② Intermittent operation of equipment, tungsten carbide or silicon carbide friction choice of specialty graphite.
5. Pairs of metal bellows portion appropriate heat treatment to eliminate residual stress caused by welding in order to avoid long-running process fatigue cracking.
6. Since at low temperatures, the impact of gasification friction face a lot of mechanical seal performance, in addition to select a more suitable material, reasonable selection of face pressure, mainly the amount of compression of the bellows, generally higher than those normally used compression amount to 15% to 30% is appropriate. In the mechanical seal elements close to the atmospheric side through the cooling water about 25 ℃ to improve Lubrication environment.
Report: cryogenic pump mechanical seal material combinations
Liquefied petroleum gas
Furan resin impregnated graphite + Stellite special alloy (light load)
Furan resin impregnated graphite + Carbide (overloading)
LOX
Graphite impregnated furan resin + metal chrome (light load)
Furan resin impregnated graphite + Carbide (overloading)
Liquid Nitrogen
Graphite impregnated furan resin + metal chrome (light load)
Furan resin impregnated graphite + Carbide (overloading)
Metal alloy liquid oxygen liquid nitrogen used austenitic steels

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