Design and Research of Mechanical Seal for Cryogenic Centrifugal Pump

1 Introduction Cryogenic liquid refers to the gaseous liquid phase of hydrocarbons with boiling points below -100 ° C for oxygen, nitrogen, air, hydrogen, methane and the like. With the rapid development of the petrochemical industry and the air separation industry, cryogenic liquids are being continuously applied in various chemical processes and processes. As a result, cryogenic liquid transfer pumps have become one of the key devices for these process units. For the cryogenic pump, one of the most important problems is the pump shaft seal, due to the restrictions of low temperature materials, easy to vaporize the working fluid and other factors, making the pump shaft seal is very difficult to solve. 2 sealed structure design For large cryogenic liquid pump, generally take vertical pump structure, which is conducive to the structural design of the shaft seal and seal protection system layout, but the shaft seal equipped with a very large system, not discussed here; Only on the flow in the 5-15m3 / h, Lift ≤ 120m tank truck filling, unloading pump and process within the scope of the pump seal for research. 2.1 sealing parameters: Pressure: 1.3 MPa Temperature: -196 ° C Speed: 2900 rpm Sealing shaft diameter: 35mm Medium: liquid oxygen, liquid nitrogen and liquid hydrocarbons 2.2 structural type and seal protection According to the characteristics of the working fluid, in the cryogenic liquefied gas pump, there are usually the following types of seals to choose from: a. Mechanical seal b. Maze inflatable seal c. Packing seal. Labyrinth air-tight seals require longer axial dimensions, which are too complex for tank filling and unloading pumps. Filler seals have a short service life and a large amount of maintenance and are therefore not considered when designing sealed designs. In accordance with the usual selection of the principle of sealing, the above three types of sealing a comprehensive comparison, the final choice of welded metal bellows mechanical seal. Taking into account the low temperature characteristics of the medium (easy to vaporize, the viscosity is small, the seal face is not easy to form a liquid film), in order to prevent the mechanical seal end face due to evaporation of the working fluid to cause dry friction, the pump inside the outlet and seal cavity open, Of the American Petroleum Institute standards API610 Appendix D "Mechanical Seal and Piping Schemes" in the API program 1; the principle is to rely on the differential pressure to flush the seal face and take away the friction heat, to maintain the sealing liquid state to maintain the seal End friction surface lubrication; in order to avoid freezing parts of the shaft seal, gland position in the mechanical seal With a nitrogen purge, a small amount of seal fluid can be taken away to ensure safe operation. As the seal on the assembly requirements are high, the assembly accuracy will directly affect the performance of the seal, seal design Into integrated unit components for quick disassembly. 2.3 sealed structural features Ultra-low temperature pump work, always in the state of the transmission of saturated liquid, prone to cavitation and vibration due to the evaporation of the working fluid; Therefore, the design of mechanical seals must consider its floating and followability. The metal bellows seal structure adopts static structure, eliminating the traditional moving ring seal, increasing the floating ring, can compensate for pump shaft corrosion and vibration caused by the deflection of the seal failure, and can pump operation And stop, always keep the seal face effective contact. Double-layer wave plate is used to drive and drive the bellows structure, and the torque is forcibly transmitted by dialing to avoid wave plate twisting. The overall structure of the moving ring can effectively reduce temperature deformation and force deformation. 3 material selection 3.1 metal materials Select the metal elastic wave plate material, we must consider the material under the conditions of low temperature brittle brittle and withstand mechanical alternating load capacity; According to the relevant test data: nickel-containing high alloy materials, copper, aluminum, at low temperatures Therefore, the ultra-low temperature metal materials are mainly made of high nickel alloy, copper and aluminum, while the metal material used in liquid oxygen is the best of bronze, which can effectively avoid the fire caused by friction or explosion. 0Cr18Ni9Ti material compared with AM-350, the former nickel content of the latter 2 times, its lower temperature characteristics are better than the latter; Finally, the use of 0Cr18Ni9Ti as a seal Wave plate material. 3.2 friction material Based on experience choose friction pair matching materials: Static Ring Material: YG8 (Carbide); Movable Ring Material: M158Sb (Graphite Dip Antimony) Can be competent high-speed, high load, low temperature working conditions. 3.3 Auxiliary sealing material Due to long-term work in the ultra-low temperature conditions, according to the current domestic materials, through screening, comparison, to determine the use of flexible graphite gasket material. It is important that all materials be cryogenic before they are processed or assembled. 4 sealed industrial operation test and failure analysis Seal design and processing of the above parameters into products, initially used in liquid nitrogen tank truck unloading pump, after 4-5 hours of operation, the mechanical seal leaked; remove the seal can be seen bellows cracking. Detection and judgment: (1) Check the factory parameters, the free height of the bellows has not changed; (2) The friction pair (narrow ring) breaks and the outer circle severely bruises; the friction pair (narrow ring) inlaid off, (3) the appearance of the bellows test Corrugated ring seat with the largest outer friction track; corrugated pipe inside and outside the weld was not scratched or scratched marks; corrugated pipe first round of weld fracture. Possible reasons for the above situation: The shaft is not in the same position as the sealed cavity; the gland thread and the shaft are not in the same center and other reasons, the movable ring and the shaft incline, resulting in the inclination of the stationary ring and the bellows ring seat attached with the gland ring and the contact of the bellows ring seat and the gland. In summary, the reasons for the seal failure is: pump installation or pump parts machining error caused by the bellows ring seat and gland cavity tilt and produce contact, resulting in sealed end face tilted open, resulting in leakage; continue to use this When the bellows ring seat at the maximum inclination of the continuous contact with the gland, resulting in the bellows to withstand very large torque during operation of the graphite ring pressure fracture damage, which led to the bellows rupture caused by the impact. 5. Conclusion According to the above analysis results, re-processing of the seal, the gland hole increases, so that the gap with the bellows seat ring seat to adjust to 0.2mm, rigorous inspection of the assembly shaft radial runout, the axial movement and The verticality of the shaft and the back cover so that it reaches the standard value. Re-install the mechanical seal running well, the average life expectancy reached more than three months, in the development of cryogenic pump mechanical seal process, has used the AM-350,0 Cr18Ni9Ti and other wave plate material, the experimental results show that r18Ni9Ti materials can be used at long-term low temperature conditions . But the domestic wave plate material, due to the different chemical composition of each batch, the welding process is more difficult to grasp, causing great trouble to the welding, the yield is low, the proposed use of imported sheet. 【Print】 【Close】