The DMF evaporative circulation pump is a key step in the purification of DMF. The pump population pressure is 0.15 MPa, the required circulation is 60 m3/h, and the pump head is about 40 m. The basic composition of the material is DMF 90%, dimethylamine, methanol, sodium methoxide (the material contains fine sodium methoxide solid particles). The temperature of the material is 160-180 °C. Compared with the reaction circulation pump, the working temperature is increased and the working pressure is greatly reduced. The shaft seal scheme of the double impeller seal of the auxiliary impeller (the inner mechanical seal is mechanically sealed with a metal bellows and the outer side is balanced with a spring mechanical seal) is also adopted.
After more than two years of use, the shaft seal scheme used is suitable. Under the premise of strictly following the operating procedures, the shaft seal replacement cycle is more than half a year. Compared with the original shielded pump or magnetic pump solution, the maintenance cost is greatly reduced, and the labor intensity of the maintenance worker is reduced.
For the shaft seal, in view of the pump operating temperature is less than 150 ° C, the conventional idea is to use a back-to-back double-end spring type mechanical seal, with pressurized methanol flushing Plan53, the flushing pressure is 0.05MPa higher than the pump seal chamber pressure, and 0.2MPa higher than the pump inlet pressure. Based on this, the double impeller seal of the auxiliary impeller (the inner mechanical seal is made of metal bellows mechanical seal and the outer side is balanced spring mechanical seal) is used as the shaft seal, and the structure is shown in Fig. 5. The basic idea is: using the centrifugal action of the auxiliary impeller to make the solid sodium methoxide particles leave the sealed cavity to purify the sealed cavity; at the same time, the auxiliary impeller can also reduce the sealing chamber pressure by 0.15 MPa, creating a good working environment for the mechanical seal. Thus, the flushing pressure is only 0.05 MPa higher than the pump inlet pressure.
In the actual use process, we found that the ambient temperature has a great influence on the working state of the pump, and the pump often has an evacuation phenomenon. The mechanical seal life is usually only about 2 months. The failure of the shaft seal is that the surface of the friction pair is cauterized and black, and a white self-polymer adheres to the outer surface of the sleeve exposed to the medium. The mechanical seal O-ring is jammed by the self-polymer, and the self-polymer on the sleeve interferes. The O-ring slides on the sleeve, and the mechanical seal must be replaced as a whole for each shaft seal failure repair. After the device was driven, although it was repeatedly repaired, it still failed to solve the problem well.
The mechanical corrugated pipe mechanical seal has no leakage of the auxiliary sealing ring, which reduces the leakage point and improves the reliability. By changing the traditional concept, a metal bellows mechanical seal can be used in certain specific room temperature media to achieve a satisfactory sealing effect. When the cost performance is better than the spring mechanical seal, it is worthwhile to promote the use of metal bellows mechanical seals.
Because under the high (low) temperature conditions, the auxiliary seal of the spring type mechanical seal will fail, such as the rubber material auxiliary seal will be aging at high temperature, the brittle crack will occur at low temperature; the PTFE auxiliary seal Thermal deformation occurs at high temperatures and cold flow occurs at low temperatures, so metal bellows mechanical seals have to be used at extreme temperatures. In fact, the metal bellows mechanical seal has no auxiliary dynamic sealing ring, which reduces the leakage point compared with the spring type mechanical seal, and the reliability is remarkably improved, and the advantage of being used as the shaft seal is obvious. However, the manufacturing cost of the bellows mechanical seal is relatively high. Due to economic factors, the application of the mechanical seal of the metal bellows is limited to the working conditions of high temperature or low temperature, and does not fully exert the advantage of high reliability.
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