Essential Series on Pump Selection and Piping(5)

                                                    Essential Series on Pump Selection and Piping(5)

7. Impact of Sealing

Leak-free operation is an eternal pursuit in chemical equipment, and this demand has driven the increasingly widespread application of magnetic and shielded pumps. However, achieving true leak-free performance still requires a long journey, such as addressing the lifespan issues of magnetic pump isolation sleeves and shielded pump shielding sleeves, material pitting problems, and the reliability of static seals. Below is a brief introduction to some fundamental aspects of sealing.

1. For static sealing, there are typically only two forms: sealing gaskets and sealing rings, with O-ring seals being the most widely used. For dynamic sealing, packing seals are rarely employed in chemical pumps, with mechanical seals being the primary choice. Mechanical seals can be further categorized into single and double mechanical seals, as well as balanced and unbalanced types. Balanced seals are suitable for sealing high-pressure media (generally referring to pressures exceeding 1.0 MPa), while double mechanical seals are primarily used for high-temperature, crystallizable, viscous, particulate, or toxic volatile media. For double mechanical seals, isolation fluid should be injected into the sealing chamber, with its pressure typically maintained 0.07–0.1 MPa higher than the media pressure.

2. Sealing materials: The static sealing of chemical pumps typically uses fluoroelastomer, while polytetrafluoroethylene is only employed under special circumstances. The material selection for the dynamic and static rings in mechanical seals is particularly critical. Simply pairing hard alloys with hard alloys is not optimal—high costs are one aspect, but the absence of hardness differences between the two is also unreasonable. Therefore, it is best to tailor the choice based on the characteristics of the medium.

8. Other Influencing Factors

In general industrial processes, the leakage in the pipeline system can be neglected for pumps, but the impact of process variations on flow rate must be considered. For agricultural pumps using open channels for water conveyance, seepage and evaporation losses must also be taken into account.

Pressure: suction tank pressure, discharge tank pressure, pressure differential in the pipeline system (head loss).

Pipeline system data (pipe diameter, length, types and quantities of pipe fittings, geometric elevations from the suction tank to the pressure tank).

If necessary, the device characteristic curve should also be prepared.