What are the sealing methods of melt pumps and an explanation of the principle of packing seals?
Among the sealing forms of melt gear pumps, the combination of packing seals and mechanical seals is a commonly adopted form. Compared with spiral seals, combined seals feature a simple structure, easy installation and removal, and can comprehensively leverage the advantages of both packing and mechanical seals.
Packing seals are often used in shaft seals, but the selection of packing material, sealing performance, and structural design all directly affect the effectiveness of the seal. The essence of packing seals lies in utilizing the soft nature of the sealing packing material. Under axial compression force, it radially obstructs potential leakage pathways for the melt. The force applied to the packing seal must generate contact stress between the packing and the sealed shaft surface, capable of blocking the pressure of the sealed melt. Therefore, it is crucial that the radial contact pressure at the bottom of the stuffing box must be greater than or equal to the pressure of the sealed melt. This is a vital theoretical criterion in structural design.
Packing seal assemblies typically consist of a stuffing box containing soft packing material and a gland. The soft packing material can be asbestos rope impregnated with oil or coated with graphite, or high-temperature-resistant carbon fiber. It is placed between the stuffing box and the pump shaft, and the gland is tightened to compress it and achieve deformation for sealing.
Conditions that packing must meet:
It must have a certain degree of plasticity, capable of generating a certain radial force and closely contacting the shaft under compression.
It must possess sufficient chemical stability, not contaminating the medium. The packing should not swell due to soaking in the medium, the impregnant in the packing should not be dissolved by the medium, and the packing itself should not corrode the sealing surface.
It should have good self-lubricating properties, be wear-resistant, and have a low friction coefficient.
The packing should have sufficient floating elasticity to accommodate slight eccentricity of the shaft.
It should be simple to manufacture and easy to install.
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