WHAT IS A SOFT SEAL?
A soft seal, also called braided packing, is the material that physically fills the gap between the shaft and the housing. The braid wound around the shaft looks like a rope.
These seals vary with the area they are used in; the selection follows the fluid, the pressure and the temperature at that point.
In broad terms, soft seals consist of yarn and a lubricant. The performance of these two materials affects how durable the seal is. The purpose of the seal is to stop the medium moved by the pump from escaping between the pump casing and the rotating shaft. Soft seals are also used to prevent air being drawn in from outside.
Another property of soft seals is that they are conductive: they carry the heat generated by friction into the housing. In some applications a flush fluid is used with the soft seal to cool the frictional heat of the shaft.
Types of soft seal
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Graphite braid: produced by braiding high-quality PTFE yarns impregnated with graphite, this packing is ideal for general-purpose use on every type of pump in industry. The natural lubricity of graphite gives it a low coefficient of friction. It is used mainly across a broad chemical spectrum, from petroleum and its derivatives to synthetic oils and from acids and bases to solvents, water, steam and dry air, where its high resistance gives long service life.
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Pure carbon: a packing braided in interlock form from pure carbon fibers impregnated with a PTFE lubricant. The lubricant used in its production keeps friction to a minimum. It is used in abrasive environments containing acidic and caustic chemicals, in mixers, autoclaves and pulp pumps. Beyond these sectors, the packing is also widely used on water pumps and hot and cold water pumps in the steel industry.
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PTFE fiber braid: made from fiber braid and a lubricant. It is braided by the interlock horizontal method and offers very high tensile strength. It offers safe use in many applications, above all with extremely corrosive chemicals at high temperature. Because it is white, it prevents color contamination. It can be used with fluids such as fuel, oil, all solvents, monomers and wastewater.
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Graflex: a pure graphite packing with a low coefficient of friction and long service life that can be used on pumps and on valves alike. It runs with low leakage, and in some applications with zero leakage. Because graphite generates very little friction, it keeps shaft wear to a minimum.
Where soft seals are used
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Agitator and mixer shafts: used to provide sealing on rotating equipment.
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Petrochemical facilities: used in systems working with acids, bases, solvents and petroleum derivatives.
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Food and pharmaceutical industry: used on lines that need chemically resistant, hygienic sealing.
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Paper and textile industry: used on rotating equipment in damp and chemical environments.
Temperature and pressure resistance of soft seals
Temperature and pressure resistance vary with the material used. PTFE, graphite and aramid-based types normally work safely between -100°C and +280°C at pressures up to 20 bar. With the right selection they give high resistance and long-lasting sealing.
Author:
Industrial Design Engineer