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Packings



Introduction

The packed gland is used primarily for sealing process valve shafts (axial movement) and for process pump shafts(rotary movement).   The packed gland has provided a low tech. convenient solution throughout the history of engineering.   In modern times the packed gland is being replaced by more exotic solutions.


Typical Packed Gland Arrangement

Pumps

The packed gland seal for pump applications is now rarely fitted to new pumps. This is due to it's high maintenance requirements.  When fitted it is mostly used in conjunction with long coupled bed plate mounted pumps.   Specific operating conditions require distinctly different types of packed gland seals.   They require regular checks and maintenance adjustments..   Proper lubrication of the gland packing requires a certain leakage rate.   Special manufacturers recommendation's are to be observed individually.   Service life expectancy is between 1 and 2 years, this can sometimes extend to several years on favourable conditions.   Extremely bad fluid conditions (sediments, additives, overheating) can however drastically cut short their service life.  Packed glands should preferably be used in conjunction with shaft sleeves in order to avoid damage to the shaft by aggressive fluids or due to improper treatment of the packed gland.





Valves

The packed gland is still widely used for stem sealing of various types of valves including gate valves, globe valves, and ball valves.   The packed gland provides a low cost option with the capabilities of sealing under a wide range of operating conditions with a wide range of fluids by selecting appropriate packing materials.   A packed gland is often used in conjunction with another type of seal or fluid containment e.g an o-ring seals, bellows containment etc.

  The gland packing generally creeps over time and therefore needs to be regularly tightened to maintain the seal.   The packing can be continuously spring loaded using disc springs or similar to extend the maintenance period.





General

The properties of a packing system required for good sealing and trouble free operation include

  • Compatibility with the working fluid at the operating temperatures
  • Plasticity to conform to the shaft under the influence of the gland force
  • Lubricant insoluble and immiscible with the sealed fluid
  • Non-abrasive to minimise shaft erosion
  • Non-corrosive to avoid damage to the shaft of housing
  • Wear resistant
  • Retain properties over time




Soft Packings

Yarn packings can be based on vegetable , animal or synthetic fibres and are based on various types of fibre.

  • Plaited
  • Braided
  • Twisted
  • Plastic




Metallic Packings

In plaited and braided packings reinforcing wires are used for operation at higher temperatures and pressures.   For high temperatures, woven types of copper and aluminium packing can be produces.  Metal foils of lead, aluminium, and copper can be used to form packings.

Combinations of packings having hard metal end rings with softer inner rings are used for higher performance duties including gases and abrasive fluids.





Lantern Rings

Annular rings are sometimes included between two sets of packings to allow introduction of lubricant, cooling or flushing fluid.





Shaft Design

The shaft/ sleeve surface in contact with the seal should have a surface finsh of 0.4 micrometres or better.   The clearances above and below the packings should be such to prevent extrusion of the packing material along the shaft. The clearance should not be too small that contact between the static and rotating metal is possible





Temperature Range For Packing Materials

Table provided for guidance only, final design values must be obtained from suppliers

Material Temperature Range Deg C
Jute, Flax, hemp 0 to 60
Cotton 0 to 70
Rubberised Cotton 0 to 80
PTFE -250 to 220
Aramid -200 to 280
Aluminium Mesh 0 to 420
Wrapped Metal foils-Varios types 0 to 450
Graphite Fibre -200 to 600
Asbestos Reinforced St.Steel or Inconel 0 to 800
Copper Mesh 0 to 800
Aluminia Silica filiment/ Inconel reinforced 0 to 1200





Surface Velocity Range For Packing Materials

Table provided for guidance only, final design values must be obtained from suppliers

Material Surface Speed Range m/s
Aluminia- silica 0 to 0.2
Copper Mesh 0 to 0.5
Jute 0 to 4
Hemp and Cotton 0 to 6
Flax 0 to 10
PTFE 0 to 12
Wrapped Metal foils-Various types 0 to 20
Aramid 0 to 22
Aluminium Mesh 0 to 25
Asbestos (Not acceptable -UK) 0 to 25
Grahite Fibre /Foil 0 to 50
   


Links Providing information on Packings
  1. Vulcan... A Sheffield company providing a wide range of rotary sealing products.
  2. James Walker ... Supplier of wide range of sealing products, including pump and valve packing.
  3. Klinger... A Packing supplier
  4. Johnson Packings ... A supplier of various pump and valve packing materials
  5. Shaft Seals -BPMA ... Concise notes from British Pump Manufacturers Association

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Last Updated 25/01/2013