All sealed up

Control valves are usually operated by an external electric or pneumatic actuator.

They respond to a control signal and act on a shaft projecting inside the valve body to vary size, and as a result the flow-rate of the fluid.

The shaft runs from the outside of the valve to its heart and to keep the media, inside the valve, an appropriate seal must be used.

Type and material of seal

The selection of the most appropriate packing for a given application depends on the media and process parameters, permitted frequency of maintenance, cost, environmental regulations and personal preference.

Valve stem sealing methods and materials can be categorised into three main groups:

• Compression packing
• Die-moulded rings
• Metal bellows

Compression/Yarn packing

This is one of the oldest types of packing. It has a rope-like form and is wrapped around the valve spindle and compressed to achieve a seal. It incorporates different types of fibre in a number of constructions including:

• Plaited

  The yarns are plaited so that the direction of the fibre length follows the outside of the ring. This allows individual fibres to wear away and retains pockets of lubricant between each plait. This form is usually suited to centrifugal pump applications.

• Cross plaited

  The yarns are interlocking and pass diagonally through the packing to provide a firm construction of consistent density and flexibility. It is used for synthetic yarn packing of valves.

• Braided

  Individual yarns are braided together to form a continuous tube and the required packing size is built up by braiding one tube on the next.

  The advantages are that they are self-adjusting, constant stem friction, high integrity seal and cost effective.

  The disadvantages include the limited operating temperature and they require replacement when leakage occurs.

• Graphite Packing

  Graphite has improved stem sealing performance in control valves. It is often braided providing leak-free operation and has high thermal conductivity up to about 650°C.

  Other forms of strengthening and reinforcement can be used such as inconel wire. A fine wire shroud helps to retain the packing and acts as a wiper to clean the valve stem.

  An occurrence known as 'pick-up' can be a problem whereby graphite from the sealing rings adheres to the valve stem.

  The effects of this are that firstly frictional drag increases, causing a significant increase in the load required to operate the valve.

  Secondly, due to the resultant loss of graphite which has adhered to the stem, leakage can develop, creating the need for further gland adjustment.

  Expanded graphite foil (also known as flexible or exfoliated graphite) is a recent application of graphite for control valves.

  Sheet produced by exfoliating and calendering natural flake graphite is cut into widths. The tape can then either be wrapped layer on layer around the centre pin of a die and pressed to form an endless square cornered ring or it can be wrapped directly onto the valve stem, slid into the stuffing box and compressed in place.

  It is available in various densities and different levels of purity, which affect its cost and performance. For valve stem sealing duties the preferred density is around 1.6 g/cc.

  This provides a balance between softer more deformable rings which spread easily in the radial direction but wear faster, and are prone to moisture absorption and those of a harder more rigid construction that are extremely dimensionally critical for them to perform well.

  Braided graphite rings and moulded graphite rings can be used in combination. The middle rings in a stack are moulded, providing sealing due to their controlled density and impermeability while a top and bottom braided ring, sometimes wire shrouded, prevents extrusion and pick-up of the inner rings.

  Galvanic corrosion may occur. It is caused by the different galvanic potentials of the stuffing box, valve stem and the graphite packing in combination with electrolytic media.

  To overcome this, graphite packing material is available with sacrificial or organic inhibitors impregnated in the material.

Metal bellows

Bellow seals may be an option when the flow media is toxic, flammable, explosive or expensive. They are used in zero emission applications. They can also be used to prevent leakage in vacuum service.

The bellows are normally welded to the valve stem at one end and sandwiched between static gaskets at the other. They expand and contract in the axial direction as the stem travels in and out of the valve during operation.

It is normally externally pressurised. An extension bonnet is usually required and a conventional set of compression packing is added as a back-up seal.

The bellows must be thin enough to reduce bending stresses in the wall which may cause fatigue, cracking and eventually leakage failure.

At the same time they must be thick enough to withstand the media pressure. To achieve this combination, multiple bellows may be used for high pressure applications.

Torsional stress on the bellows is prevented by a hexagonal section of the stem passing through a hexagonal bushing or by a pin extending horizontally from the stem, which slides in a vertical guide slot.

Bellows require very little maintenance and are suitable for high temperatures. The life ranges from 2000-100,000+ full travel cycles, depending on the application.

Factors affecting selection

Factors that may affect the selection of an effective stem seal include:

1. Compatibility with the working fluid at the working temperature;
2. Valve style - reciprocating or rotary;
3. Physical limits of valve size and geometry;
4. Low friction to reduce positioning hysteresis;
5. Non-abrasive to minimise stem wear;
6. Non-corrosive to avoid damage to the shaft or housing;
7. Wear resistant to minimise gland adjustment;
8. Ability to mould to the stem and housing under the gland load;
9. Ability to adjust to any thermal cycling effects;
10. Suitable for maximum and minimum valve design pressures;
11. Suitable for maximum and minimum valve design temperatures;
12. Media properties (corrosive, toxic, low viscosity, explosive, abrasive) may determine degree of seal integrity;
13. EPA regulations may determine degree of seal integrity.

Conclusion

Selecting the most appropriate packing involves a number of choices that usually exist for a given application.

The selection will depend on the media and process parameters, permitted frequency of maintenance, cost, environmental regulations and personal preference.