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Steam, Air, Water Equipments

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        Valves

Bellow Sealed Valves

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Leakage at various points in pipelines found in chemical plants creates emissions. All such leakage points can be detected using various methods and instruments and should be noted by the plant engineer. Critical leakage points include flanged gasket joints and the Valve / pump gland packing, etc. Today the chemical process industry is gearing itself towards safer technology for better environmental protection and it has become every process engineer's responsibility to design plants that limit damage to the environment through the prevention of leakage of any toxic chemicals.

There are two main types of bellow: the Forged Bellow and the Welded Bellow. Formed-type bellows are made from rolling a flat sheet (thin wall foil) into a tube which is then longitudinally fusion welded. This tube is subsequently mechanically or hydrostatically formed into a bellow with rounded and widely spaced folds. The welded leaf type bellow is made by welding washer-like plates of thin metal together at both the inner and outer circumference of the washers - like plates. A welded leaf bellow has more folds per unit length as compared to forged bellows. Thus, for the same stroke length, forged bellows are two to three times longer than their welded leaf counterparts.

Reportedly, mechanically forged bellows fail at random spots, while the welded leaf usually fails at or near a weld. To ensure full penetration of bellow ends and end coller welding it is advisable to fabricate using micro plasma welding.

Bellow materials

The most popular stainless steel bellow material is AISI 316Ti which contain Titanium to withstand high temperatures. Alternatively, Inconel 600 or Inconel 625 improve fatigue strength and corrosion resistance as compared with stainless steel bellows. Similarly, Hastalloy C-276 offers greater corrosion resistance and fatigue strength than Inconel 625. Fatigue resistance can be improved by using a multiply bellows system and reducing the stroke length; this can significantly increase the bellow service life.

Heat Transfer media: hot oil is commonly used in industries such as synthetic fibres / POY (Partially Oriented Yarn). However, there is always a risk of fire due to hot oil spillage on highly inflammable chemicals. Here, bellow seal Valves can stop the leakage.

Vacuum / ultra high vacuum: some applications require a vacuum pump to continually extract air from a pipeline. Any conventional Valves installed on the pipeline can allow external air to enter the pipeline thorough the Valve stuffing box. Hence the bellow seal Valve is the only solution to prevent air from passing through the stuffing box.
Highly hazardous fluids: for media such as chlorine (see Figure 2), hydrogen, ammonia and phosgene, the bellow seal Valve is an ideal design as leakage through the gland is totally eliminated.
Nuclear plant, heavy water plant: in instances where radiation leakage is to be prevented at all times, the bellow seal Valve is the ultimate choice.
Costly fluids: in some applications leaks need to be avoided simply because of the high cost of the fluid. Here, an economic assessment often favours the use of bellow seal Valves.
Environmental standards: around the world, standards regarding emissions and the environment are getting more stringent day by day. It can therefore be difficult for companies to expand within existing premises. With the use of bellow seal Valves, expansion without additional environmental
damage is possible.

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