In Bellow sealed valves , conventional gland packing is replaced by a bellows cartridge welded to the valve bonnet and stem. A bellow is a long accordion-like tube. As the valve stem strokes, the bellows expands or compresses with the stroke movement. There are no sliding or rotating seals through which process fluid can pass.

Types of Bellows

There are two main types of bellows, welded bellows and formed bellows. The latter is made by a relatively recent method in which a uniform bellows is produced from a thin tube. The shaping of the metal is carried out in a collapsible die under high fluid pressure. This is the most rational method of producing weldless bellows. Valvola prefers this formed bellows over the welded type. The following table will show just why...

Type	Formed Bellows	Welded Bellows
Shape of Convolution
Durability	Reliable durability because bellows produced from a single piece of metal. Guaranteed 5,000 Cycles or more life cycle.	Of doubtful reliability because of a number of a small welded Spots. Plies may have been affected by heat and thus life is hard to predict
Suitability for use in Valve	Because of wide opening between plies of bellows, bellow function will not be impaired by bits of foreign matter in fluid.	Wedge-shaped opening between plies will trap foreign matter in fluid, leading to bellow failure.
Quality Control	Easy to test for quality. Bellows can be produced of uniform quality.	Very difficult to inspect welded spots for quality, hence quality difficult to control.

A bellow is the heart of this type of valves. A formed bellows is made by rolling flat sheet into a tube and longitudinal fusion welding. This tube is then mechanically or hydraulically formed into bellows with rounded and widely spaced folds.

In welded leaf- type bellows. Washer like plates of thin metal are welded together. Welding is done on both inner and outer circumference of of the washer like plate. A welded leaf bellows has more folds per unit length. Both formed and welded leaf bellows have the same travel per fold. Thus, for the same stroke length, mechanically formed bellows are two to three times longer than their welded leaf counterparts. Reportedly mechanically formed bellows fail at random spots, while the welded leaf usually fails at or near a weld. That is why Valvola uses only formed bellows.

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Why Bellow Sealed Valves?

Leakage from the valve gland is often a problem for the maintenance engineer. Losses incurred due to this type of leakage are considerable. Steam at 150 psi escapes through a gland with a clearance of 0.001 in. at a rate of 25 lb/hr. This means a loss of $1.2/8-hr shift and $1,100/yr. A tiny drop of 0.4 mm diameter/sec results in a waste of about 200 l/yr of costly oil or solvent.

Apart from this high cost of energy and material loss, the leakage also results in serious environmental, ecological and oftne hazardous pronlems. The leaking fluid can be corrosice, explosive or poisonous. Ina any case, it has to be checked before it leaks. A bellows seal valve can be a simple solution to all these problems. The following table proves this:

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Comparison with Conventional and Piston Valves

	BELLOW SEALED VALVES	PISTON VALVES	CONVENTIONAL VALVES
Primary Stem Seal	Metallic Bellow	Piston ring	Gland Packing
Secondary stem Seal	Gland Packing	None	None
Stem Leakage	Not Possible.Metallic Bellows are designed to last several thousand cycles	Occurs as soon as the rings(which are made of gasket mtl) wear out	Very common.Due to friction between stem and the gland, leakage occurs within a few cycles no matter which make valve or what quality of gland packing
Packing Replacement	Not applicable	Rings need to be changed frequently	Packing needs to be replaced very often
Replacement Cost	Not applicable	Very high	High
Media Loss	Zero Loss.As the isolation is by metallic bellows,leakage (even in ppm) is not possible	Large in case of leaks	Considerable amount through gland leakages. Normally large amount when leakage remains unattended.
Equipment Downtime	Nil	Very high for replacing rings	Very high for replacing gland packing
Maintenance Cost	Nil	High as rings need to be changed at least once in six months	Very High. Gland packing needs to be replaced/ repacked often. This cost along with equipment down time, man hours spent etc. is very high
Valve Life	High - in years	Low	Very Low - in months. Due to leakage through gland, certain parts erode, making the valve irreparable after some months. Valve needs to be replaced
Safety	Can be used safely for almost any media	Can be used for limited media like steam, hot water etc.	Highly unsafe when the media is hazardous /poisonous
Cost	Comparatively higher initially but lowest total cost of ownership	High compared to the benefits viz a viz problems	Initially low but very high after some years if cost of packing, down time, man-hours spent, number of valves replaced etc. are taken into account

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Features Of Bellow Sealed Valves

• Bellow Seal Valve comply with full condition at high temperature and high pressure.
• Bellow Seal prevents leakage of sensitive media which constitute high fire risk.
• Our bellows are capable of withstanding 20,000 cycles and above without failure under pressure.
• Steam at pressure of 150 p.s.i.escapes through a stem / gland clearance of 1 / 1000th of an inch at a rate of 24 lbs per hour.
• Gland packed valves often demand continual maintenance - inaccessability creating particular difficulties.
• Best suited valve for all critical applications.