There are a number of limitations that can be expected from a bellows valve. They can be summarized as follows:
Cycle Life Bellows have a finite life when used in valves that expose them to pressure fluctuations and full compression and expansion. BS-5352 requires that Bellow Seal Gate valves have a 2000 cycle life and that Globe valves have a 10,000 cycle life. The cycle life limitation may not allow their use in some highly stroked valves whose cycle life exceeds the bellows life. The cycle life limitation will require a quality backup packing system to overcome a leak in the bellows.
Pressure Retention Capability Since bellows are designed with the intent that thin wall formulas be used, bellows with membrane thickness that provide flexibility and low spring rates are the outcome. It is highly unlikely that bellows can be designed to interface with larger high pressure valves that have traditionally used the thick wall formulas in designing of the pressure boundary. It may not be possible to design bellows for use in gate and globe valves that can retain high pressures while providing flexibility and a low spring rate.
Size Limitation The operating torque of a standard gate or globe valve is influenced by the area of the stem and the operating pressure acting on this area. The load on a bellows valve stem is a function of the bellows inside/outside diameters. The size of the bellows required to accommodate the stem, pressure and stroke of larger valves may lead to a bellows with a spring rate and unbalanced area on the valve stem so large that it will be impractical to operate when used in a gate or globe valve.
Corrosion Control The selection of the material for the bellow will be critical. Matching of service to the bellow material will require greater caution because unlike the pressure boundary of a valve, there is no inherent corrosion allowance in the bellow of a Bellow Seal Valve. Bellows are typically available in 316Ti, lnconel625, Incoloy825,Hastelloy C276 material.
Dimensions Bellow Seal Gate valves will have a much greater height than the conventional gate valves. The bellows must be designed to accommodate the high stroke requirements of a gate valve. This leads to a long bellows that requires the bellows gate valve to be extended, impacting on piping configuration and layout.
Bellow Seal Valve Costs It is expected that it will be anywhere from three to ten times more expensive than their standard packed valve equivalent. This higher cost is primarily due to the cost of the bellows. The higher price tag for reliable Bellow Seal Valves still may be economically attractive when reviewed in regard to emerging EPA requirements and incentives.
SG Brand Bellow Seal Valves use weld Roll Hydraulically formed multi-ply bellows and Seamless bellows. The bellow is welded into a bellows subassembly to ensure that the corrosion resistance of the membrane-thick bellows is maintained. They are designed to ensure that no torque is placed on the bellows during operation and that the bellows does not experience any angular or lateral offset. The design permits in-line compression and/or expansion of the bellows only. All SG Brand bellow sub assemblies are tested before assembly into the valve with a helium mass spectrometer to detect helium leaks in the 10-6 standard cubic centimeter per second range. Such a small leak would not generate a bubble in an air-under water test. It would take years for 1 cc of leakage to occur.
All SG Brand Bellow Seal Valves use long-life flexible graphite as the backup packing. A full size packing chamber is used. The valves are also designed with backseats to provide additional insurance that the stem/packing leak path can be eliminated even in the event the bellow and packing should fail.
SG Brand Bellow Seal Valves pressure / temperature rating scheme is in accordance with ASME/ANSI B16.34, and they can be used within the full pressure / temperature scheme of the applicable class of ASME / ANSI B16.34.
The life cycle of SG Brand Bellow Seal Valve testing is specifically designed to detect any leakage at the seal welds and valve closure elements