Contributed by Alain Dahmani – Flowlink, France 

The seal of a diaphragm valve is made through the controlled deformation in a Hastelloy® diaphragm across a Kel-F® seat. The stroke, or deformation travel, of the diaphragm between open and closed positions is in the order of 0.2mm - 0.3mm.

The seal of a formed bellows valve is made by the displacement across the axis of the bellows via the expansion of the undulations. One such undulation is equivalent to about two diaphrams.

Example:

	Number of undulations	Total stroke at opening	Stroke for 1/2 undulation
Diaphragm	1/2 (1x 1/2 undulation)	0.3mm	0.3mm
Bellows	20 (10x 2x 1/2 undulation)	1.2mm	1.2mm / 20 = 0.06mm

So, for an opening 4x greater (1.2mm vs 0.3mm) of the bellows valve, the unit stoke is 5x smaller (0.06mm vs 0.3mm).

As a consequence, for equivalent dimension valves, the bellows valve will have a better Cv and a longer life expectancy over a diaphragm valve. Therefore, in a highly automated process where the gas valves are cycled continuously, it would stand to logic to give preference to a bellows valve.

    

The corrosion-resistant Hastelloy® alloys have become widely used by the chemical processing industries. The need for reliable performance has also lead to their acceptance and growth in the areas of energy, health and environmental, oil and gas, pharmaceutical and flue gas desulfurization industries. The attributes of these alloys include high resistance to uniform attack, outstanding localized corrosion resistance, excellent stress corrosion cracking resistance, and ease of welding and fabrication.
Kel-F® homopolymer of chlorotrifluoroethylene has the lowest vapor transmission rate of any plastic. It is specified in many aerospace valve, seal and gasket applications. It also has excellent thermal characteristics and maintains an operating temperature range of 33°Kelvin to 204°Celsius. It can be machined to precise dimensions. Compression, impact and tensile strength are high over a wide temperature range. Non-wetting and zero-moisture absorption are inherent in Kel-F®.
Both materials find wide use in high quality precision gas valves.