Is it all valves subject to vacuum conditions shall have water seals or metal bellow seal as an optional? Generally no, but you have to specify with details to get a proper answer: exact vacuum (figures), application, industry type, valvetype, valvesize, other requirements, and eventually local regulations.Most important is that the valve selected are constructed and tested for ALL conditions that you have, both vacuum, overpressure, thightness (under given conditions) and have references to prove troublefree operation over time. If a control valve will see vacuum under all process conditions then you can install V-ring packing upside down.If the valve will see both pressure and vacuum then you can install "double packing" where half the chevron rings are inverted.I recommend bellows if air will contaminate the process.

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