valves in vacuum conditions 3

[chungming]

Is it all valves subject to vacuum conditions shall have water seals or metal bellow seal as an optional?

 

[gerhardl]

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.

 

[NGiLuzzu]

I agree with gerhardl; most of all, I would underline again that the "exact vacuum (figures)" must be specified.

Unluckily, many Customers use the expression "full vacuum", which is meaningless (unless you define it with a maximum absolute pressure...)!

Several "vacuum grades" are conventionally defined, based upon the (absolute) pressure level, and each grade may require its own specific types of pumps, pipe and gasket materials, sealing systems, etc...

See, for example:
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http://en.wikipedia.org/wiki/Vacuum#Quality;

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http://www.absolute-vacuum.com/resources_whatisvacuum.php;

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http://www.internationalvacuum.com/glossary2.htm;

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http://www.themeter.net/pressioni1_e.htm?Submit=flow+-+vacuum;

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http://www.hydraulicspneumatics.com/200/FPE/Pneumatics/Article/True/6460/Pneumatics;

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http://vacuum-systems.com/.

Hope this helps, 'NGL

 

[gerhardl]

Anegri: Just what I meant! Thank you for highlighting!

 

[ione]

chungming,

What above outlined by gerhardl and anegri is undoubtedly correct.

If I may add, when you have to deal with other conditions beside vacuum service, such as corrosive media and high temperatures, the use of bellows seals is probably more than an option. Bellows are usually made of 316 stainless steel and they both prevent air infiltration and protect part of the bonnet from damages caused by corrosive agents and high temperature. Finally I would also suggest to foresee a safety chamber with a device (pressure gauge) that could advise when bellows seats leak.

 

[Scotsinst]

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.

 

[bcd]

Inverting chevron v-rings does not enhance the sealing for vacuum. Vacuum is simply atmospheric pressure in the opposite direction. This amount of pressure does not "energize" the v-rings any more than force from the gland bolting. The pressure force is negligible when compared to the force from gland bolting.

Inverting the packing for vacuum is a myth. Don't bother.

 

[ione]

For vacuum applications there is a particular arrangement which foresees a double V-ring packing (twin seal), making this configuration an alternative to seal bellows.

 

[gerhardl]

A V-ring sealing, either way, is more prone to mechanical wear, abrasion and agening processes over time, than a bellow sealing.

A soft bellow sealing, or e weak low-quality bellow sealing of metal might develop leakage in the 'folds' over time, but a top quality bellow sealing will have advantages and be a better solution in my opinion over time than a general sevron-ring sealing.

The bellow sealing might be the highest priced one, but not necessarily, or the difference in price need not be significant between the two construction.

If there is a higher cost for bellow sealing it might well be a cheap buy for the longer troublefree run of the process.

The cost has to be compared anyway to the importance of keeping the given vacuum value.

As an example a few percent under normal atmospheric pressure to suction away some non-toxic gases does not need high-qualified sealings, while a high vacuum for some chemical processes might be critical.