Valve Ratings: Vacuum vs. Applied Pressure 2

[mgwilkinson]

Hello All,

I am an engineering student, so forgive me if this seems basic or is otherwise indicative of my inexperience with valve design and selection.

I am curious as to how valves are rated if they are going to be used in a vacuum application. This is to say, if the valve is separating a vacuum chamber operating at a high vacuum from something else, like a vacuum pump, would the required ratings for the valve be any different than a valve operating where the items on either side are at atmospheric pressure? In this case, the valve is controlling the flow of air between the chamber and pump. Is a purpose-built valve required, or, provided a solid seal is achieved, would any correctly sized, correctly mated valve do the job?

Thus far, the standards I've come across for valves (ANSI 150, ANSI 300 and B16.34) don't specifically address this. Most of the spec sheets for valves I've looked at list a psig rating, but nothing indicating their ability or inability to operate in the vacuum pressure range.

Any help you all could offer would be greatly appreciated.

Kind Regards,
MG Wilkinson

Worcester Polytechnic Institute
Mechanical Engineering

 

[MikeHalloran]

Vacuum is a whole 'nother world.
My near-zero experience with it says this:

Generic valves often contain grease or plasticizers that would evaporate and foul a high-purity system.

Vacuum systems are often cleaned by 'bakeout', which may harm a generic valve's seals.

Some seals that you might find in generic valves don't do well with the pressure reversed. Rolling diaphragms, e.g., will survive very few excursion cycles after being partially everted.

I.e., buy valves that are rated for the service in which you intend to use them, and documented as such.


Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

As Mike said "high vacuum" is a completely different world from plant valves and piping even where vacuum may still be involved. There are a different set of companies that supply products to that market.

 

[gerhardl]

Following advice:

1. Please remember that 'vacuum' as one common technical description is iften used, but technical completly wrong, when specifying either usage or components. If not a specific underpressure is known, at least divide in four along lines often used in the 'vacuum equipment branch' :

a) Course vacuum from (about) 1013 mbar down to 1,33 mbar
b) Fine vacuum less than 1,33 mbar down to 0,00133 m bar (1,33 10 in minus third power)
c) High vacuum less than b, down to 1,33 10 in minus 7th power.
d) Ultra-high vacuum from less than c.

2. Within a) you will find that some 'ordinary' valves will be guranteed or specified from the suppliers down to a certain (smaller and upper) part of a, depending on fluid and temperature and requirement to leakage rate. This will for instance occur for specific solenoid valves, but also ordinary (manual or motorized) larger and smaller valve

3. Whithin 'engineering specifying according to the scale above' you will also be astonished at how many people are talking about 'vacuum' but actually describing something an ordinary poor vacuum cleaner could supply.

4. In adddition: follow advice already given by others! Under- or overspecifying of equipment is equally likely to crush budgets!

Good luck!

 

[bcd]

Hey, you are at WPI. Just go over the hill to Metso (Jamesbury factory) and get the valve you need. Jamesbury makes ball and butterfly valves rated for vaccum service and can get you all the information you need. If you need a valve, they will prbably give you one to support local education. You could not have been any closer to an answer without going on the internet.

 

[Drexl]

Generally, this is a good question - for valves there are no vacuum pressure ratings (to my knowledge). Usually what I do is send a technical specification to the suppliers, where I would state a minimum and maximum design pressure, say -1 / 3 bar(g). Based on this I would just assume that the valves the supplier offer are suitable for the specified purpose, at least for small bore valves.
Many systems where vacuum appear (such as condensers..) you don't want leakage into the system, thus picking valves from a catalog without checking vacuum properties might be a bad idea.
Shell design strength is usually not a problem but in some special cases I think we have even required a strength calculation.

Drex