Paul,
you're partly right. Design of instrument air systems is not a clear science, and there are myriad ways to approach it (and most "chicken out" and overdesign, because it isn't too costly and is never a major cost factor in the design or installation of the plant). Although it is indeed pressure-based (typically at least 6 bar), each valve actuator or other device requires some volume of air at pressure to actuate it, and some devices might be something like "normally open/fail closed", so they require constant but very small flow during normal operation (that situation is quite rare, but does occur). In a large system, there might be 100s of devices, each requiring 0.5 cu m of air per cycle. Then one needs to get a count of number of devices that might need to acutate at any time or over a shor span, and provide for some diversity. Therefore, the total air flow can be determined (or at least estimated). The flow could be 100s of normal cubic meters per hour, but again only over a 10 second period. Then, it is usually not cost efficient to run 75 or even 100 mm pipe for such a short duration need. The usual approach is to use and install reservoir tanks to hold the volume nearer the use points and run smaller supply lines (such as 25 mm). The only real line size issues are 1) adequate structural strength to support itself, and 2) being able to deliver flow fast enough to replenish the reservoir tanks, given many minutes to do so.
Air to open (ATO) means that air pressure (and the small flow) are required to open the valve; air to close (ATC) means that air pressure must be applied to close it; fail open means that when air fails, the valve opens (like spring close, but not necessarily needing a spring to close) - not related to ATO or ATC.
That's a start. The rest starts to cost, because it starts to take time from paying clients.....
... Bruce D. Bullough ...
Sebesta Blomberg & Associates, Inc.
2381 Rosegate
Roseville, MN 55113 USA
+ 651-634-7344
www.sebesta.com
-----Original Message-----
From: Paul Bowers [mailto:pbowers@pipingdesign.com]
Sent: Friday, January 28, 2005 9:03 PM
To: <a href="/group/PipingDesign/post?postID=gSD8Y4eYI16i1dUBKMtVzyUBkSgFSm97ZLi7fGMBAvfhvQXzzAy_M6Yi1dzEcEiyNCfJaOosn_a6VTSNN7GRKJIXfcjUJg">PipingDesign@yahoogroups.com</a>
Subject: [PipingDesign] Instrument Air Pipe Sizing - Dumb Question
[bcc][faked-from][mx][spf]
Hi guys,
Since instrument air systems are pressure-based and not flow-based, how are the line sizes calculated? There typically is little/no flow loss for the operation of valves.
For preliminary pipe size calculations, Pie are squared is used, n'est-ce pas?
Given good fail-safe design (all control elements default to a non-nasty natural state) how is a proper IA system sized?
Some other piping/control/valve terminology: air to open, air to close, fail open, fail closed. I used to vaguely understand all this but my brain has gone soft with all this computer stuff.
It'd be nice if someone could summarize since I tried to explain it to a intermediate-level piping design modeller and found myself wondering if I had it right. I told him I'd get a definitive answer so I'm appealing to you guys - I don't want to steer him in the wrong direction due to my own possible ignorance.
Paul "I are not an engineer" Bowers
<a href="http://www.pipingoffice.us/">http://www.pipingoffice.us/</a> =========================================Main site: <a href="http://www.pipingdesign.com">http://www.pipingdesign.com</a>
Yahoo! Groups Links Received on Mon Jan 31 12:56:00 2005
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