Instrument air consumption calculation

[bowman30]

Hi guys,
As you know, on/off valves consume air in start-up / shutdown condition. My supervisor believes because these valves will not work simultaneously, considering 10% of them in air consumption calculation are enough.
But I think because they are "fail to close" and have "three way solenoid valve" after total SD, all of them consume instrument air simultaneously. So I consider 100% of them in instrument air consumption calculation.
What do you think?

Thanks much!

 

[IRstuff]

Well, what is the statistical likelihood of those occurrences and are you willing to pay for supporting those scenarios?

TTFN
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[roydm]

I would guess your supervisor is right, most valves will be spring to the fail position so won't use any air on power failure or total shutdown.
After a shutdown they won't all need to open at the same instant.
You also have a lot of capacity in the airlines and air reciever.

 

[bowman30]

thank you both!
maybe i'm not right! but i think "three way solenoid valve" consumes air when it doesn't energize on/off valve!

 

[roydm]

No, a 3 way opens from supply to cylinder to move the actuator compressing the spring, when it switches the other way it blocks the air supply and vents the cylinder to atmosphere. Note, I didn't say which way the valve goes when energized, it can be either.
A 4 way solenoid will use air in both directions these are used on double acting valves with no spring.

 

[ccfowler]

The instrument air receiver tank capacity should be large enough to cover all demands for shut-down or brief periods of unusually high demand plus some reasonable reserve margin. Oversizing of compressors is a very effective means of wasting energy, capital, plant space, and O & M costs.

If you are using reciprocating compressors at more than about 50 psig, be sure to use 2-stage compressors with intercooling to save energy and maintenance costs. In my experience, single stage compressors for 90 to 100 psig systems always seem so attractive for initial cost, but their unavoidably high operating temperatures always result in much-greater-than-forecast operating and maintenance costs along with resultant reliability problems. In contrast, for nominal 90 to 100 psig systems, 2-stage reciprocating compressors rarely require anything beyond normal, scheduled maintenance for trouble-free reliable operation.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[bowman30]

thank you!