Instrument Air Utiity Report

[techman81]

Guys,

I need your advise

I am preparing utility report for instrument air. In the form I have a column for flow rates,but I don't know how spesify flowrate for pneumatic actuators ?

 

[danw2]

We happen to have Moore 760 and Siemens Sipart PD2 positioners, so I have those spec sheets around. You'll have to find spec sheets for your own brand positioners.

The spec sheet for the Moore M760 (page 2) says:
Air consumption
Standard Spool = 0.5 scfm
Low Gain Spool = 0.5 scfm
High Flow Capacity Spool = 1.0 scfm (typical)

The Siemens PS2 uses air only on active movement, it uses no air in a static state, so its consumption is negligible.

PS2 Spec sheet says
Power consumption in the
controlled state
< 3.6*10^-2 Nm³/h (0.158 USgpm)

which is appears to be babel translation from German,
so let's assume they mean air consumption, not power consumption and one doesn't measure air consumption in units of gallons.
Assuming the original metric value is correct,
3.6*10^-2 Nm³/h

meter^3 per hour = 35.314667 feet^3 per hour

0.036 m3/h = 1.27 cfh = 0.021cfm

 

[dcasto]

Air compressors and instruments are is STANDARD cubic feet per minute (SCFM). And the instrument vendor should list this and it assumed that the air is supplied at a fixed pressure, typically 30 psig.
Your air compressor will have the same kind of rating of standard cubic feet per minute at a fixed discharge pressure, typically 90 to 100 psig. If you are at high altitute or in an area where you have extra capacity air filters on the air compressor, you may loose some capacity.

As danw2 states yousome instruments only bleed air while in motion. For those you will have to assume some percentage of the time they will move. For a report I'd assume 30%, for design I'd assume 90%. add up all your SCFM's for your users and compare with the air compressor SCFM to get utilization rate.

 

[sunh]

Generally instrumentation will estimate air consumption based on valve counts, like CVs, On-off valves,analyzers, etc. Each type of instrument consumes different amount of air, and you may need to check with your instrument supplier for those. As dcasto pointed out, you need to assume some percentage of them modulating simultaneously. If the percentage is too high, you'll overdesign the system. If it's too low, then....My experience tells 30-40% would be adequate.

After all above steps, you calculated only instrument air. Some margins need to be added over the top for Air leakage, Future development, and plant air only if plant air comes from the same compressor. 10-15% for air leakage and 30- 40% for Future aren't unusual. If insufficient info available, assume plant air identical to instrument air. Plant air less than 200 SCFM is unusual!

Sun