Flow Through Given Orifice

→ Perry, "The Chemical Engineer's Handbook", any edition.

If you are flowing to atmosphere Q scfm = 14.5 x d^2 x (P psig + 14.7) x Cd

Cd (coefficient of discharge) may vary from .97 for well rounded to .65 for sharp edge orifice. If the Cd is .85 the flow at 100 psig with a 1" orifice would be 14.5 x 114.7 x.85 = 1424 SCFM or approximately 350 HP.

If you have a limited pressure drop across the orifice

Q scfm = (d^2/.0336 x [Delta P x ((P1 psig - Delta P) + 14.7)]^.5 / 1.024) x Cd

eg. At 20 psid Delta P with a 1" dia orifice with Cd of .85:

29.76/1.024 x (20 x 94.7)^.5 x .85 = 1075 SCFM (269 HP)

Note for this formula a Delta P of P1 x .47 is the maximum. Past that flow is sonic and does not increase unless primary pressure is increased.

At a Delta P of 60 psi the flow would be 29.76/1.024 x (47 x (53 14.7))^.5 x .85 = 1393.3 scfm (348 HP) about the same as the orifice to atmosphere.

Without complicated technics, here's what happened in my place.

I 'invented' and constructed a device which needed a high volume discharge of high pressure air.

The result was two(2) 12mm orifice's at 120psi.

A 275cu ft/min diesel compressor just coped with this load.

Just to give you an indication.... Cheers, Stu.

Stu what was the pressure drop across your orifices, how long did the flow last and how would you rate their shape? Calculations indicate that you would use from about 600 to 900 SCFM if dumping to atmosphere. Tom

Thanks for your comments,

The figures that you have given would indicate that our shop compressor that is only capable of 40 cfm is inadequate for the job. I already had this suspicion but needed to understand the theory. We are using a 45 degree chamfer in the orifice plate that is placed within a 2" dia pipe open to atmosphere. We are using the Δp to calculate flow for a particular application.

Your help has been appreciated.

Regards