Pneumatic pipe line sizing 2

[sonyduo]

I am looking at a new pneumatic application, being a hydraulic engineer at first I thought that this would be an easy transition to pneumatic equipment but it seems that the flows are huge in comparison to hydraulics. As such the standard pipe sizing with limits on hydraulics being 5-7 m/s on high pressure supply lines is not applicable as it would seem that 100's of litres can pass though a line which should only handle 10's of hydraulics litres.

Struggling to find what the maximum velocity recommendation would be and how to calculate this for pneumatic equipment along with any factors which would need to be taken into account. Maybe the hydraulic world has tainted my view but pneumatics seems at first to be a guessing game with little information available to allow you to calculate the correct line sizes for your application?

Please could someone help and point me in the right direction for some guides and calculations which will allow me to ensure that the pipe sizing is correct.

 

[BigInch]

30 to 60 fps may not be extreme, whereas over 100 fps usually is.

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[LittleInch]

Your problem also is that this refers to actual velocity, which keeps varying for the same standard flow as the stuff keeps expanding and contracting..... Always make sure you know what the volume figure you've been quoted is based on as it can vary a lot - search cfm on this site and you'll see a lot of posts.

Short term, short distances (10 to 20m) 20m/sec is probably ok, but longer distances, "normal" flow - 10-15 m/sec is more usual as a first pass for sizing. As said, working out lowest pressure / max actual velocity is the key.

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Also: There's usually a good reason why everyone does it that way

 

[zdas04]

Both of the Inches have the middle 80% of the actuation cycle, but not the first 10% and the last 10%.

You always start out with an evacuated line and acuator. When the valve opens to send pressure, you initially have choked flow (call it 1300 ft/sec), usually this lasts for about 5% of the fill time. As downstream pressure passes the critical pressure you shift into transonic flow for the the next 5%. The next 80% is incompressible flow so you can use D'Arcy-Weisbach to calculate (but you need to break the line up into pretty short segments and recalculate friction factors every meter or so of pipe length). For the last 10% of the flow your actuator is moving so the volume of the system is changing and I've found D'Arcy to miss the flow pretty badly when that happens. Working in velocity terms leads you in circles with this problem.

When I work a pneumatics problem I pretty much ignore velocity in the downstream piping and determine the mass flow rate through the choke or control valve. That mass flow rate is going to be reasonably constant for enough of the flow time to be useful. You know what work you are asking the actuator to do. All you have to find is the ΔH that matches that work (plus a healthy safety factor). Then find the Δm that matches the ΔH and see how long it takes for your mass flow rate to give you that Δm. If the time is too long, you need bigger choke or more upstream pressure.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat