How to Calculate Pressure Drop in a Multi-Valve Air System?

[5150racecar]

Hello, I’ve seen a few posts regarding this topic but I can’t seem to grasp the concept of it. I have a main steel pipe x diameter providing 100psi of air to 6 smaller lines attached to it. At the end of these lines are valves. Is there a way to calculate the drop in pressure if I opened up 1 valve ranging from 0-60% if I know the orifice diameter? Moreover, could I find the drop in pressure if I open up multiple valves? Much help is appreciated.

 

[NdotA]

I am new to this forum and I do not know what has been posted as yet. Out of my practice as mechanical engineer I would answer your question with a clear yes and no and it depends...

Yes means, of course you can compute the pressure drop, provided you have all the data needed. And it depends on the precision you want to achieve.

There are a lot of textbooks around, that give you equations and parameters for the loss in pressure for flow through pipes and T-pieces of pipe other connections, valvesand such. But I found the results give some idea on what to expect for the actual pressure loss you see in real life depends on many params that usually are not modeled in the textbook's params. Like make of T-pieces, their radii, their finish. Or the power of the source to maintain the nominal pressure at increased flow. And ... and ... and...

I guess to have some minimum precision you will have to perform some tests on the pressure loss coefficients of the components of your system.

 

[5150racecar]

Thanks for your reply NdotA. I'm not necessarily looking at absolute numbers. I'm just looking to see what effect opening and closing valves have on the system, and the other pipe lines. I unfortunately do not have access to textbooks, I was hoping to find a good website talking about this topic, if you could please direct me to one that would be very helpful.

 

[NdotA]

If you do not have access to printed books, I think best would be to start in Wikipedia. Look for the article 'Pneumatics' there, which gives anumber of leads to other references.

 

[OffTheRecord]

Someone correct me if I'm wrong, but I think this summarizes the general approach:

Lets say you have a pressurized cylinder or a compressor up the line. If you just turn it on without letting a valve open, it will just build up pressure until it can't do any more. When you let a valve open, you'll get some airflow through the pipe, and some friction resisting it. The more friction, the more pressure loss. Friction ultimately depends on the flow rate of air, and the type and dimensions of the pipe. That flow rate, in turn, depends on the amount of pressure you've got in your system. Now, if you've got different sized pipes in series, then you just figure out the pressure drop in each section, then add up all the respective pressure drops.

You can find a lovely chart here:
http://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.html

The key is figuring out what flow rate to use. The units of flow rate will be CFM (english units). Flow rate is loosely governed by the ideal gas law PV=mRT (where V is flow rate, m is mass flow rate, T is temperature, and R is a constant, found on a chart), but in all reality you just have to look at the specs on the compressor. The compressor should have a rating on it telling what the flow rate is at each pressure. You can probably use the starting pressure (100) to figure that out. If the resulting pressure drop is more than about 5%, you may want to use that answer to back-calculate the flow rate, and re-calculate.