How to determine the pressure drop in pipe diameter sizing

[inkblotch]

TL;DR Summary

How is the pressure drop determined when calculating pipe sizes?

Hi all, I'm a tinkerer with no formal engineering background, but enjoy read about physics and engineering in general.

I'm reading up on fluid flow and I'm a bit grasping at the concepts. As far as I understand it, flow rate, pressure drop and pipe size are all interrelated, where one affects the the other.

However in real life, I'm not sure how it's done - say I want to know the pipe size needed to carry compressed air from the compressor to the shop. The flow rate required is known based on the expected usage - but what about the pressure drop, which is dependent on the pipe size?

Is it simply done by determining an acceptable/maximum pressure drop value, and then calculate the diameter required to achieve the required maximum acceptable pressure drop?

 

[Dullard]

That's about right. The distribution pressure also matters.

 

[anorlunda]

Yes, and there are online calculators that can help you. It's not just pipe size, every bend and turn also contributes pressure loss.

Here's an example calculator.
https://www.engineeringtoolbox.com/pressure-drop-compressed-air-pipes-d_852.html

There are other calculators for water piping.

 

[gmax137]

Usually it is an optimization problem. Increasing pipe diameter increases cost but lowers the pressure drop. Raising the pump (compressor) discharge pressure increases the pump cost and the running cost (electricity for the pump motor).

Then other practicalities may enter into the analysis. Larger diameter may increase the system volume, reducing the cycling of the compressor and leading to less maintenance cost.

If you are designing a large industrial air system, the compressor vendors can help. If you're putting air in your home shop, use 1/2 or 3/4 inch pipe like everyone else. Use copper or galvanized pipe. I would not use PVC.

 

[inkblotch]

Thanks for the replies.

I'm not actually planning to make one, I just find it easier to apply the knowledge to simple real world examples to better understand it, thanks.

So it's pretty much:

1. Get the required flow rate
2. Set an acceptable pressure drop (i.e. the min pressure at the outlet/use point)
3. Calculate pipe diameter
4. Optimise for cost

Of course, assuming different media, operating temperatures, etc would factor in as well, but I get the general idea now.

 

[gmax137]

If you can get your hands on something like Crane-410 "Flow of Fluids through valves, fittings, and pipe." It has a table "Flow of Air through schedule 40 Steel Pipe" showing the pressure drop per 100 ft of pipe for various pipe sizes. This should be useful for sizing. There are also tables showing "equivalent lengths" for standard valves and fittings (pipe elbows, branches, etc.)

The Crane 410 is a widely used source in engineering calcs of piping systems. It is very worthwhile for practical real-world work. I'm not sure how much they sell it for today. Mine was $8 in 1986. I see them in used bookstores, especially if there is an engineering school or a lot of industry in the area. EDIT - the Crane site sells it for $75. Amazon shows it from $100 to $300, that makes no sense to me.