# Estimate pipe diameter based on flow rate

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1. Jan 6, 2015

### antonclaeys

I'm designing a testing unit where I need to estimate the smallest diameter of a short complex pipe (around 1 meter, three bends and some diameter viariations). Since the pipe is being cooled down in a water bath after production, my idea is to force water with a known pressure in one end of the tube (other end is at pressure of the water bath) and measure the flow rate. I'm assuming that when the smalles diameter of the pipe is much smaller than normal due a construction fault, the flow rate will be lower due to higher pressure buildup in the smallest section of the pipe (like what happens if you almost close the end of your garden hose). However, I'm having troubles proving this. Are my assumptions correct and can someone get me started to prove this?

2. Jan 6, 2015

### Staff: Mentor

Pressure drop per length increases if pipe diameter goes down. There is a formula for constant pipe diameters (at least within some length) and laminar flow, but the basic idea stays the same independent of those conditions.

"Large diameter everywhere apart from a spot with small diameter" can still have a better flow than "medium diameter everywhere" - flow does not get a hard limit from some narrow section.

3. Jan 6, 2015

### Staff: Mentor

I don't understand: if you are manufacturing the pipe, don't you already know the geometry?

4. Jan 7, 2015

### antonclaeys

@mfb:
so you're saying that I probably won't be able to measure this?

@russ_watters:
I'm trying to build this system as a test to verify if the pipe diameter isn't smaller than normal; I'm trying to check for construction faults because sometimes the pipe can be almost fully closed and this won't get noticed early enough in the production process.

5. Jan 7, 2015

### Staff: Mentor

What is "this"?

You cannot measure the smallest diameter along the pipe just based on flow measurements.

6. Jan 7, 2015

### antonclaeys

Which variables do I need to measure to be able to tell if the pipe has internal obstacles due to production errors (and thus has a diameter which is smaller than normal)? Which kind of measurement setup do you propose?

7. Jan 7, 2015

### Staff: Mentor

Take a ball with the required minimal radius, try to get it through, see if it gets stuck?