This morning I surfed the web trying to find a friction loss chart for funny pipe. You don’t see them at the back of any irrigation catalogs and rightly so. You never want to design laterals using funny pipe. It’s a swing joint, and every pro sprinkler guy has seen amateurs overuse funny pipe, add too many heads, etc. That said, there are always special circumstances where the most seasoned and crusty sprinkler guy must bend some rules.

In this case we are converting an existing spray/ rotor shrub zone (yes, mismatched precip, sloppy install) to Netafim™ Techline® drip. One obvious way to do this is dig up the lateral between the valve and first head and re-route a new header. BUT, sometimes you’d rather just go a faster easier route. Why not pull up some conveniently located heads, remove, and convert right from the swing joint?

No problem, but we should know what our maximum flow rate from a swing joint is going to be. We can guess it’s below 5 gpm, the most you would get out of a 1/2″ PVC pipe. I should mention here to the non-crusties we base the maximum flow on a 5 feet per second velocity threshold. If water travels faster than that through any sized sprinkler pipe, the performance of the head or emitter starts to suck.

Ok, so it’s lower than 5 gpm, but what is it exactly? I gotta know, man! I’ve got options here. Netafim™ has all kindsa different flow rates. On this particular bed, I’d like to use the Techline® EZ .26gph 6″ spacing emitter. I’ve measured the bed and figured my row spacing and have calculated the zone to need around 7 gpm.

On the internet search I stumble across an amusing forum conversation about friction loss through funny pipe. One nerdy guy asks for help in  his painstaking calculations of every elbow, tee, forty five, valve, swing joint, etc. for friction loss. Another nerdy guy provides him the answer in detail about funny pipe and funny fittings. Then a Crusty type jumps in and yells, “WHO CARES?! Why are you wasting your friggin’ time?!!” Then the predictable admonishments, name-calling, childish behavior, and final apologies ensue.

I typed in a different search for my needs and came across How To Convert Pipe Size to GPM . First I had to find out what the inside diameter was for funny pipe. Using our killer caliper tool I find it is, in fact 0.5 inches. Exactly half inch.

Half inch poly and PVC is actually .622″ inside diameter. Iron pipe sizes are nominal. Nothing in this world is as it seems. According to the eHow page I found, the first step is finding the area of our open inside pipe. Pi are squared.  Wait, lemme Google the right way to type that:  πr²

Okay so r= .5/2=.25

3.14 x .25 x .25= .19625 sq. inches area for the .5 diameter opening.

Let’s find the volume for 5 feet of funny pipe. We eventually want that amount of water per second to find our flow rate.

.19625 x 60 (inches in five feet) = 11.775 cubic inches

divide by 1728 (a constant) to get our cubic feet = .0068 cf

convert cubic feet to gallons by multiplying by 7.48 = .051 g

Now to get gallons per minute per feet per second.

One foot per second converts to 60 feet per minute

60fpm/ our 5 feet of pipe = 12

12 x our .051 gallons in 5 feet of funny pipe =.612 gallons per minute at one foot per second.

.612 x 5 feet per second= 3.06 gallons per minute.

HUZZAH! The maximum flow from funny pipe not exceeding 5fps is 3.06 gpm!

There’s the answer to the ellipsis in the title of this post. Going back to our shrub bed with our knowledge we know we have to tie in to 3 heads to get the 7gpm total.

7 divided by 2 heads=3.5, we’re over the 3.06. We might succeed, but why take a chance?

7 divided by 3 heads= 2.3, awesome. Comfy. We could add more tubing later if needed.

So, the way I’ll go about choosing the heads is two from the extremes of the zone and one in the middle. Cap the rest. The goal is to avoid a dead ended lateral with trapped water that may freeze in winter. My flush valve will go somewhere in the middle, but opposite the middle head tie in- the water is all looped, but I’d like a central exhaust. I’m babbling now, this isn’t important.

What’s important is that now we know we know a spec for funny pipe we didn’t know about an hour ago! Yay. If my math is wrong, please write me and tell me why. Thank you.

POSTSCRIPT:

I found a much easier velocity formula in the Netafim book, duh!

Velocity in feet per second= .4085 times  Q (gallons per minute) divided by d^2 ( inside diameter squared). V=.4085(Q/d^2)

5=.4085 (Q/.25)

5/.4085= Q/.25

12.24=Q/.25

12.24 x .25= 3.06