Friction Loss In PVC Fittings
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Recently this picture showed up in my Facebook feed from a swimming pool professionals group. The picture is from a recent Pentair training seminar of fluid hydraulics. From the second I saw this picture I knew that something was amiss but it was not for another 24 hours that I had a moment to look into it further. During this time nobody mentioned anything about the problem, and even it was referenced that the information shown was correct. I could tell logically what was wrong with the picture right away...I just needed a few minutes to actually spend looking into it to find the source of the problem.
In this example, using two 45 degree fittings causes less flow restriction that even a sweep elbow - a product that was developed exclusively to reduce flow restriction. The logical problem here is if you set out to make a product with the least flow restriction, a sweep elbow, but the end result performed less well than a pre-existing solution, two 45's, then why would you make a sweep elbow at all? You would just make two 45's glued together permanently and call it a "good ell" (Goodale, or whatever your name might be) because that is the best for flow. So there is the problem staring me right in the face and before I even start to look I know that these numbers are wrong...but they are being taught by Pentair during hydraulic training - you would kind of assume that this information is being presented correctly. It is my belief that this is a mistake, and this information has accidentally been taught to many pool service technicians who now defend this erroneous information. Let's get to the bottom of this and clear up some things.
Sweep Elbow
- A sweep elbow is the longest and least disruptive to flow of the available fittings for negotiating a ninety degree turn. It has a slip connection on both ends and a smooth and continuous contour to the inside to minimize turbulence, and ultimately losses, in a plumbing system.
Short Radius Elbow
- Also sometimes just called an ell, elbow, or a "ninety", this most common pool fitting is not a sweep elbow, but does have a similar continuous contour to the inside, albeit a shorter and more disruptive radius than the sweep fitting. This fitting is a slip connection on both ends. While this is the most common way to transition a 90 degree turn, these are actually intended for areas where clearance to use the preferable sweep elbow do not allow.
Street Elbow
- Also called a street ell, this fitting has the most disruption to flow and increase in turbulence as the water negotiates the turn due to a squared off design. Essentially there is no effort made to help water transition the contour, and the original path of flow simply meets a flat, dead end, at which time the water experiences turbulence as it chaotically negotiates the turn. The required hallmark of a street ell fitting is that it is slip socket on one end, but the other is a spigot socket, sized to fit directly into a slip socket. This allows for the minimal possible clearance in order to transition 90 degrees, however it is also, by far, the most disruptive to flow.
It is critically important that we define these fitting qualities as the source of this misinformation, I believe, is incorrect terminology when discussing the friction loss in these fittings. Both in the Pentair training information, as well as multiple online sources, incorrectly apply known friction loss values to the wrong fittings. The numbers are right...just that the numbers and the pictures do not line up. Let's clarify a few things here:
Friction Loss In 2" PVC Fittings
2" short radius 90 degree ell = 5.5 feet
2" 45 degree ell = 2.8 feet
2" long radius 90 ell (sweep elbow) = 4.3 feet
2" street ell (slip x spigot) = 8.6 feet
The confusion here comes from a few online sources that site only 90 degree elbow, or incorrectly site sweep elbows when they actually are referring to standard (short radius) elbow fittings. If you cross check enough sources you should also be able to confirm as I have that for 2" schedule 40 PVC friction loss the values are as I have listed them above. Here is one source for you to verify these numbers. And here is another source.
Logically this makes sense as well. A sweep elbow should be (slightly) better than using two 45's to transition a turn...and it is. But two 45's sure will get you pretty close to the same thing, and either are a massive improvement over short radius elbows or (gasp) street elbow fittings. Most of you who follow me know that I have been campaigning aggressively against the use of street elbows for years as evidenced in my 50 part video series review of pool equipment installations.
In this instance what has happened is the values shown here for short radius elbows are actually the values for street elbows. 5.5 is actually a short radius ell, which is pictured as currently showing 8.6. The 8.6 is actually a street ell (not pictured) friction loss rating (slip by spigot). So does any of this matter? I mean, yes, it does. I know this is just an honest mistake but Pentair represents some of the highest quality equipment in the industry and their training programs should aim to be equally as impressive. I wonder how many pool technicians are out there now thinking that two 45's are better than a sweep ell...
In all fairness, this is just a small error and not an indication of overall quality of the education in any way. Still I would like to see this amended so that we can be training the highest caliber of pool technicians for the future possible. BTW - Pentair I would be happy to join your team to help create and distribute training material. Set it up with Phil or Darren.
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