What Is A Sweep Elbow?
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A very common question from swimming pool owners relates to the choice of efficient PVC fittings for the pool plumbing system. Sweep elbows represent the best, most efficient method for water to negotiate a 90 degree turn in your system. While there are an assortment of other options (other than sweep elbows) to negotiate corners, the sweep ell is the most hydraulically efficient choice to make. This is an important distinction since pool plumbing systems will need multiple elbow fittings, with some more advanced systems needing upwards of a dozen or two 90 degree fittings. Choosing the wrong fittings for turning corners in your PVC system can easily add up to an appreciable amount of inefficiency in the way your system moves water.
It is important to understand that laminar water flow moves very efficiently, but introducing turbulence and friction into the water flow will disrupt this efficiency. In order to maintain laminar (most efficient) flow in your plumbing system you need to observe the maximum water velocity of six feet per second. Any water movement in excess of these speeds will cause an increase in friction and turbulence, and ultimately inefficiency of the water you are pumping. In addition to the water velocity you also must consider the amount, and type, of plumbing fittings that you are using.
Limitations of laminar water flow - The limit for laminar water flow in a PVC pipe is equal to six feet per second of water velocity. In a 1.5" pipe this is equal to 38 GPM, a 2" pipe equal to 65 GPM, a 2.5" pipe equal to 90 GPM, a 3" pipe equal to 138 GPM and a 4" pipe equal to 238 GPM. It is important to understand that pipes of this size, especially when paired with a powerful
pump motor, can well exceed the amount of flow that you see listed here. These are not limitation of flow in the pipe, but instead limitations of laminar flow so in an ideal sense these GPM numbers represent the ideal maximum you should operate your pool system, at least from a flow efficiency perspective. The size of the trunk line through your equipment set is generally the limiting factor for efficient water flow.
Why is a sweep elbow better than other 90 fittings? - Simply put, the sweep elbow allows for a smooth transition of the water from one direction to another. It is designed to create the least amount of turbulence, friction, and ultimately restriction to flow. The amount of restriction in your plumbing lines is measured in feet of head. Every twist, turn, fitting and valve you install in your plumbing system will add to the total resistance to flow of the system. Since many 90 degree fittings are used in pool installations, using a less efficient fitting than a sweep elbow will add up to be major efficiency losses by the time you have the full system installed. A sweep elbow allows for the least possible transmission losses in the pipe versus other, less efficient fitting options.
PVC fitting options - When it comes to turning a corner with your PVC pipe your main options will be a street elbow, a short radius 90 and a long radius 90 (called a sweep elbow). A street elbow is a slip by spigot connection, and transitions the corner in the least possible space. While this is great for space saving installations it is unfortunately a pretty big problem for flow efficiency. Each street elbow has many times the resistance to flow of a sweep elbow. If you look inside a street elbow you will see that it meets in the corner in a 90 degree sharp bend, and there is no attempt to have the water transition the corner smoothly or without turbulence. Avoid street elbows wherever possible for your plumbing system. A short radius 90 is the next smallest fitting for negotiating a corner, and largely it is acceptable to use short radius 90 fittings as needed for your installation. While a sweep elbow may have better flow dynamics than a short radius 90, the space needed for sweep elbows can add up, especially on the equipment pad where you might need a dozen or two 90 degree fittings. Commonly short radius 90's are used to maximize space on the equipment pad without resorting to using street elbows. So in short, use sweep elbows wherever you have enough room to allow for, but in tight spaces it is definitely acceptable to use short radius 90 fittings. You should avoid street elbow fittings wherever possible.
Pressure rated sweep elbow fittings - Unfortunately pressure rated sweep elbow fittings are still pretty rare and the average pool owner shopping at a local hardware store is unlikely to find a suitable pressure rated sweep ell fitting. Instead you will likely find DWV rated sweep elbow PVC fittings. To the untrained eye these look like the right thing, but they are definitely not. DWV sweep elbow fittings are common and available almost anywhere, but pressure rated sweep elbow fittings are certainly not. The way you can tell, other than the plastic saying DWV embossed right on the fittings, would be to look at the length of the glue slip joint. Drainage rated fittings have a very short glue slip joint length, less than one inch long, where pressure rated PVC fittings have a much longer glue slip joint, commonly a minimum of 1 3/8" in length. If you are unsure about whether you have DWV sweep elbows or pressure rated sweep elbows simply compare the length of the glue socket. Never use DWV fittings for a swimming pool installation. If you are having trouble finding pressure rated sweep elbow fittings you should try a swimming pool wholesaler who very likely will keep these uncommon fittings in stock.
It is not always practical to use sweep elbows, because they are hard to find, quite expensive, and take up a lot of room when you install them. For this reason the goal is to not use sweep elbow fittings for every 90 degree transition, but instead use short radius 90's as your standard fitting, and use sweep elbows where possible to reduce on the total resistance to flow of your system. Small, incremental improvements in efficiency will pay dividends to you over the lifetime of operating your swimming pool, so choosing the right versus the wrong PVC fittings is definitely important.
If you are interested in learning more about friction loss, laminar flow and how much (precisely) the difference is between street elbows, short radius 90's and long radius 90's then you can read this additional article about friction loss in PVC fittings.
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