Friction Loss Explained

Friction loss is the loss of pressure in tubing or pipe caused by friction between water and the inner walls of the tubing. The factors that influence friction loss in irrigation tubing include: flow rate, the materials that make up the tubing, the length of the tubing and its diameter.

Essentially, the more flow you have going through the tubing, and the faster it is moving, the greater the loss of pressure will be. Likewise, longer lengths of pipe and smaller diameters of tubing can expect to experience great losses of pressure from friction. Too much friction loss can cause pressure to drop to such a degree that the system may not operate as it should.

Flow

In regards to pressure loss, you can break flow down to two separate variables, velocity and volume. Velocity is measured in feet per second (FPS) and flow is measured in gallons per minute or gallons per hour (GPM / GPH). Larger volumes and faster movement of water incur greater losses of pressure from friction.

Materials

The material construction of the tubing influences friction loss. The rougher the material, the greater the friction. In general, plastic tubing types (polyethylene, PVC, PEX, etc) experience less friction than metal (copper, steel, etc) types.

Tubing Length

The longer the distance the water must travel, the more friction it will encounter. This is usually expressed in linear feet. Fittings (tees, elbows) and turns also cause friction loss. Most DIY’ers can disregard the loss from fittings, but larger systems seeking to be as efficient as possible should account for losses from fittings in their planning.

Tubing Diameter

Larger tubing inside diameters experience less friction. This is because a lower percentage of the water comes into contact with the walls of the tubing. For the most part, it is this contact with the walls that causes frictions, thus a larger diameter tubing enjoys lower rates of pressure loss from friction.

Importance of Calculating Friction Loss — Costs

Performance of the system is the main reason to calculate friction loss in a system. If too much pressure is lost from friction, there may not be enough to operate the emitters further down the line. Even if there is sufficient pressure to operate the emitters, it may not be enough to keep them uniform with the rest of the system; this can cause issues like plants further down the line not receiving enough water (or plants closer to the water source receiving too much if run time is increased).

Another factor, though not as critical as the above, is also significant: cost. If you do not calculate friction loss, you may end up purchasing a tubing diameter that’s larger than what you need and thus end up paying more than necessary. On the other side of that, if you purchase tubing that’s too small in diameter, it will likely have to be replaced. The costs of replacing tubing are significantly higher, as there is the lost cost of the initial roll of tubing, the time spent to install it, and possibly losses from the crop suffering from poor irrigation system performance.

Planning ahead of time will not only save you money, but also help ensure good system performance.

Protection of Components

Another reason to account for friction loss is protection of components. High water velocity can cause damage to valves, fittings, emitters and other components in an irrigation system. If the water velocity is too high you can experience what’s known as “water hammer.” It is an apt name, as any time you hear that sound it is the water hammering into a component, be it a valve, fitting, the walls of the tubing, or even a sprinkler or emitter.  This hammering causes damage over time.  High velocities can also displace the tubing due to movement; this displacement can cause damage to the outside of the tubing.  Very high velocities can even reach erosion levels and cause damage quickly. This won’t be a concern for most DIY’ers unless the system is larger, but still something to account for to ensure the longevity of the parts in the system.

A general rule is to make sure the velocity is not much higher than 5’ per second (5 feet per second). This is somewhat arbitrary (in that you won’t find anything official) but 6’ per second or less also improves the aesthetic in that the sound of the water moving through the tubing is lower at this velocity. This is one reason why you see recommendations in keeping the flow rate in ½” tubing to 200 GPH or less – 200 GPH going through ½” poly tubing has a velocity of about 5’ per second.  This recommendation, though not a hard and fast rule of the industry, is designed to keep friction loss and velocity reasonable for both performance and longevity.

It’s important to note that, in much larger systems using a very large diameter of tubing or pipe, the velocity is also important for personnel safety. A high velocity of water in a 10” pipe could even cause significant injury, particularly if it is a metal pipe.

Calculating Friction Loss

Googling “how to calculate friction loss” brings up an overwhelming amount of information, from general guidelines (like the 200 GPH maximum for ½” tubing) to complex mathematical formulas. Performing the math yourself could be beneficial if you’re designing a large system, but if you’re a home DIY’er the friction loss calculator we have on our site will be more than sufficient; all you need to know to utilize the calculator is the system flow rate in GPH, the total linear length of the tubing in feet and the diameter of the tubing. Here is the link to the calculator: Pressure Loss Calculator.

If you’d like to calculate the velocity, you can use Washington State University’s calculator. They have two separate calculators; one to determine velocity in feet per second and another to determine what tubing size would be needed to keep velocity at 5’ per second or less. I recommend using the second calculator as it will be more accurate in regards to what tubing size is needed. For example, if you use the velocity calculator and put in 3.33 GPM (200 GPH) on the flow rate line and 0.5” on the tubing diameter line, you’ll get a result of 5.44’ per second. This is because they are using an actual ½ inch tubing size on that calculator, but ½” tubing tends to be larger than an actual half inch – our ½” tubing, for example, has an inside diameter of .600”, which is a very common inside diameter for irrigation tubing labeled 1/2.”.

Using the second calculator will give you an inside diameter to use. For example, if you input 8 GPM on the flow line, it will report that you need a 0.8” inside diameter tubing to keep velocity at 5’ or less.  0.8”, while not an actual tubing size, is close to common ¾” tubing sizes. Our ¾” tubing, for example, has an inside diameter of .820”.  Here is the link to WSU’s calculator: WSU Pipe Water Velocity and Minimum Pipe Diameter Calculator.

Hunter Industries, a manufacturer of premium irrigation supplies, has created a short booklet that provides charts and formulas for friction loss in several schedules and classes of IPS PVC and Polyethylene.  For anyone looking to truly account for friction loss, this resource will prove invaluable.  It can be found at this link: Hunter Industries Tech / Friction Loss Tables.

Concluding Thoughts

Accounting for friction loss is important to save on costs, time and to help ensure proper system performance.  For the home DIY’er, following the general guidelines will typically be more than sufficient; larger commercial growers, who have the most on the line, stand to gain the most by calculating and planning for friction loss.  Online calculators can do the math for the end-user with just a little bit of input that should be easy to attain.