Joe Evans, Ph.D

http://www.pumped101.com

FRICTION AND FRICTION HEAD

Friction occurs when a fluid flows in or around a stationary object or when an object moves through a fluid. An automobile and an aircraft are subjected to the effects of friction as they move through our atmosphere. Boats create friction as they move through the water. Finally, liquids create friction as they move through a closed pipe.

A great deal of money has been spent on the design and redesign of boats, aircraft, and autos to reduce friction (often referred to as drag). Why? Because friction produces heat and where there is heat there is energy, wasted energy that is. Making these vehicles "slippery" reduces friction therefore reducing the energy required to get them from point A to point B.

As water moves through a pipe its contact with the pipe wall creates friction. As flow (or more correctly velocity) increases, so does friction. The more water you try to cram through a given pipe size the greater the friction and thus the greater the energy required to push it through. It is because of this energy that friction is an extremely important component of a pumping system.

Laminar flow describes the flow of a liquid in a smooth pipe. Under conditions of laminar flow, the fluid nearest the pipe wall moves more slowly than that in the center. Actually, there are many gradients between the pipe wall and the center. The smaller the pipe diameter, the greater the contact between the liquid and the wall thus the greater the friction. The figure below shows laminar flow through two cross sections of a pipe. The vector lengths in the right-hand drawing are proportional to the velocity of the flowing liquid.

Figure 9

Friction or Friction Head is defined as the equivalent head in feet of liquid necessary to overcome the friction caused by flow through a pipe and its associated fittings.

Friction tables are universally available for a wide range of pipe sizes and materials. They are also available for various pipe fittings and valves. These tables show the friction loss per 100 feet of a specific pipe size at various flow rates. In the case of fittings, friction is stated as an equivalent length of pipe of the same size. An example is shown below.

Figure 10

It is evident from the tables, that friction increases with flow. It is also evident that there is an optimum flow for each pipe size, after which friction can use up a disproportionate amount of the pump output. For example, 100 GPM for short lengths of 2" pipe may be acceptable; however, over several hundred feet the friction loss will be unacceptable. 2 1/2" pipe will reduce the friction by 2/3 per 100 feet and is a much better choice for a 200-foot pipeline. For systems that operate continuously 3" pipe may be the appropriate choice.

Many friction tables show both friction loss and fluid velocity for a given flow rate. Generally, it is wise to keep fluid velocity under 10 feet per second. If this rule is followed, friction will be minimized.

The friction losses for valves and fittings can also add up. 90 degree turns and restrictive valves add the most friction. If at all possible, straight through valves and gentle turns should be used.

Consider this problem: What head must a pump develop if it is to pump 200 GPM through a 2.5" pipe, 200 feet long, and to an elevation of 75'? Use the table in Figure 10.

Elevation = 75'

Friction 2.5” Pipe = 43' / 100ft or 86'

Total head = 75' + 86' = 161' TDH

Use of 3" pipe in the above problem will reduce friction head by 30% and although it will cost more initially, it will pay for itself in energy savings over a fairly short period of time.

The “Hot and Cold” Puzzler delves deeper into fluid friction and its consequences.

About the Author

Joe Evans lived in beautiful Rhododendron Oregon and retired from Pump Tech Inc on 12/31/15. Since entering graduate school, a continuing interest has been one of computer control of mechanical and electronic systems. It began with the introduction of the minicomputer, in the late sixties, and continued with the advent of the PC and PLC in the eighties and nineties. He accidentally entered the pump industry in 1986 and has been trapped there since. He is passionate about the sharing of knowledge and its ability to replace memorization with understanding.