Last month we examined two of the three factors to be considered when determining the pressure requirements of a pump - household working pressure and lift pressure. The third factor is pressure drop due to friction loss. Friction loss is the energy lost due to friction when two objects move in relationship to one another. When you drive your truck down the road at 30 mph, the pressure caused by the friction of the air against your truck uses up a certain amount of horsepower. As your speed increases, more horsepower is required to overcome the friction. This is friction loss, and it works against you in a piping system just as it does on the highway system.
As the water travels through the pipes of a water system, friction causes the pressure slowly to drop so that the pressure at the outlet is always lower than the pressure at the inlet (if the system is level). This is called the friction loss or pressure drop, and it varies depending on several factors:
- The viscosity of the fluid. Viscosity is the measure of a fluid's resistance to flow. The higher the viscosity, the more pressure it takes to push it through a pipe. Molasses is harder to pump than water, but since we are only considering water systems in this article, all the tables and factors presented here are calculated for the viscosity of water.
- The smoothness of the inside of the pipe. A pipe that is coated with calcium deposits and rust will cause more pressure drop than a clean smooth pipe. The tables on this page are for new smooth pipe, so if you suspect the pipes in your system will be coated with deposits, double the pressure drop in your calculations.
- The inside diameter of the pipe. The smaller the pipe, for a given flow rate, the greater the pressure loss.
- The flow rate or velocity of the fluid. The faster it flows, the higher the pressure loss.
- The number of valves and fittings in the system. When a fluid has to change direction or flow through an irregular object, more pressure is lost than when it flows through a straight section of pipe. The amount of flow restriction for valves and fittings is expressed in terms of equivalent lengths of pipe. Fortunately for us, the engineers have done their work and have created a set of tables for determining the pressure loss of water in different types of valves and fittings. For instance, a 1-inch diameter copper elbow causes the same amount of pressure drop as 3 feet of 1-inch copper pipe. Table 1 shows equivalent lengths of some common fittings used in pumping systems. Fittings made from different materials have varying pressure drops, so be sure to use the table for the type of fittings in your system, i.e., copper, PVC, steel, etc. The Uniform Plumbing Code has tables of pressure drop in fittings for various materials, as do engineering manuals from most pump manufacturers.
We commonly speak of friction pressure loss in terms of pressure drop per 100 feet of pipe. Tables depicting pressure drop are called dynamic pressure drop tables because they show the pressure loss due to the moving (dynamic) water through the pipes. The other kind of pressure loss, which we talked about last month, is called static pressure loss, which is the loss due to differing elevations in the system. Table 2 shows pressure loss in terms of feet of head and pounds plus velocity in terms of feet per second for copper pipe. It is important to size your piping system with velocity in mind. The National Hydraulic Institute recommends 7 ft./sec. as a maximum design velocity for optimum hydraulic efficiency and minimal erosion. As you can see in Table 2, velocity goes down dramatically as pipe size goes up for a given flow rate. Contact your pump or pipe supplier for similar tables for PVC and steel pipe.
Next month we will use this information to calculate the pressure drop in the piping system in a typical residential water system.