Copyright Goodheart-Willcox Co., Inc. 188 Fluid Power Institute (ANSI). This standard uses schedule numbers ranging from 10 to 160 to rate pipe. Hydraulic sys- tems typically use schedule 40, 80, or 160 pipe. Sched- ule 40 pipe is generally used in lower-pressure parts of the systems, such as pump inlet lines and system return lines. Schedule 80 or 160 is used in the working lines, depending on the maximum system pressures expected. Figure 8-11 shows typical burst pressure rat- ings for the three commonly used pipe schedule rat- ings. Burst pressure is the highest internal pressure a component can withstand without failure. Typically, the maximum recommended operating pressure is 25% of the burst pressure. Tubing Tubing is a relatively thin-walled, semirigid fl uid con- ductor. It is available in a number of different materi- als, ranging from plastic to stainless steel. Tubing can be easily bent, which allows relatively easy conductor installation with a minimum of related fi ttings. The end result is a system with fl uid conductors that provide proper fl ow characteristics and involve a minimum Pipe used in hydraulic systems should be seam- less, black pipe. It may be either hot or cold drawn. The internal surfaces of the pipe must be free from scale and any other contaminants. Galvanized pipe should not be used in a hydraulic system. The chemical ingredients in oil and its additives react with the zinc coating. A nominal sizing system is used for pipes. With this system, the stated size, or nominal size, is neither the actual outside nor inside diameter of the pipe. For example, the outside diameter of a 1″ pipe is 1.315″, while the inside diameter varies depending on wall thickness. Figure 8-9 includes all of the common nomi- nal pipe sizes. It indicates the actual outside dimen- sions and the inside diameters of a number of the pipes rated by wall thickness. Pipes are available in a variety of wall thicknesses, Figure 8-10. This is an important consideration as it is a key factor in establishing the safe maximum operat- ing pressure of the conductor. The pipe with the thick- est wall will withstand the greatest system pressure, if all other conditions are the same. Wall thickness of pipes is identifi ed by a schedule number established by a standard of the American National Standards (Double Extra Heavy) Schedule 160 Schedule 80 (Extra Heavy) Schedule 40 (Standard) Goodheart-Willcox Publisher Figure 8-10. The wall thickness of a pipe is indicated by schedule numbers that are part of an American National Standards Institute (ANSI) specification. Nominal Size (inches) Outside Diameter (inches) Inside Diameter (inches) Schedule 40 Schedule 80 Schedule 160 Double Extra Heavy 1/4 0.540 0.364 0.302 — — 3/8 0.675 0.493 0.423 — — 1/2 0.840 0.622 0.546 0.466 0.252 3/4 1.050 0.824 0.742 0.618 0.434 1 1.315 1.049 0.957 0.815 0.599 1 1/4 1.660 1.380 1.278 1.160 0.896 1 1/2 1.900 1.610 1.500 1.338 1.100 2 2.375 2.067 1.939 1.689 1.503 Goodheart-Willcox Publisher Figure 8-9. Pipe size is stated as a nominal dimension, which is neither the actual inside nor outside dimension of the conductor.