In many systems, the expansion and compression of such air may take place even if the injection valves do not open. An insufficient fuel supply may result from a defective or inoperative part in the system. Such items as a closed inlet valve in the fuel piping or an empty supply tank are more likely to be the fault of the operator than of the equipment. Usually these cracks occur on threaded pipe joints at the root of the threads. Such breakage is caused by the inability of the nipples and pipe joints to withstand shock, vibration, and strains resulting from the relative motion between smaller pipes and the equipment to which they are attached.
Metal fatigue can also cause breakage.
Each system should have a systematic inspection of its fittings and piping to determine if all the parts are satisfactorily supported and sufficiently strong. In some instances, nipples may be connected to relatively heavy parts, such as valves and strainers, which are free to vibrate. Since vibration contributes materially to the fatigue of nipples, rigid bracing should be installed. When practicable, bracing should be secured to the unit itself, instead of to the hull or other equipment. Breakage can also cause leakage in the high-pressure lines of a fuel system.