Factor of Safety – What Is Piping

The factor of safety or safety factor is a very important term in engineering design. While designing any Engineering product or component, safety is of utmost importance. To ensure the safety of those items, each component is designed to bear more loads than its actual operating loads. So, there will always be some margin or cushion as compared to its operating capabilities. This is ensured during the design stage by considering a suitable factor of safety. The factor of safety is defined as the ratio of ultimate stress of the component material to the working stress. It denotes the additional strength of the component than the required strength. Importance of Factor of Safety A factor of safety is related to the safety of people. It reduces the risk of failure of a component by adding some cushion in design. Also, there always will be some hidden circumstances or unknown parameters that can not be considered accurate in design. At the same time, the reliability of material is not 100% and the load used in design calculation is not the maximum load. So the factor of safety or safety factor provides protection against those unknown events to some extent.Although all components are designed with a factor of safety, it's always suggested to use those components within their design limit. Factors Affecting Determination of Factor of safety Component design codes & standards normally provide a minimum factor of safety for that component. However, choosing the exact safety factor is dependent on various parameters like Material type Ductile vs BrittleLoading type Static vs dynamicWhether cyclical loadsThe intensity of stress concentrationUnforeseen Misuse of the componentAccuracy and stress complexity during a calculationEnvironment and design temperature and pressure.Impact of failureCost of component or materialCorrosion rateMaintenance frequency Factor of safety equation As defined in the first paragraph that the factor of safety is a ratio of two loads or two stresses. Mathematically factor of safety can be expressed as Factor of safety=Ultimate Load (Strength)/Allowable Load (Stress) As understood from the above equation the allowable stress is always less than the ultimate failure stress. Hence, the factor of safety is always greater than 1. The ultimate stress for brittle material is considered as ultimate tensile strength and for ductile material is considered as yield strength. Also, as the equation for factor of safety is the ratio of two stress or load values, it is dimensionless. The difference between the Factor of Safety and 1 is known as Margin of safety. So, Margin of safety= (Factor of safety-1). Examples of Factor of Safety Typical examples of the factor of safety for a few materials depending on application and loading types are listed below in table 1. However, please note that the table only provides a typical range or typical values to provide an example. Actual values for the factor of safety must be taken from the design standard of the relevant component. For example, the factor of safety for pressure vessel design must be taken from the ASME Sec VIII code. Sr NoMemberTypical Factor of safety range1Structural Members in building services22Pressure Vessel3.5 to 43Automobiles34Aircraft and Spacecraft1.2 to 3.05Aircraft components1.5 to 2.56Boilers3.5 to 67Bolts8.58Cast-iron wheels209Engine components6.0 to 8.010Heavy duty shafting10.0 to 12.011Lifting equipment - hooks ..8.0 to 9.012Pressure vessels3.5 - 613Turbine components - static6.0 to 8.014Turbine components - rotating2.0 to 3.015Spring, large heavy-duty4.516Structural steel work in buildings4.0 to 6.017Structural steel work in bridges5.0 to 7.018Wire ropes8.0 to 9.019Highly reliable materials where loading and environmental conditions are not severe1.3 to 1.520Moderately reliable materials where loading and environmental conditions are not severe1.5 to 221Ordinary materials where loading and environmental conditions are not severe2 to 2.522For use with less tried and for brittle materials where loading and average environmental2.5 to 323For use with materials where properties are not reliable and where loading and environmental conditions are not severe, or where reliable materials are used under difficult and environmental conditions3.0 to 4.024Impact Forces3.0 to 6.025Brittle Material1.0 to 6.0Table 1: Typical Factor of Safety Range Factor of safety calculator Various factor of safety calculators is available online to calculate the values of the factor of safety with known ultimate strength and allowable stress values. Let's take an example to calculate factor of safety for the following situation: The yield strength of a ductile material is 240 MPa. If the material is subjected to a loading condition that generates the maximum allowable stress of 140 MPa. Then the factor of safety of that material is 240/140=1.7. However, note that both stress values are in the consistent unit before calculating the factor of safety.