Factors Affecting the Factor of Safety for Pressure Vessels

The factor of safety (FoS) for pressure vessels is a design criterion that ensures that the vessel can safely handle loads beyond what it is expected to experience during normal operation. The factor of safety is defined as the ratio between the material’s ultimate tensile strength (or yield strength) and the allowable stress.

The specific factor of safety for a pressure vessel varies depending on:

The Code or Standard: Different countries or industries may adhere to different design codes and standards for pressure vessels. Commonly used standards include the ASME Boiler and Pressure Vessel Code (ASME BPVC) in the United States, the Pressure Equipment Directive (PED) in Europe, and others.

Service Conditions: The required factor of safety can vary based on the intended service conditions, such as whether the vessel is intended for lethal service, contains toxic substances, or operates at elevated temperatures.

Material: Different materials have different mechanical properties, and the factor of safety may vary depending on the material chosen for the vessel.

Design Life and Fatigue Considerations: For vessels subjected to cyclic loading, additional safety factors might be applied to account for fatigue.

Unforeseen Operating Conditions: 

To account for unexpected scenarios or uncertainties in operation, material properties, and loading conditions.

For the ASME BPVC, Section VIII (which governs the design of pressure vessels), typical factors of safety are:

For materials with a clearly defined yield point, the minimum factor of safety is usually taken as 3.5, meaning the allowable stress is set at about 1/3.5 = 28.6% of the yield strength.

For materials without a clearly defined yield, like some stainless steels, the factor of safety might be taken as 4 based on the material’s tensile strength.

However, these are general values, and the specific factor of safety for a particular vessel would depend on various conditions and criteria set by the relevant design code or standard. It’s essential to refer to the appropriate code or standard and work with qualified engineers when designing or assessing a pressure vessel.

Sure, let’s delve deeper into other considerations related to the factor of safety (FoS) and its implications for pressure vessel design and operation.

Joint Efficiency:

When designing a pressure vessel, especially a welded one, the quality of the welds is paramount. Welded joints can introduce discontinuities and potential weak points in the vessel. To account for this, design codes often use a joint efficiency factor. If full radiography (a method of inspecting the integrity of welds) is employed, a higher joint efficiency can be used, reducing the factor of safety’s impact. Conversely, vessels with welds that aren’t fully radiographed might need a lower joint efficiency, leading to a thicker vessel wall for the same design pressure.

Temperature and Material Behavior:

The strength and behavior of materials can vary significantly with temperature. At elevated temperatures, materials might lose strength, and at very low temperatures, they might become brittle. This temperature dependency is considered in pressure vessel design codes by providing allowable stress values for various temperature ranges. As such, the effective factor of safety might vary depending on the operating temperature.

Corrosion Allowance:

Over time, the walls of pressure vessels can thin due to corrosion, especially if they handle corrosive substances. To account for this anticipated loss in thickness, an additional “corrosion allowance” is often added to the wall thickness during design. While not a “factor of safety” in the traditional sense, this allowance serves a similar purpose: ensuring safety and integrity over the vessel’s service life.

Dynamic Loading:

While the primary design criteria for pressure vessels usually involve static pressures, some vessels might be subjected to dynamic loads, such as pulsating pressures or external forces (e.g., from earthquakes or machinery vibrations). In such cases, additional safety factors or design considerations might be applied to account for the dynamic nature of the loads.

Importance of Inspections and Maintenance:

The factor of safety applied during the design phase provides a buffer against unforeseen conditions and potential degradation over time. However, this does not eliminate the need for regular inspections and maintenance. Over time, factors like corrosion, fatigue, or physical damages can compromise a vessel’s integrity. Regular inspections can identify potential issues before they become critical, ensuring the vessel remains within its safe operating limits.

Evolving Standards and Best Practices:

As our understanding of material behavior, fabrication techniques, and failure mechanisms improves, design codes and standards evolve. New research, technological advancements, and lessons learned from past incidents are continually integrated into updated versions of pressure vessel codes. For organizations, staying updated and compliant with the latest versions of these codes is crucial for ensuring safety.

While the factor of safety is a fundamental concept in pressure vessel design, it’s only one piece of the puzzle. Comprehensive vessel safety involves a synergy of robust design, quality fabrication, regular inspections, and diligent maintenance. By understanding and respecting these multiple layers of safety considerations, industries can ensure the safe and efficient operation of pressure vessels, safeguarding lives, investments, and the environment.