WIND EFFECT ON PIPING SYSTEMS

Wind loads are critical in designing industrial piping systems, especially in regions where wind intensity can reach high levels. These loads can significantly affect the overall structural integrity of a piping network. This blog post will discuss the impact of wind on piping stress, the necessary load cases for wind analysis in Caesar II, detailed calculation methods, and relevant international codes.

1. Effects of Wind on Piping Systems

Wind-induced forces on piping systems can result in the following effects:

• Lateral Deflections: Wind pressure can cause horizontal displacement in above-ground piping systems, affecting support alignment and introducing unexpected stresses.

• Dynamic Vibration: Strong winds can cause dynamic vibration or flutter in the piping, which can potentially lead to fatigue failure, especially in long, unsupported runs.

• Increased Stress at Supports: Due to wind pressure, supports can be subjected to increased loads, which can result in higher reaction forces and overstress.

• Risk of Buckling: High wind speeds, especially in tall structures or elevated piping, could induce buckling of slender pipes or support structures.

2. Load Cases for Wind in Caesar II

Wind load analysis can be modeled in Caesar II by defining specific wind load cases. These are based on wind velocity and exposure conditions according to international standards.

Common load cases for wind calculations include:

• Sustained Load Case (W+P+T): Where wind loads are combined with operating loads such as pressure and temperature.

• Occasional Load Case (W+P+T+Wind): Wind loads are considered as occasional loads and are typically combined with operating loads during extreme conditions.

• Alternative Load Case (W+Wind): In some cases, wind loads may be considered without the full operating load, especially if wind is the governing condition for the design of the pipe supports.

In Caesar II, wind load cases can be defined under:

• Wind Parameters: Input the wind speed, direction, and applicable height from the ground.

• Wind Profiles: Define the wind profile according to site-specific conditions or standards.

• Wind Load Factors: Factor in conditions such as topography, terrain category, and exposure, which will influence the wind pressure calculations.

3. Wind Load Calculation Methodology

Wind load calculations on piping systems involve several steps, typically following the simplified formula derived from international standards like ASCE 7 or EN 1991-1-4 (Eurocode 1):

Fw=Cf×qz×A×G

Where:

• Fw​ = Wind force acting on the piping (N or lb)

• Cf​ = Shape factor (depends on pipe geometry)

• qz = Dynamic wind pressure (Pa or psf) at a given height z

• A = Projected area of the pipe perpendicular to wind direction (m² or ft²)

• G = Gust factor (accounts for short-duration wind fluctuations)

The shape factor (Cf​), also known as the drag coefficient, accounts for the influence of an object's shape on how it interacts with wind forces. It determines how much aerodynamic drag or resistance the wind exerts on an object, such as a pipe, depending on its geometry and orientation relative to the wind direction.

The shape factor is essential in industrial piping because different shapes interact with wind forces in different ways. For cylindrical piping, which is commonly used in industrial settings, the shape factor is typically determined based on empirical studies and is standardized in design codes.

Typical Values for Shape Factor Cf​:

• Circular Cylindrical Piping (exposed to wind):
  For long, smooth cylindrical pipes (which are common in industrial piping systems), the shape factor is generally between 0.6 and 0.7. This value may vary slightly depending on the diameter of the pipe and the surface roughness.

• Rectangular or Flat Surfaces:
  The shape factor is generally higher for flat surfaces or box-type structures, often around 1.2 to 2.0, since flat surfaces experience more resistance from wind compared to cylindrical shapes.

Influence of the Shape Factor:

• Lower CfCf​: More streamlined shapes like cylindrical piping tend to have lower drag coefficients, indicating less resistance to wind and, therefore, lower wind-induced forces.

• Higher CfCf​: Flat or sharp-edged structures have higher drag coefficients because they disrupt the wind flow more, leading to higher wind loads.

Key Variables:

• Dynamic Wind Pressure (qz): This is dependent on wind speed and air density. The wind speed at the height of the pipe must be considered, and this can be calculated using the logarithmic wind profile or simplified methods from codes.

qz=0,5ρ*Vz^2

Where:

• ρ= Air density (1.225 kg/m³ at sea level)

• Vz​ = Wind speed at height z (m/s)

4. International Codes for Wind Calculations

Several international codes govern wind load calculations on industrial piping systems. These codes provide methodologies for calculating wind pressures and forces and guidance on load combinations.

• ASCE 7-22 (Minimum Design Loads for Buildings and Other Structures): This code from the American Society of Civil Engineers provides comprehensive guidelines for calculating wind loads on structures, including exposed piping systems. It includes procedures for static and dynamic wind loading.

• EN 1991-1-4 (Eurocode 1: Actions on Structures - Wind Actions): The European standard offers guidelines for wind actions on structures, including pipes and exposed equipment. The code includes calculation methods based on wind exposure, terrain category, and structural height.

• ISO 13703 (Petroleum and Natural Gas Industries - Design and Installation of Piping Systems): This standard gives guidance on wind loading considerations for above-ground piping systems in the oil and gas industry. It suggests designing wind speeds based on site location and includes methods for calculating forces on piping systems.

• API 560 (Fired Heaters for General Refinery Services): Although specific to heater stacks, this code offers insights into the effects of wind on vertical piping systems and chimneys, which can apply to other elevated piping.

• NBC (National Building Code of Canada): The Canadian standard includes provisions for wind load calculations on structures and applies to piping systems in industrial facilities exposed to extreme wind conditions.

5. Practical Considerations for Wind Analysis in Piping Design

• Support Spacing and Type: Wind can induce significant forces on piping supports, especially in long runs. In windy areas, reduce the spacing between supports or increase support size to accommodate wind forces.

• Guides and Anchors: Adding guides or anchors along the piping route can help mitigate lateral deflections due to wind forces.

• Elevation and Topography: The higher the piping system, the stronger the wind forces. Consider terrain effects and elevation when calculating wind loads. Wind speeds generally increase with height above the ground.

• Flexible Joints: In cases where excessive stresses are predicted due to wind loading, flexible joints or expansion loops may be used to reduce stress levels.

• Dynamic Effects: For regions with high wind speeds, such as coastal or mountainous areas, dynamic wind analysis, considering wind gusts, vortex shedding, and oscillatory forces, may be necessary.

Conclusion

Wind calculations are essential for ensuring the safety and integrity of industrial piping systems. Using tools like Caesar II, engineers can effectively model the wind load cases, considering both static and dynamic wind forces. Applying international codes such as ASCE 7, Eurocode, and ISO 13703 ensures that wind forces are accurately represented in piping stress analyses, mitigating risks related to structural failure and operational hazards.

This analysis underscores the importance of understanding the local wind conditions and correctly applying wind load factors to achieve a robust piping design that withstands the effects of wind-induced stresses.