Wind Loads on Piping systems

Wind Loads requirements as per ASME B31.3

As per clause 301.5.2 of ASME B31.3, The effect of wind loading shall be taken into account in the design of exposed piping. It may be noted that wind load is considered as an occasional load. The wind loads on small bore piping are usually insignificant and are generally disregarded for any stress analysis. The wind loads on piping systems larger than 10” can be significant and should be considered in the piping stress analysis.

Determination of wind loads on piping systems

The analysis considerations and loads may be as described in ASCE-7-05 “Minimum Design loads for Buildings and other Structures”. Authoritative local meteorological data may also be used to define or refine the design wind loads.

The site-specific design parameters can be used to refine the design wind loads. For a typical project, the details could be usually specified in the Structural Design Basis for the project. A typical project wind velocity data is provided below for reference and shall not be used as generally applicable for all cases of wind loads:

Example of Wind load data for Plant XYZ or Project XYZ

Basic wind speed V = 145 km/hr

Importance factor I = 1.15

Exposure = D

The applied Wind Force “F” is determined by the basic equation:

F = q_zGC_fA_e

where:

q_z = Velocity pressure at height z above ground

G = Gust effect factor = 0.85

C_f = Net force coefficient

A_e = Projected area normal to wind

The velocity pressure is computed as follows:

q_z = 0.613K_zK_ztK_dV2I (N/m²)

where:

q_z is based on K_zt of 1.0

K_d = 1.0, Exposure Category C

Importance Factor I of 1.15 and

Velocity V in m/sec of 3-sec gust wind speed

Note: A wind directionality factor K_d = 0.85 shall be applied to all the above calculated q_z values when wind loads are used with service and factored load combinations.

Force coefficient C_f

Beams, bracings and columns 2.0

Pipes 0.70

Cable trays 2.0

Tributary Area for Piping

The tributary area for piping should be based on the diameter of the largest pipe plus 10% of the width of the Valve Access Platform. This sum is multiplied by the length of the pipes to determine the tributary area.

Considerations of wind forces are normally not necessary in the longitudinal directions because friction and anchor loads will normally govern in the longitudinal direction.

Partial Wind Load (Wp) shall be based on the requirements of SEI/ASCE 37-02, Section 6.2.1, for the specified test or erection duration. The design wind speed shall be 0.75 multiplied by the Basic Wind Velocity according to SEI/ASCE 37-02 for test or erection periods of less than 6 weeks. For test or erection periods longer the six weeks refer to SEI/ASCE 37-02

Application of Wind Loads in Stress Analysis

In the Operating Case of stress analysis, the sum of the longitudinal stresses, S_L, due to sustained loads, such as pressure and weight, and of the stresses produced by occasional loads, such as wind or earthquake, may be as much as 1.33 times the basic allowable stress given in Appendix A of ASME B31.3. Wind loads shall be considered as acting along the horizontal axes (both in the +ve and in the –ve directions, i.e. along North, South, East and West directions), but not acting simultaneously. Wind and earthquake forces need not be considered as acting concurrently.

A typical operating case with wind acting is illustrated below.

where

W = Dead weight of piping system

P1 = Design Pressure

T1 = Design Temperature

WIN1 = Wind Load in +X direction

WIN2 = Wind Load in -X direction

WIN3 = Wind Load in +Z direction

WIN4 = Wind Load in -Z direction

Vortex Shedding due to Wind Loads

In the case of wind loads on piping, the fluid is flowing past a right-circular cylinder causing vortex shedding. This flow frequently produces vortices that are referred to as Karman vortices, and shed in a regular pattern over a wide range of Reynolds numbers. The Reynolds number is given by

R = (vdØ)/μ

in which v is the fluid velocity, d is the diameter of the cylinder, Φ is the mass density of the fluid, and μ is the absolute viscosity of the fluid. The vortices shed alternately from opposite sides of the cylinder with a frequency f. This causes an alternating pressure on each side of the cylinder, which acts as a sinusoidally varying force F perpendicular to the velocity of the fluid before its flow is disturbed. This force is given by

F = [(CΦv²A)/2]sin(2πft)

where C = drag coefficient (dimensionless) (C ≈ 1 for a cylinder)

v = fluid velocity (ft/s, m/s)

A = projected area of cylinder perpendicular to v (ft², m²)

Φ = mass density of fluid (lb•s²/ft⁴, kg/m³)

Experimental studies have confirmed that the frequency of the vortex shedding described above can be expressed as

f = Sv/d

where

S = Strouhal number (dimensionless S = 0.20 for structures of circular cross section)

d = diameter of cylinder (ft, m)

v = velocity of fluid (ft/s, m/s)

f = frequency of vortex shedding (cycles/second)

If the flow velocity is not perpendicular to the structure interrupting the flow but has an angle of inclination θ, the vortex shedding frequency becomes

f = (Sv sin θ)/d

in which vsin θ is the component of velocity perpendicular to the structure.

The natural frequencies of piping elements and structures shall be checked to ensure that they do not coincide with the vortex shedding frequency. Insulation applied to piping increases its overall projected pipe area and shall be included in the wind load calculations.

As per Caesar II Technical Reference Manual: Winds of 20 to 40 mph can cause vortex shedding and excitation in the 30 Hz and higher range that can cause fatigue failure in smaller line sizes particularly susceptible to fatigue type failures. To analyze vortex shedding, harmonic analysis methods can be used.