Types of Loads Considered for Piping Stress Analysis

Piping Stress Analysis is the practice and discipline carried out behind the scenes of any piping design project to ensure structural and operational integrity, and optimal design. All piping systems are inevitably subjected to various kinds of loads in their design life cycle. These loads develop stresses in the arrangement. Pipe stress engineers typically analyze such systems for stress using reliable software such as Caesar II to check that the stress levels are within the permissible limits as defined by the codes/standards. 

The following are some of the most common type of loads considered during stress analysis:-

Wind Load:
The majority of all piping system installations are indoors where the effects of wind loading can be neglected. However, there are sufficient numbers of outdoor piping installations where wind loading can be a significant design factor. Wind load, like dead weight, is a uniformly distributed load that acts along the entire length, or that portion of the piping system that is exposed to the wind. The difference is that while dead weight loads are oriented in the downward vertical direction, wind loads are horizontally oriented and may act in any arbitrary direction. Since wind loads are oriented in the horizontal direction, the regular dead weight support system of hangers and anchors may have little or no ability to resist these loads. Consequently,when wind loading is a factor, a separate structural evaluation and wind load support system design is required.

Determination of the magnitude of the wind loading's is based upon empirical procedures developed for the design of buildings and other outdoor structures. Analysis of piping system stresses and support system loads is accomplished by using techniques that are similar to those applied for dead weight design.

Snow and Ice Loads:
Snow and ice loads, like wind loads, need to be considered in the design of piping systems which are installed outdoors, particularly if the installation is made in the northern latitudes. Since snow and ice loads act in the vertical direction, they are treated the same as dead weight loads. In design, they are simply added as distributed loads in the dead weight analysis.

a) SNOW LOADS :-
ANSI/ASCE 7–95, Minimum Design Loads for Buildings and Other Structures, provides recommendations and data for developing design loading's due to snow. The methods used in this standard are generally applicable to sloping or horizontal flat surfaces such as building roofs or grade slabs.

b)ICE LOADS:-
Ice storms are sporadic in the frequency of their occurrence and in their intensity. Weather records dating back to the turn of the 20th century for a typical mid western state relate instances of ice storm deposits of 1/8 in (3.2 mm) to 4 in (102 mm) in thickness. The American Weather Book 10 cites examples of ice accumulations of up to 8 in (203 mm) in northern Idaho (1961) and 6 in (152 mm)
in northwest Texas (1940) and New York State (1942).

Seismic (Earthquake) Loads :-
Under certain circumstances it is necessary or desirable to design a piping system to withstand the effects of an earthquake. Although the applications are not extensive, piping system seismic design technology is well developed and readily accessible.Many currently available piping stress analysis computer programs are capable of performing a detailed seismic structural and stress analysis, in addition to the traditional deadweight and thermal flexibility analyses. Most of these programs run
on desktop microcomputers. Because of the higher construction costs and design complexities introduced by the application of seismic design criteria, this type of work is normally done only

in response to specific regulatory, code, or contractual requirements.

Sustained Loads :-
Sustained loads exist throughout the plant’s operation. These mainly consist of internal pressure and dead-weight. Dead-weight is generally from weight of pipes, fittings, components such as valves, operating fluid, test fluid, insulation, cladding, lining etc. These are to be considered as follows:

• Design pressure
• Weight of Pipe and associated components such as Flanges, Valves, Strainer, Sight glass etc., mounted on the Piping System.
• Weight of Fluid/contents in the piping
• Insulation and cladding weight
• Hydro test loads, if applicable
• Snow load, if applicable

Different international piping codes have set the limits that are also called “allowable stresses for sustained loads”, on these axial stresses caused by dead-weight and pressure in order to avoid “failure by collapse”. To achieve the allowable stresses for sustained loads, it is often required to support the piping system vertically.

Occasional loads :-

These type of loads are imposed on piping by occasional events like wind, earthquake etc. Wind loads are considered for lines with external diameter 14” NB (including insulation) or above and at elevation 10 meters or higher from the ground level. Wind normally blows in the horizontal plane and to protect piping from wind, it is standard practice to attach lateral supports. In case of an earthquake, the earth seems to move vertically and to protect the piping against both horizontal/vertical movement, some resting supports might be constructed as integral two way lateral and vertical restraints. Normal operating temperature is used when analyzing occasional loads.

PSV Reaction Force :-
If the piping system in question includes PSV (pressure safety valve), then the reaction force due to PSV operation is considered as applicable. While analyzing PSV connected stress systems, the reaction force needs to be calculated. A dynamic load factor equal to 2.0 must be applied on the valve reaction force value. But in case of high reaction force values, the DLF value calculated as per appendix-II of ASME B31.1 should be used. This needs to consider valve data provided by vendor.

Slug Force :-
Slug force has to be considered in stress analysis for lines that have slug flow regime. It is calculated as follows:

Fslug = (ρ) (A) (V2) [2(1 – cos θ)]1/2 DLF

Where,

Fslug = Force due to slug in Newton.

ρ = Density of the slug in Kg/m3

A = Inside area of pipe cross section in m2,

V = Velocity of moving slug in m/sec.

θ = inclusion angle at elbow or change of direction

DLF = Dynamic Load Factor (DLF) equal to two shall be used, unless more accurate value is available.

Faxial = (ρ) (A) (V2) DLF

Forthogonal = (ρ) (A) (V2) DLF

Slug properties can be obtained from Process group.
The equivalent static analysis in Caesar-II is performed to simulate slug loading in piping system using the above formula for calculating slug force.