1.0 Introduction to Stress Analysis
Stress Analysis is a subject, which is more talked about and less understood. The objective of pipe stress analysis is to ensure safety against failure of the Piping System by verifying the structural integrity against the loading conditions, both external and internal, expected to occur during the lifetime of the system in the plant. This is to be undertaken with the most economic considerations. Hence the objectives of stress Analysis could be listed as
1.1 Objectives of Stress Analysis are to
1.1.1 Ensure that the stresses in the piping components in the system are within the allowable limits.
1.1.2 Solve dynamic problems developed due to mechanical vibration, acoustic vibration, fluid hammer, pulsation, relief valves etc.
1.1.3 Solve the problems associated due to higher or lower operating temperature such as
a) Displacement Stress range
b) Nozzle loads on the connected equipment
c) Pipe displacements
d) Loads and moments on the supporting structures
When piping is connected to strain sensitive equipment, the flexibility required to satisfy the acceptable limits of nozzle loading on the connected equipment (‘b’ above) overrides all other considerations.
1.2 Classifications of Piping Systems
The piping systems are mainly classified into three main categories and then again subcategories. The main categories are the
1.2.1 Hot Systems.
1.2.2 Cold Systems.
1.2.3 Cryogenic Systems.
The fundamental reason for this classification is that hot lines and cryogenic lines must undergo Flexibility analysis to determine thermal forces, displacements and stresses. These systems are further divided into.
i) Small bore lines
ii) Large (Big) bore lines.
As a general practice those pipe lines with nominal diameters 40mm (11/2″) NB and under are classified as small and 50mm (2″)NB and above as larg. Further, piping system could be classified based on the regulatory codes under which the system is designed. Certain codes require more stringent analysis than others.
1.3 Steps involved in the stress analysis can be listed as
1.3.1 Identify the potential loads that the piping system would encounter during the life of the plant.
1.3.2 Relate each of these loads to the stresses and strains developed.
1.3.3 Get the cumulative effect of the potential loads in the system.
1.3.4 Decide the allowable limits, the system can withstand without failure.
1.3.5 After the system is designed, to ensure that the stresses are within the safe limits.
1.4 Types of loads
All the American code for Pressure Piping classifies the loads mainly into three types.
1.4.1 Sustained Loads: Those due to forces present during normal operation.
1.4.2 Occasional Loads: Those present during rare intervals of operations
1.4.3 Displacement Loads; Those due to displacement of pipe
This analysis is most commonly called as the Flexibility Analysis. Further those conditions stipulated in the regulatory code ASME B 31.1 and ASME B 31.3 only are considered hereafter.
1.5 Conditions of Acceptability of Piping System
The Piping Engineer has the following choices to establish that the required flexibility has been provided in the piping layout.
1.5.1 As per clause 119.7.1/319.4.1 of the code ASME B 31.1/13 31.3, no formal analysis is required in systems which
I) Are duplicates of successfully operating installations or replacements.
ii) Can readily be judged adequate by comparison with previously analyzed systems.
iii) Satisfy equation specified in clause 119.7.1(A3)/ 3194.1 (c)
1.5.2 Analyzing the layout by an approximate method.
Approximate method shall be applied only if they are used for the range of configuration for which adequate accuracy has been demonstrated.
1.5.3 Carrying out a comprehensive analysis.
ii) Model test
iii) Chart method