Pipe Stress Analysis: Introduction & Objective

Basics of Pipe Stress Analysis: Introduction & Objective

Introduction

          Pipes are the most dedicate component in any process plant. They are also the busiest entities. They are subjected to almost all kid of loads – intentional/unintentional. It is very important to take note of all potential loads that the piping system would encounter during operation as well as during other stages in the life cycle of a process plant.

          Ignoring any such load while designing, erection, hydro testing, start-up, shutdown, normal operation, maintenance, etc can lead to an inadequate design & engineering of the piping system.

The step involved in stress analysis

The approximate step involved in stress analysis can be listed below.

1. Identify potential loads that would come onto the pipe or piping system during the entire life.
2. Relate each of these loads to the stress and strains that would be developed in the crystal grain of MOC of the piping system.
3. Decide the worst three-dimensional stress states that MOC can withstand without failure. 
4. Get the Cumulative effect of all the potential loads on the 3D stress scenario in the piping system under consideration.
5. Alter the piping system design to ensure that, the stress pattern is within the failure limit.

       The Goal of analysis and qualification of the piping system is to provide safe design through the erection, hydro testing, start-up, shutdown, normal operation, maintenance, etc. A piping designer would have a number of alternatives to choose from giving the most suitable & economical solution.

Pipe Stress work process diagram.

The objective of Stress Analysis

         The objective of piping stress analysis is to ensure safety against the failure of the piping system by verifying the structural integrity against the loading condition, both external and internal, expected to occur during the lifetime of the system in the plant. Thus, the Objective of the Stress analysis could be listed below.

1. Ensure that, the stresses in the piping components in the system are within the limits.
2. Limiting the sagging of the piping system within allowable limits
3. Directing the line movements so as protect sensitive equipment against overloading
4. Locating supports, guides, anchors, compressor hold downs, and hanger rods
5. Resisting pipe system to collapse in case of earthquake, wind or shock loadings.
6. Providing pre-spring, cold spring, clearance required for line expansion, and additional line flexibility.
7. Solve dynamic problems developed due to mechanical vibration, acoustic vibration, fluid hammer, pulsation, relief valve, etc.
8. Transmitting and handling stress sketches in complete compliance with procedures outlined in Piping Engineering Practices.
9. Solve the problem associated due to higher or lower operating temperature such as

·         Displacement stress range

·         Nozzle loading on connecting equipment.

·         Pipe Displacements

·         Load & Moments on supporting structures.

        Basically, Piping stress analysis is a discipline that is highly interrelated with the piping layout and support design. The layout of the piping system should be performed with the requirement of piping stress & pipe support in mind. i.e.

1. Sufficient flexibility for thermal expansion.
2. Proper pipe routing so that simple & economical supports can be constructed.
3. Piping material & section properties commensurate with the intended services, temperature, pressure & anticipated loading.
4. If necessary, the layout solution should be iterated until a satisfactory balance between stress & layout efficiency is achieved.
5. Once the piping layout is finalized, the piping support system must be determined.
6. Possible support location & type must be iterated until all stress requirements are satisfied & other piping allowable are meet.
7. Piping support is then designed base on the selected location and type and the applied loads.

       Basically, Piping Designer is not responsible for the flexibility calculations nor the validity of such calculations made by the Stress Engineer. Designer is responsible for any calculations or flexibility analysis that was undertaken and resolved without Stress Supervisor's approval

Causes of pipe stress

       The two common causes of pipe stress are weight and thermal loads which cause loads on equipment nozzles.

1.Weight

       Weight causes the pipe to sag, which puts stress into the piping material

and forces onto equipment nozzle. It includes the weight of the pipe, the weight of the insulation, the weight of valves, instruments, etc.

2.Thermal 

       When the temperature of the pipe is higher the size of the pipe increases which causes the nozzle loads to increase and the nozzle loads are further increased when the supports restrain the pipe from moving. Thus improperly stress the analyzed system will cause very high loads on connecting equipment nozzles.

3.Occasional Loads

       The other causes of the pipe stress are the occasional loads caused due to Wind, earthquakes, dynamic loads due to equipment operation like Reciprocating Compressor, Pilot safety valve reaction force, Slug flow, etc.

Governing Codes and Standards for Stress Analysis

      Codes and Standards specify minimum requirements for safe design and construction (i. e. provides material, design, fabrication, installation and inspection requirements.)

Following are the codes and standards used for Refinery Piping:

·       ASME B31.3: Process piping Code

·       ASME B31.1: Power Piping Code

·       Centrifugal Pumps: API 610

·       Positive Displacement Pumps: API 676

·       Centrifugal Compressors: API 617

·       Reciprocating Compressors: API 618

·       Steam Turbines: NEMA SM23/ API 612

·       Air Cooled Heat Exchanger: API 661

·       Fired Heaters: API 560

·       Flat Bottom Welded Storage Tanks: API 650

·       Heat Exchangers: TEMA/ Vendor Specific.

·       Vessel/Column: Vendor Specific

Few more piping resources for you...

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Stress Analysis Of Column Piping System - 02

Stress Analysis Of Column Piping System: PIPING &COLUMN Modeling IN CAESAR-II

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