All you need to know about Piping Stress Critical Lines

Apr 8 14:33 2021 Judith Morrison Print This Article

This article defines the pipe stress critical line, then describes the factors for defining the basis of piping stress critical lines and the criteria for defining the basis based on different categories.

Stress analysis is the most vital activity in any process unit. The Piping Stress engineer carries out a flexibility review for a piping critical line. It is based on the weight,Guest Posting temperature, external loading, supporting arrangement, external loads, lines connection, vibrations and more.

A piping critical line is essential for a flexibility review to be carried out by a Piping Stress Engineer because of temperature, weight, supporting arrangement, external loading, lines connection to strain sensitive equipment, vibrations, etc.

Did you know: The lines 50 mm (2") NB and smaller are generally not considered critical.

There exists a large number of lines that flow from one location to another. Examining all lines will be time-consuming task and cost-intensive. So, every organization in this field has defined guidelines for deciding which lines are to be stress analyzed with software such as Caesar II, Autopipe, CAEPIPE, START-PROF or Rohr II.

Each piping line shall be identified based on crucial features such as pipe size, material, design parameters, and loading conditions. The word lines in the below section mean lines 65 mm (2 1/2") NB and above, unless otherwise stated.

Different Types of Stress Critical Lines in Piping

Equipment Critical Lines

Lines connected to critical static and rotary equipment fall in this category. For instance, lines associated with pumps, compressors and turbines are considered stress critical and need analysis using the software.

Support Critical Lines

The lines that require special care and consideration during engineering support fall in this category. It consists of supports requiring spring hangers, non-ferrous lines, pipes with stainless steel or super duplex stainless steel material. Computer analysis is not required for lines that come into this category.

Relief Critical Lines

The pipes undergoing relief loads come into this category. It consists of the relief valve, blowdown valve rupture disks, and more.

Temperature Critical Lines

Lines carrying fluids having high temperature falls in this group.

Service Critical Lines

Piping systems carrying category M fluid service, hazardous fluid service, severe cyclic condition, and more falls in this category.

Let's have a look at the primary factors to decide critical lines:

  • Line design/operating temperature
  • Pipe thickness
  • Pipe and equipment material
  • Pipe condition
  • Pipe thickness
  • Fluid category
  • Severe cyclic condition
  • Design/Upset pressure

Though every organization has its own guidelines and it varies from one project to another. But a few standards for determining stress critical lines are as follows:

  • The size of all pumps (Centrifugal- API/ANSI, gear pump, Screw pump) suction and piping should be 4 inches and larger.
  • Centrifugal compressor inlet and outlet piping
  • Lines to and from steam reactors and turbines
  • Process lines to and from strain-sensitive components such as pumps, compressor suction and discharge piping.
  • Air cooler inlet and outlet piping with size 3" and larger.
  • Lines classified in category M as defined in ASME B31.3
  • Piping that requires expansion joints or other proprietary expansion devices.
  • Inlet and outlet piping of steam and gas turbine
  • Two-phase flow piping – blowdown and flare


The piping system is heart of any process plant facility. The analytical study of piping systems is done through the process piping code ASME B 31.3. If you're looking for stress analysis services for piping systems and piping flexibility analysis, consider hiring an engineering firm that adheres to international codes and standards.

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About Article Author

Judith Morrison
Judith Morrison

Judith Morrison is an expert in the field of industrial engineering and writes articles related to piping, civil, equipment engineering related articles.

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