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Piping Stress Critical Lines: Procedure Defining the Basis For Piping Critical Line List

Below procedure defines the basis to establish piping critical line list from line list, P & I Diagrams and Piping Specification.

What is Piping Critical Line:

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



Line 50 mm (2”) NB and smaller are not considered critical.  Therefore, in the following definitions the word “lines” means lines 65 mm (2 1/2”) NB and above, unless otherwise stated.

CATEGORY 1:  Stress Critical Lines

CATEGORY 2:  Equipment Critical Lines

CATEGORY 3:  Support Critical Lines

CATEGORY 4 :  Relief Critical Lines

CATEGORY 5:  Category M Fluid Service Lines

CATEGORY 1:  Stress Critical Lines

These lines are identified based on the temperature to which line is subjected Normal basis for temperature is maximum operating / design temperature.  In absence of maximum operating temperature, design temperature shall be considered.  Operating temperature shall be considered only if client ask for.

Following are the criteria to establish critical lines under this category:

  • a) Lines 80 mm (3”) NB to 125 mm (5”) NB at temperature 2600 C and above or at (-) 2000 C and below.
  • b) Lines 150 mm (6”) NB to 600 mm (24”) NB at temperature 1200 C and above or at (-) 1000 C and below.
  • c) Lines 650 mm (26”) NB and above at temperature 600 C and above or at (-) 400 C and below.
  • d) Non-ferrous lines operating above 1000 C.
  • e) Jacketed Lines – All sizes.
  • f) Lines for which a probable requirement for an expansion joint (e.g. bellows) has been pre-determined by process group, and shown on the P&ID, as a result of their experience on a similar plant for accommodating thermal expansion. Bellows shall not be used unless approved by stress department.
  • g) Any line for which formal calculations are specified, either by client or local authorities. For example IBR-370 calls for flexibility analysis of IBR controlled piping.  However same temperature criteria shall be followed as highlighted in a, b, c.  IBR lines analysis shall be done using ANSI B31.1 code.
  • h) Lines that although not in themselves critical, tie-in to critical lines and have a significant effect on their flexibility.As a general rule such lines are added to the Critical Line List at the initial review stage if their nominal size exceeds one-third of that of the critical line (and they exceed 50 mm NB).  The remainder shall be established, and added to the critical line list, during the design stage of the contract.These lines cease to be considered critical beyond the nearest anchor to the tie-in-point.
  • i) Lines having very long straight run and are subjected to harsh solar temperature (above 500 C).
  • j) Any other lines that the Piping Engineer and/or Lead Stress Engineer consider Critical. Formal computer Analysis shall be carried out for a,b,c,d,e,f,g,h.

CATEGORY  2 :  Equipment Critical Lines

Lines connected to strain sensitive equipment fall under this category :

  • a) Piping connected with centrifugal pumps having size 65 mm (2-1/2”) NB and above with maximum operating temperature of 1200 C and above or minimum operating temperature of (-) 1000 C and below and 150mm (6”) NB and above size with maximum operating temperature of 600 C and above or minimum operating temperature (-) 400 C and below.
  • b) Rotating machinery, other than centrifugal pumps, e.g. steam turbines, centrifugal compressors and fans.
  • c) Fired heaters and reforming furnaces.
  • d) Reciprocating compressor.
  • e) Air cooled exchanger.
  • f) Vessels fabricated from fragile construction such as graphite, glass etc.
  • g) Vessels with linings of a brittle nature, e.g. glass, brick or refractory materials or non-metallic material shall have piping connection based on vendors recommendation.
  • h) Special items of equipment for the manufacturer specifies low nozzle loadings. These shall be added at a late date.
  • i) Lines connected to any vessel or tanks which may undergo settlement in course of its working life are also classified critical in this category.

Lines that are classified as critical solely because they are connected to strain sensitive equipment cease to be considered critical beyond the anchor nearest to the equipment concerned.

Nozzle loading for such equipment shall be generally as per following codes.  However, Lead Engineer in the beginning of the project should ascertain that the equipment is going to be bought as per what code ?




Steam  Turbine                        –           NEMA  SM23

Centrifugal Pumps                 –           API 610

Centrifugal Compressor       –           API 617

Air Fan Coolers                      –           API 661

Fired heaters                           –           API 560

Formal computer analysis shall be carried out for lines under a, b, c, d, e, h only.

CATEGORY 3 :  Support  Critical  Lines

Lines which requires special care and consideration during supporting falls under this category.

Following are the criteria to establish support critical lines:

  • a) Lines to which field welding is prohibited such as stainless steel, alloy steel or thick carbon steel which require stress relieving after welding.
  • b) Lines subject to two-phase flow.
  • c) Lines connected to reciprocating compressor
  • d) Non-ferrous lines
  • e) Non – Metallic lines shall be supported as per vendor’s recommendation.
  • f) Lines with wall thickness schedule 160 or greater.
  • g) Lines 500 mm (20”) NB and larger thickness less than Standard Weight wall thickness.
  • h) Lines having coating of lining of a brittle nature e.g. glass, concrete.

Computer analysis need not be carried out for lines under this category.

CATEGORY 4:  Relief  Critical  Lines

Lines upstream and downstream of pressure relieving devices such as relief valve, blowdown valve etc. fall under this category.

Following are the criteria to establish relief critical lines:

  • a) Relief valve / control valve having:
    • i) Inlet lines 65 mm (2-1.2”) NB and above having set pressure 5.0 kg/cm2 g and above.
  • b) Lines downstream of bursting discs having pressure and size criteria as per above norms of relief valves.

CATEGORY 5:  Category  M  Fluid  Service  Lines

These are lines 40 mm (1 1/2”) NB and larger, at any temperature, with piping spec. Codes identified in the Contract Piping Specification as ANSI B 31.3 Category “M” Fluid Service.

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