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when do we need to perform flexibility analysis of our piping system made from carbon steel (API 5L Gr.B)
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- #2
Following is the basic practice I use. Other more specific criteria have been developed and shared on the COADE (Caesar II) discussion forum (
A. All piping connected to rotating equipment nozzles require computer analysis. Period. No exceptions.
B. For all other piping, the following groups apply:
Group 1 -- Visual inspection (by knowledgable/experienced piping engineer) or manual calculations (simplified or approximate methods) are acceptable, or computer analysis:
1½” NPS and less, all temperature ranges
2" NPS, from -20°F to 400°F
3" thru 6" NPS, from -20°F to 200°F
Group 2 -- Comprehensive manual calculations are acceptable, or computer analysis:
3” thru 6” NPS, from 200°F to 400°F
8" NPS, from -20°F to 400°F
10" and 12” NPS, from -20°F to 300°F
Group 3 -- Computer analysis required:
All piping not covered previously(A. or B. Groups 1 or 2)
Note: Usually, the system I need to analyze contains piping of size/temperature range that falls into Group 3 as well as some of the other areas, so I end up doing a computer analysis on everything even if the diameter and temperature of a specific leg of piping by itself might be able to be done by less rigorous means.
Hope this helps.
A. All piping connected to rotating equipment nozzles require computer analysis. Period. No exceptions.
B. For all other piping, the following groups apply:
Group 1 -- Visual inspection (by knowledgable/experienced piping engineer) or manual calculations (simplified or approximate methods) are acceptable, or computer analysis:
1½” NPS and less, all temperature ranges
2" NPS, from -20°F to 400°F
3" thru 6" NPS, from -20°F to 200°F
Group 2 -- Comprehensive manual calculations are acceptable, or computer analysis:
3” thru 6” NPS, from 200°F to 400°F
8" NPS, from -20°F to 400°F
10" and 12” NPS, from -20°F to 300°F
Group 3 -- Computer analysis required:
All piping not covered previously(A. or B. Groups 1 or 2)
Note: Usually, the system I need to analyze contains piping of size/temperature range that falls into Group 3 as well as some of the other areas, so I end up doing a computer analysis on everything even if the diameter and temperature of a specific leg of piping by itself might be able to be done by less rigorous means.
Hope this helps.
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- #3
The criteria of kstaylor are quite comprehensive, but I would add consideration for some more points:
a) the presence of (correctly positioned and supported) expansion bellows may withdraw any requirement for flexibility analysis
b) if the piping is in cyclic service (more than 7000 cycles) the requirements on when to do a detailed analysis should be more severe (this is the case of condition A. for kstaylor, but other situations may require attention)
c) if a piping system is similar to another one that performs well, or when the designer may infer from preceding analyses that the current system will perform well, the detailed analysis may not be necessary
d) ASME B31.1 gives a quantitative criterion on when to do flexibility analysis (it is not valid under severe cyclic service, it remains quite indicative, requires judgement from the designer and should be applied between any two subsequent anchor points or restraints):
DY/(L-U)2>0.03
D=nominal pipe size in inches
Y=resultant of end movements in inches
L=developed length in feet
U=anchor distance (straight line) in feet
prex
motori@xcalcsREMOVE.com
Online tools for structural design
a) the presence of (correctly positioned and supported) expansion bellows may withdraw any requirement for flexibility analysis
b) if the piping is in cyclic service (more than 7000 cycles) the requirements on when to do a detailed analysis should be more severe (this is the case of condition A. for kstaylor, but other situations may require attention)
c) if a piping system is similar to another one that performs well, or when the designer may infer from preceding analyses that the current system will perform well, the detailed analysis may not be necessary
d) ASME B31.1 gives a quantitative criterion on when to do flexibility analysis (it is not valid under severe cyclic service, it remains quite indicative, requires judgement from the designer and should be applied between any two subsequent anchor points or restraints):
DY/(L-U)2>0.03
D=nominal pipe size in inches
Y=resultant of end movements in inches
L=developed length in feet
U=anchor distance (straight line) in feet
prex
motori@xcalcsREMOVE.com
Online tools for structural design
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- #4
The new prEN13480 (European piping code - still preliminary) includes this criteria which is roughly similar to that listed by prex:
No formal analysis of adequate flexibility shall be required for a piping system which meets one of the following criteria:
-duplicates or replaces without significant change a system operating with a satisfactory service record
-can readily be judged adequate by comparison with previously analysed systems
-is of uniform size, has no more than two anchors and no intermediate restraints or other movement controlling devices, and is designed for a service of not more than 7000 full cycles (or in the case of fuel gas piping, 1000 full cycles) and satisfies the following empirical equation
OD*Y/(L-l)^2<= 208.3
Same equation as prex' - only in metric dimensions i.e use 200 for an 8" pipe
Also checkout the NORSOK Standards homepage at
This includes a guideline for Critical Line selection used in the norwegian offshore industry:
*****************************************************
As a general guidance, a line shall be subject to comprehensive stress analysis if it falls into any of the following categories:
All lines at design temperature above 180°C.
4" NPS and larger at design temperature above 130°C.
16" NPS and larger at design temperature above 105°C.
All lines which have a design temperature below -30°C provided that the difference between the maximum and minimum design temperature is above:
-190°C for all piping
-140°C for piping 4" NPS and larger
-115°C for piping 16" NPS and larger
Note: These temperatures above are based on a design temperature 30°C above maximum operating temperature. Where this is not the case, 30°C must be subtracted from values above.
Lines 3" NPS and larger with wall thickness in excess of 10% of outside diameter. Thin walled piping of 20" NPS and larger with wall thickness less than 1% of the outside diameter.
All lines 3" NPS and larger connected to sensitive equipment such as rotating equipment. However, lubrication oil lines, cooling medium lines etc. for such equipment shall not be selected due to this item.
All piping subject to vibration due to internal forces such as flow pulsation and/or slugging or external mechanical forces.
All relief lines connected to pressure relief valves and rupture discs.
All blowdown lines 2" NPS and larger excluding drains.
All piping along the derrick and the flare tower.
All lines above 3" NPS likely to be affected by movement of connecting equipment or by structural deflection.
GRE piping 3" NPS and larger.
All lines 3" NPS and larger subject to steam out.
Long vertical lines (typical 20 meter and higher).
Other lines as requested by the stress engineer.
All production and injection manifolds with connecting piping.
Lines subject to external movements, such as abnormal platform deflections, bridge movements, platform settlements etc.
*****************************************************
Regards
Mogens
No formal analysis of adequate flexibility shall be required for a piping system which meets one of the following criteria:
-duplicates or replaces without significant change a system operating with a satisfactory service record
-can readily be judged adequate by comparison with previously analysed systems
-is of uniform size, has no more than two anchors and no intermediate restraints or other movement controlling devices, and is designed for a service of not more than 7000 full cycles (or in the case of fuel gas piping, 1000 full cycles) and satisfies the following empirical equation
OD*Y/(L-l)^2<= 208.3
Same equation as prex' - only in metric dimensions i.e use 200 for an 8" pipe
Also checkout the NORSOK Standards homepage at
This includes a guideline for Critical Line selection used in the norwegian offshore industry:
*****************************************************
As a general guidance, a line shall be subject to comprehensive stress analysis if it falls into any of the following categories:
All lines at design temperature above 180°C.
4" NPS and larger at design temperature above 130°C.
16" NPS and larger at design temperature above 105°C.
All lines which have a design temperature below -30°C provided that the difference between the maximum and minimum design temperature is above:
-190°C for all piping
-140°C for piping 4" NPS and larger
-115°C for piping 16" NPS and larger
Note: These temperatures above are based on a design temperature 30°C above maximum operating temperature. Where this is not the case, 30°C must be subtracted from values above.
Lines 3" NPS and larger with wall thickness in excess of 10% of outside diameter. Thin walled piping of 20" NPS and larger with wall thickness less than 1% of the outside diameter.
All lines 3" NPS and larger connected to sensitive equipment such as rotating equipment. However, lubrication oil lines, cooling medium lines etc. for such equipment shall not be selected due to this item.
All piping subject to vibration due to internal forces such as flow pulsation and/or slugging or external mechanical forces.
All relief lines connected to pressure relief valves and rupture discs.
All blowdown lines 2" NPS and larger excluding drains.
All piping along the derrick and the flare tower.
All lines above 3" NPS likely to be affected by movement of connecting equipment or by structural deflection.
GRE piping 3" NPS and larger.
All lines 3" NPS and larger subject to steam out.
Long vertical lines (typical 20 meter and higher).
Other lines as requested by the stress engineer.
All production and injection manifolds with connecting piping.
Lines subject to external movements, such as abnormal platform deflections, bridge movements, platform settlements etc.
*****************************************************
Regards
Mogens
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