B31.3 Piping Checklist and Rules of Thumb March 1, 2008 Program Version Paulin Research Group s

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1 Paulin Research Group s PRG B31.3 Piping Checklist and Rules of Thumb Program User Guidelines Contents 1.0 Program Objective 2.0 Summary 3.0 Getting Started 4.0 Main Topics Addressed 5.0 Report Discussion Quickstart: When the piping checklist form appears, check the boxes that apply to your system. Fill in any other information that is known and click on one of the report buttons below. A report or rating screen should be shown that reflects the characteristics of your system. Paulin Research Group Houston, Texas 1

2 Paulin Research Group s B31.3 Piping Checklist (and Rules of Thumb) 1.0 Program Objective: Version : March 1, 2008 The B31.3 Piping Checklist (and Rules of Thumb) are intended to help designers evaluating piping systems. Notes and warnings from the 2007 B31.3 piping code are included, in addition to situations where the B31.3 Code rules require special consideration. Safety, Analysis, Applicable B31.3 References, Risk, SIFs, Flexibiliites, and probability of failure estimates are presented in individual reports. Each report is discussed in detail in the body of this guideline. The reports can provide a record of the evaluation procedure and considerations that went into the piping system design. Per 300(b)(2), The designer is responsible to the owner for assurance that the engineering design of piping components complies with the requirements of the B31.3 Code and with any additional requirements established by the owner. The calculations made in the B31.3 Piping Checklist are intended to assist the designer in satisfying that responsibility by drawing the designer s attention to possible areas of concern. In all cases it is the designer s responsibility to validate any conclusions or recommendations made by the B31.3 Piping Checklist and to verify that all items of concern are properly considered. 2.0 Summary: The B31.3 Piping Checklist (and Rules of Thumb) program performs a piping evaluation based on general system characteristics and determines the following: Expected Safety Factors B31.3 Applicable Code Notes Variations in Stress Intensification Factors (SIFs) Risk Associated with System Operation Probability of Failure Recommends if Expert Analysis is Needed Buried Pipe Rules of Thumb Critical Flaw Size and Transition Temperature Estimation Effect of Flexibilities on Load and Stress Determination Effect of d/d ratio on Load and Stress Determination Effect of Environment and Corrosion B31.3 Fluid Category Min Pipe Wall Thickness Criteria for Severe Cyclic Service Formal Analysis Requirements ( ) Common Expansion Joint Code Criteria Creep Temperature Ranges Potential need for FEA ( ) Needs for Safeguarding (B31.3 Appendix G) Piping checklist reports can be saved, edited and printed to serve as a record of the mechanical concerns that went into the safety, analysis and design of the piping system. Paulin Research Group Houston, Texas 2

3 Output is organized into the following categories: System Specific B31.3 System characteristics are evaluated and B31.3 Code notes that apply to the piping system are printed. Over 200 notes are included in the B31.3 Note database. Short and Detailed versions of the report are printed. Short versions include more important items, and Detailed versions are more thorough in reporting system specific Code notes. General B31.3 General B31.3 notes of general interest to the designer are printed. General notes are typically not duplicated in the specific notes. General notes can be generated in a Short or Detailed format. Most System Specific B31.3 and General B31.3 Code notes are modified to include all references so that readers can get the full implication of any particular directive by reading only a single note. Designers are always responsible for their own interpretation and validation of any program results. Analysis Notes Addresses items of interest for the analyst of the piping system. Cautions, rules-of-thumb, SIF and Flexibility notes, probabilities of failure and injury and risk are presented. Safety Evaluation Fluid categories, min wall thicknesses, SIFs, Flexibilities, Critical Flaw sizes, etc. are addressed where it is believed that these parameters will effect the safe operation of the piping system. Checklist A list is provided of items that should be considered for the evaluation of the piping system. The list contains Code related notes, possible effects of SIFs, flexibilities, expansion joints, high pressure, Category M fluids, and others. Ratings Two graphs are presented. The top graph gives probabilities of failure based on the accuracy of SIFs and flexibilities in the system. These probabilities are compared to the probability of being injured while driving an automobile. The bottom graph rates: Analysis Difficulty, Dynamic Considerations, Cyclic Considerations, Consequences of Failure, System Complexity and the Need for an Expert Analysis. A typical ratings result is shown below. Output reports containing input listings and recommendations can be printed and checked. A premium version of the B31.3 pipe checklist is available. The premium version includes options for saving input under different file names, support and enhanced documentation and program upgrades when they become available. Paulin Research Group Houston, Texas 3

4 Defaults are provided for required text entries and where defaults are entered, the background text is yellow. Values entered by the user have a green background. The ability to use defaults can be deactivated by check the No Defaults box in the upper left corner of the screen. This checkbox is shown below. The defaults checkbox is stored in the registry. For user s that employ the program frequently, they should check the No Defaults check box, as the use of defaults can easily lead to mistakes in input. An option is available for support and input transfer to Paulin Research Group for evaluation. options can be accessed through the button on the main form: is only available via a Microsoft compatible service on the users s computer. The form is shown below: B31.3 Piping Checklist Form The first and fourth options are only available in the PREMIUM PACKAGE (not the shareware version). Product support is not available for the shareware version of the program. To receive information for upgrading to the PREMIUM VERSION of the B31.3 Pipe Checklist program, please fill in your name and address and then press the bottom button on the form. Additional product information can be obtained by filling in the name and address text cells and pressing the second button. Information describing training courses and free webinars offered by Paulin Research Group can be obtained by filling in the name and address text cells and pressing the third button. Paulin Research Group Houston, Texas 4

5 3.0 Getting Started: All that s required is to fill in any checkboxes or values that apply to the piping system of interest and then to select the report of interest. First time users should probably select individual reports and review them to determine the type of information available. Experienced users will likely go to the Grouped Reports, select the reports and input of interest and then press the yellow Generated Grouped Reports button. Many experienced user s simply print these reports and then make sure that each item is individually checked off when the piping analysis is performed. The input control checkboxes and buttons are shown below: Details and Notes for Individual Reports are given in Section 5.0. General analytical topics and discussions are included in Section 4.0. Paulin Research Group Houston, Texas 5

6 4.0 Main Topics Addressed: The philosophy and nomenclature used in the B31.3 Piping Checklist is described below on a per subject basis. A discussion of unique properties of each report is given below. Nomenclature: N Number of Cycles D Diameter of Pipe (Header pipe when d and D are used.) d Diameter of branch pipe. T Thickness of Pipe (Header pipe when t and T are used.) Senior Analyst Recommendations: When it is recommended that a senior analyst review the system, this is because there are one or more characteristics of the model where errors are typically made or where safety factors in the Code are small. These areas tend to involve problems that: 1) Indicate some dynamic analysis is required, or 2) where large D/T piping is involved, or 3) where the number of thermal and or pressure cycles are approaching a critical level, where a critical level can be taken to be from about 4000 to 10,000 cycles depending on other circumstances. In these cases, the actual safety factor is relatively small, (compared to system having only a few hundred cycles), and so accuracy in the analysis is more important, or 4) where some part of the pipe is buried and warm, or 5) where corrosion or some other combination of characteristics exist, that taken together indicate that a senior analyst should be involved. Pressure Fatigue: The B31.3 Code does not directly address pressure fatigue in the equations provided in the Code. Where pressure cycling is a significant contributor to the stress state, a pressure fatigue analysis of some kind should be considered. This can be performed by use of the rules in ASME Section VIII Division 2 and/or FEA. FEA should be considered where significant cyclic external loads interact with significant cyclic pressure loads as these loads may not occur at the same point and so their interaction insofar as fatigue is concerned should be considered. Inspection Programs: Where Code rules, load specifications, or any other uncertainty exists with respect to the piping system an inspection program should be considered to help identify cracks or thin areas before they become a problem. Approximate critical flaw sizes and minimum temperatures are given for materials that have Charpy samples taken and for materials that do not have Charpy samples taken. Fracture strength and nil ductility temperature profiles are significantly lower for materials where Charpy samples are not taken. Fatigue and Stress Intensification Factors (SIFs): The fatigue calculations for the piping system depend on the accuracy of the SIF and the allowable. Life safety factors are based on the ratio of the expected failure stress as predicted by the Markl equation (490,000)N -0.2 to the allowable stress. Typical safety factors are hoped to be around 1.6 or greater. The safety factor applies to the stress. If the safety factor for a particular component is 1.6 for example, then if the calculated stress is 1.6 times greater than a value equal to 100% of the allowable for that component, then at the end of the component life, it would be at the mean of it s failure life, i.e. at the end of the system life, half of the number of parts made would survive, and half would fail. A 50% probability of failure is too great. Typically survival probabilities greater than 99% are desired. If any safety factors are predicted to be lower than 1.6 the following should be noted: Paulin Research Group Houston, Texas 6

7 1) Predicted safety factors are affected by environmental and geometry factors. Geometry factors are generated when laterals or hillsides connections are present. 2) Worst case safety factors assume that the calculated system stress at the weakest component in the system is 100% of the allowable and that the moments act in a direction that exacerbates any weakness in the B31.3 Code calculated SIF or stress (S E ). In the case where system d/d ratios are less than 1.0, then the 2007 B31.3 Appendix D notes that the equation for unreinforced fabricated tees may be nonconservative. If this nonconservatism occurs along with an environmental factor, a large number of cycles, and a large D/T ratio, then safety factors may be considerably lower than 1.6. In these cases an experienced analyst is recommended. Flexibility Recommendations: Reductions and increases in load are based both on experience and on approximate calculations for span lengths. The span lengths are taken as the approximate length in between spans that may interact with the nozzle. Approximate lengths entered will be used if they are smaller than the approximate span lengths calculated. The flexibility reductions suggested are by no means minimum values. If the pipe attached to the nozzle is stiffer than what has been assumed and limited geometrically, i.e. planar with short offsets, then a much greater load reduction than was is predicted might be experienced. The only way to know what load reduction will be experienced when a proper flexibility is included in the piping model, is to include one and make the appropriate calculation. Formal Analysis Requirements ( ): The formal analysis requirement discussion of Ev Rodabaugh in WRC 492 was used as a basis for the range of values given in the pipe checklist. Since Rodabaugh evaluated a variety of geometries this seemed realistic. The results of the analysis are shown as a unity factor in the pipe checklist where Dy/(L-U) 2 / K1 = Unity Factor and must be less than 1.0. Buried Pipe Calculations: The buried pipe calculation assumes two burial depths: 2 ft. or 2D whichever is greater, and 12 ft. or 4D whichever is greater. Once these depths are found the following properties can be calculated assuming the soil density is lb./cu.in. 1) Virtual Anchor Length 2) Fully restrained longitudinal stress 3) Displacements at 90 degree buried long radius bends. 4) Lateral bearing lengths at bends 5) Stresses in buried pipe bends. The piping designer must review the results of these two range calculations and estimate where his piping system is with respect to these limiting values. Probability of Failure: The probability of failure in a process plant environment can be found by modifying general probabilities for incident in industrial environments and including the contribution from higher probability of failure events that may occur in the non-passive environment of a pressurized piping system. There is some contribution due to events that include additional unknowns, such as environmental effects, but in general the most significant effect is due to the relatively known standard deviation associated with welded fatigue failures. Given this standard deviation, and the proximity to the mean failure line provided by the calculated safety factor a known probability of failure can be found. A normal distribution is assumed even though beyond a 50% probability of failure even though the distribution of failures does not follow a normal distribution in this range. Predictions for probability of failure that are greater than 50% will be somewhat exaggerated for this reason. The approach in Metal Fatigue in Engineering, by Abelkis on p.436 is used to evaluate the probability of failure to stabilize the approach for the typical small probabilities of failure that stresses below the allowable and separation of operating and design conditions often provides. It is not uncommon to see large probabilities of failure in some piping systems, as the desired safety factor on Paulin Research Group Houston, Texas 7

8 stress is about 1.6 and the errors in calculated stresses due to the error in SIFs can be of the same order when the number of cycles is significant. When a high probability of failure occurs and the risk is high, the user is urged to pay particular attention to the components in the system that will show sensitivity to the SIF problems, and keep stresses on those components small. Risk Assessment: Risk is the product of probability of failure and cost of failure. The cost of failure is estimated from a baseline of leak repairs and then is increased based on both the degree of additional equipment involved, i.e. rotating equipment and the risk assessment parameters included on the form activated by the Risk button. Increased consequences exist when a failure of the piping system shuts down the refinery or unit, is close to a heavily populated facility, and may release a significant volume of dangerous material, and is associated with the loss of life. Leak Before Break: The leak before break analysis is based on assuming a hemispherical thru wall crack. The K IC value is calculated from the crack and converted into an equivalent Charpy value by an empirical relationship. Providing the equivalent Charpy value is less than the available Charpy value, then the system is deemed to leak before breaking. The temperature corresponding to the required Charpy value is printed. The pipe designer should review these temperatures and make sure that the system cannot be pressurized or loaded at temperatures below those reported. Critical Flaw Size: All flaw sizes will be assumed to have semicircular shape. Given the critical stress intensity at ambient conditions (70F, 21C), and the relationship between stress intensity and flaw length, a depth can be calculated. If that depth is much greater than the thickness of the pipe then the critical flaw size is very large, and plastic failure of the section would occur. When the equivalent depth is less than 90% of the pipe wall thickness it is reported as a critical flaw size. These flaw sizes are calculated assuming the system can be pressurized at ambient temperature. Uninsulated Pipe Rain Effects: When uninsulated pipe at less than 350F is exposed to rain, there is a possibility of thermal bowing as the rain cools the outer, and top of a horizontal pipe. An approximating equation can be used to estimated the temperature at the top of the pipe and thermal bowing computed assuming the temperature gradient from the top to the bottom of the pipe is linear. If the pipe is insulated, or if the interior of an uninsulated pipe can always be assured to have a sufficient heat flux to the pipe wall to maintain its temperature, then this calculation can be ignored. Creep-Fatigue Interaction: The B31.3 and ASME Section VIII codes do not specifically address creep fatigue interaction. Where a system in the creep range must undergo cycling, methods in ASME Section III Subsection NH may be consulting to address this concern. The PRG MatPRO program includes the NH rules as part of a calculator. User s may take CAESAR (or any piping program results) and plug them into MatPRO to perform a creep-fatigue interaction analysis. NozzlePRO and FE/Pipe FEA programs perform this evaluation automatically. Severe Cyclic Conditions: A condition defined in B31.3 and applies to specific piping components or joints in which S E exceeds 0.8 S A, and the equivalent number of cycles exceeds 7000; or other conditions that the designer determines will produce an equivalent effect. If the user wishes to avoid severe cyclic condition notes the checkbox for the S E, S A relationship can be checked although the user must verify that this stress vs. allowable relationship is valid. Pressure and thermal cycles are used to determine if a severe cyclic condition exists in the checklist. Paulin Research Group Houston, Texas 8

9 5.0 Report Discussion: 5.1 B31.3 Code Specific Notes produces Detailed or Short B31.3 Interpreted Notes that apply to specific features of the specified piping system. The Detailed and Short input frame is shown below. For simple piping systems, the System Specific B31.3 reports can be 1 to 2 pages long. For complex piping systems, the reports can be 4 to 5 pages long. An example of the system specific Code notes is shown below: Example Page from: System Specific B31.3 Report Users may print, save, copy, cut or paste from or to the report. Users may also add their own notes or delete notes in the present report that do not apply. B31.3 Code Specific notes can be generated alone or included in Grouped reports and printed, edited or saved. For relatively novice B31.3 or pipe stress program users, the System Specific B31.3 detailed report should be printed and reviewed for each system until the user immediately recognizes benign from dangerous systems. (See the Ratings reports for quick ways to know if expert analyses are needed for a piping system analysis.) Once the user recognizes critical systems easily, only notes for critical systems should be reviewed so that pertinent guidelines are not missed. Paulin Research Group Houston, Texas 9

10 5.2 B31.3 Code General Notes produce Detailed or Short B31.3 Interpreted Notes that apply to general features of most piping systems. The Detailed and Short input frame is shown below. The short report can be from three to six pages long. Definitions, warnings, loading requirements, etc. are given. These reports are intended to provide the user with most B31.3 notes that should be reviewed before attempting a pipe stress analysis. The intention by providing this section is so that pipe stress novices could find all mechanically applicable notes for the analyst in one place. This section was also intended to provide the expert with a refresher on the expanded topics discussed in the 2007 version of B31.3. An example of the general Code notes is shown below. Note how basic definitions and requirements are given: Example Page from: General B31.3 Code Report Users may print, save, copy, cut or paste from or to the report. Users may also add their own notes or delete notes in the present report that do not apply. B31.3 Code Specific notes can be generated alone or included in Grouped reports and printed, edited or saved. Paulin Research Group Houston, Texas 10

11 5.3 The Checklist report includes items that should be considered when evaluating the piping system for mechanical stress and integrity. Items are printed in a checklist format so that each item can be reviewed and either considered or disregarded. The user is recommended to go through each item in the checklist carefully and discard those that do not apply, evaluate those that do apply, and discuss with colleagues or experts those that are not well understood. The Checklist report can be edited to add comments and remove sections that do not apply to the system. The removal process is particularly important where the checklist is to serve as an historical record of the mechanical evaluation of the piping system. An example portion of a checklist report is shown below: Example Page from: Piping Checklist Report Depending on the system parameters, the piping checklist reports are typically one to three pages long. Paulin Research Group Houston, Texas 11

12 5.4 Probability and system ratings screens are generated. Examples are shown below. Character Rating A variety of experienced based parameters are quantitatively evaluated to determine a rating for the six system properties included. Probability of Failure Ratio The probability of failure ratios give the probability of failure of the piping system with respect to the probability of being injured in an automobile accident. (The odds of being injured in an automobile accident are approximately 3/10,000. Three out of every 10,000 people in the United States will be injured in an automobile accident in any one year.) Ratios are provided so that: 1) a comparative unity check can be given that is more meaningful than a probability. 2) a plant failure probability can be rated to a failure probability that is well understood. The four probability ratios given are described below: 1) Standard Probability of Failure No SIF, environment or flexibility adjustments 2) SIF Sensitive Probability of Failure SIF, environmental and system adjustments to the safety factor are included in the calculation. 3) Load Reduced Probability of Failure - Flexibility affected probability of failure. The reduction in load due to flexibilities are included with the standard probability of failure. 4) Load Increased Probability of Failure Flexibilities can also increase loads in other parts of the system. This increased probability of failure ratio is intended to include this effect with the standard probability of failure. Paulin Research Group Houston, Texas 12

13 5.5 Fluid categories, min required wall thicknesses SIFs anomalies, Flexibilities effects, Critical Flaw sizes, etc. are evaluated and discussed from the perspective of how they will effect the safe operation of the piping system. The safety reports attempt to focus on the characteristics of the system that can have the largest impact on the accuracy of any analysis. Typical safety reports range from one to three pages depending on system characteristics. A portion of an example safety report is shown below: Example Page from: Safety Report Paulin Research Group Houston, Texas 13

14 5.6 Information is provided that may impact the approach used when performing a pipe stress analysis of the system. Example Page from: Analysis Notes Report Analysis notes reports are system dependant and typically vary from two-to-four pages in length Paulin Research Group Houston, Texas 14