Sample questions on piping stress analysis By S Koley, QEI-Piping Technical Knowledge: Q1. What is primary and secondary stress? What are the code allowable stresses per B31.3? ANS: Primary stresses which are developed by the imposed loading are necessary to satisfy the equilibrium between external and internal forces and moments of the piping system. Primary stresses are not self-limiting. Therefore, if a primary stress exceeds the yield strength of the material through the entire cross section of the piping, then failure can be prevented only by strain hardening in the material. Secondary stresses are developed by the constraint of displacements of a structure. These displacements can be caused either by thermal expansion or by outwardly imposed restraint and anchor point movements. Under this loading condition, the piping system must satisfy an imposed strain pattern rather than be in equilibrium with imposed forces. Local yielding and minor distortions of the piping system tend to relieve these stresses. Therefore, secondary stresses are self-limiting. ¾
Limit of Primary Stresses. The sum of the longitudinal stresses SL due to pressure, weight, and other sustained loads must not exceed Sh (basic allowable stress at maximum metal temperature). The thickness of pipe used in calculating SL shall be the nominal thickness minus mechanical, corrosion, and corrosion allowances.
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Limit of Secondary Stress range. The displacement stress range SE shall not exceed SA SA= allowable displacement stress range = f(1.25Sc + 0.25Sh) = f[1.25(Sc + Sh) - SL] when Sh > SL Sc = basic allowable stress at minimum metal temperature, psi f = stress range reduction factor per Table B4.2
Q2. Why and where Spring support is used in piping system? Is it used to lower the thermal stress in the system? ANS: In piping system when pipe lifts from its one support point due to thermal expansion then total dead weight is redistributed on the other supports or at connecting nozzle point. To reduce that nozzle load we use spring support at that lift-off support location to take the hot dead load of the piping. No. It may increase/decrease system stress depending upon the routing of the piping system. Q3. From where we get allowable loads for Centrifugal pump nozzles? Why suction and discharge nozzle loads both needs to satisfy the API-610 requirement to safe guard the pump? Ans: Allowable loads for the centrifugal pump are taken from API-610 STD table-4. To safe guard the pumps, two effects on nozzle loads are considered (Per API-610) a) Distortion of the pump casing. (Bending of casing due to equivalent loads and moments) b) Misalignment of the pump and driver shafts (leads to enormous wear between rotating and static part) So value and point of act of Equivalent loads (combining the piping loads at suction and discharge nozzle) are the governing parameter for pump design. That why both suction and discharge nozzle loads simultaneously needs to under allowable limit. Q4. What is the basic difference between WRC-107 and WRC-297 Ans: Both are used to calculate local stress with some limitations as below: For WRC 297: a) d/D <= 0.5 Where D=OD of vessel, T=thickness of vessel b) ) D/T<=2500 d= OD of nozzle, t= thickness of nozzle c) d/t <=100 d) Calculates vessel stress and nozzle stress, e) Used for cylindrical nozzle on cylindrical vessel For of WRC 107 a) d/D < 0.33 b ) Dm/T>50, where Dm = D – T, mean vessel dia. c) Calculates vessel stress only.
Sample questions on piping stress analysis By S Koley, QEI-Piping
Practical Knowledge: Q1. For column piping (connected at top and coming down to ground to control station) where you place spring support? And why? Ans: Spring support is taken at the vertical leg near to the elbow at top if rigid rest support is inactive or semi-active at that point and spring should be taken from Column shell so as to minimize the differential thermal movement between pipe and shell. There are two common behavior observed in column piping : ¾ If column temperature is higher than connected piping: Due to the higher temperature of the column, vertical upwards movement of Support-Shell junction point for support taken from Column Shell is higher than the point of support on pipe. So column will push the pipe on upward direction. Again vertical upwards movement of the column from Support-Shell junction point to Nozzle point is greater than the vertical upwards movement of pipe form Support point to nozzle point. So Column is pulling the pipe in expansion case. The support on pipe will lift depending on the combination effect of these Push-Pull events. This is a common behavior of the column piping. Sometimes support is not lifting but releasing some percentage of dead load in expansion case and this load is accumulating on the nozzle which leads to high nozzle load in vertical forces and longitudinal moments. By using spring support at that point Push-Pull event is minimized and nozzle loads are drastically lowered. ¾
If column temperature is lower than connected piping:
Due to the lower temperature of the column, vertical upwards movement of Support-Shell junction point for support taken from Column Shell is lower than the point of support on pipe. So support at pipe will lift from it’s point. Again vertical upwards movement of the column from Support-Shell junction point to Nozzle point is lower than the vertical upwards movement of pipe form Support point to nozzle point. So Column will restrict the upwards movement of pipe in expansion case and the thermal growth will flow towards the down and it will try to put back the support at its point. The support on pipe will lift depending on the combination effect of these Push-Pull events. Sometimes support is not lifting but releasing some percentage of dead load in expansion case and this load is accumulating on the nozzle which leads to high nozzle load in vertical forces and longitudinal moments. By using spring support at that point Push-Pull event is minimized and nozzle loads are drastically lowered. Q2. How a control valve assembly is supported and why? Ans: In a control valve assembly kinetic parameters of the fluid are controlled and due to that the whole assembly is very susceptible to vibration which adversely effects on the accuracy of the controlling. To minimize the vibration amplitude a 3-directional translational stop at the upstream elbow with trunnion and a guided rest support at downstream elbow with trunnion are considered to support the whole assembly. Sometimes additional rest support is required between the elbow to carry actuator weight for heavier control valves. Q3. What is the objective of stress analysis? Ans: 1. To ensure that the stresses in piping components in the system are within allowable limits. 2. To solve dynamic problems developed due to mechanical vibration, fluid hammer, pulsation, relief valves, etc 3. To solve problems associated due to higher or lower operating temperature such as a) Displacement stress range b) Nozzle loading on connected equipments c) Pipe displacements d) Loads & moments on supporting structure. Q4. What are the steps involved in stress analysis (or any stress package carries out)? Ans : 1. Identify the potential loads that the piping system would encounter during the life of the plant. 2. Relate each of these loads to the stresses and strains developed. 3. Get the cumulative effect of the potential loads in the system. 4. Decide the allowable limits the system can withstand without failure as per code. 5. After the system is designed to ensure that the stresses are within safe limits. 6. Ensure the loads at different restraint and nozzle/Tie-in points are within reasonable range. Q5. What is desired life cycle for Piping in operation? Ans: Desired life cycle for Piping in operation is 20 Years. Assuming one cycle per day, the normal no. of cycles for which the displacement or thermal stresses are designed is = 30 (day)x 12 (month)x 20 (years) = 7200 cycles ~7000 cycles.
Sample questions on piping stress analysis By S Koley, QEI-Piping Software Knowledge: Q1. How do you calculate the operating load with WIND Loads ? (Only mention the load cases.)
Ans: 1. W+P1+T1+H+WIN1 2. W+P1+T1+H+WIN2 3. W+P1+T1+H+WIN3 4. W+P1+T1+H+WIN4 Where W = Dead Weight of the piping system with fluid and insulation. P1= Internal pressure T1= Temperature H = Hanger load at T1 operating case(If have) WIN1/2/3/4= Wind force towards (+X)/(-X)/(+Z)/(-Z) directions respectively.(assuming +Y is vertical) Q2. What is ESL data? Ans: The ESL Menu gives access to utilities which interact with the External Software Lock. • Show Data—Displays data stored on the ESL. ESL is used at the time of changing/opening/saving/Running of any module in CAESAR-II. Q3. How thermal movements (say for equipment nozzle) at any point are fed in CAESAR-II? Ans: This auxiliary screen ‘Displacement’ is used to enter imposed displacements at up to two nodes per spreadsheet. Up to nine displacement vectors may be entered (load components D1 through D9). If a displacement value is entered for any vector, this direction is considered to be fixed for any other non-specified vectors. Leaving a direction blank for all nine vectors models the system as being free to move in that direction. Specifying “0.0” implies that the system is fully restrained in that direction. Q4. What is ‘Kaux’ menu? Ans: The Kaux menu provides some miscellaneous items. • Review SIFs at Intersection Nodes—Allows the user to run “what if” tests on the Stress Intensification Factors of intersections. • Review SIFs at Bend Nodes—Allows the user to run “what if” tests on the Stress Intensification Factors of selected bends. • Special Execution Parameters—Allows the user to set options affecting the analysis of the current job. Items covered include ambient temperature, pressure stiffening, displacements due to pressure (Bourdon effect), Z-axis orientation, uniform loads in ‘g’s, etc.
