Questions for Stress Analysis

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