Trunnion – What Is Piping: All about Piping Engineering

Trunnion supports are one of the most frequently used pipe supports in process piping industry. This support is widely used in piping industry due to its ease of construction and erection. The construction and erection of a dummy supports is very easy because you have to simply weld a pipe (normally one or more size less than the parent pipe to which it is to be welded) with the parent pipe. However the load bearing capacity of this supports are not as comparable to civil supports. So every stress engineer must check the weld point from failure viewpoint and investigate the ability to carry the piping load (mostly the tangential and longitudinal load and corresponding moment). The chances of weld failure increases with increase in trunnion length or trunnion height. The load carrying capability of trunnion mainly depends on the following factors: Parent pipe and trunnion/dummy pipe diameter: With increase in pipe size the load carrying capacity increases. Parent pipe thickness: With increase in pipe thickness the load carrying capability increases. Parent pipe material: With increase in parent pipe material allowable strength (Sh) the load carrying capability increases. Design temperature: With decrease in design temperature the load carrying capability increases. Parent pipe corrosion allowance: With decrease in corrosion allowance the load carrying capability increases. Design pressure: With decrease in design pressure the load carrying capability increases. Trunnion/dummy pipe height: With decrease in trunnion height the load carrying capability increases. There are various ways in which trunnion checking can be done. However the Kellogg Method of trunnion checking using excel spreadsheet is the most common among EPC organizations. In some organization trunnion checking by WRC method is prevalent. In this article i will try to explain the steps and formulas used while trunnion checking using Kellogg method. Steps for Trunnion Checking: First of all run the static analysis in Caesar II to obtain the load values at trunnion nodes from output processor. It is better practice to take the maximum value from all load cases (Sustained, operating, design, upset, hydro etc) After that we need to calculate the bending stress generated on the pipe shell based on the following Kellogg equation: Sb=(1.17 * f * √R )/ (t^1.5) ......(1) Here, Sb=bending stress in pipe shell R=Outside radius of pipe shell t=Corroded pipe thickness (actual pipe thickness-corrosion allowance) plus thickness of re-inforcement pad f=loading per unit length Now from Caesar we will get three forces with respect to each trunnion; longitudinal forces, circumferrential forces and axial forces. So accordingly we will have to calculate three f values as mentioned below: Loading due to longitudinal bending, fL=ML/ (Π r^2 ) ......(2) Loading due to circumferrential bending, fC=MC/ (Π r^2 ) ........(3) and Loading due to axial force, fA=P/ (2Π r)...........(4) Where, ML=Longitudinal force obtained from Caesar * trunnion effective length MC=Circumferrential force obtained from Caesar output * trunnion effective length P=direct axial force obtained from Caesar II output. and r=outside radius of trunnion. Next step to to calculate all bending stresses using equation (1) for longitudinal (SL) , axial (SA) and circumferential (SC) forces as calculated from equation (2), (3) and (4). Now Calculate longitudinal Pressure Stress (SLP=PD/4t) and Hoop Stress (SCP=PD/2t). Now combine all these forces for proper load cases as shown below and compare the combined value with allowable stress value (Normal industry practice is to take 1.5 times Sh value as the allowable stress value where Sh is the basic allowable stress at design temperature from code ASME B 31.3). SL+ SA + SLP <= 1.5 * Sh SC+ SA + SCP <= 1.5*Sh And Trunnion Stress<=Sh Here trunnion stresses should be calculated as=[{32*Trunnion OD*√(ML^2+MC^2)} / {Π*(Trunnion OD^4-Trunnion ID^4)}] While checking trunnions or dummies you can find that major chunk of trunnions fails due to circumferrential loads. So orient or place the trunnion is such a way that the circumferrential force on the trunnion becomes very less to permit/allow greater trunnion heights. Otherwise try to reduce trunnion height or increase trunnion size if possible. In my future post I will post one practical case study explaining the trunnion calculation methods. Hope you like this post, If you have any confusion/comment please inform in comments section.