Trunnion Support (or Dummy) Check
Trunnion supports are among the most frequently used pipe supports in the process piping industry. This support is widely used in the 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 these supports is not as comparable to civil supports. So, every stress engineer must check the weld point from a 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 increase with an increase in trunnion length or trunnion height.
The load-carrying capability of a trunnion mainly depends on the following factors:
• Parent pipe and trunnion/dummy pipe diameter: With the increase in pipe size, the load-carrying capacity increases.
• Parent pipe thickness: With the increase in pipe thickness, the load-carrying capability increases.
• Parent pipe material: The increase in the allowable strength (Sh) of the parent pipe material increases the load-carrying capability.
• Design temperature: With the decrease in design temperature, the load-carrying capability increases.
• Parent pipe corrosion allowance: With the decrease in corrosion allowance, the load-carrying capability increases.
• Design pressure: With the decrease in design pressure, the load-carrying capability increases.
• Trunnion/dummy pipe height: With the 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 an Excel spreadsheet is the most common among EPC organizations. In some organizations, trunnion checking by the WRC method is prevalent. In this article, I will explain the steps and formulas used while trunnion checking using the Kellogg method.
Steps for Trunnion Checking:
• First, run the static analysis in Caesar II to obtain the load values at the trunnion nodes output processor. It is better to practice taking 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^1/2 )/ (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 the thickness of reinforcement pad
f=loading per unit length
From Caesar II, we will get three forces with respect to each trunnion: longitudinal, circumferential, and axial forces. So we have to calculate three f values as mentioned below:
Loading due to longitudinal bending, fL=ML/ ( r^2 ) ……(2)
Loading due to circumferrential bending, fC=MC/ ( r^2 ) ……..(3)
and Loading due to axial force, fA=P/ (2*pi*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.
• The next step is to calculate all bending stresses using equation (1) for longitudinal (SL), axial (SA), and circumferential (SC) forces as calculated from equations (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 the 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)} / { *(Trunnion OD^4-Trunnion ID^4)}]
While checking trunnions or dummies, you can find that a major chunk of trunnions fails due to circumferential loads. So, orient or place the trunnion in such a way that the circumferential force on the trunnion becomes much less to permit/allow greater trunnion heights. Otherwise, try to reduce the trunnion height or increase the trunnion size if possible.
