Trunnion Checking or Dummy Checking during stress analysis of a piping system

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.
TRUNNION
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. 

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

18 thoughts on “Trunnion Checking or Dummy Checking during stress analysis of a piping system

  1. Hi Anup,
    The articles are very usefull to all Piping engineers. Every one uses trunnions for supporting the piping.
    I am stuckup with one problem on How to calculate temperature for Trunnion support Or Shoe support?
    Generally we ambient temp. for modelling trunnion in CAESAR. But i am looking for article on how to calculate trunnion element OR shoe element temperature. Please provide your valuable details on it. Can we use Skirt temp calculation for calculating the trunnion temperature?

    Thanks & Regards..
    Prakash Mahajan.
    Jacobs Engineering India Pvt. Ltd.

    1. The best method for calculating trunnion/shoe temperature is performing an FEA analysis to get realistic results.

      For approximate methods normally process piping people use 40 degree centigrade of temperature drop per inch (25 mm) of length in case of non insulated trunnion/shoe (part outside insulation) and 4 degree centigrade drop per inch inside insulation.

      Anyway i will try to find out more on this topic.

    2. Dear Prakash,

      Can u please send me the ‘ Skirt temp calculation ‘ method ?
      I want it for Distillation column piping Analysis in CAESAR II.

      Your reply in this regard is highly appreciated.

    3. Dear Prakash,

      Can u please send me the ‘ Skirt temp calculation ‘ method ?
      I want it for Distillation column piping Analysis in CAESAR II.

      Your reply in this regard is highly appreciated.

  2. Hi Anup, Please clarify below point please. 1)The last formula “as=[{32*Trunnion OD*√(ML2+MC2)} / {Π*(Trunnion OD4-Trunnion ID4)}]” is not relevant to MW Kellogg.

    2)As looking at this formula’ s unit, we found out N/mm3 and it is not a stress unit.
    Regards
    Han

    1. Dear HAN,

      Stress in Trunnion is as per Flexural Stress Formula S=Mc/I ,

      where:

      M=combined moment (ML^2+MC^2)^0.5

      c=distance from centroidal axis to outside fiber(OD/2)

      I=moment of inertia about the centroidal axis (pi x (OD^4-ID^4)/64.

      The outcome unit would be N/m^2 which is a unit of stress.

  3. i’m very thankful to ur such informative topics. i used to visit this site daily and gaining knowledge a lot. keep on educating me like beginners.

  4. I have a question about allowable stress.
    Your guide is as below.
    SL+ SA + SLP <= 1.5 * Sh
    SC+ SA + SCP <= 1.5*Sh
    And Trunnion Stress<=Sh,
    But Table B-"Total allowable stresses" in the Design manual – Piping Mechanical by The M.W.Kellogg, I can choose total allowable stress according to the design conditions and any cases.
    Normal operating – 2.0Sh
    Short time operating – 2.4Sh
    Normal thermal only – 1.25Sc + 0.25Sh
    Short time operating with thermal – 1.5(Sc+Sh)
    Test – 2.4Sc

    What is different your guide and design manual?

  5. Dear Sir,

    thanks for your usefull text

    In our Organization we set the unreinforced SIF for intersection point of parent pipe & trunion and model the trunion
    then we check the stress at this point at sustain and expansion load case and compare with allowable stress of material.

    in your opinion is this true?
    so why whould we use the kellog method?

  6. I have come across an excel sheet for Trunnion Strength Calculation which is as per Kellogg’s method. It has 4 case study methods defined which are transit, 200 year, 10000year and blast, based on these types allowable stress changes. Please let me know which one I should pick for onshore O &G Piping.

  7. I have the same question as Jack,

    Why “Normal industry practice is to take 1.5 times Sh value as the allowable stress value” instead of using 2,0Sh for normal operation and 2,4Sh for occasional, as MW Kellogg say?

    Regards

  8. Hi Need some clarification, If i relate all parameters.

    1. Did not see the Sb=(1.17 * f * √R )/ (t^1.5) ……(1) is compared with allowable and used in acceptance criteria
    2. Parent Pipe OD or ID is not appearing anywhere in formulation, Does it means any pipe size there is no relation and effect of parent pipe, unless it affects dimension interfence

  9. Hello All,

    My question is on piping designer aspect.
    Can we have trunnion support on 3D/5D bends?
    As wall thickness will be different from Pipe bends & forged elbows does its advisable for critical piping?

  10. If the trunnion is welded to the base then the attached parent pipe wall also experience moments. Hence stresses due to external moment also need to be added. This has not been mentioned in this topic.

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