Pipe Trunnion or Dummy Support and Their Stress Calculation

What is a Pipe Trunnion?

Piping Trunnion supports are one of the most frequently used pipe supports in the 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 pipe trunnion or dummy support are very easy because you have to simply weld a pipe (normally one or more sizes less than the parent pipe to which it is to be welded) with the parent pipe.

A pipe trunnion is defined as an additional pipe of similar or lower size welded to an active piping system to provide physical support. However, trunnion supports with the same size pipe are usually avoided due to profile cutting and welding difficulties during construction.

The load-bearing capacity of trunnion supports is usually less and not as comparable to civil structural supports. So, every pipe 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. Usually, trunnions with a height of more than 1500 mm are not suggested.

Sometimes structural steel members in place of pipe are also added to the parent pipe to form trunnion support. The material of the dummy or trunnion to be attached to the parent pipe should be compatible with the parent pipe material.

Factors affecting load-bearing Capability of Trunnions

The load-carrying capability of a trunnion mainly depends on the following factors:

• Parent pipe and trunnion/dummy pipe diameter: With an increase in pipe size the load-carrying capacity increases.
• Parent pipe thickness: With an increase in pipe thickness the load-carrying capability increases.
• Parent pipe material: With an increase in parent pipe material allowable strength (S_h) the load-carrying capability increases.
• Design temperature: With a decrease in design temperature the load-carrying capability increases.
• Parent pipe corrosion allowance: With a decrease in corrosion allowance the load-carrying capability increases.
• Design pressure: With a decrease in design pressure the load-carrying capability increases.
• Trunnion/dummy pipe height: With a decrease in trunnion height the load-carrying capability increases.
• Reinforcement Pad thickness at the weld interface: Adding a reinforcement pad at the parent pipe and trunnion pipe increases the load-carrying capability of trunnions to a huge extent. However, adding RF Pads at elbows is difficult to construct and hence suggested to avoid.

Applications of Trunnion or Dummy Supports

Wide application of trunnion or dummy supports is found in control valve assemblies. In control valve stations, trunnion or dummy supports are usually welded from parent pipe elbows as shown in Fig. 1 below:

Trunnion or dummy supports are quite common to use as adjustable supports on a pump or other rotary equipment for alignment purposes.

Typical Trunnion Support Configurations

A variety of support configurations can be made by welding trunnion supports on pipes. Examples of some widely used typical pipe trunnion support configurations are provided in Fig. 2:

Trunnion Calculation Method

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 recent times, Trunnion or dummy support checking using FEA calculations has been increasing among EPC organizations. Various extended modules of stress analysis programs like FE-Pipe/ FE-bend or other FEA programs are used to calculate stresses in the junction point and check if those are acceptable.

In the next paragraphs, I will brief the steps and formulas used while trunnion checking using the Kellogg method.

Inputs Required for Pipe Trunnion Calculation

The inputs that will be required for pipe trunnion calculation are:

• Pipe Support loads from stress analysis software.
• Parent pipe OD and thickness.
• Pipe trunnion OD and thickness.
• Parent pipe corrosion allowance.
• Parent Pipe material to get stress values.
• Pipe design temperature and pressure.
• Pipe trunnion height.
• RF pad thickness if required/provided.

Steps for Trunnion Calculation by the Kellogg Method

1. First of all run the static analysis in Caesar II/AutoPipe/Rohr II/Start-Prof/Caepipe/Other Pipe Stress Analysis Software to obtain the load values at trunnion nodes from the output processor. It is better to practice taking the maximum value from all load cases (Sustained, operating, design, upset, hydro test, occasional, etc.)

2. After that, calculate the bending stress generated on the pipe shell based on the following Kellogg equation:

S_b=(1.17 * f * √R )/ (t^1.5) ……(1)

Here,

• S_b=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

3. Now from piping stress analysis software, we will get three forces with respect to each trunnion; longitudinal forces, circumferential forces, and axial forces. So accordingly we will have to calculate three f values as mentioned below:

• Loading due to longitudinal bending, f_L=M_L/ (Π r² )    ……(2)
• Loading due to circumferential bending, f_C=M_C/ (Π r² )   ……..(3) and
• Loading due to axial force, f_A=P/ (2Π r)………..(4)

Where,

• M_L=Longitudinal force obtained from Caesar  * trunnion effective length
• M_C=Circumferential force obtained from Caesar output * trunnion effective length
• P=direct axial force obtained from Caesar II output. and r=outside radius of the trunnion.

4. Next step is to calculate all bending stresses using equation (1) for longitudinal (S_L), axial (S_A), and circumferential (S_C) forces as calculated from equations (2), (3), and (4).

5. Now Calculate longitudinal Pressure Stress (S_LP=PD/4t) and Hoop Stress (S_CP=PD/2t).

6. In the next step, combine all these forces for proper load cases as shown below and compare the combined value with the allowable stress value provided in terms of the S_h and S_c values as defined in the ASME B31.3 code.

• S_L+ S_A + S_LP <= k * S_h
• S_C+ S_A + S_CP <= k*S_h And
• Trunnion Stress<=S_h

Here trunnion stresses should be calculated as=[{32*Trunnion OD*√(M_L²+M_C²)} / {Π*(Trunnion OD⁴-Trunnion ID⁴)}]

The values of “k” are dependent on loading types. The allowable stress values as suggested in the Kellogg Method are produced below in Fig. 3 for reference.

Options to reduce stresses while Trunnion Support Checking

There are various ways by which the calculated stresses on the trunnions can be reduced so that the dummy calculation qualifies with respect to allowable stresses. Some of such tips are

• While checking trunnions or dummies, it is found that a major chunk of trunnion supports fails due to circumferential loads. So, orient or place the trunnion in such a way that the circumferential force on the trunnion becomes very less to permit/allow greater trunnion heights.
• Reducing the trunnion height or increasing the trunnion size also will reduce the calculated stresses.
• RF pads are required to be added to increase the junction thickness and thereby increase load-carrying capabilities which decrease stresses.
• Increasing the parent pipe diameter will also qualify the trunnions. However, increasing parent pipe size needs process confirmation.

What is the difference between a Trunnion and a Dummy Leg?

Even though the terms “Trunnion” and “Dummy Leg” are used interchangeably, there is a slight difference between the two pipe supports.

Dummy pipe is welded onto the pipe elbow to extend the line to reach the next support while Pipe trunnions are welded to vertical lines. Usually, two short pipe pieces are welded to vertical pipe for better stability for trunnion application. It’s easier to provide an RF pad on trunnion supports but the same is difficult for dummy legs as the pipe profiling and welding at pipe elbows are usually difficult.

Online Course on Pipe Support Engineering

If you want to learn more details about pipe support engineering then the following online course is a must for you:

• Pipe Support Engineering for Industrial Piping Systems
• For other online relevant courses click here