CHAPTER 2: BASICS OF PIPING STRESS ANALYSIS WITH CASE STUDIES

Piping failure in a running plant is always a major issue that leads to shutdown and production loss. Two interesting piping failure case studies with basics and solutions will be discussed in this chapter. Failure normally occurs in cases where there is a major obstruction to the line thermal movement or basic engineering principles are not followed. Both cases discussed here are practical encountered problems.

CASE STUDY – 1 Obstruction to pipe thermal movement

For small-bore lines, many companies end up ignoring basic stress analysis requirements, just because it’s a small-bore line. It is observed that even large companies sometimes ignore stress analysis due to time constraints or to save on small engineering costs.

In the two cases discussed here, the line is supposed to thermally grow vertically upwards, but unforeseen obstruction is created which leads to over-stressing the line and flange joint. As always, the weakest link in the system will fail first, i.e. either the pipe will fail or support will fail whichever is weak, here the flange joint is weakest, which fails and leaks.

There is a 3” NB line which comes out of an 8” Line. The 3” vertical valve flange joint showed frequent leakage. Frequent shutdown and gasket replacement with different types were tried but failure kept recurring. The line was never stress analysed and was never studied by a stress professional. If a line is allowed to expand freely the line and the support both will be safe, however any restriction on movement will lead to over-stressing.

Here in this case, you will find the vertical length (BC) is around 6 m and the temperature is around 200 degrees, net expansion upwards at point “C” will be around 2 x 6 = 12 mm, So a minimum 12 mm clear space is required for the pipe to move upward for safe operation.

In the shown two cases

  1. Line movement is blocked by a U-Bold leading to flange leakage.
    Solution: Remove U Bolt
  2. Line movement is blocked by the structure beam, leading to flange leakage.
    Solution: Create min 30 mm space above the top of the pipe by cutting the pipe,
    removing 30 mm of pipe spool and re-welding.

In both the above cases, very high compressive loads are generated in piping (BC), leading to overstressing of pipe (red zone in pipe) and crushing of gaskets leading to flange leakage

CASE STUDY – 2 Secondary support design

Here we will discuss, how a faultily designed structural member can lead to vibrations.

Primary Support: Normally insulated lines are supported by attaching shoes to the pipes, this is called primary support.

Secondary Support: The TEE or the goal post used to support this shoe is called the secondary support.

To save on engineering costs, some companies prefer to go by the thumb rule and don’t go for a formal design. Here we are talking about the design of secondary supports for the piping loads. The rigidity of the secondary support plays a very important role in the stability of piping. A highly under-designed support post and its anchor bolts and baseplate may lead to heavy vibrations in the line making it difficult to run the plant. Many times it is considered to be flow-induced vibration, even though there is no source of vibration in the line.

We faced a similar problem in a turbine exhaust line which started vibrating heavily during commissioning. We were told to have a look into our analysis and give a solution. However, since there was no source (like two-phase flow, reciprocating machine flow, etc.) of vibration, we were sure that the issue was not related to stress analysis. As the structure and secondary support design was in the client’s scope, we asked for the secondary support design calculations to check rigidity. The client was not able to present and we were told that secondary supports were installed without design with thumb rules.

We suggested applying bracings to all secondary supports in both horizontal directions for TEE posts, to limit deflection if any and increase rigidity. This worked and the vibration stopped.

A similar case was observed in a reciprocating pump line where the column supporting the pipe was almost resting on the floor. The anchor fastener used for the base plate was very small, 8 mm in size, and there were only two anchors on the base plate. This made the base plate moments free in one direction, which led to heavy vibrations.

Solution: The steel member (post) and the base plate were re-designed for piping loads with 4 anchor bolts. For new members selected, the natural frequency was also checked with respect to the machine frequency to ensure sufficient rigidity. The same analysis approach is applicable to all the secondary supports of the lines and the supporting structure. For large bore reciprocating lines, we recommended RCC columns for pipe supports to limit deflection and vibrations and it worked.

All the solutions, as discussed above, are after a detailed analysis of software, since any change that is recommended to the client needs a back-up by supporting document. In case of failure of piping, many other aspects also need to be checked before suggesting modifications, i.e. availability of flexibility and support piping loads.

When we come across a line failure where there is no piping analysis done, the line is completely checked by stress analysis on software and a modified line is also analysed to ensure that it does not pose new problems.

We may now move to a more complex failure analysis in piping from the next chapter onward. Please give your feedback at [email protected] on this article and kindly suggest which topic you would like me to cover in future.

Author

Ashok Gupta

ND Engineering Services

www.ndesindia.com

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