After 2 years of commissioning a tube bundle dryer shows cracks on the headpiece. A considerable part of the annual world production of a certain plastic runs through the production line in South America. When the dryer is switched off for repairs, costs amounting to millions are incurred within a few days. The repair welds are completed in one week, but after half a year cracks appear again. The problem is not solved, only postponed.

Operational hectic begins. There are different opinions in the room, which has led to the damage. Experts are called in who are initially certain that the damage was caused by their special field, e.g. materials engineering, welding technology or vibration. But later it is a little more complicated …

The pressure increases the longer it takes to find the cause or search for suitable repair measures.

The newly designed head piece is already in production, but does it really or are the cracks back after a year? The end customer has become involved in finding a solution and is carrying out his own investigations.

I have often encountered stories like these in my more than 30-year professional career. With the help of numerical simulation, we could show our customers in almost all cases,

1. what the cause of the damage is.
2. how the damage can be repaired.
3. how to avoid similar cases of damage in the future.

A case of damage is usually associated with high costs. It not only costs money, but also causes considerable damage to the company’s image. The longer you fish in the mud, the more expensive it is.

Therefore, the time factor is decisive for the 3 steps above.

Saving time means doing the right things in the right order.

But what are the right things?

Causes of damage can be:

• Misinterpretation
• Construction errors
• Manufacturing defects
• Material defects
• Welding defects
• Resonant vibrations
• Static overload
• Dynamic overload
• Overloading of the base material
• Overloading of the base material
• Thermomechanical overload stationary
• Thermomechanical overload during heating and cooling
• Deformations too high
• Creep effects
• Shock loads
• Vibration loads
• Operating errors
• Transport
• Not considered or inadmissible load cases
• Uneven pressure distributio
• etc.

First, simulations (CAE, FEM, CFD) must be used to find out which causes of damage are possible. I often encounter the approach of our customers to pack everything into a simulation model, because they might forget something. To evaluate results here and draw conclusions is impossible. The simulations are complex and therefore take too long.

It is also wrong to want to examine the optimized model without analyzing the damage.

I successfully pursue a different approach:

From simple physics for simple models to complex physics for complex models, but only if necessary!

Observations must be plausible and fit together.

From a simple static FEM investigation with unit load cases and the determination of the natural frequency, different causes of damage can already be excluded or confirmed.

With appropriate checklists I make sure that I do not forget anything.

I assume that the information provided by the customer or the operator is not always all correct. Especially in the case of operating errors or design faults, both sides often try to bend the truth in their favour. This just does not help, because it does not solve the problem.

Furthermore, I use TRIZ tools such as the cause effect analysis or the functional analysis to better isolate the causes of damage.

If the cause of the damage is clearly known, repair measures can be derived, and experience can be used for new designs.

I have learned that there is always a root cause for damage. It is never the unfortunate concatenation of several circumstances.

Even Murphy’s Law is not completely out of the picture: The measure best suited to repair or redesign the structure always fails for some reason. However, the perspective of those responsible can also change as the level of suffering increases.

Summary of procedure

1. Collect data and mark questionable data with a question mark.
2. Always examine the configuration in which the damage occurred first.
3. Make simple FEM models or CFD models to get a first impression.
4. Exclude certain causes of damage if possible.
5. Concentrate on the remaining possibilities and make the FEM and CFD models more complex where necessary.
6. Clearly determine the cause of the damage.
7. Consider how to avoid the problem.
8. Define and analyze repair measures and evaluate the implementation possibilities.
9. Establish guidelines to avoid the mistake of redesigning.

By the way, the cause of damage to the dryer above was an inadmissible load case: When cleaning the dryer, it was filled with water and left to run for several days. The 40t rotating bending was not considered in any consideration.

Sometimes it helps to talk to the operators of the machine and ask stupid questions. 😉

Yours Stefan Merkle

PS: We assume that you are doing everything correctly and that no damage will occur.

PPS: If, contrary to expectations, problems should arise, we will be happy to help you professionally. Please send me a mail to s.merkle@merkle-partner.de.

You can find more information about damage analysis here.