Design Margin of Pressure Vessle 10

[mechengineer]

Enthusiast and expert of pressure vessel design.
Let us discuss what the actual design margin is in the pressure vessel design and fabrication.
1. The constant sustained design load are the operating pressure, design pressure and MAWP in pressure vessel design.
2. In terms of primary membrane stress, the design margin (I call 'safety factor') for the allowable stress is the design margin given by the design code.
3. The design margin for the design loads is the difference between the operating pressure and design pressure, furthermore the operating pressure and MAWP.
3a, the design code does not give how to determine the design pressure from the operating pressure.
Whether the allowable stress in the design code is conservative for short period load?
3b, the design pressure often comes from process engineer. My experience is that mostly the design pressure is very far from the operating pressure.
Discussion: Over the past years the code has carefully and rigorously reduced safety margins. However, due to item 3 above not being standardized and rigorous it makes the effort to reduce the design margin by code may be meaningless. Hence I think the code shall not only give the design margin for the allowable stress, but also shall give the rule to determine the design pressure from the operating pressure.

 

[DriveMeNuts]

"The mechanical engineer should have a clear understanding of what actually is the maximum operating pressure and how it was derived by the process engineer."
The mechanical engineer doesn't need to know anything about the Process design. If the vessel does not have cyclic service, the mechanical engineer doesn't even need to know anything about the operating conditions. The mechanical engineer only needs Design conditions, where there is no cyclic service.
As a Mechanical Engineers at a fabricators I was only given Design Temp and Pressure, and no operating conditions.
As a Mechanical Engineers at a chemtech company, I occasionally interfaced with a Process Engineer, but it was not a necessity. I don't need to know how the Process Engineer derived the operating pressure. All I needed from the Process Engineer was a datasheet with the operating pressure specified on it. The process design is none of my business.

This has to be the most muddy and pointless thread that I have ever participated in.
For some reason, the OP is mishmashing vessel code design margins with multiple unrelated margins from various departments into a single discussion, and then making the cardinal sin of calling them 'safety' factors.

 

[mechengineer]

@LittleInch, thank you very much for your input, you understand what I am concerned about much better.
Over the past years the code has carefully and rigorously reduced the margin of allowable stress, for example, the design margin from 4 to 3.5, the allowable stress shall be the smaller of UTS/3.5 or Yield/1.5, VIII-1, design by rule. However, due to item 2 above not being rigorous and it makes the effort to reduce the design margin by code may be meaningless. As the result of the final thickness, the design margin reduced of the allowable stress from the code may be still within the design margin from the OP to DP.
The final thickness is determined by both DP and allowable stress. Code only considers the allowable stress without the consideration of the limit for the DP from OP, how to know the real benefit from the reducing the design margin of the allowable stress? In another word, what is the basis that code reduced the design margin of the allowable stress?
Thanks,

 

[mechengineer]

@Snickster, please ignore my first post as I can't edit and remove it, so I said sorry for the confusing by 2nd post.

 

[Snickster]

Drivemenuts

It can be that the process engineer sets the design pressure if that is how the design company operates. That would work too . I am used to the process engineer providing the maximum operating pressure and then the mechanical engineer takes over as there are a lot more factors in deciding the design pressure. I think the mechanical engineering department is the best to control the design pressures and temperatures of the vessel because they are also controlling the design pressure and temperatures of the piping.

 

[Snickster]

In fact my experience is that once the process engineer develops the PFD and mass balances he gives to the mechanical engineers who then take over the ownership of the development of the entire P&ID's in consultation with the process and instrumentation engineers. The process engineers move from job to another developing the process and do not have time to get into the fine details of the implementation of how to design the system to accomplish the process requirements.

 

[Snickster]

Attached is a sample of the process stream data I am typically provided by the process engineer. Then I take this information and develop the P&ID's in consultation with process and instrumentation engineer. I believe this is a Hysis output.

 

[Snickster]

And here is the associated PFD provided by the process engineer for the flow streams of the previous post. I do see that the design pressure is specified for the equipment at top of PFD at approximately 10% above maximum operating pressure. However this is the final PFD that went through many iterations. It has been a few years so I can't remember the exact work flow as to how the design pressures were determined and by who. In any case whether the process engineer sets the design margin or the mechanical engineer, this is what is provided to the vessel fabricator so there is no more added design factors above this.

 

[Snickster]

And sometimes all the process engineer gives me in the very beginning is this - just a Hysis output diagram, and I have to develop the P&IDs from this. See attached.

 

[TGS4]

I don't think that I went off on a tangent. The OP was asking a specific question about one small element with respect to the overall design margin against one of the many failure modes. It is my opinion that this excessive focus completely misses the mark when it comes to the design of pressure vessels.

It is not for the mechanical engineer to make the determination about the design conditions - our job is to ensure that the equipment is suitable for all of the design and operating conditions. Let the process engineers make those determinations - noting that there may be more than just the design in front of them - they may be looking at redesigns 5-15 years down the road and future re-rates (or avoiding the need for that).

I will note that recent changes to the Code have incorporated such aspects as using the operating pressure to be coincident with a wind or seismic event (rather than the design pressure). These help to sharpen the pencil.

But all of this means nothing if the materials selection is incorrect, or the corrosion allowance is insufficient, or the design is actually governed by fatigue (Note that I have designed a few vessels specifically for fatigue, resulting in a wall thickness 3-4 times that necessary for pressure-retention only).

 

[Trestala]

The mechanical engineer doesn't need to know anything about the Process design. If the vessel does not have cyclic service, the mechanical engineer doesn't even need to know anything about the operating conditions. The mechanical engineer only needs Design conditions, where there is no cyclic service.
As a Mechanical Engineers at a fabricators I was only given Design Temp and Pressure, and no operating conditions.
As a Mechanical Engineers at a chemtech company, I occasionally interfaced with a Process Engineer, but it was not a necessity. I don't need to know how the Process Engineer derived the operating pressure. All I needed from the Process Engineer was a datasheet with the operating pressure specified on it. The process design is none of my business.

This has to be the most muddy and pointless thread that I have ever participated in.
For some reason, the OP is mishmashing vessel code design margins with multiple unrelated margins from various departments into a single discussion, and then making the cardinal sin of calling them 'safety' factors.

No, unless your work only focuses on the Code calculation or as a fabricator, you still need to be aware of process and system design as a vessel designer including operating conditions and what happens inside the vessel. Operating conditions are very relevant to material selection, thermal design, painting, insulation, internal design, use of wear plates, use of impingement plates, load combinations where operating condition is considered rather than design, etc. Those will affect how you judge margins, safety, and amount of risk you are taking in different aspects or failure modes on your design and if it aligns with the same principles as the Code you are following.

 

[r6155]

Some things influence and can modify the design pressure.
Nobody mentions NDE and testing which are generally not considered by the process engineer.
-A vessel may be designed and Code stamped for more than one pressure/coincident maximum metal temperature condition.
-Lethal service require butt welds and 100% RT
-Vertical vessels being tested in the erected position, whether shop or field, shall have consideration given to the additional pressure and weight due to the fluid head.

Regards

 

[Rubje]

@DriveMeNuts

"For ASME materials, the guaranteed minimum Yield and UTS of a material which is mandatory in design calcs, corresponds to plate which is ~100mm thick. However, 20mm plate has material properties superior to 100mm plate, and therefore there is waste material in 20mm thick vessels. European material codes reduce this waste by specifying different Yield and UTS for different thicknesses."

I didn't realize this until now!