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# Design Margin of Pressure Vessle10

## Design Margin of Pressure Vessle

(OP)
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.

### RE: Design Margin of Pressure Vessle

The code is not a substitute for the engineer's good judgment.
Yield and ultimate strength of the material shown in the material test report acts as a design margin.

Regards

### RE: Design Margin of Pressure Vessle

There is no such thing as safety factor.
Or at least safety factor is just one of many multiplication factors which make up the design margin.

### RE: Design Margin of Pressure Vessle

Think of the reasons that there are safety factors.
You don't know the mechanical properties of all of the materials at every point.
You don't know the quality of the welds at every point.
You don't have a way of actually knowing the applied stresses at every point.
You are responsible for the health and safety of the plant and the public.
Given all of these factors we need margins to assure that we can get our 40+ years of safe service out of our equipment.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Design Margin of Pressure Vessle

The "safety factor" is an "ignorance factor."
One of the biggest risks is "The Inspector.".

Regards

### RE: Design Margin of Pressure Vessle

The safety factor for stress is the allowable stress.

The operating pressure is the normal maximum operating pressure.

The design pressure considers the absolute maximum operating pressure in upset conditions such that only on very rare occassions will the relief valve go off. Most companies set a standard % above maximum normal operating pressure. The design pressure is what you request the minimum pressure to be designed to by the vessel supplier as shown on the data sheet in your provurement specification.

The MAWP is the actual maximum pressure the vessel is good for in the as-built condition considering that it can only be manufactured with standard wall thickness and flange ratings. Therefore you may ask for a design pressure of 200 psig but in the end the vessel may really be able to withstand 275 psig. If it is really rated for 275 psig then you would want that to be shown on the nameplate - not the 200 psig you asked for. That way it can be used at a higher pressure if process conditions change.

### RE: Design Margin of Pressure Vessle

The OP is so far from the mark, I don't know where to start, to unpick the false premise and lack of knowledge of the OP.
Perhaps TGS4 will wade into this topic for the sixth time.
Or you could search the forum archives, and learn from previous discussions.
In short, a 'safety' factor is a 'I don't know what I am doing, so I will just add some additional random margin and hope no one notices' factor.

### RE: Design Margin of Pressure Vessle

" My experience is that mostly the design pressure is very far from the operating pressure."

Often, there is not much margin between maximum operating pressure - MOP, and process design pressure. What you perceive to be a large margin is probably due to insufficient information on the full operating pressure range of the equipment. For example, a HP flare knock out drum in an oil/gas plant or oil refinery may have a design pressure of 10barg and you may only often see an operating pressure of say 1-2barg at this drum. What may set the design pressure is the max or peak pressure seen during a total plant emergency blowdown, or a large emergency relief load when some unusual combination of operating circumstances all coincide with one or more operating failures, or flashback pressure on the drum due to purge gas failure. These cases which determine design pressure of equipment is often found in some process design report prepared during FEED or concept engineering. Lead process engineers, senior Operations staff, process design managers in the Operating Company and HAZOP leaders all play a key role during design reviews and audits.

### RE: Design Margin of Pressure Vessle

2
Yes I’m going to chime in on this.

The OP is definitely far from the mark.

First off, there exists no such thing as a “safety factor”. What we have are design margins against specific failure modes.

The pressure vessel codes all recognize that there are multiple failure modes that must be protected against. And although this is most explicitly described in the Design By Analysis method in ASME Section VIII, Division 2, Part 5, it is nevertheless still there in the other Design by Rules sections. The failure modes particular the pressure vessels are:
- Plastic collapse (burst)
- Local failure
- Buckling
- Fatigue
- Ratcheting
- Creep
- Brittle fracture
- Corrosion
- Stress corrosion
- Corrosion fatigue
- Material degradation over time (think HTHA, neutron embrittlement, etc)
- Leakage of non-welded connections (flanges)
- etc

For each and every one of these failure modes, there are different approaches and hence different design margins against that failure mode. Some of the failure modes have design margins as low as 1.0, and others may be as high as 20.

Some of the points that the OP makes is particular to only one failure mode, but there are very small issues around the edges that tend to distract the design and operating engineer from the important issues, which is to focus on the totality of all of the failure modes.

### RE: Design Margin of Pressure Vessle

(OP)
Thanks for all inputs and apologize for the unclear post. Please allow me revise my post as below,
In terms of primary membrane stress, what the actual total design margin is in the pressure vessel design and fabrication and how to control the design margins from points 2 & 3 to avoid giving the over design margin in total.
I think that the total design margin shall combine the three points below,
1. The design margin from the code allowable stress given by the design code.
2. The design margin from the design pressure given by process engineer.
3. The design margin form the final thickness given by design engineer.
As the design engineer, he is able to control the final thickness, but don't know the design margin from the design pressure wich given by process enguneer.
It may be the material waste in the design margins from the points 2 & 3 more than the material saved by the decreasing the design margin year by year from point 1.

### RE: Design Margin of Pressure Vessle

mechengineer, it looks like you are referring to the Plastic collapse failure mode.
For ASME VIII Division 1 the Design margin for Plastic collapse is 3.5x, where the material UTS is divided by 3.5 to get the maximum allowable value for the general primary membrane stress.

For your point number 3, plate thickness used tends to be thicker than the calculated minimum thickness because material stockists only supply specific thicknesses. The result is that the MAWP of the actual vessel is greater than then the Design Pressure specified on the process datasheet. I would view this gap between MAWP and DP and the associated excess Margin (above 3.5x) as 'waste', not safety. It is the job of a good engineer to minimise the gap between MAWP and DP, to minimise waste.

There are many examples of this waste:
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. Even though the ASME code prohibits making use of such waste, I still don't refer to it as safety. TGS4 provides a good idea for what to do with such excess margin here. Perhaps, he still stands by it: https://www.eng-tips.com/viewthread.cfm?qid=345631

For point 3a and 3b of your original post, Design Pressure and Operating pressure are often unrelated. DP may be governed by an extreme one off upset condition (i.e. plastic collapse failure mode) specified on the Process datasheet, while operating pressure is governed by cyclic production cycles (i.e. fatigue failure mode). DP and OP are usually as unrelated as an apple and a bicycle. While the equipment is operating at operating conditions, it is messy and unhelpful to describe the DP divided by OP as a margin because upset conditions could occur at any time.

Sometimes large vessels need to be beefed up because of the transport stresses from fabricator to plant, which results in an excessive MAWP. Sometimes a vessel in a plant is connected to other vessels which have much higher DP's and OP's, and during the plant commissioning stage the entire plant needs to be pressure tested together, therefore the low DP vessels need to have the same DP as the highest DP vessel.
Some engineers view these cases of excess margin as fortunate safety, some view it as unavoidable waste. Either way, it allows you to breathe a little easier during your spare time.

### RE: Design Margin of Pressure Vessle

(OP)
My major concern is the design margin from the design pressure given by process engineer.
"DP and OP are usually as unrelated as an apple and a bicycle" Before seeing your words, I just doubted about the significant difference between DP and OP without knowing the reason as the design engineer. I did not know this is an accepted fact. In China design code and Technologic Supervision Regulations on Safety of Pressure Vessels, it gives the guidance how to determine the design pressure for pressure vessel according to different cases, such like with the safety vale/rupture disk, the medium is liquid, gas, or liquefied gas, ......
I hope it may close the gap between the pressure vessel design engineer and the process engineer. In fact, it should be simple thing, the pressure vessel design pressure made by process engineer shall agree (sign) by the design engineer. May ASME code make an effort for it? give the guidance for determination of the design pressure, like China code.

Regards,

### RE: Design Margin of Pressure Vessle

(OP)

#### Quote (Some engineers view these cases of excess margin as fortunate safety, some view it as unavoidable waste. Either way, it allows you to breathe a little easier during your spare time.)

On the contrary, I can't sleep because of the waste without knowing the truth.

### RE: Design Margin of Pressure Vessle

Thanks for all inputs and apologize for the unclear post. Please allow me revise my post as below,
In terms of primary membrane stress, what the actual total design margin is in the pressure vessel design and fabrication and how to control the design margins from points 2 & 3 to avoid giving the over design margin in total.
I think that the total design margin shall combine the three points below,
1. The design margin from the code allowable stress given by the design code.
2. The design margin from the design pressure given by process engineer.
For the companies I have worked for the process engineer provides normal and maximum operating pressure/temperatures (with PFD and material balances for every flow stream) not the design pressure. The mechanical engineer is responsible for pressure/temperature rating of vessel such that drsign pressure exceeds the maximum operating pressure by at least 10% - this is a mechanical engineers responsibility not process engineer. Usually client specs have requiremt for how much the design pressure should be above the maximum operating pressure and reqiurements for margins for design temperatures and MDMT. In any case you should be given the maximum operating prssure by the process engineer not the design pressure
3. The design margin form the final thickness given by design engineer.
For the companies I worked for as a mechanical design engineer of process equipment and piping systems, the design engineer would set the design pressure and would specify this design pressure to the vessel fabricator. The vessel fabricator designs the vessel performing all calculations and determining the wall thickness. It is up to him to choose the most economical wall thickness based on the desigm parameters you give to him. If you are going out for a minimum of three different suppliers then you do a bid evaluation on the proposals and you see that they will all have same wall thickness or if not you question them why.
As the design engineer, he is able to control the final thickness, but don't know the design margin from the design pressure wich given by process enguneer.
See comment above and if not then why cant you just discuss with the process engineer what margin does he have or think there should be.
It may be the material waste in the design margins from the points 2 & 3 more than the material saved by the decreasing the design margin year by year from point 1.

### RE: Design Margin of Pressure Vessle

"2. The design margin from the design pressure given by process engineer."
Setting the DP by adding 10% to the Process specified OP is not a vessel code requirement. An executive Technology manager may have made a risk based call to add another 10%, for many reasons. This has nothing to do with code rules, or the vessels designer. Knowing the reasons behind the 10% won't enable the vessels engineer to improve the fabrication of the equipment.

Any margins which the process engineer adds to the operating process design relate to the accuracy and variability of the process. It is the process department's job to manage and minimise their process margins. These margins have nothing to do with the mechanical design of the Pressure Vessel.

When developing new products, Vessels, Process and Technology, Commercial and many other departments get together as a team to co-ordinate to develop the product. After that, the scope of work for each department including boundaries is precisely defined, and each department sticks to their respective scope. After a project is executed to fabricate a product, huge amounts of schedule and money are wasted if the vessels engineer insists on sticking his nose in the Process department's business to re-open the development of the product half way through a project. Improvements of the product are ideally best done between projects.

Don't you have a mentor or manager, who can teach you this stuff?

### RE: Design Margin of Pressure Vessle

2
I'll have a go here but think Snickster has it about nailed.

The issue for mechengineer seems to be how to get the design pressure from the operating pressure.

This is normally contained in client design guidelines or engineering guidelines issued by the design house or the client (typically large Oil and Petro chem companies)

ASME VIII is agnostic about these things - it just tells you how to design a vessel to meet a specified set of conditions - Pressure, temperature, volume, fluid etc - defined by the engineer.

As said by many above you start withe normal operating conditions then move to extreme operating conditions. The often quoted 10% margin is really a practical margin for simple spring relief valves with enough space to add in some alarms or trips before you get to design pressure. All your reliefs, trips etc are set at design pressure for a pipiing system or the MAWP for a PV.

Now MAWP is at or more than the design pressure due to the way PV's are designed. It is though not really possible to use MAWP before you have done the design work on the PV. So this becomes a certain additional margin above the design pressure for the PV itself. However all the connecting pipework will just be designed to the design pressure so you're no further forward usually.

So yes, as described above, the design flow goes Process engineer - mech engineer - PV designer. now if the PV MAWP or the design pressure set by the mech engineer just exceeds some fixed value which adds cost, the original ma operating pressure and temperature can be challenged to set the design pressure under a fixed value which reduces cost, e.g. a flange class rating or thickness of material jumps up but an design pressure say 3% less would bring it under.

but essentially going back to the OP, you are correct. The PV design code does not include design manuals and procedures, but lets everyone do their thing based on their own particular circumstances. Is that good? maybe, maybe not, but it isn't going to change anytime soon....

Then you start to look at fault conditions such as blocked exit etc or extreme conditions such as fire. These set the relief flow rates or sometimes yu can show that the event ( say blocked discharge with a pump running) cannot exceed the MAWP and therefore there is no need for a relief flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Design Margin of Pressure Vessle

"2. The design margin from the design pressure given by process engineer."
Setting the DP by adding 10% to the Process specified OP is not a vessel code requirement. An executive Technology manager may have made a risk based call to add another 10%, for many reasons. This has nothing to do with code rules, or the vessels designer. Knowing the reasons behind the 10% won't enable the vessels engineer to improve the fabrication of the equipment.

Most companies set a specific margin for the desing pressure above the maximum operating pressure provided by the process engineer. The maximum operating pressure sould consider expected excursions above the normal operating pressure. There are also excursions above the normal operating pressure not normally expected by process engineer - hence why a margin is provided. Most companies provide a 10% marging from design to maximum operating pressure when using conventional relief valves since these valves will start to simmer at less than 10% set point is reached, and 5% margin when using pilot operated relief valves.

Any margins which the process engineer adds to the operating process design relate to the accuracy and variability of the process. It is the process department's job to manage and minimise their process margins. These margins have nothing to do with the mechanical design of the Pressure Vessel.

Any margins the process engineer wants to add if he thinks that there will be frequent pressure spikes above the normal operating pressure he can by bumping up the stated maximum operating pressure. But the final design pressure is determined by the mechanical department in consultation with the process department but the mechanical department in the end has ownership of all design aspects of the pressure vessel as he is writing the specificatio/data sheet and responsible for selecting the successful bidder. The mechanical engineer does not do all this unilaterally though but in consultation with process engineer and client. There are cases when you may need to bump up flange/pressure rating due to mechanical considerations not process. So the mechanical engineer must keep control over the pressure vessel design.

When developing new products, Vessels, Process and Technology, Commercial and many other departments get together as a team to co-ordinate to develop the product. After that, the scope of work for each department including boundaries is precisely defined, and each department sticks to their respective scope. After a project is executed to fabricate a product, huge amounts of schedule and money are wasted if the vessels engineer insists on sticking his nose in the Process department's business to re-open the development of the product half way through a project. Improvements of the product are ideally best done between projects.

All decisions should be made up front as to the pressure temperature ratings required by the equipment and piping systems. The mechanical engingeer should have a clear understanding of what actually is the maximum operating pressure and how it was derived by the process engineer. It should be a joint decision as far as setting it but the mechanical engineer should eventually have ownership. Once the specification for the vessel is issued it should not be changed.

Don't you have a mentor or manager, who can teach you this stuff?

I am retired with 40+ years in oil and gas, refining and chemical plant design as lead mechanical design engineer for major design consulting firms. I might not get everything exactly right because alot of questions involve things I have done 20 years ago or more and not since so sometimes my memory is a little blurred but in general I think my recollection is correct and what I state is how I recollect it is.

### RE: Design Margin of Pressure Vessle

And the reason I chimed in on this question is because you and TGS4 did not answer the question the OP asked and went off on huge tangents about unrelated things that were more confusing than clarifying. You can see by OP last post that he was still not clear on anything.

### RE: Design Margin of Pressure Vessle

To add to the posts above a 10% margin wouldn't leave much margin in absolute terms for a low-pressure vessel. From what's I've seen the margin between the Design Pressure and the and the Maximum Operating Pressure is usually more complicated than just adding 10%. Sometimes the margin is set as the greater of some fixed amount (such as 25/50 psig) or 10%/15% of the Maximum Operating Pressure. And on top of all of this the Design Pressure usually rounded up to the next 10 or 25 psig increment.

The margin for the Design Temperature is often around 50°F above the Maximum Operating Temperature, rounded up to the next 50°F increment.

Sometimes even higher pressure/temperature margins are allotted if it's deemed economical.

-Christine

### RE: Design Margin of Pressure Vessle

More mud in the water: The difference between Design Pressure and MAWP for pipe-sized vessels can be quite high sometimes...

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

### RE: Design Margin of Pressure Vessle

"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.

### RE: Design Margin of Pressure Vessle

(OP)
@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,

### RE: Design Margin of Pressure Vessle

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

### RE: Design Margin of Pressure Vessle

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.

### RE: Design Margin of Pressure Vessle

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.

### RE: Design Margin of Pressure Vessle

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.

### RE: Design Margin of Pressure Vessle

2
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).

### RE: Design Margin of Pressure Vessle

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.

### RE: Design Margin of Pressure Vessle

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

### RE: Design Margin of Pressure Vessle

"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!

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