truthseaker
Mechanical
When the customer requires MAWP calculations. Is there an industry standard that dictates which component (heads, shells, nozzles) is allowed to be limit the MAWP for ASME code pressure vessels?
thanks
thanks
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MAWP Limiting Component 6
- Thread starter truthseaker
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Just customer spec's and good engineering practice. There is no code (Section VIII-1) requirement which would indicate that certain components should not limit MAWP.
The typical spec would say something to the effect of minor components shall not limit MAWP. "Minor" is generally considered to be nozzle necks, opening reinforcement, sometimes flange rating. Body flanges are often considered "major".
jt
The typical spec would say something to the effect of minor components shall not limit MAWP. "Minor" is generally considered to be nozzle necks, opening reinforcement, sometimes flange rating. Body flanges are often considered "major".
jt
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- #3
truthseaker
Mechanical
But if the customer has asked for the limiting component of a design and no other specs exist then I believe it is still acceptable to limit by "Minor" components (nozzles, reinforcement, etc).
I cannot see the difference in limiting between a minor or major component because in the end you are still maximizing a design.
I cannot see the difference in limiting between a minor or major component because in the end you are still maximizing a design.
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I have seen the idea discussed before that reinforcement should not be the limiting factor in order that the vessel could possibly be uprated at some time in the future. The problem with this line of thinking is that when you specify that minor components not limit the pressure, you will likely get thinner major components rather than heavier minor components. (It is fairly common to adjust joint efficiences or head geometry to intentionally eliminate repads, for example.) To me, it makes a lot more sense, if you think you might uprate the pressure in the future, to just specify a higher pressure to begin with, rather than specifying other criteria and hoping they somehow maybe result in higher pressure ratings.
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It is normally the customer who asks for MAWP in a new installation then the supplier designs the product based on this requirement including EVERY component. This design pressure exists in every related calculation and component selection sheet. Therefore, and typically, every component is likely a limiting factor. It is not just because they want to save material and remain competitive also because that safety devices such as safety valves are always required and installed so nobody needs to design anything to withhold a higher pressure.
It comes many times that customers want to see if they can increase MAWP's of existing products then the engineers have to look at EVERY single pressure part calculation to see if there is still room. Even if found possible to increase MAWP and proceed to do it, it is then called 'alteration' and the pressure part has to be re-calculated and re-certified, inspected, stamped, registered, etc. the whole nine yards with no exceptions.
If only talking about limiting components on the material cost side, I would say normally they are the large items and the ones which consume largest quantities of materials. For example, steam drums and vessels (weakest parts are shell thicknesses), furnace tubes (weakest are thicknesses at bends), piping and components (limited by pressure ratings), etc. The design engineers are very 'frugal' in the calculations and selections because of the highest cost contributions. For example, nowadays, the shell thickness of a steam drum typically allows only 0.02 inch of corrosion otherwise it will not satisfy MAWP. For a furnace tube the number is 0.04 or 0.08 depending on conditions. Smaller items such as nozzles, fittings, etc., like Vesselfab says, one can normally find extra strengths on them and they are not the limiting components. No matter how, like said, every thing needs to be checked for sure.
Boilerone
It comes many times that customers want to see if they can increase MAWP's of existing products then the engineers have to look at EVERY single pressure part calculation to see if there is still room. Even if found possible to increase MAWP and proceed to do it, it is then called 'alteration' and the pressure part has to be re-calculated and re-certified, inspected, stamped, registered, etc. the whole nine yards with no exceptions.
If only talking about limiting components on the material cost side, I would say normally they are the large items and the ones which consume largest quantities of materials. For example, steam drums and vessels (weakest parts are shell thicknesses), furnace tubes (weakest are thicknesses at bends), piping and components (limited by pressure ratings), etc. The design engineers are very 'frugal' in the calculations and selections because of the highest cost contributions. For example, nowadays, the shell thickness of a steam drum typically allows only 0.02 inch of corrosion otherwise it will not satisfy MAWP. For a furnace tube the number is 0.04 or 0.08 depending on conditions. Smaller items such as nozzles, fittings, etc., like Vesselfab says, one can normally find extra strengths on them and they are not the limiting components. No matter how, like said, every thing needs to be checked for sure.
Boilerone
- Thread starter
- #7
truthseaker
Mechanical
Boiler mentions in his closing statement that "extra strength can be added to the nozzle". If I have numerous nozzles on my vessel of the same size that limit MAWP why would I want to add a re-pad to each one of them so that they are no longer the limiting component? Adding this extra material will increase the cost of my unit along with increasing fabrication time.
This is why I believe that it is acceptable to have a nozzle as a limiting component for an any MAWP design. After all as stated above, most systems have a safety device like a pressure relief valves and if the system is lacking a one then it probably does not make a difference if your head or your nozzle is the limiting component because over pressurization is going to be an issue.
This is why I believe that it is acceptable to have a nozzle as a limiting component for an any MAWP design. After all as stated above, most systems have a safety device like a pressure relief valves and if the system is lacking a one then it probably does not make a difference if your head or your nozzle is the limiting component because over pressurization is going to be an issue.
I agree with vesselfab and most of the companies I have worked with have stated that the MAWP shall be limited by the Head, Shell or Transitions and that it shall not be limited by other minor components such as nozzles or repads.
The intent is that heads, shells and transitions are the most costly part of the vessel and once these have been selected the customer wants to maximize the rating they can get from them and will uprate the minor components as necessary to take advantage of this potential for increased rating.
There is nothing in code that requires this practice and unless it is specified in the Customers specifications, there is nothing to say that a fabricator has to do this. Consequently, I have also seen many vessels where very minor nozzles have limited the MAWP.
Some end users do prefer to allow the "minor" components to limit the rated MAWP and then to use the extra thickness on heads, shells and transitions as additional corrosion allowance on those components. My only concern with this is that if you need additional corrosion allowance on the heads, shells and transitions, don't you also need it on the minor components?
The intent is that heads, shells and transitions are the most costly part of the vessel and once these have been selected the customer wants to maximize the rating they can get from them and will uprate the minor components as necessary to take advantage of this potential for increased rating.
There is nothing in code that requires this practice and unless it is specified in the Customers specifications, there is nothing to say that a fabricator has to do this. Consequently, I have also seen many vessels where very minor nozzles have limited the MAWP.
Some end users do prefer to allow the "minor" components to limit the rated MAWP and then to use the extra thickness on heads, shells and transitions as additional corrosion allowance on those components. My only concern with this is that if you need additional corrosion allowance on the heads, shells and transitions, don't you also need it on the minor components?
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- #9
Not allowing minor components limit MAWP is a good practice.
The users will get higher MAWP rating by specifying this requirement. Otherwise, fabricator will use design pressure as the basis to design nozzles and use the extra thickness in shell and heads for area reinforcement.
I disagree that nozzle flange rating cannot limit MAWP. Fabricator does not pick flange rating. The users or their engineering contractors select the flange rating. It does not make sense to change flange rating of all pipes connected to the vessel just because the fabricator used extra thick plates on vessels.
The users will get higher MAWP rating by specifying this requirement. Otherwise, fabricator will use design pressure as the basis to design nozzles and use the extra thickness in shell and heads for area reinforcement.
I disagree that nozzle flange rating cannot limit MAWP. Fabricator does not pick flange rating. The users or their engineering contractors select the flange rating. It does not make sense to change flange rating of all pipes connected to the vessel just because the fabricator used extra thick plates on vessels.
TRUTHSEAKER
generally speaking, if the client is asking for MAWP, he will have in his specifications that the mawp whall not be limited by nozzle reinforcement.
if that is not in his specifications, leave your calcs like they are. if it is in there, you must adjust your calcs.
If you are using commercial software, there is normally a switch for this.
If you are designing by hand, find mawp of major stuff first, the design the nozzles.
NOW, if you are intent on keeping the nozzles the same, you can reduce the thickness of the top head or bottom head to a decimal equivalent to reduce the mawp. that is if you need .7837 for design thickness and you select .8125" min with a 7/8" nominal, you can call out a smaller decimal required thickness of head with the same size nominal to make your nozzle reinforcing design work out. May have to change top nozzle calcs, but somethings got to change.
generally speaking, if the client is asking for MAWP, he will have in his specifications that the mawp whall not be limited by nozzle reinforcement.
if that is not in his specifications, leave your calcs like they are. if it is in there, you must adjust your calcs.
If you are using commercial software, there is normally a switch for this.
If you are designing by hand, find mawp of major stuff first, the design the nozzles.
NOW, if you are intent on keeping the nozzles the same, you can reduce the thickness of the top head or bottom head to a decimal equivalent to reduce the mawp. that is if you need .7837 for design thickness and you select .8125" min with a 7/8" nominal, you can call out a smaller decimal required thickness of head with the same size nominal to make your nozzle reinforcing design work out. May have to change top nozzle calcs, but somethings got to change.
I think jamesl put it well. Given a DP, a shell thickness can be established. Given a tmin for the shell, opening reinforcement can be designed. If this opening reinforcement is allowed to govern the design of the vessel, then by definition the MAWP has become DP. A vast majority of the shell in effect has extra thickness - which cannot be used.
I find it interesting that it appears that there is a distinct difference in perspectives here - I get the impression that most of the folks arguing for no limiting component restrictions are fabricators and that they speak as though they know what's best for the user.
Folks - why don't you identify your role in the game? Preferrably by going to the "my stuff" tab and choosing "my profile" and filling in the little box titled "tell other members about yourself." You can see mine by clicking on the "jte" at the top of this post. The second paragraph lets you know my background. Its only fair in these discussions to clearly identify what side of the fence your personal income is coming from. Let's not pretend that has no influence on your perspective!
jt
I find it interesting that it appears that there is a distinct difference in perspectives here - I get the impression that most of the folks arguing for no limiting component restrictions are fabricators and that they speak as though they know what's best for the user.
Folks - why don't you identify your role in the game? Preferrably by going to the "my stuff" tab and choosing "my profile" and filling in the little box titled "tell other members about yourself." You can see mine by clicking on the "jte" at the top of this post. The second paragraph lets you know my background. Its only fair in these discussions to clearly identify what side of the fence your personal income is coming from. Let's not pretend that has no influence on your perspective!
jt
I think my handle clarifies my side of the fence.
But I said minor components normally do not limit mawp
funny how that works isn't it?
but i did not see anyone really saying a minor component should/could limit MAWP....other than the original poster.
truthseaker...the reason behind this is to let the major parts limit the vessel because of changes in service later. the vessel limits itself. if the vessels is good for 200 psig and a nozzle is only good for 150 psig (the design pressure), the the vessel has a hard time time upgrading with out NBIC repairs to nozzles.
If the fabricator spends a few bucks for a pad based upon the 200 psig for shell, then the vessel can be upgraded to 200 psig later with no NBIC repairs/alterations.
We do all our vessels this way, by the way. whether required or not...just good practice
But I said minor components normally do not limit mawp
funny how that works isn't it?
but i did not see anyone really saying a minor component should/could limit MAWP....other than the original poster.
truthseaker...the reason behind this is to let the major parts limit the vessel because of changes in service later. the vessel limits itself. if the vessels is good for 200 psig and a nozzle is only good for 150 psig (the design pressure), the the vessel has a hard time time upgrading with out NBIC repairs to nozzles.
If the fabricator spends a few bucks for a pad based upon the 200 psig for shell, then the vessel can be upgraded to 200 psig later with no NBIC repairs/alterations.
We do all our vessels this way, by the way. whether required or not...just good practice
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- #14
truthseaker
Mechanical
Vessel / heat exchanger fabricator here.
I was thinking more along the lines of:
Design Pressure: 346 psig
Calculated MAWP (to be stamped on nameplate): 498 psig
Limiting Component: Nozzle
I understand that sometimes units require re-rating as time goes on. But re-rating involves many other issues such as thickness readings of all components which I will not go into at this time. My question is: will the customer really being given an inferior product with the above design if I limited the MAWP to a nozzle? Don't want to give people the impression that the nozzle has to always be designed to the design pressure.
I was thinking more along the lines of:
Design Pressure: 346 psig
Calculated MAWP (to be stamped on nameplate): 498 psig
Limiting Component: Nozzle
I understand that sometimes units require re-rating as time goes on. But re-rating involves many other issues such as thickness readings of all components which I will not go into at this time. My question is: will the customer really being given an inferior product with the above design if I limited the MAWP to a nozzle? Don't want to give people the impression that the nozzle has to always be designed to the design pressure.
Christine74
Mechanical
I'm not certain what the objective of the special "limiting component" requirement is, but I assume that it's one of the following:
1) Safety (if the relief valve fails, a "major" component such as a shell/head will blow apart before a nozzle shoots off).
2) Cost-effective maximization of MAWP (don't let inexpensive items such as nozzle reinforcement limit the MAWP).
I don't think adding this requirement accomplishes either objective, because 1) a component's calculated MAWP per Code tells you little about what pressure it will actually fail at, and 2) fabricators deliberately make major components such as heads thinner in order to make it the "limiting" component.
If you want a vessel to be able to withstand a higher pressure, specify a higher design pressure on the data sheet. It's that simple.
-Christine
1) Safety (if the relief valve fails, a "major" component such as a shell/head will blow apart before a nozzle shoots off).
2) Cost-effective maximization of MAWP (don't let inexpensive items such as nozzle reinforcement limit the MAWP).
I don't think adding this requirement accomplishes either objective, because 1) a component's calculated MAWP per Code tells you little about what pressure it will actually fail at, and 2) fabricators deliberately make major components such as heads thinner in order to make it the "limiting" component.
If you want a vessel to be able to withstand a higher pressure, specify a higher design pressure on the data sheet. It's that simple.
-Christine
jte, you make this statement: "A vast majority of the shell in effect has extra thickness - which cannot be used."
But the question is, does removing that extra thickness and adding repads instead provide any benefit to the owner?
The assumption is being made that major components will remain the same thickness reqardless of how openings are designed. That may or may not be the case, depending on the circumstances.
If you want that extra thickness AND want to be able to re-rate to a higher pressure, specifying a higher design pressure will accomplish that. Specifying that minor components not limit the MAWP may or may not accomplish that- it just depends on who designs it and how.
But the question is, does removing that extra thickness and adding repads instead provide any benefit to the owner?
The assumption is being made that major components will remain the same thickness reqardless of how openings are designed. That may or may not be the case, depending on the circumstances.
If you want that extra thickness AND want to be able to re-rate to a higher pressure, specifying a higher design pressure will accomplish that. Specifying that minor components not limit the MAWP may or may not accomplish that- it just depends on who designs it and how.
""fabricators deliberately make major components such as heads thinner in order to make it the "limiting" component.""
AS A FABRICATOR: it is our requirement to furnish a vessel that meets the design parameters, specifications, purchase order, and ASME code.
If we specify a lower minimum thickness on a part that is formed and will have thining, we can do so as long as our minimum thickness meets the design calculations and the head manufacturer certifies that the head meets the specifified minimum thickness.
do you somehow feel like this is a bad practice?
AS A FABRICATOR: it is our requirement to furnish a vessel that meets the design parameters, specifications, purchase order, and ASME code.
If we specify a lower minimum thickness on a part that is formed and will have thining, we can do so as long as our minimum thickness meets the design calculations and the head manufacturer certifies that the head meets the specifified minimum thickness.
do you somehow feel like this is a bad practice?
Christine74
Mechanical
vesselfab: Nope, it makes perfect sense from a fabricator's perspective.
However, I'd bet that if more owner/users were aware of this practice, they would drop their "limiting component" requirements in a hurry.
-Christine
However, I'd bet that if more owner/users were aware of this practice, they would drop their "limiting component" requirements in a hurry.
-Christine
JStephen-
Yes... having more additional nozzle reinforcement benefits me as an owner/user. I generally don't get "increase the MAWP" type requests. Most of the time I get "We have a thin spot! Now what?" issues. More added opening reinforcement tends to help me with thin shells near openings. On the other hand, I don't agree with the 100% reinforcement crowd who won't allow any credit for excess thickness in the shell. I think that's taking things too far.
jt
Yes... having more additional nozzle reinforcement benefits me as an owner/user. I generally don't get "increase the MAWP" type requests. Most of the time I get "We have a thin spot! Now what?" issues. More added opening reinforcement tends to help me with thin shells near openings. On the other hand, I don't agree with the 100% reinforcement crowd who won't allow any credit for excess thickness in the shell. I think that's taking things too far.
jt
Suppose you have two options for the same identical vessel:
Option A, use 0.75" thick shell plate with MAWP limited to 300 psi by a nozzle.
Option B, use 0.625" thick shell plate with MAWP limited to 300 psi by the shell plate.
Now, if I'm understanding correctly, Option B would actually be preferred, even if Option A was cheaper to buy?
Option A, use 0.75" thick shell plate with MAWP limited to 300 psi by a nozzle.
Option B, use 0.625" thick shell plate with MAWP limited to 300 psi by the shell plate.
Now, if I'm understanding correctly, Option B would actually be preferred, even if Option A was cheaper to buy?
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