Hydrotest Pressure for high temp vessel
Hydrotest Pressure for high temp vessel
(OP)
Hi,
I have a vessel made of SA-516-Gr70N, the vessel max deign temperature is 500°F, the flanges located ont he vessel are ANSI 300.
Now the drawings show a MAWP of 600 PSIG and a test pressure of 780 PSIG.
ASME states that the test presure will be 1.3 times the MAWP times the ratio of the allowable stress at ambient over the stress at max temperature.
For this grade of material that ratio is 1 (20KSI/20KSI)
What has me concerned is the flanges on the vessel, they derate with temperature, but the hydrotest is not going to be completed at temperature.
So this is where i get fuzzy, I believe the flanges and fittings on the vessel would ahve been hydrotested int he factor before they were purchased, but im not
I have a vessel made of SA-516-Gr70N, the vessel max deign temperature is 500°F, the flanges located ont he vessel are ANSI 300.
Now the drawings show a MAWP of 600 PSIG and a test pressure of 780 PSIG.
ASME states that the test presure will be 1.3 times the MAWP times the ratio of the allowable stress at ambient over the stress at max temperature.
For this grade of material that ratio is 1 (20KSI/20KSI)
What has me concerned is the flanges on the vessel, they derate with temperature, but the hydrotest is not going to be completed at temperature.
So this is where i get fuzzy, I believe the flanges and fittings on the vessel would ahve been hydrotested int he factor before they were purchased, but im not





RE: Hydrotest Pressure for high temp vessel
To finish up im not sure if i need to test the vessel at a higher pressure to compensate for the fact that the fittings and flanges on the vessel derate with a temperature increase. if i test the vessel at the derated pressure but not at the derating temperature i have not really rested the fittings and flanges have i?
RE: Hydrotest Pressure for high temp vessel
I you are really worried about it you could check the MAP (new & cold) of vessel and hydro to that per ug-99 after reviewing reinforcement on openings.
but you are doing exactly per code so why worry.
what is operating and design pressures/temp?
RE: Hydrotest Pressure for high temp vessel
But it is being tested at 780PSIG and ambient temperature, maybe 65°F
So for the vessel itself it is fine, but im kinda concerned about the fittings and flanges, those are not really being tested as they derate with temperature... If the shell material derated with temperature then you would increase the test pressure accordingly, but when its the fittings and flanges i dont know if this is necessary.
RE: Hydrotest Pressure for high temp vessel
UG-99 (b) "... a hydrostatic test pressure which at every point in the vessel is at least equal to 1.3 times the maximum allowable working pressure to be marked on the vessel multiplied by the LOWEST ratio (for the materials of which the vessel is constructed) of the stress value S for the test temperature on the vessel to the stress value S for the design temperature..." (Emphasis mine)
No reading this tells me that i need to look at the stress ratio for all parts of the vessel and multiply the lowest ratio by test pressure.
What concerns me here is that say for some odd reason you fabricated the shell out of SA-516-70N and the heads were fabricated out of SA-240-348, and your operating temperature was 500 °F in this case the lowest ratio of stresses would be from the 516-70N with a ratio of 1 (for those temperature ranges) however the 24-348 has a ratio of 1.33.
Does this not mean that the test pressure is not really testing the head material as it should be tested at a higher pressure due tot he differences in the stresses?
I hope ive misintepreted the code in some way here.
P.S. please note ive used these materials as a random example, I am not necessairly saying that this exact situation would happen, im just saying that the code seems to overlook this potential in a very serius way, or ive misunderstood the code.
RE: Hydrotest Pressure for high temp vessel
RE: Hydrotest Pressure for high temp vessel
RE: Hydrotest Pressure for high temp vessel
TGS4 is correct in the interpretation of the code, but I believe his information to be incomplete. When determining the hydrostatic test pressure, you must consider the *limiting* component. In your example of dissimilar materials in the head and the shell, your calculation for the vessel should be based on the known weakest component. While not explicitly stated in the code, this falls under the code requirement of "competent engineer." I believe Compress performs this check by default. Come to think of it, I'm not sure there is a way to turn it off.
Fegenbush
RE: Hydrotest Pressure for high temp vessel
I completly agree with you, but i must say this seems like a very big error in the code. by stating to multiply by the lowest ratio, the code is explicitly telling you to design for the stronger material, and ignore the weaker material.
For myself i know i will never sign off on a vessel that does that, however i would love to see ASME address that section of the code as it is very misleading, hell its wrong as far as im concerned.
RE: Hydrotest Pressure for high temp vessel
RE: Hydrotest Pressure for high temp vessel
The standard flanges and fittings are fine to use as part of the vessel, but dont they then become part of the vessel once they are welded on, and hence need to be included when hydrotesting the vessel, as they may be the weakest part of the vessel? the flanges and fittings are probably tested at the factory, but just like with a pressure piping system, that does not relieve the responsibility of testing once fabrication is complete, so i think it still leaves the same problem with the code intepretation, by using the lowest ratio the flanges and fittings are still not being tested to the maximum stresses they will see in service, its kind of relying on any factory test to have caught defects.
P.S. im not trying to be argumentative, im just ahving trouble wrapping my head around this one and i want to make sure im proceeding correctly, cause i know once i bring this up im going to face alot of internal questions as to whjy im saying the vessels need to be tested to higher pressures in some cases.
RE: Hydrotest Pressure for high temp vessel
you can calculate the MAP (new & cold condition)
and hydro at 1.3 times that per ug-99
have you looked at doing that if you are so concerned about testing the flanges at higher pressure?
RE: Hydrotest Pressure for high temp vessel
That would be my preference, my main issue is at this point the fact that the vessel was not hydro-tested to the flange limits, but the shell limit, the code seems to say this is correct but it seems like a mistake to me, but since the vessel is already fabricated and tested id like to have all my ducks in a row before i go making waves saying that i think its wrong.
Right now the code seems to say that what was done is correct, but as fegenbush pointed out there is an expectation that the vessel will be designed by a competent engineer and that competent engineer would test to ensure the weakest parts of the vessel are sufficient.
Really im looking for somethign to support or disprove my thoughts on this matter, at this point it is not a fabrication issue to solve expediently, rather its a go forward design issue to hopefully avoid in the future.
RE: Hydrotest Pressure for high temp vessel
But first of all let me observe that what you are stating for the flanges could be equally right for any other vessel parts (e.g.nozzle necks) if fabricated with a material whose high temp properties decay faster than the material used for the shell (or even viceversa, a nozzle neck with flatter properties could dominate over a shell with faster decay!).
The point I think you are missing is that the hydrotest pressure is necessarily the result of a compromise: we want to test at the highest possible pressure, as this is safer, but with a confidence close to 100% that we won't ruin one vessel after another one, because of excessive distortion.
That's why the code requires the lowest ratio of cold/hot allowables. That lowest ratio is the maximum that insures no damage is incurred by the vessel due to hydrotest.
Here I don't follow you and I think you are misinterpreting the function of us engineers: we are not here to decide what is safe and what is not, after all an hydrotest factor of 1.35 would be safer than 1.3, no? Where do we stop? In fact our duty is to certify that our vessel is compliant with the rules of the code enforced by law, and that code is supposed to be, based on experience and on the discussions over decades that led to its formulations, simply the transposition of a sufficiently safe (not more not less) set of rules.
prex
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RE: Hydrotest Pressure for high temp vessel
You have a very good point, and i agree its not the position of the engineers to keep making the requirements more and more stringent, and im not suggesting that because the vessel has flanges and fittings that it be tested to 1.5 as per the ASME B31.3.
But it does seem to me that we should test to the weakest part of a vessel, not the strongest. If there is a defect in the flange on a vessel i would rather find it during hydro than during operation.
My understanding of hydrotesting is that it is meant to simulate the worst possible occurence, because those conditions actually happen. In this situationt he vessel shell has been tested to 1.3x the MAWP, but the flanges have only been tested to 1.05x MAWP.
to me that just seems to be alot of safety margin cut out.
RE: Hydrotest Pressure for high temp vessel
Note also that the primary mode of failure of flanges is leakage rather than rupture. It's for this reason that many owners specify that they'd like their designs limited by flanges rather than by shell or nozzle thickness etc.
If you look at the temperature/pressure rating curves for B16 flanges, you'll notice that they're steeper than the stress value versus temperature curve for the materials the flanges are constructed from. There's good reason for that too.
RE: Hydrotest Pressure for high temp vessel
what I see is that a 300# flange has a pressure limit per ANSI Stds,
(this flg maybe good for the 600# design-(I do not have the Code with me),
and no, not all flanges and fittings are factory tested for pressure.
you are forgeting the ASME safety factor, means that the flange is good for up to 3.5 times its calculated pressure.
an example: some smls pipe is Code good for 850 psi the same Code diff. Sect: the factory test should be at 2,500 psi not hot but cold,
What is this telling us?
ASME Code is also based in Eng experience, if it is mfd to Code, it will not blow up.
RE: Hydrotest Pressure for high temp vessel
Here is footnote 37.....
37 When a pressure vessel is expected to operate at more than one
pressure and temperature condition, other values of maximum allowable
working pressure with the coincident permissible temperature may be
added as required. See UG-20(b).
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RE: Hydrotest Pressure for high temp vessel
I thought of what you are saying, but given that we dont test piping systems to 3.5x a B31.3 system will be tested to 1.5x. so the idea that the flanges are designed for so much more therefore we dont need to actually test them to the service conditions seems kinda wrong to me.
I do totally understand that for all practical considerations this likely makes no difference, but still it keeps bugging me that you have two parts of a vessel, one part is rated to 740 PSIG @ 100°F and 600 PSIG, @ 500°F
and yet when you pressure test you only test @ 600 x 1.3 @ 100°F when the actual operating condition is @ 500°F
look at it this way if the flange were to be tested alone for those service conditions you would test it @ 740 PSIG x 1.3 @ 100°F, but because we are welding it on to a vessel now it only has to be tested at 600 PSIG x 1.3 @ 100°F ????
it just seems wrong to me, welding the flange to the vessel does not make the flange stronger, more reliable or less prone to defects which is why we test it in the first place.
RE: Hydrotest Pressure for high temp vessel
did you ever check the map new and cold.
if you did the vessel would most likely get up to around 700-725 maybe 740 psi.
if you do this....then you could test at 1.3 X that map
otherwise....the fabricator did exactly what the code requires unless otherwise instructed per spec to test per ug-99(c)
without checking the head, shell, and reinforcement of the vessel, you can not test higher than 1.3 x mawp X stress factor.
it is the way it is in vessels.
in pipe...it is known that the pipe and fittings are much higher rated than the flanges and the flanges set the maximum test.
in vessels....we do not make those assumptions and are not allowed to test per the flange ratings unless the vessel is checked.
in this case the hot flanges set the mawp of the vessels.
check it all cold and uncorroded and you may get your wish to retest.
I doubt very seriously if the "user/owner" of the vessel will see it you way...but you can always go in his office and jump on his desk and say this vessel is designed and tested incorrectly.
RE: Hydrotest Pressure for high temp vessel
in pipe...it is known that the pipe and fittings are much higher rated than the flanges and the flanges set the maximum test.
That's NOT always true, for certain. There is no requirement in B31.3 for instance that piping design be limited by flanges.
Again, as far as I understand it, the ratio of stresses for the test is for the strongest material at temperature- even if this is the flange BOLTING.
Yes, the hydro can be a truly wimpy test- for some of the components. But there's no point in putting some components into yield during the test in order to test other components at something closer to what they'll experience in service at temperature.
RE: Hydrotest Pressure for high temp vessel
I have never seen a pipe spec written by an experience engineering company, for refinery/chemical use that the flanges were not the limiting factor.
But then again, I have not seen every pipe spec written.
would love to see an example of one...attache one please.
I never said that was a requirement in B31.3. I was talking practicallity and reality
RE: Hydrotest Pressure for high temp vessel
I know there are situations where a vessel or part may be designed to yield, as the yielding of the materail gives you different properties and such.
But aside from that situation, when you have a vessel that is not intended to yield under operational conditions, wouldn't you want to find out during test if your weak link is actually a weak link? id rather have the inlet flange yield during test than find out during operation that it was a defective flange.
I guess though from the responses to this thread i am far more conservative about this than most people are, but i just cant help thinking about the what if scenario, and since i know the flange WILL see a more severe pressure/temperature combination than the test simulates it is a concern to me.
RE: Hydrotest Pressure for high temp vessel
If you cause the yielding of a flange during the test, then your flange will have a beautiful chinese hat shape, and you'll have to cut it (if it's not a cover) and replace it: so what's that for?
prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads
RE: Hydrotest Pressure for high temp vessel
I guess in that case you'd consider my company not to be an "experienced engineering company for refinery/chemical".
We, like the code, permit any component to limit. We also call out what component limits.
RE: Hydrotest Pressure for high temp vessel
RE: Hydrotest Pressure for high temp vessel
http://www.osti.gov/bridge/basicsearch.jsp
If you want further information specific to your question, you may find it within this search engine.
Here is an excerpt from one of the documents I found from the following document:
http:/
.............Sections I and VIII-1 of the ASME Boiler and Pressure Vessel Code, and B31.1 Code for Power Piping, are all based on the principle of maximum primary membrane stress, whereby the wall thickness is established by limiting the hoop stress to a predetermined maximum value based on the yield strength, ultimate strength, or creep rate of the material at temperature. Secondary stresses are taken care of in the pressure vessel codes by design rules which specify certain dimensional requirements which hold such stresses at a safe level consistent with experience. Sections I and VIII-1 do not call for a detailed stress analysis but merely set the wall thickness necessary to keep the basic hoop stress below the tabulated allowable stress. They do not require a detailed evaluation of the higher, more localized stresses which are known to exist, but instead allow for these by the safety factor and a set of design rules. The simplified procedures of Section VIII may be in error by being over conservative on one side and on the other side being inapplicable for the more severe types of service. Detailed analysis of almost any Code vessel would show where the design could be optimized to conserve material.
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RE: Hydrotest Pressure for high temp vessel
Are you concerned about the bolting too? see proposal....
http://cstools.asme.org/csconnect/pdf/R08697.pdf
should there be separate requirement for a range of materials/types/items etc.....?
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RE: Hydrotest Pressure for high temp vessel
My thought is id rather replace a flange after test than an hour after start up, how is it better to replace it then?
But still my point is not that i want to test the flanges past their design point, but that iw ant to test the flanges based ont he operating conditions.
Let me try again to outline the scenario im looking at, maybe i ahvent explained it clearly, god knows thats common enough an occurence for me.
Vessel
Design Pressure 600 PSIG
Design/Operating Temperature 500°F
Now the flanges are rated for 740 PSI @ 100°F
However at 500°F (the operating temperature) the flanges are rated to 600°F, this is as per B16.5 because i would presume the flanges are weaker at higher temperatures.
The shell material is A-516-70N
This particular material does not derate due to temperature until it is past 500°F
Now the test pressure is 1.3 x 600 (780 PSIG)
BUT the test temperature is 100°F
Because this flange is attached to a shell that does not derate with temperature the test pressure at the test temperature was 780 PSIG.
BUT if the flange were attached to a vessel witht he same temperature derating as the flange the test pressure would have been 1.3 x 740 PSIG = 962 PSIG
As i understand it part of the reason for a hydrotest is to discover faults int he materials or fabrication before the vessel goes into serviceand for this reason you test the vessel at a pressure to simulate the operating pressure with a specified safety factor.
If the vessel is made of material that derates with temperature you test to a higher pressure since the testing is done at ambient temperature, and not the elevated operating pressure.
But for some reason the code states that if you have one part of your vessel that does not derate with temperature, and another that does, you do not ahve to account for the deration in the test pressure.
I really cant help but feel like i am missing somethign glaringly obvious, or just not explaining myself properly.
Maybe an extreme silly example would make my point a little clearer, if you had some material X that you use for the head or flange or whatever of a vessel, that is rated to 10,000 PSIG at ambient temperature, but is rated to 100 PSIG at 500°F and you plan to operate at 500°F and 100 PSIG.
If another part of that vessel is 516-70N then by the wording of the code you only ahve to test at 130 PSIG.
Now suppose for a minute that there was a mistake in the fabrication of the vessel head made of material X and it can only hold half its rated pressure, 5,000 PSIG at ambient. you have just tested it to 130 PSIG, and it passes with flying colours.
But then a week later you put it into service at 500°F and 100 PSIG, and the vessel ruptures because of the defect it could only hold 50 PSIG at temperature.
Basically the end result is you tested that material X under conditions it was very likely to pass in, but then operated it under conditions where it was much mroe likely to fail.
I know this is an exaggerated example, but i believe the point still applies.
In my real world example if the flange were to be considered by itself then the test pressure for that service was 182 PSIG lower than it should ahve been (780 vs 962) but because its welded on to a 561-70N shell we say that the 780 test pressure is good even though it does not simulate the operating conditions withthe appropriate safety factor?
RE: Hydrotest Pressure for high temp vessel
please don't try that unless you are very thick skinned
RE: Hydrotest Pressure for high temp vessel
I know im kind of alone here in my thoughts on this, and im not trying to be difficult, i really do appreciate and value everyones opinions on this, i just may be too stubborn/conservative in the end here.
RE: Hydrotest Pressure for high temp vessel
Regards,
Mike
RE: Hydrotest Pressure for high temp vessel
Most of the owner specs I've seen consider flanges to limit- until you consider the valves in the spec...
We design, fabricate and test to code, using engineering judgment to fill in the blanks. Our specs make clear what we're doing, and what we're NOT doing.
RE: Hydrotest Pressure for high temp vessel
http://
Perhaps any Owner/User with the same concerns as yourself can follow-up w/FEA if not comfortable with the Code criteria as written. or follow the interpretation below as ASME has recognized this will happen....
Interpretation: VIII-79-13R
Subject: Section VIII-1, UG-99(b) and UG-99(d)
Date Issued: December 29, 1982
File: BC78-847*, BC81-433*
Question (1): Is it the intent of UG-99(b) that the highest ratio of stress values should be used provided that no other part of the vessel would be subjected to a stress exceeding 90% of the material yield strength?
Reply (1): The requirements of UG-99(b) call for the minimum hydrostatic test pressure to be 1-1/2 MAWP multiplied by the lowest ratio of stress values. The requirements of UG-99(d), however, do say that Section VIII, Division 1, does not specify an upper limit for hydrostatic test pressure.
Question (2): Concerning the "lowest ratio of stress values" of UG-99(b), may the lowest ratio be applied even for vessels for which the ratios of stress values are significantly different as could result from using different materials or different design temperatures for various parts?
Reply (2): Yes.
Question (3): Assume that the ratios of stress values are significantly different as described in Question (2) and that it is desired to test to 1-1/2 times the MAWP multiplied by the highest ratio of stress values permitted by UG-99(d). Should the manufacturer consider the need to increase the thickness of parts having lower ratios of stress values?
Reply (3): Yes.
Note that this interpretation states an incorrect hydrostatic test pressure multiplier. The current UG-99(b) multiplier is is 1.3 instead of 1.5.
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RE: Hydrotest Pressure for high temp vessel
You are awesome, this completly addresses my question and at least my intepretation of their response is that the wording of the code does leave a potential problem, hwoever as others ahve pointed out there is an expectation of a competent engineer reviewing the desing such that if there were to be a MAJOR issue arising from different materials being used than the designing engineer may at their discretion test to higher pressures.
In short follow the code but dont do so blindly.
RE: Hydrotest Pressure for high temp vessel
In fact you could think that codes and standards are there only or mainly to enforce safety, but that's not exact. If safety was the only concern, there would be no need for codes: designers would take all the margins they deem necessary, and that would possibly give a safety higher than what we are accustomed to. But of course vessels would cost much more, and here comes the point.
In fact the true reason for the existence of codes and standards is economy: they are there to find an acceptable compromise between cost and public health. In fact what IMHO you miss, is that our world cannot be perfect (100% safe) because perfection simply does not exist. Our modern societies may exist and continue to grow (till when is a completely different issue) only if economic constraints are well respected, and that's the main function of codes.
So you should be prepared to accept the compromise implied in the code and be satisfied with that.
In other words: you are worrying about a component that could have a defect that is not shown by a too mild hydrotest. But you should also worry, on the other side, of the possibility of destroying many other vessels with a very conservative test, where those other vessels could have serviced correctly: this is economics!
prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides
http://www.levitans.com : Air bearing pads
RE: Hydrotest Pressure for high temp vessel
Thats a good point and i guess i was not really looking at it quite that way.
It really does come down to engineering judgment, if for example the vessel shell was a standard plate thickness rolled and that standard thickness was more than enough to accomodate the higher test pressure then i cant see any reason why you would not test to the flange limit, on the other hand if this was a very large vessel where you would ahve to go up tot he next plate size potentially adding tens of thousnads in materials and labour costs then it would make much more sense to test as per the code witht he knowledge that the flanges most likely are more than adequate for the service.
RE: Hydrotest Pressure for high temp vessel
-345.4.2 Test Pressure. Except as provided in para.
345.4.3, the hydrostatic test pressure at any point in a
metallic piping system shall be as follows:
(a) not less than 1.5 times the design pressure.
(b) when the design temperature is greater than the
test temperature, the minimum test pressure, at the point
under consideration, shall be calculated by eq. (24).
When the piping system contains more than one material
or more than one design temperature, eq. (24) shall
be used for every combination, excluding pipe supporting
elements and bolting, and the MAXIMUM calculated
value of PT is the minimum test gage pressure.
This is of course limited by
(c) if the test pressure as defined above would produce
a nominal pressure stress or longitudinal stress in
excess of the yield strength at test temperature or a
pressure more than 1.5 times the component rating at
test temperature, the test pressure may be reduced to
the maximum pressure that will not exceed the lesser
of the yield strength or 1.5 times the component ratings
at test temperature.
May i add a question (new thread? sorry..first post here)
-are we correctly testing B31.3 products?
-It seems impracticle to always calculate max test pressure for all components then recalc all components pressure stresses to limit test pressure if needed...
RE: Hydrotest Pressure for high temp vessel
For piping systems first of all by my understanding you are doing your tests correctly, secondly it has been my experience that this si much less of an issue with piping than it is with pressure vessels.
typically you will be working with a piping specification and that is already taking all of these factors into consideration, what i ususally see in a piping system is 300# design, you test to 4960*1.5 kPa = 7440 kPa then of course you set your design pressure to whatever the maximum is for that pressure class at the temperature your operating at. If your working with a B class pipign spec, (300#) then just test it to the max of the spec at ambient, everythign in the spec is designed for that pressure, then you are done nice and simple.
I guess my point is ive seen few (read none personally) situations where the piping is limited by a component like that, the specs are usually cohesive, whereas with a pressure vessel each one can ahve different materials and the fittings and flanges themselves are from a piping spec, not from a "vessel spec" so you run into this more with vessels.
RE: Hydrotest Pressure for high temp vessel
RE: Hydrotest Pressure for high temp vessel
RE: Hydrotest Pressure for high temp vessel
(c) if the test pressure as defined above would produce
a nominal pressure stress or longitudinal stress in
excess of the yield strength at test temperature or a
pressure more than 1.5 times the component rating at
test temperature, the test pressure may be reduced to
the maximum pressure that will not exceed the lesser
of the yield strength or 1.5 times the component ratings
at test temperature.
I now see our interpretation of 2006 was not right. I guess thats why they changed it!
RE: Hydrotest Pressure for high temp vessel
The B31.3-2008 testing requirements are new, and stricter than those in section VIII.
Paragraph c) from 2006 that you've quoted above is still there in the 2008 version- it's just going to be relied upon a lot more than it once was due to the change in b) noted above. b) will now call for higher test pressures for multi-component hot piping, and c) will now determine the maximum test pressure quite often in our business!
RE: Hydrotest Pressure for high temp vessel