Hydrotest Pressure for high temp vessel 10

[jmiles]

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

 

[moltenmetal]

vesselfab said:

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.

 

[vesselfab]

REALLY?

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

 

[jmiles]

Moltenmetal,

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.

 

[prex]

Again, can't follow you, jmiles.
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

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http://www.levitans.com

: Air bearing pads

 

[moltenmetal]

vesselfab:

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.

 

[vesselfab]

can you attach a copy of spec with names removed?

 

[CodeRef]

I have found the following site to be a valuable resource.

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://www.osti.gov/bridge/servlets/purl/4031361-6sUmgy/

.............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.


FAQ731-376

 

[CodeRef]

jmiles,
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.....?

FAQ731-376

 

[jmiles]

Prex,

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?

 

[vesselfab]

well the designing engineer could always spec out the use of stab in immersion heaters for test fluid to get it up to design temperature if he had enough money in the project to pay for it.

please don't try that unless you are very thick skinned

 

[jmiles]

lol no i cant see ever specifying out heaters for a hydrotest, that would be pretty silly.

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.

 

[SnTMan]

jmiles, Code specifies minimum test pressure only, you are free to test as high as you like. Carefully I hope

Regards,

Mike

 

[moltenmetal]

vesselfab: no, I won't be posting any of our specs online!

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.

 

[CodeRef]

Does this from PVEng help in any way? ...........

http://www.pveng.com/m/content/article.php?content_id=283

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.




FAQ731-376

 

[jmiles]

Code Jackal,

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.

 

[prex]

jmiles, I think that you are still missing a point, that I already recalled in my first post above: it is compromise.
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

 

[jmiles]

Prex,

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.

 

[JAYDEE23]

I have been on this subject myself in B31.3-2008. We currently test to lowest ratio of all components for B31.3 construction however para 345.4.2 states

-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...

 

[jmiles]

Jaydee,

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.

 

[moltenmetal]

jaydee: thanks for noting a VERY important change between B31.3 2006 and 2008!