ASME B.31.3 Pressure Test Limits

[Giuss]

Hi,
ASME B31.3 calculation for Hydrostatic Pressure Test can be limited follows:


(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. [See paras. 302.3.2(e) and (f).] For
metallic bellows expansion joints, see Appendix X, para.
X302.2.3(a).
Similiar reccomendations are made by other piping design codes (e.g. EN13480).

Which thickness do you use to verify that induced stress by test pressure will not exceed the yield strength on a line?

a) Nominal Thickness of installed pipe
b) Nominal Thickness of installed pipe without mechanical allowances
c) Nominal Thickness of installed pipe without corrosion allowance and mechanical allowances

or

d) Pipe Wall Thickness determined by Design Code Formula considering the Design Condition of the line to be tested
e) Pipe Wall Thickness determined by Design Code Formula considering the Design Condition of the line to be tested plus mechanical allowances
f) Pipe Wall Thickness determined by Design Code Formula considering the Design Condition of the line to be tested plus mechanical allowances plus corrosion allowance

not considering tolerances, I Grouped the abovementioned approaches in two main group,
the first one considers Installed Thickness, the second one is based on ''the minimum thickness by calculation i could have specified '' i think the code is not clear on this matter.

If Pipe to Pipe Branch Connections are present on the circuit you're going to test, which is your approach?

Please,
Let me see as many approaches and considerations you have

Thank you in advance

Giuss

........

 

[BigInch]

I've always considered "mechanical allowances" as something extra to precise code determined wall thickness for those loads that are sometimes hard to quantify with a design load case scenario, but you know you'll get them someday, ie. like a fork lift hits it during some maintenance operation, etc.

Pressure (allowables and/or for test) are determined based on outside diameter and thickness of the wall of any pipe in the system, or segment to be tested and are without tolerance considerations and without mechanical allowances, minus corrosion allowance.

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[Giuss]

BigInch,

for mechanical tolerance i meant thickness fabrication tolerance accoridng to fabrication std(e.g. +/- 12.5% for ASTM A 106) or component std (e.g. +/- 12.5% for Buttwelding fittings according to ASME B.16.9) or threading depth if any.
With reference to the last part of your comment, i'm sorry but sincerly i didn't understand what you wrote.
Anyway Thank you

Giuss

 

[BigInch]

you said two things, "allowance" and "tolerance". I don't consider them the same things. Allowance is wall thinckness I add, because I want to. Tolerance is a variation the code allows in pipe dimension.

Use the listed wall thickness,
do not consider tolerance and
subtract the corrosion allowance.

Was that clear?

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[BillBirch]

To cover the worst case stress for your selected size, you should use the largest OD (upper tolerance of OD) and the lowest wall thickness (nominal less 12.5%). Or, you could use actual measured values, but I would not as you cannot be sure that you have found the thinnest section. My first suggestion also allows you to plan your test before delivery of pipes and fittings, knowing that any component delivered that is to specifications will be better than your "worst-case" assumptions.
Regards,
Bill

 

[Giuss]

Ok,
now it's clear why i didn't understand:

I have not considerations about definitions of allowance or tolerance as you have, i simply use those indicated in the design code i'm referring in the post subject, designing according a code is not a philosophical matter,

from Asme B.31.3:
302.4 Allowances
In determining the minimum required thickness of
a piping component, allowances shall be included for
corrosion, erosion, and thread depth or groove depth.
See definition for c in para. 304.1.1(b).

Manufacturer's Fabrication Tolerance is not a variation allowed by design code, infact the design code (ASME B.31.3 in this case) clearly states you have to consider it in pipe wall thickness calculation:

304 PRESSURE DESIGN OF COMPONENTS
304.1 Straight Pipe
304.1.1 General
(a) The required thickness of straight sections of pipe
shall be determined in accordance with eq. (2):
tm = t + c (2)
The minimum thickness, T, for the pipe selected, considering manufacturer's minum tolerance, shall be not
less than tm.
(b) The following nomenclature is used in the equations
for pressure design of straight pipe:
c p sum of the mechanical allowances (thread or
groove depth) plus corrosion and erosion
allowances. For threaded components, the
nominal thread depth (dimension h of ASME
B1.20.1, or equivalent) shall apply. For
machined surfaces or grooves where the tolerance
is not specified, the tolerance shall be
assumed to be 0.5 mm (0.02 in.) in addition to
the specified depth of the cut..............................
..................................

So it's impossible not to consider it(one could follows the approach suggested by BillBirch, but it's impraticable as i'm referring to a plant with more than 3000 piping lines).
Perhaps, on the contrary, i could non to consider Corrosion Allowance effect and so not to subtract it, as Hydrostatic Test wil be performed during precommisioning.

Anyway not considering, tolerances, corrosion allowance and so on, the main question is another:

ASME B31.3 Formula for Hydrostatic test pressure is as follows:

P=1.5*Pdesign*(Allowable Stress at Test Temperature/Allowable stress at Design Temperature)

but the Code says i can lower it in case the value calculated as per above will produce an induced stress in my piping system equal or above the yield strenght.
Now if the calculated pressure will not cause that induced stress for the thickness i selected (e.g. 12.7mm) but it would cause it for the thickness calculated as per asme B31.3 according to the line Design conditions (e.g. 7mm) may i lower the test pressure???????
I mean, as i could have installed a 7mm thickness pipe according to the Design Code, may i reduce Hydrostatic Pressure Test???

Thank you again

Giuss

 

[BillBirch]

Giuss,
You are tackling this the wrong way around. My earlier suggestion was based on the assumption that you wanted to test one problematic pipe. Now that you tell us you want an approach that satisfies the whole plant, here is the way forward.
Determine the design pressure of every line. This design pressure is used to size every component in the line. (Each component may contain more or less stress compared to each other but it does not matter - all components are safe to meet the design pressure).
Test each line at 150% of its design pressure, adjusted for temperature.
Your earlier approach of trying to determine a limiting test pressure based on calculating stress due to actual thickness is unproductive. All it means is that you try to nudge the test pressure closer to its allowable limit - but for what purpose? The 150% of design pressure is high enought to indicate any problems in your piping.
Another important consideration is your test crews do not have to be supplied with individual test pressures per line.
It is good that you are actually reading the Code as many presume certain things in it, but it appears that you are in need of some experienced guidance as it seems that you are trying to approach this task from first principals when it is a commonplace activity.

 

[BigInch]

Sorry for the confusion about mechanical tolerance. I used th term as something other than how it is actually defined in B31.3, which I eventually explained.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Giuss]

Guys,

i apologize if i could seem arrogant, but BillBirch my problem doesn't deal with code knowledge, it's a very unique comment i received by a client.
Sincerly, i'd like to stop polemics and i thank you (BigInch and BillBirch) for your comments, and i'd like to explain you in more detail which is the question and why i asked it:

We have a Piping Material Management software which includes Mechanical Line List and Mechanical Checks and calculation.
So we make the Pressure Test module run and the software writes on mechanical line list the value of regular and maximum Hydrostatic Test according to the design code,
Running the calculation according to ASME B31.3, the software limites the maximum hydrostatic pressure test to that value which would induce a stress equal to yield considering as thickness that deriving from the calculation according to Code considering line conditions;
As on Mechanical Line List are also indicated the thicknesses as specified per Piping Class (and they are obviously greater than those calculated for a single line, becouse there are a lot of other reasons to specify pipe wall thickness), my client commented that according to the installed thickness (those according to the Piping Classes), the pressure test was limited in a too conservative way, in fact the maximum value they found on mechanical line list would not induce a stress comparable with yield considering the thickness indicated on the same Mechanical Line List (those per piping class).

Sincerly, i don't know if he's right, the code doesn't specify which thickness i should consider, but, if we consider the hydrotest basically as a Trial Test (i'm not sure it's the right wording in english) probably he's right even if i don't understand why he'd like to test with a so high pressure.

What do you think?

Thank you again.

Giuss

 

[BigInch]

It sounds to me like this software is trying to do things it shouldn't be doing. The calculation procedure is backwards. Its trying to calculate test pressure based on wall thickness that, as you have found, may have been chosen by me

Design temperature and pressure should be specified according to the necessary process and control conditions.

Now you already know the required hydrotest pressure as 1.5 X.

From those conditions the MINIMUM wall thickness should be determined by code equations for each pipe in that system + maybe some for the engineer's experience factor wall "thickness fat" required. Let's say I chose to add 0.5 inches just because I think some bullet is going to hit it some day. (Happens a lot on x-country pipelines)

How's this software going to handle my arbitrary addition of 0.5"?

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Giuss]

BigInch,
your understanding is correct;
the software calculates again the wall thickness and uses it to verify code limits instead of considering the selected thickness.
Your advice is ok, the problem is i can't modify the software (or in case it would take too much time) which is customized within the group (i am in a multinational company), so if i'd be sure our approach is not against the code, the easiest would be to answer the client that his comment is not accepted.

Thank you again

Giuss

 

[BigInch]

Great! That's my customary response too. Not too many clients with acceptable experience these days either.

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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain