PLEASE CLEARING MY DOUBT FOR HYDRO TEST CALCULATION VARYING TEMPERATURE 4

[sathish kumar dhanasekaran]

i am conducting in asme a106GR B 3/4 INCH PIPE SCH 160-S hydro testing operating temperature of pipe is 7kg/cm2 and 170 DEGREE CENTIGRATE but design pressure is 20kg/cm2 and temp 330 DEGREE CENTIGRATE how to calculate test pressure. AS PER ASME SEC B31.3 CALCULATING OF PRESSURE MAXIMUM 1.5 ... BUT HERE TEST PRESSURE IS 35KG/CM2 HOW TO THEY ARE CALCULATING I DONT KNOW...

 

[LittleInch]

They are allowing for the temperature at the design temperature / operating temperature by increasing the pressure at test temperature.

See b below

345.4.2 Test Pressure. Except as provided in
para. 345.4.3, the hydrostatic test pressure at every
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 using
eq. (24).

Pt = 1.5 x P x (St/S) (24)
where
P = internal design gage pressure
Pt = minimum test gage pressure
S = allowable stress at component design temperature for the prevalent pipe material; see
Table A-1 or Table A-1M
ST = allowable stress at test temperature for the
prevalent pipe material; see Table A-1 or
Table A-1M

As St is more than S this will increase test pressure.

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[661660]

Dear LittleInch, thanks for your wonderful explanation,
please explain , how code has derived factor 1.5 ?

Regard's
Aaksh

 

[LittleInch]

I don't know for sure, but I think it came from pressure vessel testing where 1.5 emerged over time as the best figure between safety and making the pipe or vessel excessively thick.

Pipe in plants has other forces other than internal pressure, wind, snow, ice and people climbing on it, valve weights etc.

Pipelines on the other hand don't and hence their pressure tests can be as low as 1.1 times MAOP, but usually min 1.25.

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Also: If you get a response it's polite to respond to it.

 

[TGS4]

That 1.5 factor is not derived. It was decided by a group of interested individual engineers (collectively known as the Code Committee), based on their experience and expertise.

 

[moltenmetal]

The 1.5 factor is also inherent in the selection of the safe allowable stress values used for design in the code, which are based on a certain fraction of UTS and yield for the materials. ASME VIII, which uses a factor of 1.3, has different safe allowable stress values as a result, unless I recall incorrectly (and I might- I defer to those who, unlike me these days, work with these codes and standards so frequently that the chapters and verses are tatooed to the back of their eyelids).

The ideal nondestructive test doesn't put any component into yield, and in fact the code mentions that the test pressure must be adjusted to avoid putting components into yield.

 

[LittleInch]

I think it is partly because B 31.3 allows up to 30% over pressure for limited periods whereas I believe ASME VIII allows zero allowable or no more than 10% ?

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Also: If you get a response it's polite to respond to it.

 

[TGS4]

moltenmetal, chapters and verses are not necessarily tattooed for me, but I do deal with it day in and day out.

The design margins for ASME Section VIII, Division 1 is as follows:
Yield - 1.5
Ultimate - 3.5

The design margins for ASME B31.3 is as follows:
Yield - 1.5
Ultimate - 3.0

The 1.3 factor on the MAWP for the hydrostatic pressure test (modified by the ratio of the allowable stresses - St/S) in Section VIII, Division 1 is a minimum factor. And please note that there is NO maximum. The sole requirement is that there is no permanent deformation deemed excessive by the inspector. Although good engineering practice would say that you should keep below the engineering yield - that good practice is only for the "general membrane" stresses. In fact, at local structural discontinuities (such as head-to-shell junctions, or at nozzles/manways), it is expected that yielding (sometimes significant yielding) will occur during the test. However, this yielding will be very localized, will not produce significant deformation, and will only have the effect of producing a compressive residual stress field.

I will note that the engineering yield value - generally stated above as "yield", is actually calculated using the 0.2% offset method, which means that a stress of that magnitude will result in a permanent strain of 0.2%. But almost all of our materials have strain hardening (cast irons and other brittle materials notwithstanding), so small amounts of permanent strains are expected (and large magnitudes above this value were likely imparted on the material with the forming.

 

[weldstan]

B31.3 was spun off from B31 (B31.1) in 1959. The 1.5 hydrotest factor was in stalled in B31.1 and still and assures that test pressure will not exceed the yield strength of the pipe materials. The hydrotest pressure should still assure that the yield strength of the material will not be exceeded in B31.3 (but barely). Of the thousands of hydrotests on B31.3 piping that I analyzed, I don't recall one where the stress induced was > than 90% of the SMYS of base material.

 

[LittleInch]

TGS4 - I believe the missing phrase here is "the lower of"....
For many pipe materials, the lower value is the 1/3 of UTS, hence it doesn't get near yield under test conditions.

It's also why high strength steels common on pipelines don't get a look in as piping because the allowable stress doesn't really go up enough to cover the extra cost of the steel as it's based on UTS which doesn't increase at the same rate as the yield stress.

And one steel pipe looks like another so the risk of using the wrong pipe goes up.



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

 

[TGS4]

LittleInch - you are exactly correct. Thank you.

 

[moltenmetal]

A great discussion on a topic one would think Eng-Tips had beaten to death ten times over already- and one which proves to me the continuing value of these forums. Thank you for your contributions, esteemed TipMasters!