Test Pressure for Equipment designed: FULL VACUUM + CREEP Conditions

[Ganfoss2]

Dear all,

in our factory we have a huge equipment designed for Internal (very Low) Design Pressure and for a Full Vacuum Condition, both at a Design Temperature by which creep phenomea occours (T Des > 580°C).

Nominal Thickness of the VEssel's Components are suitable for the Vaccum Condition and are more/less twice more than the Required Thickness for an Internal Pressure and this lead to a MAWP (Max. Allowable Working Pressure) for the INTERNAL PRESURE well above the Internal Design Pressure.

The Design Code of the Vessel is ASME Div. VIII Sect. 1 + PED 97/23/EC and the value of the (Hydro)Test has been evaluated in full accordance with the requirements stated in UG-99(b) in leu of PED requirements. Further more we have "ASSUMED" the MAWP EQUAL to the Internal Design Pressure, as allowed by the VIII-1 Code.

The Final User of the Vessel disagree to perform the test in accordance with UG-99(b) with the Design Pressure in lieu of MAWP and want to test it at the HIGHEST value of test pressure as by PED 97/23/EC (using the MAWP for the INTERNAL PRESSURE).

We have demostrated that such value of the Test Pressure is not prudential since we risk to damage the Equipment at that pressures. Our value are in accordance with the ASME Code and we can give an adequate demostration of strength of the Vessel.

Please note that with "DESIGN PRESURE + UG-99(b)" the Test Pressure is 3 time the Internal Design Pressure! Further more, the FULL VACUUM Design prevail on the INTERNAL PRESSURE DESIGN Design!

With UG-99(f) we have resolved to test the equipment for the FULL VACUUM Design while we are not able to convince the Final User that we are at the right side!

I tried to look in the INTERPRETATION and in the CODE CASE, but ASME VIII-1 do not SPECIFICALLY address the requirements of UG-99(b) compared with the Design with Creep.

Do somebody has an Idea?

Many Thanks in advance.

ANN



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I have also used this position, but it wal unsuccessfull:

In the evaluation of Test Pressure, the MIN or MAX value of the Stress Ratio play a key role: this ratio may lead to VERY HIGH TEST PRESSURE due to the fact that the allowable stress at high design temperature take in account the of the Creep Phenomena and to avoid a failure of vessel's component in SHORT TIME, the allowable stress at such temperature drammatically fall down (The allowable stress is based onthe stress that lead to a rupture after 100.000 hr or that do not cause high permanent deformation rate - it is not more based on the SHORT TIME Strength considerations tied to the Yield Stress at Temperature).

With the European Codes (PD 5500 Par. 5.8.5 and EN 13445-5:2002+PrA1:2006 Par. 10.3.3.1.(d)) has been reconnaised by a TEchnical Commission that the level of the test pressure has NO RELEVANCE to the safety of the vessel with respect to creep behaviour.

Under this "aspect" the value of the allowable stress at the design temperature in the Stress Ratio should be replaced to a TIME INDIPENDENT value: a criteris is to get/evaluate the yield stress (Sy) at the design temperature and divide it by the Code Safety Factor (Ex. 1.50).

This lead to a lower value of the Test Pressure and reduce the risk to damage the equipment during the Test Phase. Meaning time it is considereed an adequate demonstration of strength.

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[prex]

Why not demonstrating by calculations whether or not the vessel will incur a damage in test conditions?
This is normally done by using as allowables 90% of yield stress at test temperature for general membrane primary stresses and 135% for membrane+bending primary stresses (limits stated in Div.2).
As you have already a full set of code calculations, that's an easy task. Only the limit on membranes should be of interest, unless you have flat closures or tubeplates.

prex

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[Ganfoss2]

Many thanks for your reply.

Already done! We have dimostrated that with ASME Test Pressure the Test is Safe, but with PED Test Pressure the Test is Unsafe for the Equipment (Damage Occours).

Whe haven't flat surfaces except for Standard ANSI B16.5 Components by which the ANSI Standar also cover the Hydrotest Pressure value (Par. 8.3) - They are not a problem.

As you have suggested, we have issued a report with the Hydrotest Verification Case, using Sm = 90% x Sy as allowable for the Main Bearing Pressure Parts (Cylindrical Shells, Transitions = Cones and Formed Heads).

We have also extended as allowable for the NON STANDARD Flanges (See App. 2, Par. 2.8).

We have also inserted the Opening (this is not required by the ASME Code - as prudential issue).

The Final User is still on its positions. In few words, under his opinion the Equipment should be designed for Hydrotest Condition, nor for the Design Condition.

 

[Greekbiker]

Your problem is that you are designing a vessel according to ASME (which is a non-harmonized code)and want to also certify it according to PED.
Although according to PED annex I, para. 7.4 (which is a law and cannot be avoided in the EU) the test pressure is the maximum of:
-"that corresponding to the maximum loading to which the pressure equipment may be subject in service taking into account its maximum allowable pressure and its maximum allowable temperature, multiplied by the coefficient 1,25"
-"the maximum allowable pressure multiplied by the coefficient 1,43"

You can try to base your deviation on the first line of annex I para.7 which states:

"The following provisions apply as a general rule. However, where they are not applied, including in cases where materials are not specifically referred to and no harmonized standards are applied, the manufacturer must demonstrate that appropriate measures have been taken to achieve an equivalent overall level of safety."

 

[rhg]

The wall thickness, in your case, is governed by full vacuum condition, hence the MAWP is full vacuum ( not internal pressure, not external pressure, full vacuum only !!!)
In your case the joint eff. is 1, and no RT performed

regards
rhg