MAP & MAWP in Hydrotest

"MAP" and "MAWP" are the same thing here. The test pressure is 1.3 times this value. What is the true problem - is it simply one of interpretation? What are the numbers involved?

Increasing the thickness of the vessel at this late stage sounds like "scrap it and start again".

Dear PWSlack,
Thank you for your reply.

Here iam having a corrosion allowance of 3mm. So the MAP & MAWP varies.
My problem is, i have selected the hydrotest as per UG-99B(1.3*MAWP).Now i have incorporate the comment 'hydrotest shall be performed as per code where MAP shall be used in leiu of MAWP"Here my hydrostatic pressure increases and vessel fails.
Iam also tried with increasing the vessel thickness. But u know, the MAP increases wrt increase in thickness with hydrostatic pressure.
I presume that you understood the scenario
Regards,
vGsh

• The design pressure is the MWP when the vessel is new.
• The MAWP relates to the corrosion of the vessel over time and its re-testing and certification at intervals under periodic re-tests witnessed by the Engineer/Surveyor from the company providing burst indemnity insurance cover on the vessel. As corrosion continues with time, the MAWP will reduce with time. At some point, renewals and replacements of parts of or the entirety of the vessel will become appropriate.
• The test pressure is 1.3 times either of the above depending on when the vessel is being tested and witnessed.
• It is far better for the vessel to fail its test with an incompressible fluid such as water than with process fluids while in use. A failed test indicates repairs or replacement need to take place before process fluids are re-introduced to the vessel.

Is there anything here that is unclear?

Dear Mr.Wijeesh,

During the Hydrotest medium of water temp. is appx. 45Deg.C i.e ambient temp. and stress value of Vessels shell/Head material is more than that of max.design temp.,for example SA 516 Gr.70 plate material's stress value at 45Deg.C is 1406.46 kg/cm^2 and 1040.78 kg/cm^2 for 400 Deg.c. So vessel will not failure until having any weld joints leakage. Please check weld joint strength before starting the hydro test.

Still you get any problem please ask actual stress value of plate material and do the strength calculation reversely.

Hope you Clear.

Thanks

Hajee

Do you mean it has failed the hydrotest? In what way? Did a welded joint fail, or something else? If the thickness and hydrotest pressure have been calculated correctly the metal stress on hydrotest would be well the UTS, so failure suggests very poor workmanship.

You're right that if you increase the thickness you also increase the calculated hydrotest pressure, so just as likely to fail the test.

Dear All

Kindly ignore the previous discussion.

Sorry, the vessel is failing for hydrotest in design and not at the shop.

Please find below the details for better understanding.

Kindly consider design as per Div-2:

Vessel ID = 3006mm.

Material - SA 516 Gr. 70

Design Pressure - 7.9 MPa

Design Temperature - 82c

Shell thickness - 84mm

Corrosion Allowance = 0

MAWP - 8.791 MPa

MAP - 9.4866 MPa

Test Pressure = 1.43 * MAWP = 1.43*8.791 = 12.571 MPa

Stress at Hydrotest = 231.17 MPa

Allowable stress = 247 MPa (0.95*Yield)

Ratio = 0.935 (Pass).

Considering MAP in place of MAWP,

Test Pressure = 1.43*MAP = 13.565 MPa

Stress at hydrotest = 249.46 MPa

Ratio = 1.01 MPa (Fail)

Hope now its clear. The same scenario is happening for vessels in Div.1 also while considering MAP in place of MAWP in hydrotest.

Kindly advise.

Regards

vGsh

If corrosion allowance = 0, how can the MAP be higher than MAWP? If you calculate a slightly lower MAP you're OK. In calculating MAP, have you allowed for allowable stress at working temperature being lower than at test temperature?

Dear Codemaster

Allowable stresses are different at design temp (162@MAWP) and ambient (175@MAP).

Iam confused about the significance of using MAP in place of MAWP.

Regards

vGsh

How is the MAP defined? I'm only aware of design pressure DP, MAWP and test pressure.

Whatever pressure you're going to multiply by 1.43 to get test pressure, you need to make sure thickness is such as to give stress at that pressure less than 247/1.43 ~ 173 MPa. Then you're OK.

Dear,

I think u r using some design sofware for calculation, dont get confuse.

lets solve it other way.

1. MAWP stand for maximum allowable working presure at corroded condition. the MAWP is obtained for the selected thickness of plate less corrosion allowance, by using the same equation of UG-27. ASME div1. ( exp;- selected thickness = 10mm, while required thickness= 7.1mm). MAWP states that suppose after 10 yrs coorossion might has taken, the vessel can stand this pressure.

2. MAP stands for Maximum allowable pressure at new and cold condition, that is the MAP is calculate for selected thickness without corroison when the vessel is new.( exp;- selected thickness = 10mm, while required thickness= 7.1mm).

so MAP will be higher than MAWP is corrosion is considered. If no corrossion then both values are same.

3. FOR hydrotest 1.3*MAWP*(St/S). just select 1.3*DP*St/S. DP is design pressure, if u select MAWP instead of DP. u will be getting the the results what u r facing now.

Read note 34 of UG 99B.

SALAAM!

A very informative discussion has been carried out above. I just want to straight out a few facts which even very experienced engineers get confused.

1. MAWP is always taken in corroded condition (Ref: ASME BPVC Sec.VIII Div.1 UG-98) That means the MAWP is not effected over the year due to corrosion( it should always be re-calculted after some repair work)

2. MAP is calculated at MDMT 'excluding' corrosion allowance like MAWP. This is very clearly mentioned in BPVC under General notes in UCS 68. let me copy paste the exact wordings here just to avoid any confusion.

"The MAP is defined as the highest permissible pressure as determined by the design formulas for a component using the nominal thickness less corrosion allowance and the maximum allowable stress value from the Table 1A of Section II, Part D at the MDMT."

I hope this will clear the stuff up.

As far as the hydro-testing is concerned, MAP is always greater than MAWP so testing the vessel at MAP with temperature higher than MDMT will always cause problems. this is the reason BPVC always uses MAWP for hydro-testing instead of MAP.

p.s: Correct me if I am wrong somewhere.