Corroded hydrotest Pv Elite

[waliq]

I was designing a vessel on PV Elite. There, in the design tab, i checked the option of Hydrotest as per UG 99 C. Then, below I also checked the option of corroded hydrotest. The idea behind doing this was, that I thought it would now test the vessel according to UG 99 C in new and cold (shop test) and also according to UG 99b (hot and corroded) based on MAWP.

My thickness based on internal design pressure was 8 mm.
C.A of 3.2 mm
When I checked the option of corroded hydrotest, my vessel got overstressed such that it was passing at a thickness of 14 mm. However, if I unchecked this option of "corroded hydrotest" and selected hydrotest as per UG 99b above instead of UG 99C, the thickness of 8 mm was sufficient.

This was opposing my initial thought that if I checked the option of corroded test below would evaluate the hydrotest on the basis of UG 99b.

My question is that what does it mean by a corroded hydrotest?
Does UG 99b not ask for a corroded hydrotest to be performed for all vessels since it says 1.3*mawp*sa/s?? Because, MAWP is calculated max pressure on the basis of corroded thickness. Am i interpreting this clause wrong?

 

[kiad]

When you consider ug-99 b, yes it uses mawp, but the hydro test stress will be obtained at uncorroded condition. Hydro test at corroded condition is something different. the decrease in thickness and increase in diameter because of corrosion will result in increase in test stress at corroded condition.

 

[waliq]

@kiad: Thanku for your info
So, corroded hydrotest does not mean the test as per UG 99b??
And the UG 99b clause stating "MAWP" can be MAWP (new and cold) to be used for shop test. Also, this MAWP can be hot and corroded to be used for field test??
If this is the case, then isnt UG 99b (in case of hot and corroded mawp being used)and "corroded hydrotest" one and the same thing??

And from this I also understand that corroded hydrotest shud not be checked unless client says so. Only UG 99b shall be used in the design of vessels on pv elite

 

[kiad]

There is no difference between the amount of test pressure between corroded and uncorroded condition per ug-99 b since the MAWP will not change.but the amount of test stress to be applied on vessel at corroded condition will be higher.you can use ug 99 b in pv elite for corroded or uncorroded condition but if you uncheck corroded hydro the software calculates vessel stress during hydro at uncorroded condition.and yes it is not required to check vessel for a hydro test at corroded condition unless client to mandate it.

 

[waliq]

@kiad: Thanku for your feedback again

Just one confusion left. MAWP is calculated Pressure from UG 27 formula when you subtract corrosion allowance from your nominal thickness.
This pressure is less than your MAP (MAWP, new and cold)
If you are taking uncorroded thickness for your hydrotest (i.e, shop test), shouldnt it then be tested at MAWP (new and cold), which is different from MAWP (hot and corroded)??

 

[kiad]

MAWP is defined at corroded condition(see ug-98 b) and ug-99 b uses MAWP.as I said in previous post you can utilize ug 99 b for uncorroded or corroded condition.

 

[SnTMan]

waliq, poking around in PVE help files I find PVE limits allowable stresses at hydro to 1.3 * ambient allowables. If hydro is per UG-99(c) and CA is deducted this allowable would be exceeded because the factor on the allowable is the same as the factor on the NEW & COLD MAP. At uncorroded thickness.

Flip the switch to set allowable at 90% of yield.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[waliq]

@snTman: I had alao checked the option of 90% allowable. Still, if i do corroded test, with allowable at 90% of yield, it overstresses the vessel.

If, I do as per UG 99C, and DO NOT check 90% of yield option, it still passes the vessel.

From this I interpret that pv elite automatically checks hydro at 90% of yield and there is no need to specifically tap the option of 90% of yield

 

[SnTMan]

waliq, I admit, I don't know exactly what PVE is doing in these various scenarios. What I do know:

Hydro per UG-99(c) (N&C) should not consider CA as deducted. This is so called shop hydro with vessel MAWP based on new, cold conditions. This hydro shall essentially be done only once at completion of fabrication. Allowables to be based on ambient temps.

Hydro per UG-99(b) (H&C) should be considered with CA deducted. So called field hydro, MAWP based on hot, corroded conditions. Vessel may be so hydro'd periodically during its service, including at or near end-of-life. Allowables to be based on ambient temps.

Allowables based on ambient tensiles may be ratio'd by hydro multiplier, typically 1.3 but may be higher, or may be based on 90% ambient yield.

I think it is mistaken to analyze hydro stresses for UG-99(c) with CA deducted, whichever allowables are used. Vessel also must be able to withstand field hydro with CA deducted.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[waliq]

@snTman: This is exactly my understanding. However, I was designing a vessel, 8mm thickness. It was overstressing at field hydrotest.
C.A = 3.2

The overstressing of the vessel is understandeable. However, this overstress was passing at thickness 14 mm. This is 6 mm thickness increased just due to hydrotest. Is it logical?? I mean, yes the vessel is being subjected to a higher pressure at a corroded thickness of 4.8 mm. But the allowable stress has been increased t 90% of yield stress. A 1mm or 2 mm increase in thickness makes some sense but I am not getting this
Material: Sa 516 70
90% of yield= 235 Mpa
Actual stress at corroded hydrotest= 315 Mpa

 

[TGS4]

Never ever set a wall thickness based on a pressure test. The 90% yield limit is to set the maximum-permissible test pressure. The minimum test pressure should never result in stresses that exceed this 90% yield limit, especially for materials such as SA-516-70 that is not yield governed at room temperature.

 

[SnTMan]

waliq, I'll say it again: Refer to the online help and documentation. Call Hexagon / Intergraph / Coade. Read thru the detailed output. Can you identify the overstressed component(s)? Can verify the results?

You have to do something other than look for error messagess. You need to find out what the software is DOING.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[waliq]

@snTman: I had gone thru the report. I am sorry I dint mention it earlier. The overstressed components were shell and both the heads (2:1 ellip). However, Pv Elite report was just showing the overstressed components, the hydrotest pressures based on UG99b, Ug99C etc
But the detailed analysis I couldnt find. I should try and ask intergraph for this

@TGS4: Yes, this is what I am unable to understand. I agree that you should never increase thickness based on hydrotest. And I have never seen it increasing also. I am new to Pv elite and had been using compress before. With 90% of yield, the thickness should not be increasing since allowable stresses are almost increased by 50% (66% to 90% of yield) while the MAWP is being increased by 30%
So, the thickness increase due to test doesnt make sense. But, still an overstress is something I aint gettin

 

[TGS4]

This sounds like a software issue. Take it up directly with PVElite.

 

[SnTMan]

waliq, yeah, PVE probably will not print a full report for the hydro calculations. Still, shell & head components are easily checked by either hand calculations or running in the Codecalc module. Using the corroded geometry and hydro pressure, calculate the stress, or other way around however you want to go at it. Confirm the PVE results. Or not.

I agree, something is not right.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Good engineering practice is to use actual (measured) thickness for each pressure element when UG-99(C) is applied.

Regards
r6155