UG-99(b) and (c)...what a mess!

[ElCidCampeador]

Hi,

My AI is questioning about some calculations of my client because he thinks that the hydrostatic test pressure reported is not correctly verified, so the vessels I'm building are wrong and I need to change thk. and flange rating.

Example of 1 of these tank that I have to U-Stamp:
Vessel is a simple vertical one with nozzles, material carbon steel, design code: ASME VIII Div.1

My client wants Hydrotest pressure=250 bar, with design conditions assigned.

I run calcs 1^st time setting UG-99(b), and calcs say that P.hydro.min=200 bar
I run calcs 2^nd time setting UG-99(c), and calcs say P.hydro=300 bar

ASME VIII UG-99(d) says that I could test vessel @ intermediate pressure, as in my case.

Problem: if I run 1^st calc, P.Hydro=250 is checked (in the way below) even if 250>200
if I run 2^nd calc, P.Hydro=300 is not checked, because 250<300 and calcs send me an error message

BUT.
In the 1^st calcs, calcs let me select hydro pressure and verify in this way: it simply uses the formula for calculaton of design pressure using P=P.hydro selected and S.yield as allowable stress. If T resulting from this formula> T.min, calcs say "ok, go". In other words, it checks if material yields @ temperature and pressure of hidrostatic pressure that I have selected. This is SOLUTION A.

Other clients, instead of using the formula above, re-run calculation supposing 250 bar equal to an UG-99(b) value of an equivalent new design pressure (higher than the first one). This is SOLUTION B.

Both solutions have been rejected by my AI, saying that it's necessary to re-run all calculation using P.des=P.hydrotest @ test condition. But in that way, all flanges and other parts have to be replaced!

I don't understand the reason behind this, because some softwares like Sant'Ambrogio, Compress and Visual Vessel Design use SOLUTION A without warning message. So all these softwares are wrong?

Please help me, thanks.

 

[Geoff13]

If I understand, the customer has specified some design conditions (not listed above), and has also specified a minimum 250 bar test pressure.

Hydrotest is an intentional overstress of the vessel to prove it's initial quality. Running calcs using P.hydrotest but also using S.design will likely show every part is overstressed (as intended by the Code). This is not reason to thicken or replace any part of the vessel.

The Code does not define what allowable stress should be used for hydrotest calcs since the Code doesn't require stress calcs to be done. Since you're trying to avoid damage to the vessel 90% of yield would be fairly typical (I believe that's what Compress defaults to, though you can reduce it if you wish). This is a "good practice" check, rather than one required by the Code.

UG-99(c) is usually a higher test pressure than UG-99(b), so I'm not surprised by your 300 bar vs 200 bar result. This higher pressure creates a greater risk, and thus this pressure is only applicable when both Manufacturer and User agree. This risk for your vessel is somewhat mitigated by 250 bar being lower than the UG-99(c) limit.

Since you did a calculation of every part to arrive at the 300 bar UG-99(c) pressure I'm not sure what other calc anyone would require to confirm 250 bar is safe. The Code certainly doesn't require any other calc, and that's all the AI should use to make their requests.

 

[ElCidCampeador]

In general I agree but

Since you did a calculation of every part to arrive at the 300 bar UG-99(c) pressure I'm not sure what other calc anyone would require to confirm 250 bar is safe.

what if pressure test required by client is even greater than UG-99(c) value? Is the "good practice" check enough?

 

[david339933]

Calculated test pressure:"The basis for calculated test pressure in either of these paragraphs is the highest permissible internal pressure as determined by the design equations for each element of the vessel using nominal thicknesses with corrosion allowances included...." App. 3-2 Definitions.

UG-99(c) - "The hydrostatic test pressure at the top of the vessel shall be the minimum of the test pressures calculated by multiplying the basis for calculated test pressure as defined in 3-2 for each pressure element by 1.3 and reducing this value by the hydrostatic head on that element".

So if you calculate the maximum pressure for each part of the vessel based on nominal thickness + CA, then take the minimum of these values, how can the parts not take the pressure?

Standard Designation: BPV Section VIII Div 1
Edition/Addenda:
Para./Fig./Table No:
Subject Description: Section VIII-1, UG-99(d)
Date Issued: 06/29/1984
Record Number: BC84-154
Interpretation Number : VIII-1-83-287
Question(s) and Reply(ies):
Question (1): In Section VIII, Division 1 construction, when hydrostatic test pressure is used to compute the thickness of a shell, is it the intent of UG-99(d) to take the yield strength as maximum allowable value in order to have no visible permanent distortion?

Reply (1): UG-99 gives the hydrostatic test requirements for a vessel which has had the thickness set by formulas given elsewhere in the Code, such as UG-27, UG-28, UG-32, and UG-33. It is not, used to compute the thickness of the shell.