API 650 11th edition Add 1 Tank Anchoring 1

[99491]

I have been running some calculations for a tank with a design internal pressure of 7.5mbar and a 90mph wind speed. According to section 5.11 the tank does not require anchor bolts however when I do calculation F 4.2 - Pmax is less than my design pressure.

The roof to shell compression area is not a problem and can withstand the 7.5mbar but the "uplift at the base of the shell" cannot. I am not able to reduce my design pressure to suit the Pmax so could I just anchor the tank instead?

I find it strange that according to section 5.11 using the same internal pressure the tank apparently does not require anchors yet it is not able to withstand the pressure and wind speed when doing the calculation in appendix F.

 

[JStephen]

I think they added the equations in 5.11 but didn't update the equations in App. F to match.

 

[99491]

JStephen..... If I am to submit the calculations to my client could I recommend anchor bolts to suit app F even though they are not required in 5.11?

 

[AK8824]

There are a few mistakes in Table 5-21a(SI).

The heading says 'Net Uplift Formula' U (N) whereas the terms W1, W2 and W3 are expressed in 'kN'. The formulae given in the tables use Mw, whereas the corresponding defined term is Mwh. You will also notice that the table in customery unit uses Mw and Mwh both.

 

[DSB123]

AK8824,
Are you suggesting that the Code has been issued without an appropriate Quality-related Proof reading just to meet an issue date no matter whether the contents are correct?

 

[AK8824]

DSB123,

Yes. That is what I thought. API committe did not carefully check before releasing the revision.

 

[AK8824]

DSB123,

You may like to go to the following link to know more about it:

http://www.ast-forum.com/ast_forum_display.asp?post=13181

 

[AshokNair]

JStephen and AK8824 are both right. Also I would like the member’s opinion on the following:

1) Equations under clause 5.11.2: The equations should have been on uplift rather than on moment (60% of uplift due to MWH + 60% of uplift due to PWR + uplift due to internal pressure < 66% of W2).

2) What is the meaning of clause F.4.2 and F.1.2 branch of the “Decision Tree, which prohibit design pressure from exceeding Pmax? This is true only for an un-anchored tank. The wording should have been "If P exceeds Pmax, provide anchor bolts and meet all other requirements of clause F3 through F6.

3) The definition of the term "P" under equation for P in F.4.1 is misleading. Defining this term as ‘Design Pressure’ gives the impression that all tanks designed under appendix F shall be designed for a pressure as calculated from this equation. But the ‘Appendix-F decision Tree’ gives an entirely different interpretation.

4) Equations for uplift due to frangibility pressure are incorrect in Table 5.21a & 5.21b. (Multiplier 785 / 4.08 missing)

5) Why a different multiplication factor (3) for uplift due to frangibility pressure compared to that for failure pressure (1.5)? Both are same!