Title: /
Maximum Stiffener Spacing per Appendix V
Date: / 05/15/2011Contact: / Name: / Jimmy Schroeder
Company: / Innovative Tank Solutions, LLC
Phone: / 651-480-0258
E-mail: /
Purpose: / Correct formula in V8.2.1.4
Source: / J. Schroeder at API Meeting 11/16/10
Revision: / 0
Impact: / Minimal
Background: / V.8.2.1.4 gives the formula for determining the maximum stiffener spacing for each shell thickness. The formula uses the value of HTS which is defined in V.8.1.4 as the total transformed shell height. V.8.2.1.2 provides the formula for determining HSafe, the maximum height of unstiffened shell permitted, and is based on tsmin. V.8.2.1.3 calculates the number of intermediate stiffeners, Ns, required based on HTS and HSafe.
If we have a tank that is 40’ high with five 8’ shell courses of ¼” plate, then HTS = 40’ and tsmin = 0.25”. If HSafe is 9’, then per V.8.2.1.3: Ns + 1 = HTS/HSafe = 40/9. Therefore, Ns = 40/9 – 1 = 4.
Per V.8.2.1.4, since tsx = 0.25” and tsmin = 0.25” the equation reduces to:
Lx = HTS/(Ns + 1) = 40/(4 + 1) = 8’. This is then the maximum spacing that can be used for the ¼” shell plate. Since the shell plates are 8’ wide, the stiffeners would fall on the circumferential seam. This is not allowed as they cannot be any closer than 6” to the seam. Since we are not allowed to exceed the 8’, we have to place the first stiffener no higher than 7.5’ from the bottom. We then place each subsequent stiffener at the 8’ spacing. However, this leaves us with the top stiffener being 8.5’ from the top of the tank which then exceeds the maximum spacing allowed. We would have to add an additional stiffener even though our HSafe (the maximum height of unstiffened shell permitted) is 9’. This then says another ring must be added.
If the above tank had been 45’ high, then Ns = 45/9 – 1 =4 and Lx = 45/(4 + 1) = 9’. We would then have been able to place our stiffeners at 9’ apart instead of 8’ apart and we would not need to add another ring.
If we go back to the DuPont standard, SG11.4S, from which Appendix V was developed, we see that it calculates the force on the shell from the external pressures. It then calculates the force on the shell based on the transposed shell height under consideration. As long as the force on the transposed shell height is less than the actual force, then the spacing on the ring is adequate. Rearranging the formulas would then use HSafe instead of HTS for determining the maximum spacing.
Proposal: / Revise the formula in V.8.2.1.4 to use HSafe.
Rationale: / The maximum spacing should not be based on the transposed shell height but on the maximum safe distance.
Notes:
Current: V.8.2.1.4
LX = HTS tsx 2.5
(Ns + 1) tsmin
Proposed: V.8.2.1.4
LX = HSafe tsx 2.5
tsmin
Current: V.8.2.1.3
Round up the calculated value of Ns to the nearest integer for use in v.8.2.1.4 and subsequent calculations.
Proposed: V.8.2.1.3
Round up the calculated value of Ns to the nearest integer for use in subsequent calculations.
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