API-650 Wind Design with I > 1 or Exposure D

API-650 provides the formulas for designing a tank for wind. However, the formulas are all based on a importance factor (I) = 1.0 and an exposure = C. If your requirements are for exposure D or an importance factor greater than 1.0, you have to modify all of the calculations. For the wind loadings you need to calculate a new horizontal load and a new vertical load using ASCE-7. For this you have to determine values for Kz, Kzt, Kd, G, Cf, and GCp from ASCE-7. From these values you can then calculate your new loadings. You also have to calculate a new horizontal loading based on the base wind velocity. To this you have to add the internal drag pressure specified in 3.9.7.1 and compare that to the base value used in the calculations for maximum wind girder spacing as well as for sizing requirements for wind girders.

The effect on your wind calculations is greatly dependent upon the height of your tank. The API-650 formulas are based on Kz = 1.04. For a short tank with exposure C, Kz could be equal to 0.85 which will give a lower horizontal wind load depending upon the importance factor. The vertical uplift from wind might increase depending upon the tank height. If you have a higher tank or a tank with exposure D these values will increase and will probably result in higher loadings on the tank.

The effect on the wind girder requirements for a short tank probably will not change as you cannot use a factor less than 1. The values will change for taller tanks, tanks with higher importance factors, and tanks in exposure D.

Because there is becoming more of a requirement for importance factors greater than 1.0 and the possibility of clients requiring exposure D, we have just released our new version of ITSdesign to include calculations based on ASCE-7 for these conditions. If you enter either a importance factor greater than 1.0 or exposure D, ITSdesign will automatically provide the calculations based on ASCE-7 requirements.

API-650 11th Edition

There are changes from the API-650, 10th edition to the 11th edition. I will be reviewing some of these changes over the next several postings. So here goes!

Added new paragraphs 1.1.4 and 1.1.5 and revised 1.1.6 and 1.1.18 to specify that Appendix L data sheets are mandatory unless waived by the purchaser, all tanks and appurtenances shall comply with the data sheet and attachments, field tanks shall be furnished completely erected, tested, and ready for service connections unless specified otherwise, and shop tanks shall be furnish tested and ready for installation.

Added Appendix W providing recommendations covering commercial and documentation issues.

Added section 1.3 to outline the responsibilities of the parties involved.

Added section 1.4 to specify documentation requirements.

References was moved to section 2 instead of being a part of section 1.

Added section 3 to provide definitions.

Material are now in section 4 and have the following changes:
1) Rimmed or capped steels are not permitted.
2) Cast iron for pressure parts or for attachment by welding to the tank is prohibited.
3) Cadmium-plated components shall not be used without the consent of the purchaser.
4) Added section 4.7 on bolting stating that (unless otherwise specified on the data sheet) flange bolting shall be A193-B7, anchors shall be threaded A36 round bar with heavy hex nuts and shall be galvanized, and all other bolting shall be A307 or A193, and A325 may be used for structural purposes only.
5) Added section 4.8 on welding electrodes.
6) Added section 4.9 stating gasket material is to be specified on the data sheet and shall not contain asbestos unless specified by the purchaser. Gaskets shall be made with integral centering or positioning device. No sealing compound, adhesive, or lubricant shall be used unless specifically allowed by the purchaser. Spare gaskets are not required unless specified on the data sheet.
7) Test gaskets must have comparable dimensions and compressibility characteristics as service gaskets. Descriptions of gaskets for temporary use only as test gaskets shall be submitted for purchaser’s approval.
8) The test gasket for joints that will not be disassembled after testing must be the specified service gasket.
9) Flange bolts and nuts used for testing are acceptable for use in the completed tank except for stainless steel bolting.

This takes us through section 4. Next time we will continue our look at the changes in the 11th edition.