API 650 12th Edition

The proof pages have been issued for review to the committee members for API 650 12th Edition.  This edition will be released later this spring and covers items approved by the committee.  There are some 27 agenda items that are being included in this edition.  It does not include items that were approved in the last couple of committee meetings.  We will be working on the changes to our ITSdesign software to include the changes that are coming.

Annular Ring Thickness Calculations

The following questions was posed to us:


1 - Current edition of section 5.5.3 of API-650 requires us to select the larger of
a) product design thickness plus corrosion allowance or
b) hydrotest thickness

2 - Note b to Table 5.1a requires us to use the larger of
a) product design stress or
b) hydrotest stress
in selecting the column to look at.

For a tank with
corrosion allowance = 1 mm  
bottom shell course thickness = 45 mm,
product design stress for bottom shell course = 220 MPa
hydrotest stress for bottom shell course = 240 MPa  :

item 2 above requires us to use the column for 240 MPa
bottom shell course thickness of 45 mm requires us to use the fifth line of Table 5.1a.
We end up with the 19 mm from among the tabulated values.

item 1 above requires us to use the larger of
a) 19 + 1 = 20 mm
b) 19 mm

If we could use the column for 220 MPa for the design conditions, then we would be comparing the value for design thickness as  a) 16 + 1 with the same b) 19 mm for the hydrotest condition.

With the way it currently reads we always end up having to add the corrosion allowance to the hydrotest thickness calculated.


Our response:

In reviewing the way 5.5.3 reads you are correct.  Since it specifies that you use the higher of the two stresses in determining which column you use and then must use the thickness shown plus corrosion allowance for the design condition, you will always be using the higher required thickness and then adding the corrosion allowance to that.  It would seem that we should be using the column that corresponds to the stress in the shell for whichever condition we are evaluating and use the whichever thickness is greater.  Since this is not how the standard currently reads, I have submitted an inquiry to the API committee for review.  It will take time to get a response and even longer if they decide to address the issue and make a change in the standard.

What questions do you have?

I have decided to do a question/answer for my blog. I would like you to email me a question you have about the API-650 standard that you would like answered on this blog.  Please ask one question per email.  Send your question directly to me at jschroeder@innovativetanksolution.com.

In addition I will try to keep you up to date as to what is happening with the standard.  I am currently getting ready for the next API standards meeting and will post a brief update as I get information.

API 2000 Venting

There is a new version of API 2000 for venting calculations. ITSdesign now includes this new edition for venting. There are two aspects to the new version of API 2000. The first is that to use calculations for venting that were in the previous edition, you must be using Annex A. Annex A does have restrictions that says the liquid stored must be similar to gasoline, the maximum operating temperature of the vapor space is approximately 120 degrees F (48.9 degrees C), the tank is uninsulated, and the capacity is less than 180,000 BBL (30,000 cubic meters). If it meets these criteria and you use Annex A, the results are the same as the previous standard. If, however, you use the base standard you will find that you need a lot more information about what you are storing. You need to know if the product stored is similar to Hexane, what is the Latent Heat of Vaporization of the product stored, what is the Relative Molecular Mass of the vapor, and what is the boiling point of the liquid at the relieving pressure. If you have insulation on the tank and you wish to reduce the emergency venting requirements because of the insulation, you need to know what the thermal conductivity is of the insulation. You also need to know what the approximate latitude is for the location of the tank installation. The answers to these questions could greatly affect the venting requirements especially for normal venting. It appears from some of our preliminary calculations that the normal venting per the base standard can be significantly higher than what is required by Annex A. We have just posted a new version of our ITSdesign software that includes these new calculations.

Insulation thickness added to ITSdesign

We have just released a new version of ITSdesign that includes entries for insulation thickness and weights. You have the option of having insulation on just the shell, just the roof, or both. The insulation thickness and weight can be different for the shell and the roof. You can also specify as to whether the insulation weight is to be used in the resistance to overturning for wind and seismic calculations. The addition of insulation thickness affects the area of the tank exposed to wind. In the printouts for wind you will see a different value for the tank diameter and height when detemining the area exposed when the tank is insulated. This increases the overturning moment and the sliding forces. For seismic you will see an increase in the weights. Insulation thicknesses and weights are also shown in the printouts including in the loadings page.

Frangible roof design of anchored tanks

At the API committee meeting in November I brought up the subject of the frangible roof design requirements. In particular I asked about the requirement if 5.10.2.6.d which says "For anchored tanks of any diameter, the tank shall meet the requirements of 5.10.2.6.a and the anchorage and counterweight shall be designed for 3 times the failure pressure calculated by F.6 as specified in 5.12." One of the requirements is in 5.10.2.6.a.5 which gives the maximum area of the compression ring that you can have and still be considered frangible. I asked if this requirement can be removed when the tank is anchored since we have to design the anchorage for 3 times the failure pressure which is based on the actual area of the compression area. After some discussion the committee as a group decided that you still must meet the area requirement of 5.10.2.6.a.5. What this means is that for smaller diameter tanks meeting the frangibility requirements is very difficult. For shop built tanks built to Appendix J meeting the frangibility requirements is next to impossible.

What is next in ITSdesign

We are currently working on adding Appendix P.2 calculations to our ITSdesign software. It has turned out to be a bigger challenge than first anticipated and so is taking longer than expected to program. I will keep you informed as to the progress on this. With regard to Appendix P.3 that was deleted from the standard, the committee is still working on the review of the equations in relation to the charts. That work has not been completed and so we do not have a timetable for when this section will be put back into the standard. Since it is not ready for balloting as yet, it will be at least a year.