Internal Design Pressure for Small Diameter API650 Tank

[MCooke12]

Hello,

I am reviewing the internal design pressures for the bulk storage tanks in our network and have stumbled across an issue with small diameter tanks. A conservative approach is being taken and the minimum allowable strake thicknesses (as per API653 4.3.3.1) are being used for the shell mass and area resisting compression in the shell to roof joint.

Now, the issue: small diameter tanks are coming back with a negative maximum internal pressure (floor deformation). One of the tanks is question has a 3.5m diameter, is 10m in height, and is located in a cyclonic region. My questions is: is there a method I can use that includes the minimum strake thicknesses for these smaller tanks, or is it unwise to take this approach (for the smaller tanks and/or in general).

Cheers,
Mitch.

 

[JStephen]

I'm not sure I understand the issue.
First off, if it is an existing unanchored tank, it may simply be that it doesn't meet the current standard for combinations of wind and pressure- and if you tried to evaluate it for maximum pressure, in that case, you could get a negative number.
Check the wording carefully as to whether you should be evaluating using the current standard or the standard at the time of construction.
It seems API has changed the overturning criteria with new addition or addendum.
Note that there are multiple sections that govern this, depending on the pressure.
Normally, I would expect a 3.5m x 10m high tank to be anchored for wind, regardless of pressure.

 

[HTURKAK]

I have read the thread a few times and still i am not sure that i understand correctly . Regarding the term bulk storage , is the content liquid? If the vacuum pressure due to wind, this is external pressure loading and you may look API 650 Appendix V .

Your quote ' Now, the issue: small diameter tanks are coming back with a negative maximum internal pressure (floor deformation...' I understand the floor plate coming up due to vacuum uplift .

What is the vacuum pressure that floor plate could not resist with its self weight and weight of minimum content?

Apparently, the aspect ratio D/H= 3.5/10 and requires anchorage to resist wind pressure as JStephen (Mechanical) stated.

And, if the vacuum pressure is due to wind external pressure, you are expected to check the roof , shell and total tank uplift. Regarding the bottom uplift check API 650 V.9 suggests=

'V.9.1 The bottom of the tank shall be evaluated for external pressure loading if either of the following conditions is applicable.These conditions do not need to be considered simultaneously unless specified by the Purchaser.

1. If the total design external pressure force on the bottom plate exceeds the sum of the weight of the bottom plates plus the weight of any product required by the Purchaser to remain in the tank when external pressure is acting, membrane stresses in the bottom must be evaluated.

2. If the area around the tank will be subject to flooding with liquid, provisions should be included in the design of the tank and its operating procedures to ensure that the tank contains sufficient liquid to counteract bottom uplift resulting from external flooding conditions. If the tank cannot be filled with liquid of sufficient depth to counteract the uplift from the liquid pressure under the bottom of the tank, membrane stresses in the bottom must be evaluated.'

 

[IFRs]

Is your calculation problem driven by a very small strake thickness? Do your calculations end up using 0.10" per 653 4.3.3.1? Are you trying to determine what the original design pressure was? Do you have the original design parameters? If you are trying to determine what the tanks can actually sustain, you'd be better off with actual thickness data, perhaps modified by actual corrosion rates, rather than the miniumum thickness based on hydrostatic loading which is what that section of 653 calculates. Can you calculate a new minimum thickness based on your required operating pressure and compare that to the actuals, or the original design less corrosion? Basically, I think your method is flawed, and the attempt to be conservative results in spurious answers.

 

[MCooke12]

Thanks for all the responses.

So origionally I was under the assumption that the tank was unanchored (due to an earlier survey), but due to JStephen's comment I asked site to confirm if this is correct. Turns out that it is indeed ANCHORED. I believe this means that the tank is not limited by the maximum internal pressure as per API650 F.4.2 - am I correct?

I will explain a little more about the aim, as I can see I was a little ambigious in my origional post.

I am calculating the internal design pressures of existing tanks as a part of a larger review for tank venting. These tanks are relativly old (30-50 years on average) and the available information on them is majorily from maintenance conducted in the last 5-10 years. Recently, a tank had its offstream inspection, during which it was found that the its venting requirements were insufficient. This, coupled with a general lack of data, sparked this review.

The conservative approach of "minimum allowable strake thickmess" was chosen as this was the worst case scenario for venting requirements (i.e. returned the lowest design pressure, thus requiring the greatest venting requirements). This case was chosen as we didn't want to change the venting requirements in the future, assuming these tanks lasted for another 20-50 years. With this in mind, I will investigate using actual corrosion rates for expected end-of-life strake thicnkesses, as suggested by IFRs.

Would you agree with this approach?

Thanks again for your responses so far.