API 650 Tank Top Angle Size 1

[aamir2003]

We have a 47 ft dia , CS tank with IFR and an Aluminum Dome Roof, Can this tank be classified as an open roof tank for top angle size determination?

As per our understanding for top angle clause 5.9.3.2 applies and we specified TOP angle as 75x75x6 mm where as client insists on using 75x75x10 mm as per clause 5.1.5.9 e

Also if client specifications call for CA for Shell, does it apply to TOP angle as well

Regards,
Aamir

 

[IFRs]

Unless the dome provides structural strength to resist wind buckling ( probably not ) the tank needs to be designed as an open-top tank with top angle and wind girders(s)and 5.1.5.9e does not apply. The code is silent on the need for corrosion allowance and I typically do not see it applied to top angles.

 

[Duwe6]

Turn the angle leg-out and properly paint it. Eliminates the need for CA - corrosion allowance. And a CA is indeed usually needed in the top of a tank - 'vapor space corrosion' of the shell and roof rafters is a fairly common problem with tanks.

 

[IFRs]

Duwe6 - Please help me find CA on top angles in the code. I am always interested in learning more about the code. The top angle is typically above the liquid level, so not on the pressure boundary and although it may be subject to corrosion I don't remember seeing it in 650. Thanks!

 

[Duwe6]

Top angle is not addressed for CA. That is why you cannot find it. It is up to the Engineer-of-Record, with hopefully a hint or three slipped in by the API-653 Inspector, to add appropriate CA's to the items that they deem one to be required for. And for a Leg-out angle on the outside of the rim, if you keep it painted, it is not going to corrode. It is readily inspectable without assistance - just look for the rust streaks running down the wall of your tank.

No probability of corrosion + readily inspectable = no CA required.

Floor is the opposite. Corrosion is very probable, and inspections are difficult and very infrequent and hard to arrange.

 

[aamir2003]

This is what our client wrote :

In real life, vapor do exist in the space as the seal and IFR is not 100% leak tight.
And atmospheric corrosion is the minimum you need to design for.

Any comments ?

Further can someone help with this requirement, we are at the time of putting the top ring when client has sent us this,

Client has requested the check on compliance against G1.3.1 and G 4.1.4?

 

[IFRs]

If it is atmospheric corrosion, paint should take care of it. There are hundreds of not thousands of tanks out there with code top angles and not corrosion allowance. I don't see much corrosion and even if there were, who cares - it is above the liquid level? The code is silent, therefore strictly speaking no requirement exists. If the client wants something that exceeds the code, they get it and pay for it. If you have determined that something is required by good engineering practice and judgment then if the client agrees, they pay, if they don't agree either you pay or don't do it. It's their tank, after all. G 1.3.1 probably won't require a heavier top angle but the top of the tank needs to be analyzed for the dome loads from the dome vendor. G 4.1.4 is all about tension which should be internal to the dome but may not be, you need dome loads from the dome vendor to know if their structure resists the tension or not ( not all dome vendors are the same in this regard ).

 

[aamir2003]

IFR, latest email from client , its a dilemma ! I am not sure what to reply to them at this stage , Dome supplier is not corporations after supply of Dome

Can you make sure Dome Supplier is complying with G4.1.4. I see the presence of tension ring in BTE drawing, but am not sure where the relevant calculation is.
- What are the loads that will be exerted by the dome roof to the tank shell and its foundation? Have you analyzed if the tank shell and foundation are good enough for the loads?

Tk-2054 was an open-top crude tank with external floating roof. Its diameter is 47m++, i.e. close to Tk-34. An aluminum dome roof was then retrofitted onto it during MPP1 implementation. Visual check shows the roof is attached to the shell via sliding shoes (see attached PICT0119.jpg), similar to BTE design. The shoe separation distance is between 3 and 4m, BTE’s is 4.14m. At every attachment point of sliding shoe to Tk-2054 tank shell, there is some form of reinforcement/strengthening at the shell, i.e. some members welded between the top girder and the top angle (see attached PIC0121.jpg). I do not know the cause of this strengthening, but I suppose it’s related to dome roof load(s).

Top angle size of Tk-2054 is 75 x 75 x 9mm.
- Corrosion allowance. Do you see those black mats in attached DSC05622.jpg and DSC05623.jpg? They covers holes on the old Tk-34 roof. There were many of such mats on the old tank, and the remaining roof areas that were not covered, were paper-thin (as described by Husaini). Do you dare to walk on the roof?

The submerged part of the tank shell was all good. Anything above it was horrible. We are applying the lesson learned and necessitate all CS parts exposed to product (liquid and vapor) be provided with corrosion allowance. In many cases, the 1st parts of the tank that experience perforation are the tank bottom (where water accumulates) and the parts that are exposed to the product vapor (upper shell courses and roof).

One of the principles of negotiation states we refer to a fair standard if the interests of the negotiating parties differ. I therefore suggest that we refer to the clarification during tender stage which indicates shell shall have 1.6mm corrosion allowance. As at least one side of the CS top angle forms a continuous part of the CS shell, that part of the top angle shall be provided with 1.6mm corrosion allowance.

I request that Owner’s experience on the corrosiveness of product vapor be respected, and the contract be honored. A formal change authorization is needed if you insist on a change.

 

[IFRs]

You have two issues that I see.

1) The above normal corrosion that has been experienced certainly mandates that some CA be used for all steel parts. The owner wants 1.6mm and this should be applied to the top angle thickness. No question here unless this information was not communicated during the bid stage. If there was not such communication, the owner should pay the difference since they would have anyway.

2) The tank needs to be determined to be suitable to handle the dome loads. For this you need the dome loads. If the dome vendor is not cooperating, the owner should tell them they will never work for them again and bad news travels fast in this industry. In my opinion, it is very unprofessional for a vendor to not provide assistance to an owner especially when it is simply supplying information that they are required to supply by code ( G.1.3.2 ). If they refuse to cooperate you will have to determine the loads yourself or with help. It is not self evident what proportion of the tension is handled internally by the dome, this will have to be determined. It is not hard to figure out the dome reactions other than circumferential tension using basic geometry. It is not hard to establish the capacity of the tank as a plain cylinder. It is not hard to project these loads to the foundation and review its capacity. Again, see G.2.3.1.

I may be oversimplifying and/or missing items, probably because of my ignorance of conditions.

Good luck!

 

[aamir2003]

Thank you.