Tank Annular Plate Inspection 3

[DefenderJ]

Does anyone have any experience with external (UT) inspection of API650 tanks for corrosion of the annular plate?
I have an ø18m tank, 27 years old, carbon steel containing hot caustic soda solution. This cannot be taken out of service without vast expense. I am concerned over annular corrosion due to experience with other tanks on this site. I need to get some quantitative data on the condition of the tank. At present I have no data, so I cannot quantify the risk.

I have seen information about Lorus. Does anyone know if this acutally works?

Thanks in advance

 

[rustbuster]

Lorus works well and will tell you if you have wasteage but it is limited in sizing the depth. It is a good indicative tool.

 

[IFRs]

Can you make a small trench under the annular plate and get thickness from the underside?

 

[DefenderJ]

Thanks guys for the info.
I think I need to use some technique that gives us some information about the condition so we can decide the course of action.
How limited is the sizing capability of Lorus - is it the further away from the probe the less accurate? Will it give good information in the critical zone?
If Lorus is not good at sizing the depth, will it give a qualitative indication ranging from good to very bad?

Unfortunately the tank, is sitting on a concrete ring beam, so excavation is not really and option, though I like the idea of a physical check.

 

[rustbuster]

DefenderJ,

I would call it semi quantitative, if there is corrosion or pitting they will find it and be albe to tell you if it is minor, moderate, severe. Don't think they can qualify bottom vs. top side corrosion. Range is limited to about 1 meter.

 

[DefenderJ]

Thanks Rustbuster.
I know it is only bottom side corrosion that is a concern for this tank.

Semi quantitative is better than no info, so I'll think I'll go for it.
I have a tank nearby where I know there is definite annular corrosion so I could use this to gauge the results from the technique.

 

[NDTENG]

There is a new technique called HOMC Guided waves which works better than LORUS. IT has been evaluated on a few tanks in South East Asia aimed at detecting corrosion in the critical zone of the tank while in service. Its sensitivity is better than the LORUS which has been used before howerver it is also semi quantitative as sizing with Guided waves is fairly limited. Sensitivity for top and under side thinning is the same. Defects under the shell have also been detected and on each job the correlation has been good. We are looking to validate the same on more tanks with known problems to expand the database of verified results before the equipment launch. A paper on the same is soon to be published in an internation jounal on NDT.

 

[DefenderJ]

NDTENG Thanks for the info.
LoRUS certainly seems a bit limited as the data is only qualitative, the HOMC sounds better.
I assume that it is not been offered on a commercial basis at the moment as it is still in development. I'll keep a look out for it in the future.

We have decided to begin our checks by excavation as IFRs suggested, though it involves breaking up concrete. We have discovered that there was a 200mm deep bed of concrete applied after the main ring beam was cast which makes it possible to remove a layer without too much difficulty and damage.

 

[NDTENG]

Should you be excavating and would allow a validation on known defects, it will be a good thing for validating the HOMC. A semi commercial tool is available for scanning work, however the same is restricted to projects where a confirmation of the result is possible such as in your case.

 

[DefenderJ]

Thanks for the further info NDTENG.
Nothing better than seeing the damage as well as the screen showing there is a defect.
Looks like a good technique.

 

[kalamani]

Hello Everyone

We are constructing 2 nos of atmospheric cone bottom liquid sulphur storrage tanks. The manway Nozzles (Not flush type) 24"/30" are fabricated using plates rolled and welded with doubleV longitudianl welds.

API 650 is not mention about the NDT requiremnts of this manway Logitudinal welds. Please guide me which section of API650 will specify the NDT and extent of NDT. OR is it usual practice no NDT ,only Visual testing is done on Longitudinal welds on manway Nozzle neck.

Is the API 650 doesnt not take into account of the joint efficiency for this joint, I too understand that the tables 5.3 and 5.4 are the only tables from which the thickness of the manway nozzles are selected.

I will appreciate your views and inputs.

 

[DefenderJ]

Kalamani,

I suggest you start a new thread as your question is quite different to the original topic.

I am not familiar enough with API 650 to help guide you through it. However I would do radiogrpahy of the longitudinal welds on workshop fabricated pipe sections.