Tank differential settlement

[purode333]

Dear sir,

As per API 650, differential settlement is 13mm per 10 metres of circumference.

Does it means that 13mm on every quadrant or increase from qudrant to quadrant.

Example: For 20 metre Dia Tank, from 0 to 180 degree, max differential settelment is 39mm.

Pls clear.
Ask if any doubt in clarification.


Regards
Prasad

 

[Dhurjati Sen]

Dear purode333,

API 650 is quite clear as below:

"If settlement measurements are specified by the Purchaser, any differential settlement greater than 13 mm per 10 m (1/2 in. per 32 ft) of circumference or a uniform settlement over 50 mm (2 in.) shall be reported to the Purchaser for evaluation. Filling of the tank shall be stopped until cleared by the Purchaser."

So,anything above 13 mm needs to be evaluated.

Regards.



DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[purode333]

Sir,

but code said as 13 mm per 10 meters.
Pls reply about my above mentioned example of 20 meter dia tank.

 

[LittleInch]

This should be in the tank forum really, but section 7.3.7.6 appears to relate the settlement to the circumference. ("13mm per 10m of circumference")

So a 20m diam tank has a circumference of 62m so I read max settlement from one side to the other as 80.6mm. But max uniform settlement of 50mm is reportable

It is written quite strangely and may mean that within any 10m of circumference the settlement shouldn't be more than 13mm??



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IFRs]

I think it is pretty clear. In any 10 meters you can have 13mm of differential settlement needs to be reported. Uniform settlement over 50mm also gets reported.

For your tank we need more information. You have 39mm of settlement in 62m of arc length. You don't tell us if the differential settlement is more than 13mm in any 10m arc length. For instance, do you have 13mm in each of three adjacent 10m sections? Do you have 39mm in the first 10m arc length and then no differential settlement the rest? The total settlement you have is less than the max but I would still consider reporting it.

 

[Dhurjati Sen]

Dear purode333,

Why do we evaluate settlement?

1. Strength considerations.

2. Slope issues.

3. Corrosion.

All codes, standards and best practices have been written based on the contributions and experience of innumerable unknown people working for many decades in their respective fields.

I do agree that at times, due to various considerations like economics, administrative, time, etc. one can always deviate from the code and normally the code also gives enough leeway for such deviations.

However, mis-interpretation of the code is never an option.

So, in your case, you get more than 13 mm differential settlement in any 10 metres of circumference, you need to evaluate it. This part of the code is talking about the slope, i.e tanµ

Also, any uniform settlement more than 50 mm, irrespective of the circumference, you need to evaluate it, too. This part of the code talks about actual measurement of the depression.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[LittleInch]

purode 333,

You have, I think, 2 issue here
If as you say from one side to the other you have a general differential settlement from one side to the other then this needs to be looked at in terms of "plumbness", section 7.5.2, i.e. no more than 1/200 of the tank height

In terms of differential settlement, sections 7.5.5 on the foundations apply to start with and then sections 7.3.6.5 state measurements every 10m of circumference once the tank is built and is being filled during testing.

Hence section 7.3.6.6 starts to make more sense and is looking for any differential settlement in a 10m section of circumference of the tank.

So if your settlement was indeed a general tilt from one side to the other then go back to the 1:200 limit for which you need to know height of your tank.

The 7.3.6.5 section is for a local settlement of one section of a tank or general (flat) settlement of >50mm.

So regardless of the diameter of the tank the 13mm is indeed 13mm in any 10m circumference of the tank which in your case is 6.2 sections.

An out of plumbness by a tilt from one side to the other is the 1:200 out of plumbness limit.

~So if you have say 6 measurements around the 20m diam tank and 5 of them are say +/-5mm of each other and the sixth is -35mm from the two either side of it then the tank fails this section. It's all about differential from one end of your 10m circumferntial arc to the other end.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.