Carbon steel wall thickness as per API 650

[minhaj17]

Dear All,

We are working on a project where there is are requirements to carry out preliminary design. I need some suggestion regarding wall thickness. As per the API 650, 5.6.1.1, the minimum plate thickness required for 36m diameter tank is 8mm. Now my question is does this 8mm includes corrosion allowance or I have to consider like 8+3 corrosion allowance = 11mm. Please advise. The tank dimensions are 36m dia x 15m height with 3mm corrosion allowance.

Regards

 

[JStephen]

For shell plate, the corrosion allowance is included in the minimum thickness, not added to it. The purchaser has the option of specifying otherwise if desired.

 

[MJCronin]

There are many factors not yet considered that will affect the final wall thickness

Please review Appendix K of API-650 ....

This Appendix contains Tables that will enable you to make reasonable estimates

MJCronin
Sr. Process Engineer

 

[MJCronin]

For a tank of your dimensions, assuming a design SG of 1.0 and a min yield of 30,000 psi, I suggest:

- Five course shell construction

- First Course 0.875 inch
- Second Course 0.750 inch
- Third Course 0.625 inch
- Fourth Course 0.500 inch
- Fifth Course 0.375 inch

let us know what your final estimate looks like..

Are there aditionsl tanks in your scope ??

Tell us more ...

MJCronin
Sr. Process Engineer

 

[JStephen]

The shell plate design will be based on product loading, hydrotest loading. It may be affected, particularly in the upper rings, by wind design. It may be affected, particularly in the lower rings, by seismic or elevated-temperature design. It may be affected by pressure or vacuum design. Thickness will vary depending on the grade of steel used. Thickness may be increased due to availability of steel or due to erection considerations.

The specific question was on the interaction of specified minimum and corrosion allowance, and that was what I was responding to above.

 

[minhaj17]

Dear All,

Thanx for your replies.

Dear MJ Cronin,

As per the data;
product design stress bottom course=184MPa, upper course=196MPa
Minimum yield strength=250MPa
Minimum tensile strength=400MPa
Minimum hydro static stress=171MPa,
product - water,

my calculation shows as, including corrosion allowance of 3mm (0.118")
1st course-18mm (0.708")
2nd course-14mm (0.551")
3rd course-12mm (0.472")
4/5/6th course-11mm, (0.433")

 

[MOHAMMED89]

As per API 650,Clause 5.6.1.1 the 8 mm minimum thickness is mentioned with a word 'nominal thickness', If we refer Clause 3.21 it is mentioned that the Nominal thickness is the ordered thickness which includes any corrosion.

For your case = Tank 36 (D)m x 15 (H)m

Assumptions:-

1.​

Total 6 Nos Shell courses each of 2.5m width.

2.​

Material A36 - Allowable stress in operating condition = 160 Mpa & Allowable stress in hydrotest condition = 171 Mpa.

3.​

Tnak is design for full liquid level (Full height).

4.​

Corrosion allowance = 3mm

For the above mentioned assumptions the required thickness (Including corrosion allowance) will be:-

1.​

Shell course 1 - 19.21 mm

2.​

Shell course 2 - 16.45 mm

3.​

Shell course 3 - 13.70 mm

4.​

Shell Course 4 - 10.94 mm

5.​

Shell course 5 - 8.18 mm

6.​

Shell Course 6 - 5.43mm = 8mm(minimum required as per Clause 5.6.1.1)

Regards,
MOHAMMED

 

[minhaj17]

Thank You Mohammed.

 

[minhaj17]

Dear Mohammed,

The Operating condition (product design stress) which you have considered is 160MPa, whereas the Client has specified product design stress bottom course=184MPa, upper course=196MPa. Based on the Client's data I guess my calculations are correct. Please confirm if you can.


Regards

 

[MOHAMMED89]

Dear Minhaj17,

The the product design stress is based on the MOC for the shell plate, In my calculation i have selected material A36M for running the calculation. So please check API 650. Clause 5.6.2 & Table 5.2a. from that table you can get the Product design stress for your particular material.


Regards.