API650 Tank Tolerance 1

[magmath]

Dear Experts.

Our client is asking for tolerances more than what is defined by API650. Much appreciated if you could advise if any one come across this and can this be achieved?

1. The maximum out-of-roundness measured 300mm above the floor to shell weld shall not exceed +/-19mm. [highlight #FCE94F]The maximum out-of-roundness at any other point on the tank shell shall not exceed +/-38mm[/highlight]

2. Flat spots shall be determined using a vertical sweep board. At no point shall the gap between the vertical sweep board and the tank shell, measured perpendicular to the tank shell, exceed the values provided in Table 1.

Table 1 Flatness Limits
Distance between measuring points (mm) : Flatness Tolerance (mm)
Less than 500 : [highlight #FCE94F]5[/highlight]
500 to less than 750 : [highlight #FCE94F]8[/highlight]
750 to less than 1000 : [highlight #FCE94F]10[/highlight]
1000 to less than 1500 : [highlight #FCE94F]15[/highlight]
1500 or greater. : [highlight #FCE94F]20[/highlight]

Thanks
Magesh

 

[IFRs]

tank diameter and height?
shell plate thicknesses and grade(s)?
construction method ( spiral, stack, jack, etc )?
field or shop erected?
designed or built to any API 650 Annexes?
foundation type and tolerance?
material fabrication tolerance?

 

[JStephen]

Normally, the out-of-roundness higher up in the shell is the out-of-roundness at the base plus the out-of-plumbness. For a tall tank, the specified tolerance will be a lot tighter than API tolerances.
I would question the need for any restricted tolerances.
And if you're out-of-tolerance using standard fabrication and erection procedures, what then?
Is there a good way to measure out-of-roundness higher up in the tank shell?

 

[Geoff13]

Watch out for the first one. You can't control it as you build the tank since it will change shape (at least a little bit) every time you add a new shell ring, move the scaffold, weld a seam, etc. You won't know what shape you've built until after you're finished, and then it's too late.

We had a customer come in after a tank was complete and put a 3D laser scanner in the centre of the tank. I believe it claimed an accuracy of a millimetre or two. We then spent a ridiculous amount of time (dollars!!) trying to round it to the customer's satisfaction. No idea who paid for this wasted of effort. The tank will be perfectly round when filled whether we'd rounded it or not.

The second tolerance can also be extracted from the 3D data. You might take a few measurements as you go, or even a few hundred if you're ambitious, but the customer will be able to extract tens of thousands of locations from the data, and find all the spots you missed.

You need to negotiate both the acceptable tolerances, as well the inspection method (how often, sweep board or 3D laser, etc.). Since API tanks don't fall down we can be pretty certain that these tighter tolerances won't result in a safer tank.

 

[magmath]

Thanks Experts.

Here are some more detail of the tank.

1. Tank diameter is 27m and height is 16m
2. Plate thickness at the lower course is 16mm and at the top course is 8mm and it got an intermediate wind girder at 7m from the top
3. Method of Construction - Jacking
4. Material grade is A36

Hi Geoff,

if the construction team put 3d laser scanner during construction and measure the tank dimensions, do you think these tolerances can be achieved?

Thanks
Magesh

 

[Geoff13]

That's for you and your construction group to answer, and I don't know how you'll decide what tolerance you can meet.

Lat's assume your tank will be 6 rings high, and you've built the top 2 rings so far (jacked starts construction at the roof). Ring 6 looks round due to the roof and ring 5 looks round because it's on the jacks, or perhaps the jacks create a matching series of bumps where they're mounted. Either way ring 5 is not in it's "relaxed" state so there's no point trying to round it out to the stated tolerance. Perhaps a couple of rings later ring 5 will relax but if it's now out-of-tolerance what are you going to do about it? Perhaps it will change again by the time you finish the shell and take the tank off the jacks? Who knows because no one has done this sort of scanning during construction to understand how a mid-construction scan compares to an after completion scan.