Compads on API650 Tanks

[Batie]

We have several storage tanks built between 1965 to 1983 ranging from 10m to 22m diameter. All are 15m tall.

All inspections are currently done using API650/653. The actual code of construction is unknown.

The majority of tanks have manholes with regular type re-enforcement plates, but none of the compads have rounded corners. Similar with compads about the shell nozzles.

What is the risk.
Yes, there is an increase in stress but how much?
Are we lowering the fatigue cycles number?

How can this be calculated to show we are at risk, rather than just think we are?

Thanks

 

[Geoff13]

Sketches or photos ??

 

[Batie]

 

[Geoff13]

I assume the hashing is a screen to keep workers from entering.

Certainly that is not a shape permitted by the tank codes I'm familiar with. API 650 has a "diamond" repad, but all the corners have big radii.

However , taking note of the fact the tanks have been in service for 40 to 60 years, I would leave them alone. Removing the repads to replace them with round ones would be a lot of work with little benefit. Trying to modify the existing pads in place would likely make things worse even if they look more rounded.

I've never heard of fatigue as a concern for flat-bottom tanks.

 

[JStephen]

Even in API-650, you're allowed to use alternate designs. I don't recall right offhand if there is anything requiring rounded corners on nozzle reinforcement. (There is a requirement for miscellaneous repads, though, but I think it's fairly recent, too.)

On an issue like this- it is undesirable to have the angular corners, due to potential (minor) stress increases.
What is overlooked, is that it is also undesirable to start cutting, welding, and reworking on an existing weld and existing plate, due to residual stresses and weld distortion.
So you can spend a lot of time and money making it look like it has lower stresses, when you may in fact be increasing the stresses by the rework.

 

[IFRs]

There is a non-zero risk that is probably low but it is hard to quantify. If there is a change of service, tank lean, foundation settlement, external (pipe) loading, observation of corrosion or other change or detail that might affect the fitting then the risk goes up. You can try NDE, PMI and FEA to lower the risk but there may be a non-zero risk that such investigation and analysis may uncover additional deficiencies.

You have identified questionable details and will own any consequences attributable to your failure to mitigate them.

This is a risk / cost / consequence balance.
The replacement cost and cost of a failure can be quantified.

You can begin a program of replacing the fittings one tank at a time and show that you were making a good-faith effort to fix the issues while spreading the cost out over a reasonable timeframe.

You have a responsibility to the environment and the local community to operate safely and without incident, with your eyes wide open and your conscience clear.

If your neighbors were harmed by a failure how do you think they would fair in court when it is found that you inspected the tank, noticed something that is not in the recognized standard being used to inspect but you decided to not fix it just because it would cost money?

Sharp corner repads are not shown nor alluded to or hinted at in any portion of API 650 or 653. All acceptable repads shown or referenced have radii.

My basic advice is: "If you touch it you own it."

If you do anything including change of service, reducing weld spacing to the corner weld, pressure testing, etc that might affect the integrity of these fittings in even some small way, cut them out and replace them. If you determine that any of the material grades or thicknesses, weld hardness, reinforcing area, weld details etc do not meet current API standard, cut them out and replace them.

If you don't go anywhere near them, and all your NDE and PMI meet the current standard consider leaving them - if one fails you can claim not only did you not do anything to compromise the fitting you did additional investigation which revealed only the one detail that was not shown in the standard.

 

[FMJalink]

Hi Batie,
If the tanks were not designed and constructed to API 650, they were most likely designed and constructed to BS 2654. Depends on what the company described and the industry you are in, e.g. fine chemicals or pharmacy normally do not describe petroleum industry equipment.
As mentioned above, modifying will most likely harm more than improve the situation. The tanks most likely are already near to their original design lifetime or have surpassed that already. Proving to be well designed and constructed.
Now old age problems as fatigue at high stress/tension points as sharp corners and welds are a higher risk. Best approach seems monitoring condition of the welds and wall at the sharp corners regularly on cracks etc.
Success

 

[JStephen]

Looking at this a bit more:
-For repads, per the wording of 5.7.1.8, you could use repads that were square or otherwise had corners, and that would fully comply with the current API-650. IE, that nozzle detail could be used right now with the current code, "as long as the reinforcement meets the area, welding, and weld spacing requirements outlined in ..."
-For insert plates, there is a requirement to radius the corners, but for rectangular insert plates only, and that requirement is waived where the insert plate is the full height of a shell plate. There is a requirement to taper the thickness. But, for an insert plate shaped like the picture above, there is no requirement to round the corners.
-Keep in mind that rectangular shell openings are not allowed in API-650, but they are allowed in API 12F, API 12D, and would presumably be much more of an issue than corners on an octagonal repad.

 

[IFRs]

Not sure if this applies for the cut edges on the periphery of of repads but I'm not sure why it would not:
5.7.1.6 Sheared or oxygen-cut surfaces on manhole necks, nozzle necks, reinforcing plates, and shell-plate openings shall be made uniform and smooth, with the corners rounded except where the surfaces are fully covered by attachment welds.

Notwithstanding 5.7.1.8 (in which it is not clear if sharp corners were contemplated) Figure 5.7 includes an arrow pointing to the lower left corner of the repad which says "Rounded corners (150mm [6in] minimum radius). This figure is for manways only. These same dimensions appear in the 1958 15th and later editions of 12C but in the 1951 10th edition sharp corners are shown as alternates (I don't have other versions that will open).

Figure 5.8 for nozzles shows Dp/2 as the radius for all corners of the repad and does not include a minimum radius note. These same dimensions appear in later editions of 12C but in the 1951 10th edition sharp corners are shown as alternates.

Clearly sharp corners were deliberately included and then removed from API 12C and were never in API 650, presumably for some reason(s).

As in other cases, how you interpret the language depends on if you read it to mean "only what is shown is allowed" or "if it does not specifically say you can't do it then you can". I tend towards the first because a decision to assume risk is above my pay grade.

 

[JStephen]

That "corners rounded" clause would apply, if, say, you had a 1/2" repad with a 3/8" fillet around it. With a 1/2" repad and 1/2" fillet, the surface falls under the "fully covered" clause.
I think the old bolted door sheet detail used sharp corners. I'm not sure how (or if?) they worked repads on riveted tanks, but that may be where the angular shape came from in the 1951 version.