API 650 Spiral Staircase Design Issues

[RobsVette]

Hi Everyone, I don't usually post in these forums as I dont get to much involved in API work but recently I embarked on a project to correct a defective installation on an oil storage tank. I was hoping you guys could answer a few questions for me about how to properly design and install the spiral staircase for a tank.

- In regards to the spacing of the steps, I am looking at the requirements on figure 5-18 and 5-19 that have all the guidelines for staircase design. The question is, are there any notes anywhere on tolerances for this? The stairs that got installed have a rise and run spacing that is about 1/2" to much. I have walked up the stairs and dont feel any unease, so I would think they are just shy of proper spacing (twice the rise plus the run = 26" max.) I guess the contactor that installed them just wasnt hugely careful. Also the steps are not perfectly level. Some of them are 1/4" off across the 28" span of the step. Is this a major problem that should need to be corrected?

- Second question is in regard to reinforcing plates, used to attach the steps to the tank. Some of these plates cover the tank seems and they are not much larger than the steps themselves. Thus they do not exceed past the seems by the required 6" called out in API-653 9.10.3. I was going to recommend that these plates be removed and the plates be replaced with proper size plates. In order to do this, they may have to combine reinforcing plates from two or three steps together. Am I being way to extreme here? Is this requirement on API 653 actually supposed to deal with staircase reinforcing plates or for actual spots where the tank is worn or cracked and needs to be reinforced?

Again, this staircase feels perfectly fine and is not by any means un-usable or uncomfortable to use. So I am wondering if you guys have any experience here. I am going to recommend that any deficiencies be corrected? Am I going to an extreme? Opinions are welcome.

Thanks,
Rob

 

[IFRs]

According to OSHA, "variations in riser height or tread depth shall not be over 1/4-inch (0.6 cm) in any stairway system." If you measure the rise x 2 plus the run at the tank shell or perhaps at the stairway centerline, is it within the API numbers? On the repads, API 650 does not require them. Is this a new tank or a repair? Section 9.10.3 is for bottoms - are you in another section for shell?

 

[JStephen]

I would assume the reference in API-653 is to reinforcing plates around shell openings, not pads at treads or other localized loads.

 

[RobsVette]

This is a newly constructed tank that is for the storage of number 2 diesel fuel. It is not in active service yet.

Where do you get the OSHA reference from. I like that rule as it seems to be correct. As stated that stairway on the tank is in compliance with the 28" rule and most of the steps are about 1/4" out of level from end to end. (Measured at the tank and then measured at the support rail side.

The measurement of twice the rise plus the run is just over the API numbers. The rises are about 9 1/2" for most of the steps and the runs are pretty solid at 8". Its probably over but not by much. Are these recommended guidelines or are they requirements? That is really my question. So far I had suggested all the steps be corrected to the proper API dimensions, but I dont want to make someone do a tremendous amount of work, when it is not required by the code governing the installation.

You guys seem to be correct in regards to my API653 reference. I was also looking at figure 9-6, but that also seems to be for tank bottom repairs. There do not seems to be any restrictions on attachment of stairs or other localized loads. In that case I think we will follow good pratice and not overlap welds / HAZ Etc...

Thanks for the input.

Rob

 

[IFRs]

The OSHA reference is to OSHA document 3124-12R 2003 which you should be able to find online. Note that OSHA does not require that you use the exact rise and run in the OSHA table, as long as the angle of the stairway is between 30 and 50 degrees with a 1 inch overlap and at least an 8 in tread. Also see OSHA 1920.24. API only cares about permanent attachment welds when the shell is above group 3. See section 5.8.1.2. API is silent on other grades of steel and therefore there are no requirements for permanent attachment weld spacing on these tank shells.

 

[JStephen]

Note that the OSHA document referenced only discusses stairways used in construction, there are other rules for permanent stairs. The 12' platform spacing is not normally required for tanks. I'm not aware of there being a minimum tread depth or overlap requirement, either.

 

[RobsVette]

Ok, I feel I'm going to leave the stair case as it is and not have them make any changes. As stated, its fine to walk up and down.

One last question - The figure I was looking at with regard to reinforcing / repair plates is API653 figure 9-1. The stairs were attached to the tank by welding the stair to a reinforcing plate and welding the reinforcing plate to the shell. However, a couple of these plates overlap shell seems. I was planning on recommending that these plate be enlarged to meet the requirements of figure 9-1. Do you guys think this is neccessary? This are only for localized staircase loads. They are no repair plates and will not be covering any weakened or defective area of the tank. I feel I may once again be overdoing it here.

What do you guys think?

 

[IFRs]

Figure 9.1 is for butt-welded patch plates. API 653 is silent on the need for repads under shell attachments.

 

[Duwe6]

My read of 650 is that "repads are repads". Your stair repads should have followed all the rules for clearances from, and crossing of, shell seams. The contractor made a mistake.

If it were my tank, I would have the seams at the repads UT'd or RT'd, in the manner of the ASME VIII rules for repads over seams. I would also determine if these seam welds were made welding uphill or downhill -- 4G-up or 4G-down; and if the filler was 'Red Rod' -- SMAW E6010. My read of the very stringent and conservative API rules regarding one weld being in the proximity of other welds is driven by the somewhat brittle nature of vertical-down 6010 'red rod' welds. If your seam welds were made with sub-arc SAW, or flux-core FCAW, I would stop with successful UT/RT results. If a vert seam that has been improperly overlaid by a repad was made SMAW 6010 4G-down, I would have that seam removed and rewelded. Air-arc - CAC -, or grind out starting 8" above the pad to 8" below the pad. Reweld using E7018 4G-up, and RT/UT the entire weld length and at least 1" beyond the tie-ins, on into the existing seam welds.

 

[IFRs]

My view is that repads are only repads if they reinforce an opening cut through the tank shell.

 

[Duwe6]

This issue might make a good Inquiry to the API-650 committee. It is possible that their intent is what IFR said; that the repad/patch seamweld-crossing rules only apply when there is a nozzle/penetration or a 'wasted' area being patched over. This makes the repad carry shear stress at the pad-to-shell weld, where there is no significant stress at your stairpad-to-shell weld, except from weld shrinkage.

My opinion is based on my 'read' of API's extreme conservatism regarding seam welds. The ASME rules make *much* better sense to me. Under ASME, you would RT/UT the seam prior to putting a pad/patch over a seam.

 

[IFRs]

I think the intent of API is pretty clear and unambiguous. Below are snippets of the code. These are INCOMPLETE quotes and should NOT be used without fully reading and understanding the sections but they convey my message.

API 650 5.8.1.1 "Permanent attachments are items welded to the shell that will remain while the tank is in its intended service. These include items such as wind girders, stairs..."

API 650 5.8.1.2 "Permanent attachments may be welded directly to the shell..."

API 650 5.8.1.2 "Permanent attachment welds may cross shell horizontal or vertical butt welds providing..."

I repeat: these are INCOMPLETE quotes and should NOT be used without fully reading and understanding the sections but they convey my message.

A lap patch is a whole different animal, which is not what the OP asked about.
A repad is a plate added to supply reinforcing usually to compensate for material removed from the shell, which is not what the OP asked about.
A plate between a permanent attachment and the shell is neither a lap patch nor a repad.

I'm not opposed to using plates between permanent attachments and the shell but I don't see them being required in the code.

 

[RobsVette]

IFRS,

sorry to resurrect an old thread, but I would like your opinion on something further.

I agree with your interpretation on the above, but the paragraph 5.8.1.2 seems to contradict itself at least by my understanding.

- 5.8.1.2 (a) permanent attachments may be welded directly to the shell with fillet welds having a maximum leg dimension of 1/2". THe edge of any permanent attachemtn weld shall be at least 3" from a horizontal joint of the shell and at least 6" from the vertical joints, interset plate joints or reinforcing plate fillet welds. Permanent attachemnt welds may cross shell horizontal or vertical butt welds provided the welds are continuous within the limits and the angle of incidence between the two welds be greater than 45 degrees.

Here are my questions
1) 5.8.1.2 seems to apply only to shell courses of material in groups IV, IVA, V and VI. I may be interpreting that incorrectly but the first sentence states that. I am not understanding what they are trying to communicate here. Can you clarify if 5.8.1.2 is to apply to all materials and groups IV, IVA, V and VI are a special condition where you ahve to take modecement of the bottom course into accountr for testing? On second review thats what I think they are saying and you seemed to indicate as much above.

2) bullet point (A) seems to contradict itself. It says all attachment welds must stay 3" and 6" from shell welds but then states that they can cross if the angle of incidence is less than 45 degrees. The way I interpret this is for parrallel welds, not any welds. Ie... if you are attaching something to the tank and the attachment weld will run in the horizontal direction the attachment weld has to be at least 6" from the horizontal seam. Is this interpretation correct?

Thank you for your assistance in this matter.

Rob

 

[IFRs]

Rob -

My interpretation is that 5.8.1.2 is only for certain groups of steel. API is silent on all other steel groups, ergo there is no similar requirement for the other groups. Apparently, the groups mentioned are different enough from other steel groups to warrant this special treatment.

My interpretation of bullet point A is there is no contradiction. API is instructing us about permanent attachments. I agree with your basic interpretation but don't see that the welds must be parallel. Simply that welds must be spaced as indicated unless they cross at the angle specified. The welds can be non-parallel but the nearest approach must satisfy the spacing requirement. Unless the weld crosses at the specified angle.