Hi Everyone,

Our company is new to the design of Pressure Vessel, but for one of our jobs, we are required to fabricate some tanks, the details of which are mentioned below.

ID: 1700 mm
Length: 2150 mm (tangent to tangent)
Ends: 2:1 ellipsoidal dish ends
Shell thickness: 6 mm
Dish end thickness: 6 mm
Material: A240 316L
Design Pressure: 2kPa
Design Pressure: 85 degC

please note that this is not a pressure vessel, because the pressure inside is too low.

It has a 20" Manway (500 mm ID and 8 mm thick nozzle). I have 2 questions, which are regarding the 20" Manway

1. Would there be a problem, if we did not provide any reinforcement pad while welding the 20" manway.
2. It would not be wise to be put a 20" NB flange, which has dimensions as per ANSI B 16.5, because that a 20" SORF Flange would be quite heavy. So we decided to make the flange and cover for the manway from plate (12 mm thk). Is the thickness ok? Does ASME Sec VIII tell us something regarding the flange and cover made from plate, and its thickness, particulary for the manway.

Regards,
Abhilash Kurian

Kuriana,

I would advise to consult ASME Code Section VIII; Div. 1, Para.U-1.
The vessel with MAWP of 2kPa is not in the scope of the Code.

If although considered within the Code, please refer to Appendix 2 for the design of the flange. In this forum we do not intend to perform design calculations, we recommend to consult an experienced designer.

By the way, if the flange is a fabricated hubbed flange, it may not be machined from plate.

DK

why don't u want to put pad for such a big nozzle

Abhilash,
1. Where do you want to locate your manway? Perpendicular to the shell axis (in the shell) or in the axis (in the dished head)?
2. What is the service? (what medium, any fluctuation in pressure and temperature)?
3. Is it possible to see partial vacuum in the vessel sometimes?
4. Are the dished heads die-pressed or spinned? Are they off-the shelf or ordered specially for this job? Can the holes be flued-out (like in Fig UG-38, only outside)?

Konrad

_________________
Konrad Anikiel

Hi Konard,

Thanks a lot for showing interest to my query.

Please find below the replies to your queries.
1. The manway is located perpendicular to the shell axis.
2. The service is basically the storage of various chemicals. The vessel has atmospheric pressure. For design purpose, the consultant has asked us to take 2kPa as the design pressure.
3. No, the vessel, will not see partial vacuum, because, it has a vent that is open to the atmosphere, ensuring that the pressure inside the vessel is always atmospheric.
4. The man hole is not related to the dishends, hence I am not providing any details of the dishends. The dish ends are specifically made for this job. They are not of the shelf. They are cold formed from plate.


My major concern is the cover and flange for the Manhole. Can this be fabricated from plate and welded on to the nozzle. As mentioned above. ASME Sec VIII Div 1 App 2 lists out all the flange connections. But what I want to know is that why we should go for a hubbed flange, in this case. Why cant we just make the flange and cover from the plate and weld the flange to the nozzle and put the cover (blind flange also made from plate), like a slip on flange welded to a pipe. Is this acceptable?

Please respond.

Hi Abhilash,
I would say, that stress concentration and fatigue issues are of the greates concern, when a vessel is under low pressure. But since it's open to atmosphere and you don't expect any pressure fluctuations- there is no problem at all. I would follow UG-36 even though the tanks are not going to be stamped, just to demonstrate that area replacement is satisfied anyway. Since it is not a nozzle (no pipeline is connected to it), no additional forces will act on it. You don't need any reinforcing pad. The only reason to use it in this case could be manufacturability. You will need to stiffen the shell during welding to ensure it won't be distorted because of heat and material expansion/shrinkage. Just think of some stiff and massive woodden frame firmly holding the inside shape to prevent from this effect, and you're not going to have any problems!

PS this is why I anticipated that these manholes will be located in the dished head- much less problems with welding distortion, and you could flue the opening (with spinning) to avoid any welding at all. Why not suggest it to the customer?

_________________
Konrad Anikiel

If ASME Sec VIII is not a mandatory design code by client specification API 650 can be used which covers storage tanks with internal design pressure upto and including 18 kPa.

API 650 allow use of plate flanges. RF pads are however required for nozzle sizes 3" and larger.

Regards

Sanjeev,
I don't have API650 at the moment, are you sure whether this requirement applies to nozzles only or inspection openings as well?

_________________
Konrad Anikiel

Please note that API 650 would not be applicable in this case because API 650 Sec 1.1 states

"This standard covers material, design, fabrication,
erection, and testing requirements for vertical, cylindrical,
aboveground, closed- and open-top, welded steel storage
tanks in various sizes and capacities for internal pressures
approximating atmospheric pressure (internal pressures not
exceeding the weight of the roof plates), but a higher internal
pressure is permitted when additional requirements are met
(see 1.1.10). This standard applies only to tanks whose entire
bottom is uniformly supported and to tanks in nonrefrigerated
service that have a maximum operating temperature of 90°C"

In our case, the bottom is not fully supported.

Konard,

I would require your suggestions for flange and the cover for the nozzle.

The issue regarding pad plate (reinforcing pad) is over.

But my concern is the flange and the cover. I referred App 2 of Sec VIII Div 1. Sec 2-4 (3) Optional type flanges, which suits my design. Am I correct in choosing Item 10 of Fig 2-4.

I have a doubt in this case, becasue there is no hub for this case.

If you refer Sec 2-4 (3) Optional type flanges, it mentions that this would be applicable if B/go is not exceeding 300, where go is the hub thickness, but as per Item 10 of Fig 2-4, there is no hub at all. What should I do?

This is a major concern, because in the near future I would have to design a tank, which is vertical cylindrical with 2:1 ellipsoidal dished end in the bottom and top is a flanged cover. The capacity of this tank is around 300 L. So, there is no need to go for a horizontal tank.
In this vertical cylindrical tank, the top head would be flanged. I am not sure, which design to follow. I was hoping to use Item 10 of Fig 2-4. But again, the hub thickness comes into play.


Please advice.

Regards,

Abhilash Kurian

Abhilash,
I don't really think that you can get a lot of benfit from calculating it as optional. Anyway, you can do it as optional, just take the dimensions like they are described in sketch (7) of Fig 2-4. The bottom line is, that when you consider strengthening effect of a hub, then your nozzle becomes the hub! Good luck with all the C's from the 3-7.1 Table

PS if you really want to sqeeze the design to the last gram of steel, consider O-ring gasket and Appendix Y- type flange. Put a lot of small screws instead of typical few big bolts. You owe me a kingfisher if it helps

_________________
Konrad Anikiel

Hi Konard,

Thanks a lot for the replies.

I guess Heniken would be a better choice than Kingfisher.

All are welcome to give their views.

I have to look into this in detail. Shall let everyone know regarding the same, once I get the design over.

Thanks,

Abhilash Kurian

Konrad,

API 650, Clause 3.7.2.1 mention "Openings in tank shells larger than required to accommodate a NPS 2 flanged or threaded nozzle shall be reinforced".

Table 3.3 to 3.14 provides requirements including reinforcement pad requirement for nozzles and manholes.

A separate RF pad may not be required if the area compensation is provided by providing extra thickness on shell/ nozzle neck above the minimum required thickness.

I hope I am not missing any thing else in API 650.

Abhilash,

I do not think that including cost for engineering/ designer manhours, you will be able to save a lot by using a custom designed flange to ASME Sec VIII. In case it is going to be a mass production, situation may be slightly different.

Regards

Sanjeev

OK, this is a requrement pertaining to nozzles, not applicable in this case

Are those requirements or provisions to be used "when required"?

Come on, you want to tell to an engineer that engineering is not the right way to go? I have designed gazzilions of flanges to different rules, and I still live. It's not harmful or anything

_________________
Konrad Anikiel

Dear Konrad,


Does it make any difference on opening except for external loadings. Area compensation requirement will be same in either case. Any way same table is referred for RF pad dimensions for manhole and nozzle.



Unfortunately could not locate words like "if/ but/ when required" for manhole RF requirements.


Neither the Intent was to undermine engineering efforts/ capabilities nor to conclude that the cost will be less. Just wanted to highlight that considering the efforts to buy custom designed flange, saving may not be significant as compare to standard flange. 20" NB Standard flanges are in regular production and may be available off the self at Competitive prices.

Sanjeev,
What I mean is that many times I saw people doing something strange. When I asked why they did it that way, the answer was frequently "because it was on a sketch in some code". Even if the sketch name was "Some acceptable...". Sometimes people just can't see the difference between a requirement and a suggestion. This is what I asked you to investigate: to ensure whether you are citing a requirement or a non-requirement.

And buying something off the shelf is the easiest way, I can bet that this option was investigated by Abhilash in the beginning

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Konrad Anikiel