hi can anyone tell me if there a

[spereira]

hi

can anyone tell me if there are any rules of thumb for designing storage tanks, I mean regarding height/diameter ratio? I have to design a 3000 m3 vessel but due to spacial reasons it can't have more than a 17m diameter. this gives me approximately an 11m vessel height. The problem is that I don't have much experience with this, so I'm not sure if this is possible or logical, or if there are macanical restrictions when chossing D/H ratios in big vessels, like storage vessels..I could also split it in two smaller vessels (maybe for process reasons it would also be logical) but then construction costs and civil works would also go skyhigh!

Regards

Susana

 

[IFRs]

This is not a big vessel, as above-ground non-pressurized cylindrical storage tanks go and your D/H is quite reasonable. API 650 covers their design. I've seen tanks 100M diameter and others 24M high. Most tanks tend to be wider than high, especially when larger than about 12M in diameter or so, but that is not any rule of thumb, just what I see around. I suspect that economics had more to do with the D/H ratio than anything unless space or height restirctions come into play.

 

[phex]

the d/h ratio has an influence on static pressure inside the tank, simply due to the higher liquid level inside the tank. using a diameter as large as possible, one reduces the liquid pressure on the lower part of the walls, which in turn reduces cost of construction. (this at least holds true for non-pressurized tanks).

just my two cents.
chris

 

[spereira]

thks guys
I'll have a look at the API650 standards.
Being an aromatic compound (aniline) I need blanketing on the vessel, a few gr of pressure, I guess. does this change any of the consideration before, regarding non-pressurized vessels?

 

[Montemayor]

Susana:

There is a valid engineering reason for the the Height/Diameter ratio used in above-gound, vertical storage tanks. It has to do with the mechanical design required when one has to design for the turning moment. Wind loading plays a big factor in this design. Seismic and other loads may also affect the over-all design. For an average, crude oil-type application the criteria mentioned by IFRs and Phex are all valid and constitute a basis for tank selection. Lower height means less hydrostatic pressure and less wall bearing pressure; however, the trade-off is that more real estate is required and the roof design starts to increase in size and weight.

A tank with a Height/Diameter ratio less than 1.00 is normally a very stable vessel and should present no civil design problems. The type of tank also has a bearing: is it cone or dome roof? or floating roof? Will it operate with a gas blanket?

Sometimes, as you note, process common sense (like a need for two vessels, instead of one) is the controlling factor in determining the size of the tanks. Otherwise, you may not be able to operate the unit under normal conditions and process needs.

Art Montemayor
Spring, TX

 

[spereira]

Montemayor, thks for the above considerations.
process needs doesn't involve the need for two tanks so I've decided to go for a 20m diameter/10m height tank. as for the roof, it's cone roof. we already have a 1500 m3 aniline tank with a 16m diameter and the roof didn't pose such a huge problem so I guess 20m doesn't seem so impractical. But this is giving me a huge retention basin!!!but i guess it would be the same if I used a smaller diameter since it must hold the complete amount of liquid held in the vessel...

 

[lastone]

I realize there are many factors in tank design, but one primary factor is also economic. I am not a design engineer, but I have read somewhere that a controlling factor is standard stock size of steel plate. Don't quote me, but I think it usually comes in standard heights of 10'X(10,20,30'). It is less expensive to fabricate a tank from stock plate than have to make extra cuts and welds. Your company should have guidelines on tank design. Also, I think you should check with other design engineers in your company too.

 

[25362]

to spereira, there is little left for anyone to add on API tank sizing. Sometimes, the use of low alloy steel for the two or three lower strakes may be economically justified.
Besides, a knowledge of the ground conditions with respect to the allowable bearing pressures may set limits to the tank height. The total and differential settlement may be important in avoiding unacceptable stress in the shell and connecting pipework. The shell-to-base welded joint is generally the most highly stressed part of the tank and uneveness in support can become critical with the passage of time. One must look for guidance on appropriate tank foundations in API 650 or BS 2654.

 

[gkarthik]

Hi,

I have a small suggestion to you. Why don't you try your storage tank in spherical shape instead of going to conventional cylider.

In the case of spherical shape you are compelled to construct the storage tank with D/H ratio = 1. This nullifies the problems posed by civil works as per the discussion.

And also spherical shapes accomodate maximum volume for a given area.

 

[spereira]

thks for the suggestion above gkarthik
I never considered spherical shape since I thought it was only used for high pressure storage (gives the minimum wall thickeness I think). But aren't construction costs higher in this situation?
regards
Susana

 

[Montemayor]

spereira (Chemical):

I believe it is nonsensical to consider a spherical tank for atmospheric, oil storage. Up to now, you have not given us the contents identification nor the operating pressure. I have to assume you are debating an atmospheric pressure and crude oil (or similar). If so, that is the reason why API 650 was formulated.

Spherical tanks are for very special conditions - like pressurized, liquefied gases (LPG, CL2, etc.) and are astronomical in cost compared with API 650 design. Your capacity would require a 19 meter diameter sphere (probably 20m, allowing for spiral staircase) whose civil design and foundation alone would probably be more costly than the equivalent API 650 design. And the field fabrication would be an additional larger cost. If you don't believe me, try it.

Of course, the sphere would look much better than the "can", but the aesthetic value added would probably deplete your bank account. Engineering logic points to the API design.

Art Montemayor
Spring, TX

 

[spereira]

Montemayor,
I beleive you!I've also reached the 19m diameter number (math is the same everywhere ;-)) and had a feeling that construction costs would be much higher (in the past we had spherical vessels for ammonia high pressure storage and that's when I learned that this shape gives us the minimal wall thickeness but never wondered about construction costs though). I was just wondering if civil works would be less in a case of a spherical tank!I guess not...
The vessel is for nitrobenzene atmospheric storage. I've asked for a quotation for two different d/h ratios (d/h 2 and d/h 1.3) and will now try to calculate the foundations for both. after that I'll be able to choose the most favourable configuration.
Regards
Susana

 

[MJCronin]

spereira,

I have a fair amount of experience with the preliminary design of API-650 tanks for various kinds of storage. I can offer you the following points to consider:

- Art Montemayer offers you solid advice in his comments above... Spherical tanks are rare and only used for liquidified gasses and/or where the tank material is extremely expensive

- The API-650 code limits the operating pressure to 2.5 psig....above that limit, the API-620 is the code commonly used... Will your system be able to operate under 2.5 psig ?

- It is my experience that the most economical API-650 tanks are those with a height to diameter ratio of about 1.0 to 1.3.....That old relic of design engineers, the "Pressure Vessel Handbook" (Megusy)had a formula to calculate this. You will not have the most economical design.....but you will be close

- Purchase the book "Aboveground Storage Tanks" by Philip Myers(McGraw Hill)..(about $125)... It will answer many of your design questions....Have you considered normal and emergency venting....level indication ....ladders ???

- No one seems to be mentioning the fact that this tank must be field fabricated....it has been my experience that the cost of field fabricated tanks will depend largely upon the cost of erection crews, the schedule, the difficulty of erecting on site, painting, coatings etc. Costs can vary widely.....get several quotes from experienced fabricators...

- Premium painting, coatings and internal linings can easily change the cost of the tank by by 30 to 100%. Since this must be done in the field, ensure that you include these costs.

- Disposal costs for the API-650 mandated hydrotest liquid (typically water) can be another large cost figure....include these costs.

Just my thoughts only.....by the way,.....what is the vapor pressure of the liquid at 100F ?


MJC

 

[spereira]

MJCronin,

thks for pointing out other points to keep in mind. I'm used to design process equipment, not storage vessels so I'm still learning and studying all alternatives. I knew that it should be field erected but haven't considered the other costs that you've mentioned like premium painting, coatings and internal linings (though I don't think internal linings will be necessary) and ladders. I've already considered foam and other emergency equipment as well all instrumentation but have forgoten about the ladder. thks.
as for the other questions the vessel is atmospheric (no blanketing system) and nitrobenzene vapour pressure at 20ºC is just 0.3torr.

regards
Susana

 

[natlus]

To get a cost effective design, I suggest you indicate the tank dimensions but give the manufacturer/supplier the option to alter those dimensions to achieve your required working capacity if necessary. If you have any restrictions in plot space, you can highlight those as well.

 

[unclesyd]

Natlus has the right idea.
Give the tank fabricator your parameters, get a civil engineer to design the pad, and please invite the tank fabricator to view the site. You will probaly be stuck with the venting. Don't forget the electrical ground requirements.
Just for information tankers used to use Pi dimensions to keep from cutting plate.

 

[Hookem]

Mention is made that the tank containment is quite large! Please reveiw NFPA. You might get a break if you combine more than one tank into a common containment area. Instead of having 110% of capacity of each tank, one only needs 110% of the larger of two or more tanks.

Mention is made of tank thicknesses. API 650 uses two methods; for these small sizes, the wall more likely than not will be the minimums of either API 650 or operating company standards PLUS applicable corrosion allowance.

Mention is made of a support pad. Pad is fine except that it should have a larger OD than the tank to 1) spread the vertical fluid pressure over a larger area to keep from getting differential soil settlement, 2) spread out the weight of the tank itself over a large area, 3) provide easy means of anchoring the tank against winds -both sliding and overturning (check UBC and ASCE 7-98), 4) provide easy means of anchoring the tank against floating in case the tank is empty - or close to it - and the secondary containment holds water, oil, or other fluids, and 5) that pad does not include secondary containment. Consider ringwall only on larger tanks.

 

[spereira]

"Mention is made that the tank containment is quite large! Please reveiw NFPA. You might get a break if you combine more than one tank into a common containment area. Instead of having 110% of capacity of each tank, one only needs 110% of the larger of two or more tanks"

Hookem, I checked this before since I wanted to do this with an existing storage vessel for benzene. in this case I would only need an extra 1500m3 containment area instead of the 3500m3. question is that by portuguese legislation the containment area should have the capacity of both vessels so there would be no advantage in doing this. other reason was that benzene is considered Class A material and can't be stored in the same area as nitrobenzene. I'm going to check european legislation to see if this "110% rule" holds. Splitting the big vessel into two would really cut civil works a great deal, apart from space requirements...