API 650 TANK 1

[picasa]

A tank 55 foot diameter, 48 foot tall needs to be designed per API 650. It is a waste water tank. It will have floating roof and a conical shaped bottom so that water can be drained without solid accumulating. I have a few question about this:

1. Can the plate material be SA 283C or does it need to be SA 516 70N? Can the roof material be Aluminum? Can it be SA 36?
2. If tank has conical bottom, does it need a foundation in conical shape or will it be supported on a ring?
3. If tank is drained through say a 3" outlet at the bottom of cone, is there any clogging risk? will it be a problem if the pipe that is used to drain cloggs? Plan is to have the tank drain on its own under static head without any pumps.
4. Any interior lining required?
4. Corrosion allowance same on the plate and bottom or different on two? I would guess we need more corrosion allowance on the bottom.

Does API 650 give any standard data sheet that we can just fill out to get the final data sheet? or we have to make a new datasheet?

 

[unclesyd]

Before we start suggesting any resolution to your questions comeback with some information on the chemical and physical properties the waste water.

You have some very serious design issues that need an experienced tank designer/fabricator to be involved. I think people will be glad to give some guidance to some basic information, but as stated the actual design of this tank will require an experienced tank designer.

 

[JStephen]

Where is the tank located? The shell material should be selected to suit the temperature requirements from API-650. First choice is usually A36, second choice A36-Mod, third choice A516-70. Normalized plate would only be required in very cold climates. Also consider Appendix A as a design option.

Roof plate and bottom plate not in contact with the floor would normally be A36. A typical roof would use a center column and radial rafters to support the roof. You could use a geodesic aluminum dome as manufactured by Conservatek, Temcor, or Ultraflote as an alternative- see the appendices in API-650.

In order for the tank to drain to the center (or to an off-center sump), it needs only a very slight slope. This would generally be achieved by sloping the sand fill inside the ringwall. A tank this size would not normally have a suspended bottom. If the tank bottom is to have a very steep slope, that would be an out-of-the-ordinary situation that would have to be examined in more detail (and is not covered by API-650).

A tank of this size would normally have an outlet larger than 3". As to whether that would clog, depends on what you put in the tank, I guess. The cost to change a 3" pipe into, say, 12", will not be too significant compared to the cost of the tank.

Water tanks are normally line with epoxy coatings- Tnemec "PotaPox", for example. For waste water, some other non-potable coatings would be acceptable as well.

Amount of corrosion allowance is optional with the purchaser. The majority of water tanks built do not have any corrosion allowance specified. With a grade-supported floor, the floor acts simply as a membrane, and can actually withstand more corrosion (in a uniform fashion) than can the shell (which IS stressed).

API 650 does include data sheets that can be filled out.

An alternative design code for the tank would be AWWA D100.

 

[JStephen]

Can the tank be built open-topped? Normally oil tanks have roofs to prevent vapor escape and product contamination, water tanks have roofs to avoid water contamination. But raw water and waste water tanks can be open topped in many cases, with savings in construction and in maintenance.

 

[picasa]

waste water has some hydrocarbon in it. water is not really corrosive. water from this tank will go to waste water treatment plant.

 

[picasa]

The bottom of the tank needs to be sloped towards the center slightly. This is to not allow solid particles to collect at the bottom. What section of API 650 give recommendations / design guideline about such slope? Is there any limit to it? at the center sump can be located as putting a nozzle might crack the welds over time. Please direct me to appropriate sections of API 650.

 

[picasa]

What sections of API 650 talk about external and internal surface preparation for coating, painting etc.

 

[picasa]

Have any body designed an API 650 tank with sloped bottom towards centerline? The reason for slope is to not allow solid particles to collect at the bottom of tank. If you put a 3" nozzle in a 55' diameter tank, will that provide effective flushing of solid particles under liquid column head? I think the slope required for effective flushing got to be far higher and I dont know if API 650 tank can have that kind of slope. Please opine.

 

[unclesyd]

It would be an interesting design with a butt welded compression ring and then a center cone bottom. It could be done but getting it by the code might be another matter.

I have been a 60' foot dia. water tank with a fairly large 6' center sump/well. It was deep too, I know because I fell in. I don't think the tank was a complete API design. The tank floor was a lapped 3/8"/1/2" thick plate to bridge the 12" wide chase for the 8" drain line. There are now three CS water storage tanks with Al roofs.
We have two large storage tanks that have man doors to facilitate cleaning at periodic intervals.

We have 2 30' dia SS API 620 tanks that have 2 separate sumps located 180? apart.

At one time our SS tanks were built with a convex bottom, somewhere around 6" rise to the center prior to the advent of putting all large SS tanks on grillage bars.

Back in the days when you could put in a false bottom on sand fill we did put a couple with slopes in ~1/4"/ft range in SS tanks whose original bottom was lost to chloride SCC.

All of the above SS tanks have nozzle arrangements that tend to keep the lower section of the tank in mixing mode due to crystal growth at any nucleation sites. The all have a center nozzle in the dollar plate to facilitate the use of rotary tank cleaning nozzles.

As stated above you could get by with some slope but I don't think enough to do any good. I think the settlement rules/level rules will get you.

You might look at putting in a several small mixing eductors to mix the tank.

 

[picasa]

What does float well in external floating roof do?

 

[JStephen]

As far as what slope is required to make particles go downhill on the bottom- can't help with that. You might check in mining literature or with the different suppliers of clarifier equipment.

I have been involved with some projects that used a scalloped bottom arrangement with radial beams to support a sloped tank floor. In those cases, floor slope was around 2:12. Those tanks were also much shallower than yours.

I'm not sure where API-650 discusses bottom slope- don't have a copy in front of me. I think Appendix A discusses foundations, and I would look there first.

The tank codes don't specifically limit the slope. I have seen 1:20 used, maybe steeper. But the design assumption used for the floor is that the floor is entirely supported by the soil. Typically, a soil report will show greater settlement under the center of the tank than at the edge. With a more or less flat floor, this is no problem, and you can have substantial deflection with minimal stresses. As the floor becomes more cone-shaped, it becomes more of a structural element, meaning differential settlement will generate larger forces. You can get an idea of the stresses by calculating the change in length, assuming the outside is radially fixed, of a differential settlement, and converting this to a strain and then a stress.

I don't believe API-650 discusses coating at all. API does have some "Recommended Practice" documents relating to coating. Also check AWWA D102. Also check with your coating supplier.

A float well in an external floating roof should be for a float-type level gauge to operate. Look up Varec level indicators, for example.

 

[JStephen]

RP 652 "Lining of Aboveground Petroleum Storage Tank Bottoms" is the only one I see in API.

Bottom slope requirements are in B.3.3 of API-650.

 

[MJCronin]

picasa, et al....

All of your questions, answers, speculations etc would be addressed with the proper tank specification.

I believe that you should reconsider your requirement for the use of API-650 in the tank specification. AWWA D103-97 (design of bolted tanks) would be much more appropriate. Most waste water tanks are of the bolted panel style and are built to this specification.

AWWAD103 tanks are commonly specified with powder coated, bolted panels, sumps, domed roofs etc. Bolted tanks can be made larger by raising the roof with more shell sections, they can be instaled in areas without any trained welders and have an extensive corrosion history available for the various coatings offered

See some of these links:

http://www.superiortank.com/prodserv.htm

http://www.bhtank.com/bolted.asp

http://www.aquastore.com/product.asp

Why wouldn't a bolted AWWA tank work in this service ?

My opinion only.....

MJC

 

[JStephen]

The original post specified a floating roof tank, for water with some hydrocarbon in it, which would tend to eliminate the bolted tanks. If there are enough hydrocarbons present for emission or fire concerns, you need to comply with API or equivalent standards.

Note that bolted tanks can only be increased in height if the thickness of the bottom shell ring permits it- which would usually not be the case on a larger tank. The "trained welder" issue is usually a non-issue- most tank companies don't hire welders onsite. You'll note that the second link above brags of the company's erection skills- doesn't sound like they pick up laborers locally, either.