POWER PLANTS: CONCRETE UNDERGROUND WATER TANKS vs ABOVEGROUND STEEL TANKS

[Kinemenematics]

PROJECT:
Country: Bangladesh
Relatively high salinity in river - located about 1 km away.
Windspeed: zero
Temperature: 16C - 33C dry bulb
Humidity(seasonal): 40% - 80%
Design criteria hierarchy: Performance > Cost

My problem: I need to present the differences between two different WATER STORAGE tanks, but I am an inexperienced engineer, so I need to know with some certainty how these two types of tanks perform under the above conditions. I am not ready to conclude if what I have researched online is comprehensive, let alone valid. Need opinions


Relative Differences between Concrete underground steel tanks & above ground steel tanks

U/G concrete

Pros:
Costs less
Space is saved above ground
lower maintenance costs
no corrosion issues
cannot be pierced
cannot be destroyed in a fire

Cons:
static head not available for pumps
tank drainage is difficult
leak repairs are difficult

A/G Carbon Steel

Pros:
Static head is available for pumps
Easy leak repair

Cons:
subject to corrosion (but we could always paint it)
Costs more
piercable
can be destroyed in a fire
shop fabrication involved
installation time is greater


Judging by the number of pros and cons, U/G concrete tanks seem to be the best option but since more A/G steel tanks exist in actual power plants in similar weather conditions, I'm keen to know.

 

[MintJulep]

Quantify.

Identify or estimate for both:

Initial Cost
Useful Life
Annual Maintenance cost
Susceptibility to damage by:
Wind (cyclones) wind speed zero in Bangladesh, really?
EArthquake
Flood
Fire
Terrorist attack
etc.
Cost to replace at end of life or if destroyed
Annual operating cost
etc. Whatever else is important to consider.

Add them all up. Lowest total wins.

 

[Kinemenematics]

to clarify: "Wind (cyclones) wind speed zero in Bangladesh, really?" i'm just limiting the scope of study. Wind speed is 28kph max.


Thanks mintjulep, any other opinions?

 

[JStephen]

What are you storing in them?
For petroleum storage, steel tanks are normally used, due to leakage issues with concrete.

 

[LittleInch]

I'll comment in depth later, but please specify size / volume, number of tanks, soil type, water table depth, product being stored (salty water? potable water, cooling water??).

Also what sort of performance are you talking about (performance>cost)??.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cvg]

U/G concrete

Pros:
Costs less (probably not)
Space is saved above ground (not really, you probably should not build anything over the top)
lower maintenance costs (not convinced this is true either)
no corrosion issues (you might have corrosion of reinforcing and high sulfate contents in soil are tough on concrete)
cannot be pierced
cannot be destroyed in a fire (well, the roof could be)

Cons:
static head not available for pumps
tank drainage is difficult
leak repairs are difficult

A/G Carbon Steel

Pros:
Static head is available for pumps(yes, but only a few feet of head and you also need to fill this tank which will require higher pressures)
Easy leak repair

Cons:
subject to corrosion (but we could always paint it) ( you will need to paint it both inside and out on a frequent basis)
Costs more (not sure that it does)
piercable(yes, but is this really a high risk?)
can be destroyed in a fire (yes, but is this really a high risk?)
shop fabrication involved
installation time is greater (not likely)

 

[cvg]

28 kph max wind speed, not likely. you cant limit the scope using un-realistic wind speeds. Design wind speeds are generally 70 - 100 mph or more where tornados and hurricanes are common, such as in bangladesh

 

[LittleInch]

I agree with cvg on most of his points. One thing I always ask people to do is where possible quantify your statements, i.e. avoid "less", "more", high, low and use figures or percent. Your interpretation of what is important may not match your readers, but without this data it becomes difficult to weigh up the issues.

One thing missing form this is any hint of the civil design. I don't know a lot about Bangladesh, but I do know it is very low lying and hence if you dig a big hole for your concrete tank you may just fill it with water or have it collapse. similarly your AG steel tank may need special foundations or piles. There's nothing like a bit of ground works to push the price up....

The space saved idea is pointless - the cost of a roof to be able to do anything on top of you buried tank would be huge compared to the benefit unless you're really short of space - again a site dependant thing we know nothing about.

Really don't know why the pierced / fire case is such an issues - this is a water tank. Would be a long way down on my list of issues

Your 28kph wind speed is way too low for design, but AG steel tanks can be designed for 100mph without great issues providing they're not too tall and slim

concrete tanks, especially ones with corners or penetrations can leak and unless you often stop and check for a long time, might be leaking for along time. If the water is benign then there may not be a big issue, but finding a hole and then repairing it can take a loooong time.

In general there is probably not too much in it between them , but you need to make it much more site specific and include schedule in this as well.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Kinemenematics]

I do not have detailed empirical data off-hand nor much time to do a deeper study (this is a tender support job). The Contractors are from China and they are trying to cut cost here by suggesting the use of concrete U/G tanks.


cvg: this is historical wind data provided by a weather study at the same site. Bangladesh is vast, and this portion of that site is not affected by cyclones, etc. (or at least they have not)

littleinch:
product being stored: raw, well water; potable water; auxiliary cooling water; condensate storage;

Performance:
1. space saving (the client is hell-bent on this). What about totally buried tanks with single storied buildings like site offices on top of them? would this be considered a normal practice?
2. Corrosion resistance (high salinity of river water but thanks for the info on the soil)
3. Safety prerformance (fire, floods, etc)
4. Ease of tank drainage
5. Ease of maintenance and repair



cvg, littleinch thanks for the info, I was thinking along the same lines too; I've already given them importance ratings, but will modify them thanks to your inputs.`

 

[Kinemenematics]

btw, are there any ready sources on the web about soil sulphate contents?

 

[Kinemenematics]

oh nevermind i found it.

yes,
salinity is going to be a problem.
floods occur every year
there is a potential of a hurricane even though there hasn't been one in the area

 

[LittleInch]

If you have different types of water to keep separately then a compartment type buried tank might start to be more attractive as one wall is then shared and the walls might act as supports for your roof. I still wouldn't put any permanent buildings over it, but maybe it could be used as a road or car park where if required you can clear the top off and get access to it. Because of the thickness of the concrete, the reinforcement required and the increase in weight to be supported by the walls of the tank, it normally doesn't make sense to roof over these sorts of buried tanks with anything that can support a lot of extra weight such as another building.

You could think about bridging over the tanks with steel beams onto separate piers or foundations if you're really short of space, but seems a lot of bother and expense to get a few square metres....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

First, this question should have been posted in the "Storage Tank" forum

Second, it is important to note that not all steel tanks are alike ! !

There are welded steel tanks in conformance with API-650 and AWWA D100 (which require a certain amount of welding skill to construct)

And there are "Factory-Coated Bolted Carbon Steel Tanks for Water Storage"..... built, inspected and constructed to AWWA D103.

http://superiortank.com/

http://www.awwa.org/store/productdetail.aspx?ProductId=6912

These offer several advantages for water storage compared to welded tanks:

- Ease of assembly - suitable for third world applications
- Quality, baked-on Factory coatings ( Virtually no maintenance for decades)
- Easy expandability -- need a larger tank ? simply remove the roof and increase the shell height !
- Possible dis-assembly and re-use of tank in another location

http://books.google.com/books?id=Bb...wAA#v=onepage&q=advantages D103 tanks&f=false

MJCronin
Sr. Process Engineer
Venture Engineering & Construction

http://www.VentureEngr.com