Effects of Briny Water on Cast Iron Tanks 1

[javry]

Hi guys - First post on this forum. I am in Aghanistan doing some project management and engineering for a clinic that is being built in a very small village in Helmand Province. The design calls for an above ground water tank made of 3/4 inch cast iron that will be charged by a local water well. Ground water here tends to be on the briny side with a salt content high enough that the locals can taste it and will generally avoid it if they can. Anyway, what I need to know is whether the brine will have a corrosive effect on the cast iron and any alternatives that you think appropriate. Thanks in advance guys.

 

[micalbrch]

Yes, you can expect corrosion of the cast iron. How much and how quick depends upon the chemical composition of the brine.

Can't you use a plastic tank? PVC will work well.

Take care of yourself!

 

[EdStainless]

If the salt level is low enough that you can taste it but it wont hurt you then cast iron should be fine. It will corrode but will last quite a while before it fails.
If you are using cast iron because it is locally available then you might consider using a liner in it.
Two coats of a good epoxy or other suitable resin will give good life.
However be warned that the surface preparation is critical. Without proper blasting and cleaning it will never stick.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[javry]

Thanks guys. The fact is, I'm using your input as ammo to recommend going with a plastic tank. I'm way out in the boonies here and bringing in a cast iron tank, IMO, is just asking for trouble. The variables are just too wide and long. The client still hasn't addressed whether it will be prefab or built onsite so that's another issue along with the ones you guys addressed. Assuming after all has been said, I have to use cast iron as designed. What precautions should I take in addition to those already mentioned?

 

[gr2vessels]

The cast iron is brittle;- don't let anyone hide behind it from bullets, it might shatter.
Perhaps a good idea would be to pain it on the exterior also. Is it drinking water or some cleaning use? The internal paint might need to be suitable for potable water. The rust will leach into the water contaminating it. The paint must be checked carefully for holidays (pin holes), which could cause rapid localised corrosion.
Why dont you use the corrugated galvanised sheet metal storaage tanks, which can be delivered in flat packages, easy on the truck and can be errected on site with hand tools. If needed, it can be lined interior with prefabricated plastic or rubber liner, you only hang it inside the shell like your laundry washings on the fence and it will stay there for decades, no corrosion problems. Besides of the joke, a bullet will put a hole on the liner, which can be patched easily like a bicycle tube, for another 20 years. Look up on the web for AWWA bolted and lined tanks, fabricators everywhere in the world, very cheap and delivery no problem.
Cheers,
gr2vessels

 

[cloa]

People should be referring to cast iron as grey cast iron. Malleable cast iron is grey cast with a long heat treatment ~24 hr and nodular cast iron is grey with alloy additions and they are both tougher than grey cast iron.

 

[javry]

Thanks for the replies guys. In general, I don't thiunk this is a very good design. But it's what I have to work with. Also, there is the issue of culture. I've attempted to add a pdf of the prfile elevation drawing. It's my first time at this so I hope I get it right. In it you'll notice a whole lot of design features that are unique to this part of the world - things that we probably would not do in the states. You'll need to zoom the pdf up to 200% to see the detail on the tank. With enough input from you guys, I'm hoping to convince the powers that be here through a Request For Information [RFI] to re-think the cast iron in favor of something more suitable, given the remoteness of the area. If at the end of it all, I have to go with cast iron, I still have a "make or buy" decision in front of me which brings on a host of other issues. Any input is appreciated guys.

 

[javry]

BTW, to answer gr2vessels question, the water in the tank will input to the clinic as white [potable] and output both grey and black water on the back end. The grey and black water lines are seperate but converge at a septic tank. Will take your advise and look up AWWA bolted and lined tanks on the web.

 

[btrueblood]

Are you sure they want cast iron? The drawing says "iron", which to some is equivalent to steel.

If iron, perhaps push to have it concrete-lined. Huge amounts of ductile iron pipe is used world-wide for potable water mains, much/most of it concrete-lined.

 

[javry]

until I get more info, I'm operating under the assumption that I will have to use an iron tank. And true enough - it may be a steel tank. I am also under the current assumption, based on input from you guys, that some sort of liner will be required. I prefer plastic or some poly based material because it's easy to work with and would be easy to replace down the road. My current concern is to get this addressed as quickly as possible because this will be long lead and most likely drive my critical path.

 

[SJones]

Wouldn't a plastic or GRP tank be amenable to delivery by chopper, or is it too much of a target? Also, don't forget that if you sit the tank on soil, an 'iron' or steel tank will suffer major corrosion from the outside (a race between thermodynamics and ballistics!!)

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[javry]

Steve- In looking over the design, my principal conclusion is that the designers were guarding against attacks and bullet penetrations - which are valid concerns for this part of the world. An iron tank with a brick and plaster housing gives them that sort of protection. I'm ok with that as long as I address the issue of how to minimize any impact from the brine in the water on the tank and visa versa. The shortest answer I can think of is to make sure the brine never comes in contact with the iron. And a ploy liner is probably the easiest way to accomplish that. Any other suggestions are welcome.

 

[rconner]

While I'm not familiar with your specific application nor locale and for whatever it may be worth, I believe at least ductile iron (tougher than gray cast iron) pipes do have some history of application as various sorts of modular or readily constructed "tanks".
In this regard, ductile iron and steel water pipes (the latter at least in larger sizes) are standardly furnished with a cement mortar lining (and it appears based on a quick internet search many users of ferrous tanks have apparently now gravitated towards mortar linings, perhaps/I suspect based on history relative to thinner polymeric linings).

 

[gr2vessels]

javry,
You said the lined light tank is the best way to go. Stick to it.
Anything else would be far more expensive, unreliable, potential of corrosion and other damages, perhaps irreparable. The rupture of the lined, bolted sheet metal tank is almost impossible, unles using heavy artilery, but even then, in two days is up again and stores water. No corrosion problems, lining with 20-30 years of life. No cement to crack, no rust to perforate the shell, assembly with normal hand tools, a little chain block for lifting panels, what could possibly be easier and less expensivethan this? However, if they want cast iron, give them cast iron. Just make sure to protect yourself from blame, if the iron turn the potable water into a red likor.

 

[rconner]

I'm not sure at least ordinary "sheet metal" would put up much of an argument with even a common say ~30 caliber rifle shot (see e.g the accounts at

http://www.thehighroad.org/archive/index.php/t-498219.html

). From what I remember of old test reports I saw many years ago, ordinary thicknesses of sizeable steel and ductile iron pipes would perhaps do pretty well, as long as the shots were not of quite high caliber and dead on. I understand the Alaskan pipeline was actually shot many times before it was eventually penetrated with a very high powered 40+ caliber bullet, from I believe a reportedly drunk shooter who somehow managed to get a solid enough hit on the roughly half inch thick and I believe specified quite good quality steel. (See the warning from a poster on the blog I referenced -- don't try this at home, it can come back atcha!)

 

[BrianGar]

Glass wool insulation between brick and metal tank. Careful it does not well up with interstitial condensation(I think thats the suitable term) and turn into a damp mess that could corrode tank from outside in. Id nearly prefer to just run with a sealed airspace rather than glass wool if you cant get your hands on some some sort of closed cell foam/non absorbing insulation, or perhaps expanded polystyrene spheres mixed with a pva solution, they would be better - If you cant convince higher powers that a sealed airspace works too. Basically, pick something other than glass wool.

I witnessed a camper company that fitted glass wool behind beauty board panels adjacent to factory steel skin. Results were not at all good after a year or two. Skin showed rust bubbles on outsides at bottoms of sections(where interstitial condensation collected). No provisions were made to allow this moisture to dry, or drain so corrosion was a certainty.

Take care,

Brian,

 

[BrianGar]

One more thing I just thought of, (I seem to be posting twice a lot of late, apologies, sitting in airports is distracting)is the possible thermal expansion of the fill and supply pipes. By the looks of the drawings, the short pieces of pipe horizontally travelling from tank seem to be pretty short, with no crush media between them and brick. The pipe directly under tank has no means of expansion provision at all. Given that I have seen a steel tube 17' long grow .004 thou with a temperature rise of 20deg, maybe this expansion could cause cycle/fatigue failure after some time, either in pipes, elbows, tank connections, or tank material surrounding connections. Im not a pipe work guy, so maybe it wont. But pipes could possibly be in sunlight, and full of non moving water for periods of time, water draw or supply, would cause a temperature drop, and therefore thermal expansion/contraction many times a day.

Brian,

 

[javry]

Thanks for all the suggestions guys. Believe it or not, we are taking a lot of them to heart. We started foundation work a few weeks ago and we are waiting on the tank to arrive. And BTW, it IS poly lined. Thanks again.