Stainless Steel Potable Water Tank 2

[JohnWeal]

We have a requirement to construct 6 x 40m diameter stainless steel water treatment process tanks. The tanks are 6m high and will be on a concrete base.
The tanks will be welded and I wish to know the following:
1. Best choice of stainless steel (raw water is from a non saline river so chlorides are very low)
2. Weld preparation requirements
3. Post weld pickling procedure

Appreciate this is a sizable construction program and I need to ensure procedures are written and strictly adhered to. The tanks must not show any sign of rusting during the 20 year asset life or be subject to any kind of corrosion due to poor quality welding procedures.

Best Regards
John

 

[metengr]

What code of construction are you using? This will determine weld preps and wall thickness.

How close to any coastline are the storage tanks?

316L stainless steel or duplex stainless steel would be my first cut.

Keep tools segregated for use on stainless steel or duplex stainless steel only to ensure no iron contamination.

Pickling, you have options.

 

[JohnWeal]

Thank you metengr.

We are not near the coast.

In some parts of the tank there are some areas which may hold a little sludge which precipitates from the additional of polyaluminium chloride.

The velocity of flow around the tank is less than 0.5 m/s so was also concerned with MIC. Hence controlling welding but also checking material. Even though this is not saline, I agree the 316 would give better protection.

Regards
John

 

[MJCronin]

Consider bolted water storage tanks, built in accordance with NSF purity and proven AWWA D103 structural standards.

They are advertised to meet a long service life, are of a proven design and have been around for decades.

Tanks can be built from a variety of materials including stainless steel panels and potable water coatings.

They con be constructed in underdeveloped areas where there are no welders, but manual labor is available.

Tank designs can be rebuilt and extended (upward) to gain more capacity for the future, or disassembled and moved should future plans require this.

http://www.nationalstoragetank.com/tanks/stainless-steel/bolted.html

MJCronin
Sr. Process Engineer

 

[bimr]

Agree with MJCronin on the bolted aspect. These may be fabricated from SS. In our experience, the newer fused glass material does not seem to retain it corrosion proof quality as the older fused glass installations.

https://www.cstindustries.com/produ...ed-to-steel-water-storage-tanks-manufacturer/

304 Stainless Steel will be adequate for a raw water system with little chloride content. Chlorides are generally not an issue until the chloride concentration reaches 200 mg/L, and 200 mg/L is a much higher than concentration than most raw water applications

http://www.worldstainless.org/Files/issf/non-image-files/PDF/Euro_Inox/Cutler_water_EN.pdf

 

[JohnWeal]

Many thanks for your valued comments.

 

[metengr]

Given your last post regarding sludge concerns, you should not consider any traditional stainless or even duplex steels. You should be using either lined carbon steel tank or FRP material, period. Having any potential for sludge especially with chlorides (concentrate in sludge) or even MIC just about kills metals.

 

[JohnWeal]

Metengr,

I was also concerned that the velocity in the tanks is low so there is little turbulence effect to scour the walls.
In all honesty, we want to go back to a concrete tank design we had previously.
But our chairman is pushing us down the SS route.

With all the valued comments you have presented, it confirms our suspicions for sure.

With poor welds leaving tints and not being pickled, that I believe is also bad for MIC.

Regards
John

 

[bimr]

A typical water treatment application of polyaluminum chloride or PAC would add only 4-5 mg/L of this chemical which is 10-20% raw strength. The sludge from water treatment processing is primarily water treatment residual (silt) not PAC. There is no reason to be concerned about chloride addition from the use of this chemical.

MIC will occur in anaerobic conditions where there is a lack of disinfection residual. Provision should be made for periodic shock disinfection. MIC is more of an issue in the water distribution system than in the treatment plant.

 

[metengr]

bimr;
The issue is under deposit corrosion from sludge, and stainless steels and even duplex stainless steel are notorious for developing corrosion pitting under these conditions. If you have no sludge not a problem. Adding the element of MIC under the deposit just exacerbates the risk.

 

[bimr]

metengr,

Understand your comments.

However, I have not seen this problem in potable water tanks. The design life of potable tanks is typically longer than 20 years.

Most of the sludge from water treatment plants is deposited in the treatment tanks, not in the post treatment water tanks. Here, the poster is talking about light amounts of post treatment solids (may hold a little sludge").

Potable water tanks typically have a concrete bottom and steel walls.