Lining for Sodium Hypochlorite Tank 5

[Razif]

Just to get a feel of what is being used out there, which is more common for this purpose: rubber lined tanks or PTFE lined tanks? The solution is of 11% concentration when freshly delivered. We have occasional plugging of the dosing lines with a black substance which we suspect is the rubber lining. Comments/feedback please.

 

[RGCook]

I just did a plant upgrade and installed a 12k gal NaOCl tank that was FRP. Also, HDPE can be used.

 

[cmtham]

I will go for FRP or HDPE as well. If I am not wrong, that's how NaOCl are shipped - HDPE drums or FRP semibulk containers.

 

[ghopkins]

FRP tanks are common in this service. Make sure that the vessel is built to the following criteria:
1) use an epoxy vinyl ester resin (DERAKANE 411 or HETRON 922)

http://www.derakane.com

or

http://www.hetron.com

2) the curing system should be BPO/DMA system instead of Cobalt/MEKP system. NaOCl will attack the cobalt and severly shorten the service life.
3) a post cure of the vessel will also lengthen the serivce life - usually 200F for 4 hours
4) use a synthetic veil in the corrosion barrier
5) stamp the vessel according to ASME RTP-1 if less than 15 psig and ASME Section X if greater than 15 psig.

This type of construction should provide 7-10 years of life. Another option is to line the fiberglass with PVC. This type of construction should provide 15-20 years of life. A PVC lining will add about 15-20% to the cost of the vessel. PVC is good to about 150F. If higher temperatures are required, I recommend to line it with Halar ECTFE (a copolymer of ethylene and chlorotriflouroethylene) which is very good in bleach and caustic environments.

see

http://www.solvaysolexis.com/Halar_ECTFE.htm

PTFE can be expensive and the quality of the welds is very poor because it is not melt processible. Some modified PTFE's are starting to come out on the market that are somewhat melt processible, but I wouldn't experiment with this type of construction yet.

Let me know if you want more information.
ghopkins@rl-industries.com
513-874-2800 x119

Hope this helps,
Greg Hopkins

 

[glennpan]

Razif,

How many gallons in capacity will your tank need to hold? Will the tank be indoors or outdoors? Here is a basic rundown of the different materials that can be used.

HDPE or XLPE: There is a lot of debate between using cross linked polyethylene and high density polyethylene. The truth is that there is not much difference in performance between them. Both are low cost options that are probably the best value for tanks that are under 3,000 gallons. XLPE is a thermoset which means that it is stronger, but is more brittle and cannot be welded to. HDPE is less expensive, can be repaired, and welded to. These are the most common types of tanks for NaOCl storage.

FRP: Many municipalities use premium grade resins for their large NaOCl storage tanks. They are extremely strong, and reasonably priced (though not as low as PE). The problem with this material is if a pinhole develops in the veil, the NaOCl can follow the winding around and develop a leak in a different part of the tank making it nearly impossible to trace. Also epoxy coating on the outside is a must, or any spill of material will eat the tank from the outside.

FRP/PVC (Dual Laminate): This is a fantastic style of tank, and will give you the best life span. However, the lining will double the price over a standard FRP tank.

Rubber or Teflon Lined: These are the most expensive methods for storage and not the best. Rubber lining can degrade over time. A pinhole, or chemical attack near a seam will lead to a catastrophic failure. If you decide to line with a Teflon product, I agree that Halar is the way to go. Look for a manufacturer that uses an electrostatic method of applying the coating. This way there will not be any seams.

I have written an article on handling Sodium Hypochlorite that includes information on tank, piping, valve, and pump selection. It is posted at

http://www.panner.com

 

[ghopkins]

Here are some links for more information:

http://www.odysseymanufacturing.com/bleach_information.doc

http://www.solvaychemicals.com/docroot/chlo_inorg/static_files/attachments/hypo_general_brochure.pdf

(takes awhile to load)

And perhaps the best resource (but you have to buy their manual) is the Chlorine Institute:

http://www.cl2.com/whats_new/naocl.html

I'd like to comment that the cost for adding a PVC liner to FRP does not necessarily double the price of a tank when compared to "straight" fiberglass. It all depends on the size and configuration. But its safe to say that the incremental cost for the PVC lining (as a percentage of the total tank cost) goes down as the size increases.

The links above all contain information from NaOCl manufacturers (or the CL2 Institute) regarding the recommended handling of NaOCl.

Hope this helps. Incidentally, Glennpan, I saw your article on the web earlier today (prior to your post) after doing a search on

http://www.google.com

for PVC and sodium hypochlorite. Nice article.

Greg
ghopkins@rl-industries.com

 

[TheTankman]

I have supplied numerous chemical storage tanks. Sodium hypochlorite is indeed the "Tankman's best friend." Due to changes in PE formulation and processes, PE is now the best "bang for your buck" for storing NaOCl. Cleanliness of the system is of the upmost importance.

Anyone wishing to discuss hypo storage, I am the guy with more experience with this application than most.

Consider tank location, gasket material, how often the tank is cycled.

I have a published article archived at

http://www.waterandwastewater.com

titled "Specifing PE Chemical Storage Tanks."

 

[chicopee]

I came across a steel tank that was relined with a rubber lining to hold sodium hydroxide. The tank cpacity was around 10000 gallons and build about 20 years ago. The tank was designed per NB code and had its upper end cut out for insertion of liner. Job was done a few years ago and liner apparently still functions properly.

 

[Graybeard]

We have a need to replace and existing lined carbon steel tank that has been used for this service for about 8 years. We are in a bit of a stalemate as one group wants to use ASEM RTP1 PVC lined FRP and the other group wants to use ASME rubber lined. Note, this is a vented tank so pressure isn't really a problem, QA is the biggest driver for the ASME requirement. I've seen much info on the web and it seems that nobody is installing lined steel these days. I tend to favor FRP myself, but I'm trying to be open minded about it. Has anyone installed lined steel and if so, why did you choose it over FRP or lined FRP? In our case for a 10,000 gal tank the cost appear to be about the same.

 

[ghopkins]

Graybeard:

I myself haven't heard of rubber lined carbon steel being used in NaOCl service. The eight years service life is quite respectable. All you can expect from straight fiberglass is about 7-10 years if its built properly, as I mentioned above. The cost for this type of construction should be similar to your rubber lined steel cost.

However, again as I mentioned above, PVC lined fiberglass should provide 15+ years of service life. We've had PVC lined tanks in this service that are going on 18 years service life.

Your request for the ASME RTP-1 stamp is an interesting one. Please feel free to contact me for a list of requirements for this type of construction.

Greg
ghopkins@rl-industries.com

 

[ghopkins]

Graybeard:

One last comment:

In your risk analysis between the two types of construction, some factor should be applied for catastrophic failure. I'm assuming if the rubber lining fails, the steel structure will not last very long and provide the potential for catastrophic failure, perhaps in a time less than the inspection period. However, with PVC lined fiberglass, if the lining fails, you still have very good protection (7+ years) from a secondary FRP corrosion barrier, not to mention the FRP structural wall. Annual or even semi-annual inspections will reveal any need for repair long before failure occurs.

ghopkins