Sodium Hypochlorite Gassing Off

[tipp79]

I'm working on a WTW upgrade at the moment and would appreciate some advice. A new secondary treatment building is to be constructed with chemical storage tanks being located below ground level. One of the chemicals to be stored in these tanks is sodium hypochlorite. A concern has been raised that chlorine gas could be generated during dosing (with dosing line and dose points at ground level) or while the storage tnaks are being filled and then for the gas to fall down to the storage room thereby creating health and safety concerns for operatives.

I have 2 questions:

[li]How likely is it that chlorine gas would be generated.[/li]
[li]If generated, would the gas fall or rise?[/li]

Thanks for advice in advance.

 

[bimr]

The off gas from sodium hypochlorite storage is mostly oxygen. You will only get chlorine if you add some type of acidic agent to hypochlorite.

Chlorine gas tends to settle. You should have the building exhaust intake located near the floor.

http://www.wrights-trainingsite.com/Sodiumhypoonb.html

http://oregonstate.edu/dept/larc/sites/default/files/pdf/chlorine-fact-sheet.pdf

There are some precautions:

1. You should have a gas monitor in case of a spill.

2. You should vent the tanks to the building exterior.

3. You should have an independent automatic overfill protection device to prevent overfilling.

4. Hypochlorite is extremely corrosive. You should have some type of provision to change out the tanks as HDPE material will only last 7-8 years.

 

[tipp79]

Thanks bimr. All those precautions are already incorporated in the design. I take it that the main issue with off-gassing from sodium hypochlorite is the potential for inadequate doisng or pipe blockages?

Do many chemicals have the potential to off-gas?

 

[77JQX]

I like to have hypochlorite storage in its own room with minimal other equipment. The ordinary off-gassing is sufficient to cause accelerated corrosion of any uncoated metal, and i like to limit this to as little as possible. I've seen visible damage within a year's time to control panels etc. when the hypochlorite is stored in a room with other equipment. The more ventilation you get, the better off you'll be.

 

[ch81pc]

Off gasing and airlocks can be a major problem, especially if your plant is in an area with warmer temperatures, in which case you may need to chill your hypo to reduce the gasing. Normally I would do everything I can to prevent negative suction pressure with hypo. Are you using diaphragm dosing pumps? Peristaltics may be better as the suction presure will be more steady.. Which ever pumps you use, ensure that you alternate duty/standby regaulrly to prevent build up of gas in the standby unit.