Sodium Hypochlorite Residual Readings all over the board

[Fuzzy]

We are switching over an existing Calcium Hypochorite system to new sodium Hypochorite system and when we switch over the chlorine readings at the analyser go all over the board 0.2mg/L to 10mg/L. We had the reading relatively stable for about a week (1.2 to 1.7) but not as stable as when using Calcium. The calcium is highly diluted before injection vs. the sodium is 12% undiluted (1 l/hr injection rate).
A Couple of notes on the construction; injection line from pump 1/2" about 100' long; reciever line pressure ~2psi.
It appeared the sodium was offgassing in the line and resulting in gas bubbles being injected into the line. pressure regulator was added near injection point and set at 15psi. This reduced fluctuation for about a week but is now off scale again. Visual check shows air bubbles being injected even with the 15psi backpressure (i.e. offgassing).
Has anyone run into this problem before? Would dilution of the sodium 50/50 be enough to stablize it to prevent what appears to be off gassing?
Any assistance would be greatly appreciated as Consultant has no idea.

 

[GeneratorGrrl]

Hi, Fuzzy

I can't offer you any help with your off-gassing, but I can offer you sympathy on the wacky-readings issue. My plant uses sodium hypochlorite 12% solution, further diluted 1:4 before injection into an 8000 L cistern. We aim for a concentration of 1 - 3 ppm. Our chlorinator readings are always bouncing around wildly, never stable -- I'd be happy if it'd stay within 1 ppm range, but it never happens. I still have to use a free chlorine reagent test, and quite often its results are completely different from what the chlorinator is saying. I don't know if this is "normal" for sodium hypochlorite, but at least its not unheard of

-==- GeneratorGrrl

"Eat well, exercise regularly, die anyways."

 

[hydrae]

Fuzzy
Are you injecting straight 12%? if so dilute it and inject at a higher rate. At lower concentrations the offgassing will lessen offgassing, a 50/50 will still offgass but not as much.

That solenoid pump does not provide a continious flow, a snubber (tiny hydropnuematic tank) will help smooth out the injection rate, set the tank pressure to 80% of the backpressure valve setting. Are you sampling after a good mixing time such as a tank or just along a pipeline, is their sufficient contact time for the chlorine to react with any demand materials (biological, iron, manganese hydrogen sulfide) before sampling?
The backpressure valve may have been solving the problem when first installed because there were gas bubbles in the line for that first week, they were acting as a snubber, when the gas bubbles escaped the pulsations from the pump where no longer dampened and the problem returned?

Have you cleaned the analyzer recently? Replace the membrane and fluid on ORP type or cleaned the measuring chamber on a photometric type.

Hydrae

 

[buzzie]

Best pump for conentrated (12-15%) hypochlorite is a peristolic type. Off-gassing typically raises havoc with centrifugal (for large flows) and many times small PD chem pumps also. Off-gassing is catalyzed by metallic piping and fittings (stainless steel and C-276). Need to use plastic pipe for transferring solution. Off-gassing also accelerated by exposure to sunlight - if 100 foot line is clear plastic and outdoors or if line gets too hot gassing will be a problem. Previous comments regarding pulsating flowrate and mixing time are important as your solution is very concentrated and requires good mixing prior to analysis.

Regards

 

[Fuzzy]

We reduced the concentration by mixing 12% 50/50 with water and increased system backpressure to 40psi in attempts to reduce the gas bubbles. The piping arrangement is highly susceptible to air entrampment (up, down, underground and back up). Hydrae your point about the snubber tank to smooth out the flow is a possibility but these pumps are designed to provide a much better continuous flow rate(Grundfos), discharge is with a full stroke regulated over time, not like typical solenoid pumps which reduce the stoke length to reduce flow. The snubber tank as you say could be placed at the highest point other than the fact the piping is arranged at the ceiling already. Is it possible to install the snubber tank with an air release along the lines of a heating system?
Buzzy there are a couple of Stainless steel fittings in the injection line which had to be used as we could not source PVC to PEX transition fittings. There were also a couple of brass fitting previously installed but have since been replaced by the S.S. for the same reason.
The system injection point is in the line and the first meter is reading after about 70 feet of straight pipe at very low system pressure (~2psi). Therefore, very little mixing. The second meter is after 140 feet and is past the high lift pumps. Note that a contact tank will be put online between the two meters after we are able get reliable chlorine reading (i.e. currently bypassed).
Thanks for your input any assistance is greatly appreciated. Also any sources for suitable fittings would also help, the local wholesalers are having trouble findings suitable fittings as well.

 

[Fuzzy]

Check system again, was operating moderately stable then simply dropped off the chart very quickly. My thoughts are that the piping arrangement is allowing any offgassing to build up at the high point. Then when the gas bubble is large enough it will be forced to "pass gas". Therefore, during the period the system is passing gas it will not be providing the sodium hypochorite solution required for as long as it takes to expell. It is also likely that once the gas bubble has formed in the line it may break-off in pockets of gas which would correspond with the wild fluctions which we saw on previous testing.

I have seen no indication of a confining pressure which would eliminate the offgassing. The only references I have seen are that the vapour pressure of the chlorine gas is around .5psi, but at what pressure would the oxygen off gassing be contained?

 

[hydrae]

Fuzzy

Even the Grundfos pumps have a period of time when the plunger is pulled back, during the pull back zero product is injected, since you do not have a mixing structure any sample taken when no chemical was injected would have a low reading with a higher reading later to balance it out. The snubber is effective anywhere in the line but is best placed near the pump, make sure the snubber is compatible with the chemical, all plastic models are available.
The visible off gas bubble are chlorine. The gas is not dissolved in the water but formulated into sodium hypochlorite which is a reversing reaction with the caustic and water.
Is there gas in the suction line to the pump causing it to lose prime? or is the pump connected via flooded suction, or a short straight up connection to the inlet of the pump, most pumps must have less than 5 feet of suction lift with the only highpoint being the pump.

Stainless steel is vulnerable to chlorine, if metal fittings are a requirement use Hasteloy or titanium. Stainless will have a shorter life than brass in sodium hypochlorite

Other brands of pumps; LMI, Pulsafeeder, Prominent can come with a 5 way valve that is mounted on the pump. I would not recommend a separate air release valve.

I have tried the peristaltic type pump, and the tube replacement interval was way too short (as short as every 2 weeks and a big mess) the longer lasting tube material was not compatible with hypo.

Are you using a quill to inject the chemical into the center of the stream?

USABluebook

http://www.usabluebook.com

has all the parts you will need.

Velocity and changes in direction will aid in mixing in the pipe, system pressure will not affect the mixing. Static mixers will also aid but are generally not needed for hypo.

Hydrae

 

[Fuzzy]

Hydrae
The pump has a suction lift of about 5'-6' straight to the pump. It has auto bleed/degassing feature and will automatically switch-over on flow sensor failure. To date we have not had a loss of prime to cause the system to switch-over. The piping arrangement starts with foot valve, 3/8" poly tubing, flow switch, pump, auto degassing/auto bleed unit, 3/8" poly tubing, 1/2" PVC skid piping, 1/2" Pex tubing (~100'), pressure regulator, injection quill.
Based on the injection rate retention time of the piping would be about 2.5hrs. I was thinking of reducing the piping size to 1/4" or 3/8" tubing and possibly further diluting the solution, though this could be an issue with owner as day tank may not be big enough for long weekend supply.