Sodium Hypochlorite Process Help

[JSchule]

Good Evening. Im a Production Technician at a small power plant in Texas. My background is in I&E but due to staffing, we are required to operate the plant as well. We bring in raw water through a 36” pipeline to be used as cooling water, service water and demineralized water for steam. We have to inject 12.5% sodium hypochlorite at multiple points for algae and condenser protection. Our vessel that stores the bleach is approximately 6500 gallons. From that vessel, a discharge line is attached to 3 manifolds which is then routed to the chemical injection pumps. One of the pumps is used to inject the bleach into a 3.9 million gallon raw water tank. That tank level stats at about 96% full. There’s about 2700 GPM of raw water going in and out of the tank for days at a time. Our set point for the bleach pump is about .60 ppm FAC. So we are constantly pumping bleach to that tank. Now, here’s the problem... that pump constantly air locks, looses prime. The head pressure of the bleach vessel should always keep the pump primed because it is below the level in the tank. The injection point is through a quill with mounted on the discharge line of the recirculating pump for the 3.9 million gallon tank. Some people believe the sodium hypochlorite is gassing off causing the pump to loose prime... that’s impossible because the tank is vented and it looses prime during winter months.. this is an issue that is driving me crazy and I am hoping that someone can help me. I can take pictures of the manifold etc and can provide more information.

 

[Latexman]

I can take pictures of the manifold etc and can provide more information.

Most pump problems are on the suction side. Start there, and a pic of the pump nameplate.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[EdStainless]

Hypo will outgas, but restrictions in inlet flow make it much worse.
My guess is that your injection pump draws in fluid quickly causing a local pressure drop that is enough to give you problems.
I presume that you are measuring actual residual active Cl levels? There are guidelines for these.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[JSchule]

Edstainless: I understand hypo will gas off at 95*F, but in the summer months, we might move 350-500 gallons of hypo in 24 hours, to multiple tanks and those pumps don’t loose prime. It seems like we move too much fluid for it to gas off. I’ve shot the OD of the piping with a temp gun, in the middle of July, and haven’t read 95*F. I may be wrong, but if the tank is vented, it doesn’t seem likely that gas could entrain itself in the liquid. I’ll send pictures in just a minute. I’m at the plant now.

 

[Latexman]

Since this pump is the only one that air locks focus on differences between the other systems. Flow rate differences, suction line size differences, piping layout and fitting differences.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[georgeverghese]

Sounds like you've one or more high point piping pockets on the suction line to the pumps where oxygen is getting trapped. The suction line from the bleach tank to the pump should be continously sloped down towards the pump to keep the suction line fully primed. Also check the orientation of the suction side of the pump - it should be free venting also.

 

[JSchule]

The pump In question is the one closest to me in the 3rd picture and far right in the 4th.

 

[JSchule]

There are vents for all six pumps, it makes no difference if it’s vented or not. If you look closely, you can see an air bubble in the clear discharge line coming off of the vessel. I don’t understand how that’s even there given the head pressure of the liquid. The skid is very questionable as far as plumbing. The problem is very intermittent. 8 days ago, I didn’t have to bleed any air. We ran the pump in auto and it would reach set point continuously. I can go to every one of those calibration columns, open the valve to fill it and get air bubbles. But this one pump is the only one that looses prime..... Georgeverghese, I’m not trying to be rude or anything, but why would you want to vent the suction side of the pumps if they’re below tank level? I cannot figure out how air is getting into the system.

 

[JSchule]

I took a picture of the motor nameplate on accident disregard that one picture.

 

[don1980]

Gas-out is a widely recognized problem when pumping bleach solutions. Apparently, that's happening in your pump suction piping, and the gas bubbles are unable to flow upstream to the tank for venting. Gas formation is very likely at the temperature you mentioned (95F). Notice that your black tank is absorbing radiant heat. It would be better if it was shielded from the sun. Possibly a roof over the tank would solve the problem. Also, you may not have the best type of metering pump for this service. Consult with your bleach supplier, and show them your set-up. Hopefully they can help you decide on the best retro-fit for solving this problem. It could be that you need to put a roof over the system and change to a better pump for this service.

 

[JSchule]

Don1980, I entertained the idea of gas-off during summer ambient temps, but I have to bleed air during much cooler ambient temps as well, 35-60F. I really think it’s just a poor design and there’s some strange things happening that don’t really make sense to me.

 

[danschwind]

JSchule,

Have you tried swapping the two pumps? If you do it maybe you can pinpoint whether this is a pump-related problem or a piping-related problem?

Daniel
Rio de Janeiro - Brazil

 

[Latexman]

What's the normal flow rates of the problem pump and the pump beside it? Do they have different or the same normal operating pressures too? The suction line goes past both pumps, comes to a closed valve, but the piping carries on and increases in size. Where does that go? Are the two pumps identical or different, spec-wise? If identical, what Daniel suggests may be worthwhile, but lets hear what you say about differences first.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[don1980]

Experience in trouble-shooting has taught me the wisdom of Occam's Razor.

 

[Latexman]

What’s up with those duct tape pipe supports?

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[georgeverghese]

There is clearly a high point vapor pocket in the suction manifold piping just before the tee off to the troublesome pump, which is visible on the 3rd photo. Bleach solutions decompose insitu at ambient temps to produce O2,so it doesnt help even if your pumps are below tank level. Bad piping design on the suction side, probably unsupervised and not reviewed by Company engineers. There is another high point in the exit piping from the bleach tank. A bleed line leading back to the tank (entering the tank at above the max normal working level) from each of these high points will help clear out these trapped gas pockets.

 

[pierreick]

Hi,
consider this resource

https://www.rhfs.com/pulse/technical-guidelines-for-sodium-hypochlorite-metering/

Good luck
Pierre

 

[JSchule]

There’s only one engineer here at the plant and he’s electrical... just look past the duct tape pipe support Latexman lol. There’s a lot of eye sores in that manifold..... so I got to work today at 5:30pm. The pump hasn’t lost prime yet. Ambient temps are around 68 or so... what’s strange is 10 days ago, the tank level was at 3500 gal, around 3.5 ftH2O, the same level as it today and it controlled in Auto and never lost prime... flow rate on that pump today is about 12-13 Gallons per hour, output pressure is about 12 psi. Max GPH is 20.6 and 102 PSI. The pump next to it is the same pump.... the only real difference between both pumps is the length of the process line to the injection point... the problem pump’s process line is much longer. All of the pumps have a valve that can be opened to common vent that is tied into the sight glass. We have a bubbler piped into the sight glass to measure DP for a level indication. All of the vents are closed because we can’t notice any difference in having them open. Would it be better if we had one vent before the manifolds where the flex ties into the CPVC? Also, what’s the chances of when they deliver the bleach, air is injected to the vessel from the transport truck?

 

[JSchule]

Thanks for the website info Pierre... hell I guess it could be gas then... I assumed we pumped too much of the liquid for it gas off in the process lines and since the tank is vented, the gas would escape at the tank.

 

[Latexman]

I know from experience Pierre's references are always helpful.

I know you said y'all hope your tank and flows was too large to be influenced heavily from heat, but wherever the sun hits, it will warm up the pipe/tank and hypo at that surface, and O2 will form. And, black surfaces are the worst.

The suction 45^o's up and the problem pump is the first one after that. I don't see this 45^o up on the other pump sets. Looks like thay always go downhill towards the suction. It could be the pics aren't focusing on that on the other pump sets. How is it?

So, all the pumps have a similar flow rate? For some reason I initially thought the problem pump flowed more than the others.

Good Luck,
Latexman
Pats' Pub's Proprietor