Changing from Chlorine Gas to Hypochlorite 3

[EddieFAC]

I am trying to move from a Chlorine Gas system to a Sodium or Calcium Hypochlorite system (due to life-safety issues). I've been reading some of the other threads concerning Na vs Ca, and offgasing of sodium etc. I now have concerns about heading in this direction. (Can I get consistent chlorination without excessive baby sitting or am I making problems for myself) We will be installing this system at four different points, all of which have existing Cl2 injection. Operating pressures are on the order of 65 - 80 psi. Any input on helping me do this right the first time would be greatly apprecitated. Also any suggestions on literature (AWWA or otherwise) would be greatly appreciated. Thanks in advance for your time.

 

[BobPE]

What is the size of your system? What are some of the sizes at the various chlorination sites you refer to? How many pounds of chlorine are you using currently?

Ca vrs. Na is largely a cost consideration. If you are using a lot of chlorine, the choice is simple, sodium hypo.

But again, some additional information on your system will help us a lot.

BobPE

 

[bimr]

There are thousands of sodium hypochlorite installations, so you should be able to get your installation to work. You are correct in that the system must be properly designed to minimize the offgassing problem.

You will need to buy a good quality pump to minimize the offgassing problem. Peristalic pumps made by suppliers such as watson -marlow have been used successfully for this purpose. I would recommend that you go out and see an installation before you specify and purchase the pump.

 

[EddieFAC]

BobPE, thank you for responding so quickly to this thread. Based on your questions I had to crunch some numbers. Sorry it took so long to get back. Our water system consist of 11 shallow wells separated into 3 distinct systems (with 3 separate chlorination points) Our average usage rates are between 100,000 to 225,000 gallons per day. The first chlorination point treats about 78% of the water used while the second treats about 18% with the remaining 4% treated at the third chlorination point. Logistically we probably can't use a Na solution at the first chlorination point. We are using about 5 lbs of gas per day, and moving sodium hypochlorite to the location would be a logistical challenge. However the smaller use areas would be prime candidates.

 

[hydrae]

Sodium hypo can be purchased in 5 gallon buckets, 15 gallon minidrums, 53 gallon drums, 330 gallon totes and bulk delivery of 1000 gallons or more. At 12.5% it is about 1 lb per gallon, so your 5 gallons per day is still reasonable in the drum or tote size, if a truck can get to the site. If it cannot, how is the gas delivered?
Anything 100 gallons and over needs to follow DOT haz mat requirements. Depending upon temperature one to three months storage is all you want on site to avoid excessive degredation.
Yes, any hypo installation is more complicated than gas.
I would use off the shelf solenoid pumps built for hypo.
The solution to off gassing is to have a short, vertical, and straight suction line from the tank to the pump or have flooded and vented suction line. A backpressure valve on the discharge of the pump is needed to provide consistant metering if connected to variable system pressure.
use of a quill at the injection point is needed.
Have spare quill, tubing, and pumps available for service

The Chlorination Handbook Gerald F Connell from AWWA is a good reference
Hydrae

 

[EddieFAC]

hydrae, Thanks for your input. I spoke with someone else that also recommended that AWWA book. On order but I've yet to receive it. In previous threads some of the members recommend peristaltic pumps. I'm relatively new at the water business so could you explain why the solenoid valve is the better choice for this application. In addition to placing in the hypo systems we are also going to put in a chlorine analyzer. Do you have any recommendations on the setup of the analyer. For example how far from the point of application should we put the sensor line? Should we use any type of mixing device prior to the sensor device? How about a type (brand) of analyzer that works well and requires minimal maintenance. Thanks in advance for your time.

 

[hydrae]

Eddie
Solenoid pump (not valve) vs peristalic pump
the solenoid pump is the most common pump used for small hypo applications, inside is a plunger powered by the solenoid and a check valve on each side of the plunger. Life of the wearing materails is on the order of years. Once the offgasing issues are handled by proper suction tube design (short, vertical, straight) they are fairly trouble free. Suction lift is limited to 4 to 5 ft.
Brands include LMI, Pulsafeeder (Walace and Teirnan) and Prominet to name a few.

Peristalic pump move the fluid by pinching a tube in rollers. The life of the tube inside the rolllers is on the order of weeks. The tubing that has longer life is not compatable with hypo. While the suction lift issue is much greater the discharge pressure limits and tubing life are the drawbacks.

Maintenance on either pump requires messing with the hypo not a pleasent task.

As for the online analysis I have worked with two types
colormetric and amperometric
colormetric: (Hach and Hanna) a small sample is drawn and a dpd solution and buffer are added to the sample it is mixed the color read by a photo cell and lamp. Maintenance includes replacing the bottles once a month, cleaning the cell from once a week to once a year using acid dropper and q-tip and replacing the tubing once a year. Maintenance is heavy but regular and easy to diangonse. the process is visable and when something is wrong you can see the problem.
Since this device contains chemicals I install an air gap or rp backflow device on the sample supply just in case.


Amperometric (many brands and price ranges) water is flowed accross a probe, the magic box reads the level, sounds simple but when it is not working try cleaning the cell, or replacing the membrane, or repalce the fluid inside the memebrane, or replace the electrode. After any maintenace wait 4 hours then calibrate. Is it working? does the number match the grab sample? Maintenance is a guess and try this if not try that.
I prefer the hach colormetric.

As for sample location if it is known that there is little or no chlorine demand (biological, iron, maganese, hydrogen sulfide) sampling just after a bend will do the trick other wise sampling after 30 minutes is recomended i.e. figure flow rate and pipe diameter install tap way down there or after the storage tank. Static mixers are generally not required. If you are using a large storage tank I would sample before and after the tank and have either two analyzers or solenoid valves on a timer to switch the sample every hour or so.

Hope this helps
Hydrae

 

[bimr]

I am going to have to respectively disagree with hydrae as to the selection of pumps. Especially in light of the discharge head that you are pumping against. Solenoid pumps are cheap and dirty solutions. Solenoid pumps have short lifetimes and are basically throwaways. If you want do not want headaches, stay away from solenoid pumps.

Also, you want to minimize suction lift on your installation. 4 to 5 ft of suction lift is too much with hypochlorite.

Conventional pd pumps have been used successfully with hypochlorite and are acceptable. The problem with pd pumps is that pd pumps are susceptible to offgassing during the influent stroke, especially since pd pumps are not pumping at a constant flow rate.

http://www.watson-marlow.com/watson-marlow/app-water.htm#

As for the online analysis, if you have adequate pH, you can and should use the reagant free analyzers.

 

[ch81pc]

I have been using two Encore 700 (Wallance and Tiernan) pd pump for 2 years now since our refit from gas to hypo. Short of the occassional oil change and one set of oil seals we have never had to carry out any significant maintenance to the pump.
We have a suction pipe run of around 18 feet which is under negative pressure at times. The pumps and tanks are in a non air conditioned building. We change pump duties every 24 hours and have very few problems with gassing, so it is possible.
All I would say is keep suction pipe length to a minimum, with as few bends or T's as possible. We have also installed vented sight glasses just prior to the pumps which we leave open to enable any gas to escape. If possible install your tanks at a slightly higher level than the pumps to give a little positive head to the pumps. Failing that try dropping from 15% hypo to 12%.

Hope this is usefull