hypochlorite feed systems 1

[dfunk]

Does anyone have experience with hypochlorite feed systems at remote re-chlorination facilities?

We are investigating the use of sodium hypochlorite feed equipment with compound loop feedback control for rechlorination at un-manned remote facilities. Such systems present several design concerns related to reliability, control of the equipment, equipment selection and capacity range of the equipment relative to existing gas chlorination.

We are proposing to use large hose pumps (peristaltic pumps) at the few large facilities, but are restricted to diaphragm pumps at the smaller facilities. Pulsafeeder sells diaphragm pumps that are designed for hypochlorite handling and include degassing mecahanisms. Does anyone have experience with these pumps?


Thanks for any help that can be provided.

Dennis R. Funk,
Gannett Fleming

 

[hydrae]

Pulsafeeder is one of the common names in the diaphragn pump installation. There is also LMI and Prominent and a few others. If you like peristaltic Cole-palmer has them as small as you will need, if they are not small enough dilute the bleach to get the correct dose.

The PID loop will take a bit to tune, is your incoming residual constant? if not then your injection rate will have to be adjusted for both incoming residual and flow rate then trimmed to outgoing residual. Also as the hypo ages its strength decreases.

Hydrae

 

[WaterWizard]

What What are the climatic conditions in your area? If the area where the hypo is to be stored is subject to high temperatures you may need to take that into consideration as high temperatures increase the rate that the hypo loses strength. You may need to limit the amount of storage and depend on frequent topping up you tanks with fresh hypo, or provide for cooling using shade, insulated building with air cooling etc. Your sensors should also be a feedback from the treated side so the feed rate will automatically adjust to the larger volume of feed needed as the hypo ages.
As to pumps, I have no preference except to say that I have had a few problems with diaphram pumps that were supposedly designed for hypo, mostly with the diaphrams breaking down after only a few months service. Might be due to working continuously pumping 10-12% hypo in the hot sun on 35 to 40 degree days.

Edd

 

[semo]

I also agree that temperature has a detrimental effect on the hypo. Similarly, UV light does the same. I had a system where the hypo was stored beneath an unshaded window and the operator constantly adjusted the feed pumps to compensate for the reduction in strength. That was until the sunlight was noticed and curtains were installed.

We typically consider the diaphram pumps as disposable units with hypo as the operators are constantly installing diaphram repair kits which sometimes cost almost as much as a new pump.

I am currently working on a project where we are going to use peristaltic pumps as a test. We feel confident they will have a longer life as no moving parts are in contact with the solution. The pumps can be sized to handle about any flow too.

You mentioned Sodium Hypochlorite which is in a liquid form. If you use diaphram pumps also stay away from Calcium Hypochlorite (HTH, etc.) which comes in a granular form. No matter how I've tried to mix it into solution, it doesn't seem to completely dissolve and particles get stuck in the pump suction filter stopping the flow.

Just my 2 cents.

 

[bimr]

As far as diaphragm pumps go, you should specify a quality pump that meets the API-675 standard. If you do that, you will not have any problems with the pump. Operators love these pumps. Also, you should get the degassing accessory.

On the other hand, if you use one of those electronic diaphragm pumps, well that's another story. Those are throw-a-ways.

You should use a pump controller capable of feedback PID control to control the pump output. These controllers typically have a RS-232 communication feature that will allow you to monitor the site remotely.

If you use a more dilute hypochlorite solution, the solution will not degrade as fast. That will minimize some of the pumping problems as well. You should be able to find a degradation curve that suits your application. Here is a reference on the web:

http://www.consulthai.com/2002/news/papers/On-site_versus .pdf

 

[BigJay]

Regarding peristaltic pumps. Do not expect them to be maintenance free. The tubing in the pump head will get worn by the rollers. 4-6000 hrs life is quoted by some manufacturers.

 

[camabee]

dfunk,
For small capacity rechlorination applications one of the most common problems is pump failure due to degassing. While many of the small pump manufacturers can supply automatic degassing designs you should look at the double diaphragm design offered by Alldos. This head is specifically desgned for this service.
More information can be found at:

http://www.alldos.com/e/html/02_produkte/11_pro_primus208doppelkammer.htm

 

[FLwater]

Stick with Prominent or Pulsafeeder positive displacement pumps. Hose pumps are made for the pharmaceutical industry- they are designed for higher accuracy but with delivery in batches. They are NOT made for continuous service.

Once the hoses wear, the pump rate fluctuates and you will spend all your money on expensive, rugged hoses every 3 months instead of having purchased a pump that will last you several years.

And dont forget to use day tanks at the remote facilities!!!

 

[senglish]

Sorry about the late reply, but:

Steer clear of auto-degassing heads if you're trying to control your dose rate with any accuracy. The way they work, they tend to bleed off a variable amount of the chemical along with the gas. In my experience, they're not very good at avoiding gas-lock of your pump head either.

Most important is to keep a flooded suction (ie storage tank above the pumps) and to vent off gas bubbles before they get to the pump.

With low dose rates it may be worth considering dilution of the chemical - the chemical degrades slower (giving off less gas, as well), the diluted chemical is less viscous (making it easier to vent off gas) and you can use a bigger pump (which will be less prone to gas-lock).

My experience with the small, solenoid-driven diaphragm pumps is that they'll only work with any reasonable accuracy if you keep the stroke length greater than 50% (whatever else the manufacturers claim ;-). Avoid having a dose range which forces the pump speed to go below about 20 strokes per minute. Have you looked into the stepper-motor pumps from Grundfos? - I haven't tried them with sodium hypochlorite, but they have some features which should give them an advantage with this chemical....

We've installed quite a lot of chemical dosing systems. If you want more help or feedback, please get in touch.

Regards,
Stéphane

 

[GrizzlyCurmudgeon]

Looks like others have covered just about everything, so I'll just add a couple items:

I prefer peristaltic over diaphragm pumps. Hydrogen off-gassing doesn't stall them, and it's easier to replace tubing than diaphragms. For small peristatic pumps, look at Watson-Marlow/Bredel. The Cole-Palmer, lab-type pumps will give out quickly. Of course, the Watsons cost about three times as much, so there's the possibility of using the cheap ones as throw-away items. Regular tubing replacement will be necessary with any peristatic, but rolls of the tubing for small-size pumping rates is pretty cheap from lab supply houses.

For onsite-generation systems, I look first to Clortec, then try to find a competitor. They have the most experience. By the way, I don't buy any of MIOX's claims of being able to produce mystery additional oxidants with their patented system. I've seen some pilot data and lab studies that give them little creditability.

Hypo storage and degradation losses are something to be evaluated. If you buy the hypo solution, you can store it neat, suffering from degradation, but having less storage. You can dilute it to reduce degradation, but you need more storage room. If you use on-site generation, the concentration is already really low, so degradation isn't much of an issue.

Hypo storage tank materials are still an issue in flux, it seems to me. The HDPE vs XLPE war rages on. They have limited lifespans, especially if any heavy metals can get into the solution, such as from valves and fittings (use absolutely nothing but PVC stuff). Rubber-lined steel tanks require occasional relining (usually done on site). Pure titanium tanks will last forever, but they cost forever, too.

 

[assco]

Diaphragm pumps work best in this application given the addition of a very inexpensive addition, a stand pipe. Ensure the suction piping slopes up to the stand pipe and down from the stand pipe to the pump suction. There are electronic stroke length adjusters to receieve you 4-20mA signal and automatically adjust your flow rate. I got these pumps in the field and haven't touched them in 4 years. I use Milton Roy pumps.

 

[senglish]

Re: Stand pipes - That's just what we did on my first project commissioning sodium-hypochlorite installations. We didn't initially have stand-pipes, but we retrofitted them (and raised the tank slightly to ensure a flooded suction, and programmed daily duty rotation to stop gas bubbles building up in the standby pump) and the result was excellent - gas locking was pretty much eliminated.

One further detail about the stand pipes - they were about 15mm bore, while the take-off which sloped downwards into the pump suction was narrow bore (6mm) hose. This may or may not have helped ensure that the gas bubbles continued rising to the surface in the stand pipe rather than being pulled into the suction hose.

By the way, take care when checking suitability of components. We had problems with components which were made mostly from PVC; but a slight leak on a diaphragm seal was enough for the chemical to attack at polyethylene retaining ring - so check that non-wetted parts are suitable as well.