Use of Carrier Water & it's Dilution Effects 1

[tipp79]

The way I understand it now is that there can be a number of reasons why “carrier water” is used:

• to actually help carry along the water i.e. to make sure the dosed chemical achieves a certain velocity range e.g. so lime powder doesn’t come out of solution and block dosing lines
• to ensure there’s sufficient mixing of the chemical and the main process stream at the dose point (this “carrier water” could be added at the dosing pump or just upstream of the dose point)
• to ensure velocities are maintained for chemicals with high freezing points as too low a velocity and the chemical might freeze
• as motive water for chlorine gas

Are there any other reasons why “carrier water” is used?

By adding carrier water, will a higher volume of the dosed chemical be required as the carrier water might dilute the strength of the chemical (in the case of the project I'm working on 22% sodium hydroxide) or is it generally accepted that carrier water does not have an effect on strength of chemicals dosed i.e. will additional volume of chemical be required to be dosed to negate the diluting effect of the carrier water?

Cheers

 

[bimr]

Carrier water will help Improve the accuracy when pumping very small volumes of chemicals.

Carrier water also helps when you have to transport a chemical over a long distance.

You do not have to worry about dilution effects because the chemical dosage is based on the main treatment flow, not the carrier flow.

 

[tipp79]

Thanks bimr. With regards to dilution, my question is this: It's 22% sodium hydroxide being dosed. The required dosing flowrate and sizing of the dosing pumps is calculated on the main treatment flowrate, the dose (mg/l) required to raise the pH of the water and the % strength of the sodium hydroxide. Once I've added carrier water, the strength of the hydroxide has reduced. Do I need to increase the size of the pump (dosing flowrate) to compensate for the carrier water addition. I've added a simple calc to illustrate what I mean. Rows 55-57 show the reduced concentration of the hydroxide at the dose point.

Thanks again.

 

[bimr]

You should add the carrier water flow (170 l/hr) to the water flow (640 l/s) and then calculate the sodium hydroxide dosage on the combined flow. However, since the carrier water flow is only 0.4% of the combined water flow, it should not make much difference. The effect of the carrier water will be minor and should be well within the range of your operating system.

 

[tipp79]

Thanks bimr. When it's shown like that, it's obvious it won't make a difference. It also illustrates how such a small volume of chemical can have an effect on a much larger volume of water.

 

[operatorjoe]

Tipp79 here are a few other things to consider for carrier water. Carrier water is great for acids but if there is any hardness in water a base will precipitate hardness then coat and plug lines. If you are using liquid Aluminum Sulfate and many other coagulants it will affect coagulation. I have used very small amounts of polyphosphate in carrier water for sodium fluoride to keep lines clear. Some chemicals such as powered activated carbon would be impossible to feed without a minimum velocity carrier water but will still coat and plug lines. We use flexible lines inside conduit so replacement of feed lines can be done quickly. Best of all situations is to install feed close as possible to point of application or supply backup feed lines to use in case of failure.