Mixing distance in a pipe

[49merc]

In a water treatment plant, I need to start injecting caustic for pH adjustment. It will be in a 30" pipe with velocities in the 1.2-2.4 fps range. How far after the injection point will the mixing be complete enough so that a pH probe will give me a good indication?

 

[dicksewerrat]

Are there some baffles in the pipe to insure turbulence?

 

[49merc]

There are no baffles or bends. Just straight pipe in an existing concrete vault.

 

[bimr]

You need about 4 turns or changes of direction to be sure that the solution is mixed. At the low of velocity that you describe, it is going to take a long distance, maybe 100 feet or so.

Have you considered using a mixer?

Not sure why you would be injecting caustic for pH control. Use of caustic (NaOH) is going to be very expensive for that capacity. Have you considered use of lime?

 

[25362]

Bimr's idea is OK as long as no unwanted calcium salt precipitates out of solution under the process conditions.

 

[munich]

You guys have bauxite industry nearby? They will love to sell you caustic since it's their by-product, but don't use lime, please...

30" pipe? that's a large TP.

Cheers
munich