Minimum Velocity in Pipes

[OilBoiler]

Hi everyone,

I have already completed a design for adding caustic piping to one of our units. The original design was to use 2'' carbon steel lines (Sch. 80), which is in compliance with our company standards. However, because of some constraints in the project, we were looking for an existing line and use it instead. We have only found a 3'' carbon steel line (Sch. 80), but this one is specified only for steam and air lines in our standards. Also, this line that is out of service, used to carry solids.

The velocity of the caustic going through the 2'' line (Sch. 80) was 1.63 ft/sec. The velocity in the 3'' line would be 0.73 ft/sec.

My question is such low velocity would be a concern if we use this 3'' line and considering it used to carry solids?

Is there a minimum velocity in pipes just as there is a maximum?

Thanks for your assistance.

 

[briand2]

The typical minimum velocity in water pipes (I work in building services) is something like 2 feet per second. This is mainly to ensure that air gets carried along to proper vent points, etc.

Regards,

Brian

 

[m777182]

Perhaps I did not quite understand your point with solids: if you want to transport solids in a liquid(slurry) you should calculate the minimum settling velocity(minimum Re number) and operate at higher velocities. Provide some reserve for low speed spots. In case of caustic take care of minimum temperature!
m777182

 

[OilBoiler]

m777182,

We don't want to transport solids, I'm sorry if I didn't explain myself. My point was that when this line was in service, it carried solids. We want to use this line for caustic instead. My concern is that if that because of the previous presence of solids in the line, we may have problems of having caustic flow if we have such low velocities. Either way, the line would have to be flushed very well.

We are using fresh 25% caustic instead of spent caustic, so the freezing temperature should be below 0ºF. This same caustic is used in other units and they haven't had problems during the winter.

Thanks!