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NaOH Pipe Sizing3

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NaOH Pipe Sizing

(OP)
What is the minimum velocity in piping used to recirculate 50% caustic soda into a heated storage tank?

RE: NaOH Pipe Sizing

You haven’t provided enough information.

My guess is that the velocity should be high enough to maintain a minimum temperature of 25oC at all points in your system, as the solubility of NaOH(aq) is 50 wt% at this temperature. Solubility varies with temperature, e.g., from 30% at 0oC to 63% at 50oC. – CRC Handbook of Chemistry and Physics.

RE: NaOH Pipe Sizing

(OP)
I want to know what minimum velocity to maintain in the piping to and from the NaOH tank to avoid solids deposition in the piping.  Is 2 feet/sec sufficient?  I amm assuming a one hour turnover time in the 8,000-gallon NaOH tank for calculating heating requirements.  Thanks for your help.

RE: NaOH Pipe Sizing

There shall be no problem even if you increase the pipe flow velocity to around 5 ft/sec. Rather, it helps to avoid any solids deposition. If the pipe length is small you may increase to 7ft/sec if you had faced the solids deposition as your operating problem. Do not oversize the pipe for such applications.

Narendra K. Roy
Gramya Research Analysis Institute,
PO box 4016, Vadodara 390015, India
Website: www.gramya.com ; www.charismaglobal.com

RE: NaOH Pipe Sizing

2
You certainly need to maintain a minimum velocity to prevent settling as well as a maximum velocity to avoid excessive pressure drop in the piping.  Pressure drop is tricky here because of the caustic is both dense (1.5 specific gravity) and viscous.  Other responders, however, have already dealt with these issues.

But you also need to be consider the temperature of both the metal and the 50% caustic.  The 50% caustic will freeze around 12°C, which makes it critical to maintain both the pipe and fluid above this temperature.  In addition you must also prevent overheating of the pipe.  Above 49°C,  caustic can crack carbon steel pipe by a phenomenon known as stress corrosion cracking.  For this reason it is important to keep the pipe wall below 49°C. Stainless steel will not crack in the presence of 50% caustic unless the metal temperature exceeds 100°C.

Because of these concerns, most piping systems are heated with self-limiting electrical elements.  This prevents freezing without overheating the pipe.  If electrical heating is not possible, then steam heating can be used, but the coils must not contact the pipe directly.  The coil offset makes the heating less efficient, but it reduces the likelihood of overheating.

Tracing the piping system will also protect the piping when the plant is not operating or is operating at greatly reduced capacity.

RE: NaOH Pipe Sizing

bngreene is referring to metal. you should as well consider HDPE or LDPE as tube material which is resistent to 50% caustic.

RE: NaOH Pipe Sizing

US1612 is correct that HDPE and LDPE are resistant to 50% caustic.  Other non-metallics such as polypropylene and epoxies are also resistant.

The problem here is that protection against freezing of the caustic is more difficult because of the lower thermal conductivity of the non-metallic materials.  Also the economics of installing a relatively small plastic or plastic-lined piping system, such as a recirculation loop, may be disadvantageous.  The economic disadvantages arise from the fact that these systems usually require a different labor pool, and the non-metallic piping often requires additional pipe supports.

Nevertheless, if the system is indoors and the skilled laborers are available, plastic piping is an option.

RE: NaOH Pipe Sizing

I used to see Crane TP410M (Old edition). The reasonable velocity for pump station and drain line is 1.2 to 2.1 m/s. I believe that the minimum velocity of 1.2 m/s can be used to prevent the deposition in the line.

Any opinions, please comment.

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