Chemical feed of NaOH

[moeszu]

I have a client that has a magnetic flowmeter with erratic readings. Recent system changes (move NaOH feed point to upstream of meter) is causing low flow readings. Testing included monitoring flowmeter without chemical and starting chemical feed with as much as a 30 percent drop in flowmeter value. Dosage rate approx. 40 ppm (50 percent concentration of sodium hydroxide fed at 2.8 gallons per 36000 gallons). My first reaction was electrical property of the NaOH affecting the field readings of the meter and then a concern over grounding of the meter to eliminate any stray voltages. Anyone with past experience of similar problems adding NaOH ahead on magnetic flowmeter?

 

[doitmyself]

Check your specifications of the NaOH, there is about 3ppm Fe, and Ni in the 25% solution. you can get an iron build-up which will change the acc of the meter. The caustic will also buildup rather quickly on the device.

 

[scorcher]

I am lookig at a similar problem with caustic flow (25%). For reliability (not accuracy) reasons, it has been decided to change from a turbine to a mag flow system.

For processing reasons, the concentration can decrease significantly from its design value, as the caustic changes to other sodium salts. (scrubbing operation). Will this have an effect on the readings?

Do the build-ups of iron or caustic occur on the electrode or the flow tube?

Can electrode or flow tube material effect this?

Thanks

 

[jdbanks]

I am seeing a similar problem with Rosemount mag flowmeters on the circulating lines in an SO2/HCl caustic scrubber. Normal addition of NaOH for pH control has no bad effect, but sudden quick addition quickly drops the flow signal. When the NaOH pulse makes its way back down the scrubber we see "echos" of low flow several more times but with diminishing error amount. Two echoes is all I have seen so far. Any comments? Any way out except prevent rapid NaOH addition or change to different type flow meter?