Conductivity of DM Water in Storage Tank

What is the temperature in the tank?

Temperature in tank is Atmospheric @ 35 Deg. C

What material is storage tank made of?

Is the tank lined with a coating?

What sources of water other than from the DM system goes to the tank?

Is the tank well grounded/earthed?

What type DM System?

Is the DM water . . . also de-ionized? (Demineralization may not de-ionize!, in fact it may ionize the water, depending on how DM is done)

Look at all fittings and connections, dissimilar metals build an ionization cell, strip ions from one and depositing on the other . . . the water becomes the electrolyte (even if very pure)

If you pump the water with a centrifugal pump . . . seals may be allowing air inleakage.

Is your water source contaminated by hydrocarbons or solvents . . . materials that may not show up on the monitoring system . . . but do breakdown into ionic contaimation?

Storage Tank is made of M.S (IS2062)

Tank is lined with coating.

Other than DM System source to the storage tank, recirculation line of DM Water feed pump is also connected to the DM Storage tank.

Tank is well grounded.

It is conventional De-Mineralization Plant.

DMF ---> ACF ---> SBA ---> DE-GASSER ---> SAC ---> MIX BED ---> DM STORAGE TANK.

DM is also De Ionized. The Conductivity of inlet DM water to storage tank is about 0.03 MicroSimens/cm.

How can we assure ionization of dissimilar fittings & connections in DM Water.

conductivity reduces in storage . better use on line as far as practicable.

While not a direct correlation to your application, this may provide a useful suggestion. During a deionized water plant upgrade for an FDA-regulated cosmetics plant, we also had a large storage tank to store our purifed water. When water was not flowing thru the tower system, we also noticed a degradation of water quality, so we implemented a recirculation loop just prior to the tank to continually polish the water quality when new water was not being processed. When the tank called for water, a valve would open and new water would be processed to fill the tank. As another note, conductivity is very temperature dependent, and instrumentation to measure purified water is typically done at ambient temperatures, not at elevated temperatures. Likewise, sampling containers can immediately add contamination to the water, so depending on how you are sampling and then measuring the conductivity from the steam, you can be introducing contamination into your sample that is giving you false readings. Making, storing, handling, and testing of purifed water is a very sensitive undertaking, and many sources of contamination / errors can be introduced.

I am from Pharmaceutical industry, and understand that for your application SS 316 L tanks which we normally use is over kill. I think maintaining the conductivity, that too below decimal point is very difficult. In our inductry we find it difficult to maintain below 1.0 micro semens/cm. We generate conductivity at about 0.06 micro siemens and by the time water reaches to the storage tank, conductivity goes to 0.7 micro siemens/cm. We do come across situation when the even glass vials for injection fluids start releasing contents in the product, not sure ehetehr epoxy will hold its integrity.

I was lookign up whetehr the Nitrogen blanket over the tank will help OR a KOH bretaher will be helpful, and came across this discussion group.

Will look for some help from group members.

The conductivity depends on the value of the pH, on the temperature of measurement and on the amount of CO2 which has been dissolved in the water to form ions.

You need to reduce CO2 from the air coming through the breather in the tank.

Using CO2 scrubber for water tanks for Pharmaceutical industry is better and safer then Nitrogen blanket. There are several types of CO2 scrubbers. Scrubbers with soda lime can not keep the stable CO2 level after scrubber and you need to change the scrubbing material often. So most preferable is automated system for water demineralization through the removal of CO2 from supply air. CO2 Scrubber for tanks is to be installed at the tank vent to prevent CO2 entrance in to the DM water storage tank. The intention is to avoid CO2 present in the air entering the tank. If you keep CO2 level in incoming air at 5 ppm, you can keep water PH stable.

Here is the abstract from one research paper :

In the pharmaceutical,semiconductor, food/beverage, and power generation industries, pure water is often exposed to air (and CO2), thereby increasing its conductivity and giving the appearance of a contaminant in the water.

In ultrapure water, dissolved CO2 from the atmosphere causes increased conductivity and decreased pH relative to the expected values of 0.0550 mS/cm and 7.00 pH at 25°C. For example, the conductivity of ultrapure water increases to approximately 1 mS/cm and the pH is lowered to 5.7 when water is exposed to air with a CO2 content of 0.033%, a typical value for pure air. The other primary components of air do not form ionic species and do not affect the water conductivity.

All the research information you can get here

http://my-mt.com/thornton/pdf_files/tech_pubs/cond_CO2.pdf

So to keep conductivity on desirable to need level you need to keep temperature lower then 25°C, you need to lower CO2 level in incoming air to 5ppm and keep it stable.