New DI tank giving high conductivity readings?

[vipereyes]

Hello all. I am looking for some advice with a DI application.

We have a process that includes quenching small metal parts. The quench solution contains a sodium nitrite inhibitor that provides corrosion protection. We purchase DI water in tanks from a local water treatment supplier. In order to maintain the correct nitrite level in our tank, I use a simple Myron analog conductivity meter to measure the conductivity daily and adjust the quench solution as necessary. Everything was going along fine until my last shipment of DI tanks. Although the green “OK” light was lit on the tanks, the conductivity readings right from the DI tank were off the scale on my meter - over 10,000 microhos when normally I get around 20. The supplier said it wasn’t possible and told me it was my meter. I went to two other DI tanks we have on other processes at our facility and each gave me a reading of under 20 microhos.

The supplier came over and used his resistivity meter and could not get an accurate reading but said that since the light was green they were fine to use. I disagreed and he did end up replacing the tanks.

I also perform chloride checks from time to time and discovered that the chlorides in the process tanks were very high during this same time frame – over 630 ppm in the process tank when normal readings are under 30 ppm. The supplier said the tanks may have been used in a water softener application previously and were not rinsed before filling with new resin?

My questions are: why would the DI indicator lamp give a green OK when the conductivity readings were so high? Is it normal to use the same tanks for water softener and DI applications?

Thanks for any help you can offer.

 

[EdStainless]

I would ask them when the last time was that they calibrated the on tank meters.
It sounds like you are in the right.
the tanks should be fine, they should have been rinsed and back flushed when they were regenerated.
This isn't your fault.

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P.E. Metallurgy, Plymouth Tube

 

[vipereyes]

Thanks for the quick response EdStainless. The light has gone red on the replacement tank. The conductivity was a little over 300 when I noticed it red checked the reading.

Also, do you think the high chloride reading will be a problem on our steel parts? I am thinking we need to empty, rinse and refill the process tank but no one has ever mentioned what "too much" chlorides actually is.

 

[tomwalz]

In my experience buying DI water for applications where it is important is a chancy operation at best.

Can you make your own?

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[vipereyes]

Well, anything is possible. We never had made our own before. We have two other processes where we use DI water that we purchase - for our salt spray cabinet and our EDM machine and never had a problem before so we thought this application wouldn't be any different.

 

[bimr]

vipereyes,

The green light measurement is probably not as accurate as your portable meter.

Most likely, the tanks (called service tanks) were not properly regenerated back at the suppliers shop. Other possibilities is that the tanks were mixed up with another order, or that the resin inside is bad. It is not the tanks, it is the resin inside of the tanks.

As tomwalz points out, you are dependent on the supplier to provide you with quality products. It is out of your control. Your only choice is to continue to test the water to ensure that the supplier is doing his job.

 

[EdStainless]

vipereyes,
It could be that these tanks were regenerated with brine (like a home softener) and then not properly flushed.

We make our own DI on site, but we do not regenerate the resin here. That is a contract service (we always get the same tanks back). But at any given time we have 6-8 resin tanks in service with continuous monitoring of both conductivity and Ca ions. We find great difference in resin tank life, perhaps due to the age of the resin or how well it was regenerated.

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P.E. Metallurgy, Plymouth Tube