Materials of Construction for High Purity Water 1

[billjg]

What should piping materials of construction be for RO, DI and distilled water? Roughly at what point (or points), in terms of TDS/mmhos, does the corrosivity of high purity water become a problem?

Thanks,

Bill

 

[bimr]

The piping materials for high purity water depend somewhat on the specific usage of the water as well as other parameters such as the oxygen content. Some applications are concerned with pickup of corrosion products and organics from the piping and others are not.

Some commercial applications can get by with PVC piping. Power plant applications would probably would use carbon steel in the boiler (with zero oxygen content) and SS. Semiconductor applications would probably use PVDF.

http://www.georgfischer.nl/index.cfm?uuid=E24C3AB6D60EB5F06E543FBB53B89F5D&o_lang_id=7

The high purity water producing equipment and all piping in contact with high purity water is made with corrosion proof piping.

Other than theses general comments is all that can be made to your general questions.

 

[billjg]

Thank you, bimr.

This application is for a cooling tower for the purpose of clean operation and no purge. I have heard about the corrosivity of "demin" water on metals but never understood the mechanism. First of all, what consitutes "demin" water? 1 mmho/cm? 0.1? 0.01? I have heard that stainless is good for deionized water (typically 0.5 mmhos/cm) and this is probably a good enough answer for my purposes.

However, it got me wondering at what point does water become corrosive as you increase purity? My guess is that 20-30 mmhos/cm hardness is probably enough to buffer the electrochemistry involved with iron/zinc/copper corrosion, but I'm not sure. This is important because I will actually have to control the conductivity up in the cooling tower to avoid corrosion there.

 

[bimr]

Not sure what you are talking about in regards to "clean operation and no purge".

If you have a closed system, then you can operate satisfactorily with normal potable water, not demin water. In fact, demin water would not be recommended.

If you are just concerned with corrosion, then you probably should have some hardness in the water. In that case, you can use the langelier index as a guide to determine the corrosivity.

If you are talking about an open cooling tower, you only need to concern yourself with the piping to the cooling tower. Once you dump demin water into the cooling tower, the demin water will be dispersed quickly into the cooling solution.

Demineralization is basically the removal of most of the dissolved salts. Not sure if there an absolute cutoff as to the determination of demin water.

 

[billjg]

It is an open cooling tower. We will probably make provisions for occasional addition of potable water to keep the hardness up, monitor conductivity and monitor corrosion with a coupon rack.

Thank you for your help!

Bill

 

[dcasto]

Water from our RO had nearly zero hardness and minerals. The water pulled Iron from our carbon steel line just to get into some equalibrium that the line corroded away within a few months. We had to use stainless and plastic lines.

 

[quark]

We used to call water at 5microsiemens/cm (or micromho/cm) as demineralized water (as per earlier USP). Makeup water hardness need not be morethan 5ppm. As you already considered, corrosion coupons are the best bet. Follow a treatment programme to increase COC.

 

[cub3bead]

Demin water is generally too costly to use as cooling tower makeup!

If you were to use it as CT makeup the CO2 in the water on the first pass over the tower would hit about 5 ppm and the pH would drop to the low 5's. This would result in serious corrosion of any ferrous material and start leaching Ca from the concrete matrix.

Sure you could neutralize the CO2 with caustic but lime would be better than caustic but cause a lot of handling problems.

CTs make very good air scrubbers so yu would have to blow it down eventually to manage the TSS in the tower.

 

[kamjoshi]

I work with Flash evaporators, RO plants, boilers and some neuclear plants De-Ionisers training. 0.1 to 70 ppm is demin, 70 to 1000ppm is potable.Though our demin is 0.1 to 10 ppm generally. Corrosion is eliminated by passing potable water via Limestone filter (dolomite), use pvc type pipework for low pressure water systems, it will not corrode or give off any particulates, remove chlorine from your system by fitting a carbon filter and if u r making demin, add calgon to your feed water.
best wishes.

 

[bimr]

kamjoshi, where did you come up with your information? Regarding....

"0.1 to 70 ppm is demin" This is not the correct range for demin water.

"Corrosion is eliminated by passing potable water via Limestone filter (dolomite)" This is something that is only done for very small household residential users, not a power plant.


"use pvc type pipework for low pressure water systems" Most power plants use stainless for this application because extensive pipe supports are required for PVC.

"if u r making demin, add calgon to your feed water" Nobody in their right mind would make demin water and add calgon to it.

 

[mxmaciek]

Definitely demin means in general below 1 microSiemens/cm, preferably below 0.5.

Use of plastic materials is restricted in practice to fittings/valves related to treating equipment, for longer distances - it's too flexible. Plastic pipes (PVC, PP, PE, PVDF - refer to i.e. Georg Fisher) have the sharp thermal/pressure related restrictions in application.

Adding "calgon" or similar antiscalant is doing nothing against corrosion, for this you shall apply the corrosion inhibitor.

At last, but not least: use of demin water in cooling system is extremaly expensive choice, what was your decision path? Normally, it should be a raw water with some corrective agents for pH adjustment and corrosion prevention.