Lubrication properties of deionised and demineralised water 3

[peteroates]

I know from practical experience of mono pump rotors that DI and DM water is a poor lubricant, when compared with regular water. It also provides poor lubrication to mechanical seal faces, causing less than expected seal life.

I have searched all over the place for some science to back up these observations, however I cannot find an explanation of why this might be. Does anybody have any ideas? or point me in the direction of where I might find the information?

 

[rnd2]

I am interested in learning what information you gather.
One thing to consider is viscosity. Water devoid of minerals and organic matter I suspect to be less viscous than water rich with either or even both. In the absence of available information it would be well worth testing.
Another thing might be the effect of DI & DM water as a corrosive agent abrading the surface.

 

[peteroates]

I have also asked a similar question in the chemical process engineering forum see

http://www.eng-tips.com/viewthread.cfm?qid=136047&page=1

for the thread.

 

[rnd2]

Very interesting. I always understood that water is the most universal solvent so it stands to reason that water in its purest form maximises solvent/corroding power.
What I meant to infer in the last sentence of my earlier post 4 Oct 05 4:21 was as DM/DI water corrodes/errodes the shaft it might cause irregular jagged edges to appear on the shaft surface and grind against the contacting seal. Water flushes the debris away and thus forms a continuous cycle of corrosion/erosion to shaft, leading to abrasive wear against seal and shaft.
Just curious, but could you find out what resin type was used to bind the faster wearing carbon seal material?
I suspect antimony plays a beneficial mechanical rather than chemical role in prolonging seal life.

 

[peteroates]

The seal manufacturer says it is phenolic resin. They will not reveal the exact composition.

Another thought on the problem on the seal faces is that the surface tension on DI water will be lower than regular water. This will provide a less efficient fluid flim.
The investigation continues.......

 

[jet1749]

My experience with DI or DM water is that it will attack a conventional carbon face with phenolic resin binder, it will remove the graphite lubricant and leave the carbon matrix behind. It can similarly affect carbon steel and cast iron. I was taught that the "pure" water attempts to recover minerals/elements from its surroundings. I have however used antimonial carbon faces with success.

 

[rnd2]

Thanks Peter for confirming the resin is phenolic. Jet1749 advises it is the graphite that leaves the matrix. So, the seal material is a dense compressed mixure of amorphous carbon and graphite in a phenolic resin matrix. Bit like a three legged chair with one leg dodgy. Phenolic resin is among the least water soluble substances therefore unlikely it is dissolving directly into DI or DM water.
It would be worth immersing a few new seals in DM & DI water with no metal contact to see whether they deteriate as a direct result of only water rather than contact with water and metal.