Lack of lubrication in deionised water. 5

[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?

 

[25362]

I pressume that since one is dealing with boundary lubrication (adsorption, chemisorption, monomolecular layers, etc.), the viscosity is of less importance than, for example, the amount of dissolved oxygen in the demineralized, deionized water. Don't you think so ?

 

[MikeHalloran]

DI water is one of the most aggressive chemicals around, possibly because lack of natural buffers allows its pH to swing wildly. Maybe it's better to think of it as ionized hydrogen and ionized oxygen, borne in a common carrier. I.e., the pump rotors are not worn away, they are etched/ corroded away. The mechanical rubbing at bearing surfaces just carries away corrosion products, exposing new substrate to attack.





Mike Halloran
Pembroke Pines, FL, USA

 

[SeanB]

De-ionized / demineralized water can wreak havoc on metals. The water has been stripped of minerals and ions. As it flows it will strip ions and minerals from any source it can. As Mike says the pump rotors are not worn away, they are being corroded away due to the water, it is not due to poor lubricity.

 

[peteroates]

Many thanks for your responses. The puzzle of corrosion vs erosion was that the non contacting parts of the rotors did not appear to be affected, however I accept that if the minerals are stripped from the surface this would certainly weaken the surface causing wear.

I can see the how the metallics were worn away, however does the same thing happen to the carbon seal faces? If you consult a seal manufacturer they will recommend Antimony impregnated carbon for DI and DM duties. In theory however won't the Di water strip the antimony from the faces? from practical experience again, we found that antimony impregnated carbons lasted longer than the regular resin impregnated carbon. The resin impregnated carbons are good for strong acids and alkalis. Can anyone explain why the antimony impregnated carbon lasts longer than the resin impregnated faces?