316SS vs 304SS pump for RO water application 3

[cames]

Hi guys,
im specing out a pump for RO water. The water it would be pumping would after the water has gone thru the RO members, watersofterner, and carbon filter. Since its RO water it will be slightly more acidic (5.4-7.4) PH)
It would pump 70 GPM @ 70psi and about 100,000 gallons of water a day. The RO water has Na,K,Si,Cl,NO3 and Ca ppms of 0-1max.

The 316SS pump is $6k and the 304SS is $3k (all other specs equal), wanted to ask the experts is the 316SS really worth the extra money for RO water application?

I know the main advantage of 316 is its durability against pitting, but at my planned usage, how significant is the rate of corrosion when comparing the two?

Thank you gents!

 

[0707]

“316 stainless steel’s material composition of 16% chromium, 10% nickel, and 2% molybdenum allows it to be used in applications that require superior corrosion resistance and stringent hygiene and cleanliness requirements. Some of the most common applications are its resistance to harmful acids, alkalis, and chloride pitting”

“304 stainless steel’s chemical makeup of 18% chromium and 8% nickel makes it a good choice for many common applications, which include:
Kitchen equipment & appliances, water piping”

For filtration uses in water osmosis maybe if chlorides, acids and alkalis are involved it will be ecconomical useful to use SS316.

 

[TugboatEng]

The Mo in 316 is primarily added for chloride resistance. Your fluid has very little chloride so I see no benefits to running 316.

 

[EdStainless]

The real question is "is either one of them made well?".
Have the castings been annealed?
Are they free from residual delta ferrite?
All high purity water applications that I have delt with have used 316L.
The Mo in 316 enhances the re-passivation kinetics and helps in all aqueous corrosion situations.
Clean water can be very corrosive as everything wants to dissolve in it.

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P.E. Metallurgy, consulting work welcomed

 

[Krausen]

Think the biggest question may be the original one - is spending twice as much on 316 vs 304 really worth it in this case, assuming both are quality castings? That can depend on the project size & many other things. Evaluating it in a vacuum without knowing any background, I'm not sure I'd throw 2x more money at a 316 pump in this case.

 

[Dhurjati Sen]

It is assumed that the RO works as per spec all the time and there is no slippage!

Ideally, SS 304 is sufficient for the purpose, but hardly do we get anything ideal.

How many RO pumps would be there? If one is required to run all the time, with another on auto and the third on standby, SS 304 is fine.

If there are two, with one running and another on auto with provision to alter the instrument logic in case of major repair and also spares are readily available, then one can take a chance with SS 304.

How much would it cost if the RO plant is out for an hour? Or a day?

Answers to such questions would lead to the right material choice for the pump.

DHURJATI SEN
Kolkata, India

 

[EdStainless]

In high purity water systems it is common to see Fe transport leaving a red staining is a lot of areas.
Part of the issue is with high purity water just being a very good solvent, another is the ease with which it absorbs CO2 and becomes acidic.

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P.E. Metallurgy, consulting work welcomed

 

[MedicineEng]

In RO systems, typically people expect to have SS316L.
Everybody else already touched important points, but I would just wanted to raise one that it hasn't been touched before.
The final use of the RO water also an important decision factor. If the RO water is going to be used, say for crop irrigation it's one thing.
If it is going to be used for human pharmaceutical uses, then it is a completely different ball game.
In pharma, at least when I was working in the industry many moons ago, the use of 316L on RO water loops and equipment was a given.

 

[Rubje]

@TugBoatEng "The Mo in 316 is primarily added for chloride resistance. Your fluid has very little chloride so I see no benefits to running 316."

I don't believe that is true. I read studies that 316 still offers better corrosion protection for non-chloride containing environments compared to 304. Also, 316 works better for pitting and crevice corrosion even in non-chloride environments.