Feasibility of replacement SS 316L Pipe with PVC pipe in vacuum piping of Distiller.

[Tomy-Met]

Dear Experts, Please advise

We are facing Stress corrosion Cracking issue in Vacuum piping of Desalination plant distillers, where temperature would be around 70-100 C and pressure -1Bar, pipe material is SS 316 L.

Stress corrosion cracking is due to chloride environment in distiller and my questions are:

Can we use GRP Wrapping on cracked pipe instead of pipe crack repair or pipe segments replacement?
Can we replace existing cracked SS316L pipe with High temperature PVC, recommended material is duplex stainless steel ( with high Ni%) but duplex stainless steels are very expensive and we are considering on PVC pipe.

Thanks in advance for your valued time.

 

[Dhurjati Sen]

Dear Tomy-Met,

1st question - Temporary repair is not advisable unless absolutely unavoidable. Refer below link.

[link thread124-461821]Link[/url]

2nd question - Has PVC ever been used for such a service? Changing the material without comprehensive study simply shifts the corrosion elsewhere.

Regards.

DHURJATI SEN
Kolkata, India

 

[Tomy-Met]

Dear Dhurjati Sen, Thanks for your reply

Yes that is valid point that such GRP wrapping or replacement with PVC pipe may shift the corrosion in other connected equipment like condenser, ejector.
PVC or GRP wrapping has chlorine in them?

 

[EdStainless]

Moving to 2205 duplex will cost no more than replacing in 316L would.
After all you will be moving to thinner walls and using a lot less metal.
That is unless you are already at the bare minimum for buckling failure.
You might be able to do this with FRP pipe, but I can't see using PVC (or any related materials) in long term vacuum service. They would have to very thick.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[moltenmetal]

PVC is not suitable for use at 70-100C, nor is CPVC. You'd be looking at polypropylene (perhaps) or PVDF or FRP.

2205 pipe and fittings at sch10S has a substantial premium in cost over 316/L. Replacing the line in duplex WILL cost more money.

 

[EdStainless]

Most thermoplastics don't like being under continuous stress. They will creep and they may be susceptible to stress cracking.
If the unit was designed with as thin of 316 tube as you can use then yes, 2205 will cost more. But in most cases people take the easy way out and just use what is handy so you may have an opportunity to reduce the wall thickness of the tubes moving to 2205.
If you are buying mill products such as plate or bar then 2205 is about 5-10% more than 316L on a per pound basis. The base price will be higher, but the surcharges are lower. The difference was zero when Ni prices were higher.
I used to make a lot of sch5 2205 tube (2"x0.065" and 4"x.0083") that was used in place of 316L sch10.
Fittings are another matter altogether.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

I'm not familiar with desalination plant metallurgy, but I do now the environment is far beyond what 316SS or any 300 series alloy can handle.
I recommend getting familiar with industry standards for design and metallurgy. Nickel Development Institute might have a monogram on the topic.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

So this is thermal desal? Which plant type?
These are the lines from the stage (effect) to the vacuum?
It doesn't really matter but I would presume that you are using steam eductors for vacuum?
Is the cracking starting on the inside or the outside of the tube?
Has this always been a problem? How old is the plant?

I have seen this both interior and exterior. If it from the outside then paint the piping.
Is it everywhere or localized? (Low spots, hot spots, places with higher stresses)
If it is on the inside and has not always been an issue then what changed that resulted in more carryover?
In many plants 316 works very well in this application.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy