RO Feed Piping Material Selection

[wardens355]

I am involved in design of an RO drinking water plant and was curious if there are thoughts on piping material upstream of the RO system. We do not want to use PVC because greensand filtration is planned as pre-treatment, and there may be pressure surges associated with valves slamming shut during backwash. Carbon steel was initially selected, but some folks have concerns that corrosion may introduce iron into the water upstream of the RO (the greensand filter removes iron/manganese to protect the RO, ideally want less than 0.05 mg/L iron), and cause scaling issues. Does anyone have thoughts on acceptable piping material for this application? Carbon steel and cement-lined ductile iron pipe are being tossed around now.

 

[bimr]

What size piping?

 

[wardens355]

Size ranges from 6" to 20" from individual skid/vessel piping to headers.

 

[bimr]

You should be looking into PVC, FRP, or Stainless downstream of the greensand filter. Carbon steel and cement lined will probably throw off too much corrosion debris which is a consideration with the RO.

FRP is probably the best choice.

 

[wardens355]

There will be cartridge filters protecting the RO membranes, so maybe the concrete-lined pipe would not be as big of an issue. We probably need to look into surge analysis for closing butterfly valves on the greensand skid to make sure we will not risk exceeding pressure rating for FRP if we went that direction. Maybe designing to mitigate surge pressures could make PVC a feasible alternative, if there is a pneumatic actuator that can close a butterfly valve slow enough.

 

[bimr]

PVC is too lightweight and fragile for this application. PVC also has temperature limitations.

The FRP piping should be somewhat equivalent in strength and design aspects to steel piping.

RO Membranes are too expensive to rely on cartridge filters for protection. You should have clean water after the greensand filters, why would you want to add contamination from corrosion products?

Pneumatic actuators should come with air solenoids that allow the valve closing times to be adjusted.

 

[wardens355]

Thanks for the feedback. We have discussed potentially moving to FRP because it is a more robust material than PVC and is also corrosion-resistant. I would like to make sure that CS and DIP are poor options before wholly eliminating them. I agree that cartridge filters should be a fail safe measure.

 

[rconner]

Another option from some well-known vendors is ceramic epoxy lined ductile iron piping, and with epoxy coated sockets and spigots for RO applications e.g.

http://www.ceramapure.com/.

 

[stanier]

You could also consider spiral wound stainless steel. This is cost competitive with FRP.

ôThe beautiful thing about learning is that no one can take it away from you.ö
---B.B. King

http://waterhammer.hopout.com.au/

 

[rconner]

[fyi

http://www.wwdmag.com/membrane-hous...ater-and-wastewater-treatment-environments-1​

and

http://www.outokumpu.com/SiteCollec...ure-piping-properties-and-experience-Acom.pdf

etc​]

 

[Watro]

The piping selection also depend on feed salinity. If Seawater is fed, FRP or RTRP is the best choice with an alternative CS FBE (Fusion bond epoxy) lined, of course the size should be >DN100. In case of brackish water, CPVC is the cheapest, only if the plant is installed indoor. 304SS is another cheapest compared to FRP/RTRP, if everything is constructed in a compact skid

Regards
Niss'