# Why Piping for Reverse Osmosis Equipment Differs From Ordinary Water‑Supply Pipes (For General Users’ Reference)
## Piping Installation and Material Selection for Reverse Osmosis Pure‑Water Equipment
Material selection shall take both internal and external factors into consideration. Internally, focus on water‑induced pipeline corrosion; externally, focus on service‑environment‑caused corrosion.
The service environment of reverse osmosis equipment determines external pipeline corrosion. For favorable operating conditions, surface coatings such as paint or galvanization are generally acceptable.
For water‑borne pipeline corrosion, parameters including residual chlorine, chemical agents, pH value and temperature shall be considered. Commonly adopted materials include polyvinyl chloride (PVC) and stainless steel.
### Basic Advantages of Stainless Steel
Stainless steel delivers good general corrosion resistance, with low risk of galvanic corrosion and stress‑corrosion cracking. However, it is susceptible to pitting corrosion and crevice corrosion.
Pitting corrosion refers to localized metal corrosion that forms surface pits. Once the chromium‑oxide passivation layer is damaged, chloride ions will attack the exposed metal and trigger pitting corrosion.
Crevice corrosion is a form of pitting corrosion caused by stagnant water trapped in confined spaces such as holes, gasket interfaces, sediment deposits and under bolt heads.
To avoid pitting and crevice corrosion in high‑pressure piping of membrane systems, the following recommendations apply:
- 304 stainless steel: applicable for general water sources;
- 316 stainless steel (carbon content ＜0.08%): recommended for raw‑water salinity of 2 000‑5 000 PPM;
- 316L stainless steel (carbon content ＜0.03%): recommended for raw‑water salinity of 5 000‑7 000 PPM;
- 904L stainless steel (molybdenum content 4%‑5%): recommended for raw‑water salinity of 7 000‑30 000 PPM.
### Characteristics of PVC Pipes
A. Excellent corrosion resistance: Compared with cast‑iron and galvanized pipes, PVC pipes feature superior acid‑alkali resistance and resist rust and scaling, eliminating red‑water issues.
B. Low flow resistance: The inner wall of PVC pipe is extremely smooth with a roughness coefficient of only 0.009, creating minimal flow resistance and avoiding excessive water‑pressure loss.
C. High mechanical strength: Water‑supply PVC pipes possess sound hydrostatic‑pressure resistance, impact resistance and tensile strength. They can withstand 110 atmospheres for one hour at ambient temperature without rupture.
D. Hygienic and non‑toxic: Adopting eco‑friendly lead‑free formulations instead of conventional lead‑salt composite formulas, PVC pipes will not contaminate water or endanger human health.
E. Lightweight for easy installation: PVC density is one‑fifth that of cast iron, facilitating transportation and handling. Joints can be assembled via special adhesive bonding or elastic sealing gaskets for fast, simple field installation.
F. Temperature adaptability for water‑supply PVC pipes: Optimal operating temperature is ambient temperature. Low temperatures increase brittleness and complicate installation; high temperatures reduce tensile strength and pressure‑bearing capacity.
G. Reliable water tightness: Bond strength reaches above 12.5 MPa 15 minutes after adhesive application, with no obvious performance degradation over long‑term service. Gaskets for elastic‑socket joints match the service life of PVC pipes, resisting ageing and leakage.
Multiple material options are available for reverse‑osmosis pure‑water‑equipment piping, such as ABS, PPR, steel‑rubber lining and steel‑plastic lining. This document focuses on the widely‑used PVC and stainless‑steel pipes, with detailed analysis based on water quality and service conditions.
Important notes for piping connection of reverse‑osmosis pure‑water equipment: Avoid unnecessary valves; minimize pipe bends; the total length of drain‑pipe connections shall not exceed 6 meters.
