Flexible Supply Line Failures: What’s Really Happening?

Article by:

Share On:


The popularity of flexible water supply lines over their rigid copper piping counterparts is without question. They have many benefits regarding ease of installation, reduced costs and less noise during installation. Despite their many positive attributes, flexible supply lines constitute one of the most common causes of water-related property losses.

Most flexible water supply lines have an outer braided Stainless Steel (SS) shell and an inner flexible polymer hose construction. Since these flexible supply lines are exposed to various water pressures when installed, the outer SS braided shell provides the necessary structural support to the inner polymer hose. Unfortunately, the SS material is not entirely inert to the environment. SS material can be susceptible to chlorides. Chlorides are found in the water supply and everyday household products/cleaners. Of the various SS alloys used in braided hose construction, 304 SS is the most susceptible compared to 300 series alloys, such as 316 and 321 SS, and is the most widely used. The sequence of events in the failure of the SS braided flexible supply lines would be the following:

  1. The release of water onto the SS braided surface (perhaps from a small leak at a connection or from an external source)
  2. The evaporation of water left behind a chlorine deposit on the SS braided surface 
  3. A build-up of deposits over time due to repeated exposure
  4. Localized corrosion of the metal strands and microcracking due to stress corrosion
  5. Eating away at the individual strands, causing them to fracture completely and unravel
  6. Exposure of the inner polymer hose once enough strands have broken away
  7. Internal water pressure causes the inner polymer hose to expand beyond its limits and rupture
  8. Water escaping from the ruptured hose

An example of a failed SS braided flexible supply line can be seen in the following photo:

There are several factors that contribute to this type of failure: 

  • Installation 
  • Materials 
  • Environment

Of the three factors mentioned above, the method of installation may be playing a larger role in the failure of these supply lines than expected. The design of these supply lines is such that the tightening of the end can inadvertently lead to the loosening of the opposite end. This loosening can lead to slow leaks across the rubber seal, passing the nut and onto the SS braiding. In this scenario, the installer must ensure sufficient tightening of both ends of the connection while not over-torquing. If the connections are loose or too tight, leaking may occur. Both situations will lead to corrosion of the SS braid, fracture of the metal strands, expansion and rupture of the inner polymer hose, and finally having water escaping and causing property damage.

Material selection is also an important factor in mitigating the potential for corrosion. For availability and lower manufacturing costs, the more common and less corrosionresistant 304 SS alloy is used in these applications. In addition, and perhaps not as highly considered, is the diameter of metal strands being used. Current failure investigations have shown strand diameters to be as low as 0.15 mm. Too small a diameter can lead to gaps in the braiding in which the inner hose becomes visible and provides less structural support strength. 

These more delicate individual strands will have a higher tendency to fracture under the corrosion conditions already discussed, as compared to thicker strands in the range of 0.25 mm. Another important consideration is the type of brass being used for the barb fittings. Depending on the alloy and manufacturing process used, brass can be susceptible to de-zincification. De-zincification or the preferential leaching of zinc from the brass alloy can lead to water migrating through the fitting cross-section if the beta phase in the alloy forms an interconnected pathway. Again, this can lead to water leakage and corrosion of the SS braiding.

The final factor to consider is the home’s environment where these supply lines are located. Under kitchen or bathroom sinks, the storing of household cleaners can create a “fume” cupboard. The chemical emissions from these household cleaners can cause the chlorine in the enclosed space air to rise. When combined with the relatively higher humidity in this area, condensation and deposition onto the SS braiding may occur, leading to corrosion. Another susceptible location is behind toilets where such water supply lines are used. These areas are often directly exposed to cleaning chemicals and moisture making them prime targets for water escapes to occur due to corrosion and rupture.

As a homeowner, it is important to be vigilant of SS flexible water supply lines currently installed; checking their appearance and knowing the age of what has been installed, if possible. If water deposits, corrosion and/or fractured metal strands can be seen, it is a good indication that these lines should be replaced. As an alternative, polymer braided flexible supply lines could be used as an added protection against potential water damage due to corrosion. In any case, ensure proper installation methods are implemented when using flexible water supply lines.