By: Michelle Knight on Dec 3, 2019 9:00:00 AM

HOW TO AVOID ENVIRONMENTAL STRESS CRACKING IN PLASTIC PIPING

Environmental stress cracking (ESC) is one of the leading causes of plastic pipe failure. It occurs when the piping material is simultaneously subjected to mechanical stress and certain types of chemicals.

ESC typically results when incompatible chemicals and external mechanical forces work together to overcome the piping material’s strength. ESC can be avoided by considering the chemicals and mechanical stresses that will affect a piping system’s performance and addressing those factors during system design and installation.

Understanding Why ESC Occurs

Amorphous polymers like CPVC get their strength from long chain-like molecules that are tangled together. CPVC will fail when enough chains break or become sufficiently disentangled that a breach develops in the solid polymer. This breakage and/or disentanglement can result from strong mechanical stresses, strong chemicals or a combination of moderate levels of both.

Most people understand the concept of a strong mechanical stress such as impact causing the pipe to break, or a strong solvent causing the pipe to soften or dissolve. ESC is different, occurring when there is a moderate stress that by itself wouldn’t be enough to break the plastic combined with a moderate chemical that alone wouldn’t be sufficient to dissolve the plastic. When there is a stress pulling on the molecules of the polymer while a chemical capable of causing some weakening of the polymer is sitting on the surface, a crack can develop on the surface of the plastic. With the stress continuing to pull on the molecules as the chemical works itself deeper into the crack in the surface, the crack can continue to propagate until it breaches the pipe wall.

How Pipe Quality Affects ESC Resistance

The quality of product manufacturing has a great influence in affecting its resistance to chemicals and environmental stress cracking. The melt mixing that occurs in the extrusion or injection molding process creates the molecular entanglements as the parts are formed.

Adequate mixing and entanglement may not be possible if processing conditions are poor (including wrong equipment) and certain vulnerable areas may not achieve adequate entanglement. Other types of defects such as voids, inclusions or score lines can create areas of localized weakness or elevated stress. When molecules are not adequately entangled during manufacturing, they are easier to disentangle by mechanical or chemical effects.

How Mechanical Stress Impacts Plastic Piping

The mechanical stresses working to disentangle the polymer molecules can come from a variety of sources. In addition to operating stress caused by pressure in the pipe, stresses may result from installation issues such as excessively tight pipe hangers, improper connections and joining, inadequate support, excessive bending, etc. Following the manufacturer’s installation instructions can help minimize mechanical stress.

The illustration below shows how failure can occur when combined stresses exceed the strength of a piping material that has already been weakened by quality issues and the effects of chemicals.

Conversely, if mechanical stresses are low and material processing has been sufficient to achieve good molecular entanglement, the plastic piping material may perform for years without problems, even if it comes into direct contact with a chemical classified as “incompatible.”

Other Chemical Concerns

Oftentimes, it’s only the major components of the process fluid that are taken into consideration when a piping material is specified. However, it’s also important to consider the low-level chemical components contained in a facility’s process fluid.

Chemicals such as solvents or nonionic surfactants may contribute to ESC of the piping even when present at levels lower than one percent. Make sure all the constituents of the process fluid are identified and ask questions regarding CPVC’s resistance to these chemicals. The Corzan® CPVC Chemical Resistance Chart shows the compatibility of Corzan CPVC with over 400 chemicals.

Piping to Minimize ESC

It is nearly impossible to predict all of the mechanical variables and the different chemicals that will end up in the interior or exterior of a piping system. The designer and installer, therefore, must take steps to control the factors that can help prevent ESC.