When a process material fails, a failure analysis can help determine corrective action and preventative measures to avoid future failures. This paper will discuss various potential failure mechanisms for rigid PVC and CPVC piping materials in terms of their distinguishing characteristics. Failure by solvation, degradation, and cracking will be addressed with particular emphasis on cracking phenomena. Visual examples will be presented, and analytical techniques which can be used to confirm the cause of failure will be suggested.
INTRODUCTION
Despite our best efforts to select and specify appropriate materials for process applications, materials will occasionally fail anyway due to inadequate chemical resistance, improper installation, pre-existing damage, or unaccounted for chemical or physical demands. When a material fails, a failure analysis is necessary to determine whether the material is improperly specified for the particular application and should be replaced throughout, or whether some unforeseen event or combination of circumstances has caused a singular failure of an otherwise adequate material, and whether corrective action can be taken to improve the system's chances for success. This paper will address the failure analysis of PVC and CPVC materials. The primary examples used will be for process piping applications, although these materials also find use in tank linings, ducting, tower packing, and many other areas of a chemical process operation.
Thermoplastic material failure can occur in four basic modes: softening, degradation, erosion, or cracking. This paper will address all four modes, but will focus most attention on cracking as this presents the most varied and least understood of the failure phenomena. A variety of factors may influence material failure. These include the chemical environment to which the material is exposed, the surrounding physical environment of the system, physical damage to the system, unaccounted-for installational stresses, and plastic part quality. Contributions of all of these factors will be examined.
MODES OF FAILURE
Softening
Failure by softening is usually obvious from the appearance of the failure. The part will appear swollen and/or distorted and will usually have failed by ballooning and ductile rupture, or by distortion of the system. Softening of the material may be caused by simple exposure to temperatures in excess of the material's general capability given its heat distortion temperature, or it may be caused by a temperature/stress condition in excess of what is recommended for the material, such as operating at a pressure in excess of the pipe's derated pressure rating for the operating temperature, or installing the piping with support spacing in excess of what is recommended for the operating temperature. An example of a ballooning failure caused by an operating pressure in excess of the derated pressure for the operating temperature is shown in Figure 1.
When failure by softening occurs due to overtemperature and/or overpressure of the piping system, the failure analysis investigation naturally focuses on the system design and control. Is the material properly specified for this application, or should a higher performance material have been chosen? Are overlimit controls working properly? Have exothermic reactions or high heats of dilution been considered?
Softening of the material may also be caused by absorption of solvents or plasticizers, either from the process fluid itself, or from the external environment. When solvents are absorbed from the process fluid, it may be a simple case of having specified the wrong material for the known conditions. However,
