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Blister formation on carbon-graphite mechanical seal faces most commonly occurs in oil and hydrocarbon service, with higher viscosity oils, and with stop-start cycling. Blisters form unexpectedly, often with catastrophic results. In this study, an experimental apparatus that closely simulates field conditions was used to form blisters on five different grades of carbon-graphite seal faces. The tests showed that blisters form at the instant of initial shaft rotation. Significantly more blisters form as the viscosity of the oil between the seal faces increases when chilled. The blisters are located randomly around the seal face and not in regular patterns. The authors conclude that blister formation is a mechanical process through which a shallow subsurface crack is formed by shear forces acting at the carbon-graphite seal face.
KEY WORDS
Mechanical Seals; Seal Face Blisters; Tribotesting
INTRODUCTION
Blistering of carbon-graphite mechanical seal faces has been investigated for at least three decades. Various mechanisms for their formation have been proffered, but these mechanisms have not been extended to a universal solution. The problem continues to plague users and manufacturers of seals. The objective of the research program underlying this paper was to develop a test methodology that would reproducibly generate blisters on carbon/ graphite seal faces and to correlate the results of the experimental seal testing program with analytical analyses. The test methodology was described in a companion paper (1). In this paper, the authors examine various mechanisms for blister formation and compare them with results from the experimental test program and analytical analyses.
BACKGROUND
Blistering of carbon/graphite seal faces degrades the surface of the seal face causing irregularities in topography, which result in poor performance - higher leak rates and higher friction. The phenomenon is observed in carbon/graphite seal faces subjected to severe service conditions. Strugala (2) found that blisters can have three different appearances, a shiny spot (Type I), a shiny spot with cracks radiating from the edges (Type II), and a pitted area with radiating cracks (Type III). He characterized the blisters with a severity index where a Type I has a relative severity of 1, a Type II has a relative severity of 2 and a Type III has a relative severity of 3. Figure 1 shows a seal face that contains several...