ASME B31.8S,

B31.8S titled, Managing System Integrity of Gas Pipelines, is a supplement to the pressure piping code of B31.8. Per the introduction of the standard states,“The Standard is specifically designed to provide the operator… with the information necessary to develop and implement an effective integrity management program …”. In most cases it succeeds in that goal. However there is one part that needs to be addressed; mainly Figure 4. This graph represents the response time vs. the ratio of failure pressure to MAOP. The intent of this graph is that if an operator has an anomaly with a given failure ratio and percent of SMYS they can determine the maximum time frame to respond to the anomaly. Figure 4 is reproduced in the next plot.

While Figure 4 provides convenience it should be used with caution. This simplification requires several assumptions that may or may not apply. Among those are that the anomaly has long uniform-depth corrosion and a linearly varying ratio of rupture pressure ratio over time. In fact, B31.8S itself has the following caveat in Section 7.2.4 Limitations to Response Times for Prescriptive Based Program.“When time‐dependent anomalies such as … corrosion … are being evaluated, an analysis utilizing appropriate assumptions about growth rates shall be used to assure that the defect will not attain critical dimensions prior to the scheduled repair or next inspection.”

Why does this section have such a all-encompassing caveat? In the next section we’ll show how using Figure 4 can get you into trouble if the assumptions are not met. In this example, it will be assumed that an anomaly was reported by an ILI tool that was 35% of the wall thickness deep and 5 inches long. The pipeline is a 24" Diameter, 0.250" wall thickness operating at 72% of SMYS. The corrosion rate is 10 mils per year (mpy). If you do the B31G calculation for this anomaly it would have a Rupture Pressure Ratio of 1.4, well above what is needed for a 10 year response time. But what happens as it corrodes over time? Figure 2 displays the predicted failure pressure of the anomaly from year 0 to year 7. The anomaly that initially had a significant factor of safety above the MAOP but by the end of year 7 it nearly fails at MAOP.

Figure 2

Figure 2

Conclusion

As can be seen from this example, B31.8S, Figure 4 does not hold up in all conditions. It is necessary to look at the actual predicted failure pressure now and in the future to see if it needs to be addressed before the next reassessment. Depending on the initial size and corrosion rate using Figure 4 alone can lead to decisions that could lead to the pipeline failing before reassessment.