CREEP RUPTURE LIFE - Tube Reliability Assessment

Remaining creep life for tubes in a heater is estimated or measured using various techniques ranging from the approach outlined in API Std. 530 Annex A, API RP 579 or to destructive creep testing of tube material.

API Std. 530 Annex A, “Estimation of Remaining Tube Life” in API Std. 530 is a common approach to assess life of in-service tubes. The calculations are based on the Larson-Miller parameter curves found in the document. API Std. 530 provides an average and a minimum Larson-Miller curve for each metallurgy. The most conservative approach is to use the minimum curve since it represents the poorest material properties of those tested. The average curve can also be used, although the specific heat of material can exhibit properties either above or below this curve.

These life assessments usually require several assumptions about the tubes’ thermal and stress history. The history needs to be established to effectively determine the amount of life expended during each operating run under different conditions. One can simplify the analysis by assuming the tubes operated under the severe conditions for their entire life. Once the life fraction has been determined, remaining life can be estimated for specific operating conditions. One can establish an operating window of temperature and pressure for which the tubes can operate where creep rupture would not be expected during the next run length.

More accurate techniques to determine remaining life require destructive creep testing. One technique is the Omega methodology, which uses strain rate data generated in a creep rupture test to determine remaining creep life. The creep rupture test can be performed at temperatures and stresses that closely approximate the actual operating conditions of the tube. This is unique to creep testing, since they require tests at either higher temperature or higher stress to shorten the tests to a reasonable amount of time. The results are then extrapolated back to operating conditions. This extrapolation can lead to inaccuracies in estimates. Only a few samples are necessary for this testing and samples can be prepared from only a small section of tube. The section of tube to be tested should optimally be taken from the location operating under the most severe conditions. Additionally, the samples should be tested in the most highly stressed direction in service. Typically, this will be a sample oriented circumferential in the hoop stress direction.

The remaining life determinations provide a means to manage tube life of a heater or boiler. The operator of the boiler or heater can understand how the operation affects tube life. For instance, tube life can be monitored throughout the run incorporating all types of operations including most importantly any high-temperature excursions. These remaining life calculations can be used to predict and plan tube replacements.

When inside the heater or boiler, tubes have historically been replaced if they exhibit an increase in tube diameter beyond a specified threshold value. Company practices range from 1% – 5% of the tubes original diameter or circumference for wrought tubes. Creep testing can be used to determine a better relationship between growth and remaining life for particular tube metallurgy in a heater. Some metallurgies exhibit more growth than others do for a similar remaining creep life. Therefore, 5% may be conservative for some metallurgy and not enough for others.