Method and apparatus for determining the health and remaining service life of austenitic steel reformer tubes and the like

Abstract

Testing methods and apparatus for testing the health of steel tubes used in reformers and other tubes and pipes used in other high temperature applications. The method includes the steps of transmitting two sinusoidal electromagnetic signals, each having a different frequency F1 and F2, into the reformer tube, receiving a response signal, and analyzing the received response signal's intermodulation frequencies to determine the state of the steel reformer tube.

We claim: 
     
       1. A method of testing an austenitic steel reformer tube comprising:
 providing a sample austenitic steel reformer tube to be tested; 
 choosing one or more testing positions on said an austenitic steel reformer tube; 
 transmitting two sinusoidal electromagnetic signals, each having a different frequency F 1  and F 2 , into a test position on the austenitic steel reformer tube; 
 receiving a response signal from said test position; and 
 analyzing said received response signal's fundamental and intermodulation frequencies to determine the state of the austenitic steel reformer tube at said test position; 
 wherein said step of receiving a response signal from said test position includes receiving an analog response signal on a receiver coil; 
 wherein said step of receiving a response signal from said test position further includes the step of converting said analog response signal to a digital response signal, using an analog to digital converter; 
 wherein said analog to digital converter has a sampling frequency F s ; and 
 wherein said step of converting said analog response signal to a digital response signal, using an analog to digital converter includes combining a multiple of samples into a single representative sample, the number of samples which are combined into said single representative sample being designated the sample size S s . 
 
     
     
       2. The method of  claim 1 , wherein said step of receiving a response signal from said test position includes receiving an analog response signal on a receiver coil. 
     
     
       3. The method of  claim 2 , wherein said step of receiving a response signal from said test position further includes the step of converting said analog response signal to a digital response signal, using an analog to digital converter. 
     
     
       4. The method of  claim 3 , wherein said analog to digital converter has a sampling frequency F s . 
     
     
       5. The method of  claim 4 , wherein said step of converting said analog response signal to a digital response signal, using an analog to digital converter includes combining a multiple of samples into a single representative sample, the number of samples which are combined into said single representative sample being designated the sample size S s . 
     
     
       6. The method of claim  5  1, wherein said sample size S s  is a is an integral power of 2. 
     
     
       7. The method of  claim 6 , wherein said sample size S s  is a number selected from the group consisting of 4096, 8192, and 16384 samples. 
     
     
       8. The method of  claim 7 , wherein said sampling frequency F s  is 44100 samples per second. 
     
     
       9. The method of  claim 5 , wherein said step of transmitting two sinusoidal electromagnetic signals includes the step of defining a base frequency F 0 , wherein F 0 =F s /S s . 
     
     
       10. The method of  claim 9 , wherein said step of transmitting two sinusoidal electromagnetic signals further includes the step of choosing said two frequencies F 1  and F 2  such that:
   F 1 =N×F 0 ;
 
   F 2 =P×F 0 ;
 
 where N and P are integers with N not equal to P, and 
 N and P are chosen such that none of the intermodulation frequencies, F(Q,R)=Q×F 1 +R×F 2  are equal to an integral multiple of F 1  or F 2  for small, non-zero, integer (positive or negative) values of Q and R. 
 
     
     
       11. The method of  claim 1 , wherein said step of transmitting two sinusoidal electromagnetic signals comprises transmitting both of said signals from a single transmitter coil. 
     
     
       12. The method of  claim 1 , wherein said step of transmitting two sinusoidal electromagnetic signals comprises transmitting each of said signals from individual transmitter coils. 
     
     
       13. The method of  claim 12 , A method of testing an austenitic steel reformer tube comprising:
 providing a sample austenitic steel reformer tube to be tested; 
 choosing one or more testing positions on said an austenitic steel reformer tube; 
 transmitting two sinusoidal electromagnetic signals, each having a different frequency F 1  and F 2 , into a test position on the austenitic steel reformer tube; 
 receiving a response signal from said test position; and 
 analyzing said received response signal's fundamental and intermodulation frequencies to determine the state of the austenitic steel reformer tube at said test position; 
 wherein said step of transmitting two sinusoidal electromagnetic signals comprises transmitting each of said signals from individual transmitter coils; and  
 wherein said transmitter coils have a larger diameter than the thickness of the sample tube to be tested. 
 
     
     
       14. The method of  claim 1 , wherein said step of transmitting two sinusoidal electromagnetic signals comprises creating analog sinusoidal electromagnetic signals using at least one digital-to-analog signal generator. 
     
     
       15. The method of  claim 14 , wherein said two sinusoidal electromagnetic signals are created by two signal generators. 
     
     
       16. The method of  claim 1 , wherein said step of analyzing said received response signal's fundamental and intermodulation frequencies comprises analyzing the first order fundamental and the third order intermodulation frequencies of said received response signal. 
     
     
       17. The method of  claim 16 , wherein said fundamental is F 2  and said third order intermodulation frequencies are 2F 1 +F 2  and F 1 +2F 2 . 
     
     
       18. The method of  claim 16 , wherein said step of analyzing the third order intermodulation frequencies comprises converting the amplitude of said third order intermodulation frequencies into decibels dB relative to the amplitude of said fundamental. 
     
     
       19. The method of  claim 18 , wherein the strength of said third order intermodulation frequencies which have been converted into decibels dB is compared to the same measurement of brand new and end of service life austenitic steel reformer tubes, said comparison providing a qualitative measure of the health of said austenitic steel reformer tube. 
     
     
       20. The method of  claim 19 , A method of testing an austenitic steel reformer tube comprising:
 providing a sample austenitic steel reformer tube to be tested; 
 choosing one or more testing positions on said an austenitic steel reformer tube; 
 transmitting two sinusoidal electromagnetic signals, each having a different frequency F 1  and F 2 , into a test position on the austenitic steel reformer tube; 
 receiving a response signal from said test position; and 
 analyzing said received response signal's fundamental and intermodulation frequencies to determine the state of the austenitic steel reformer tube at said test position; 
 wherein said step of analyzing said received response signal's fundamental and intermodulation frequencies comprises analyzing the first order fundamental and the third order intermodulation frequencies of said received response signal; 
 wherein said step of analyzing the third order intermodulation frequencies comprises converting the amplitude of said third order intermodulation frequencies into decibels dB relative to the amplitude of said fundamental; 
 wherein the strength of said third order intermodulation frequencies which have been converted into decibels dB is compared to the same measurement of brand new and end of service life austenitic steel reformer tubes, said comparison providing a qualitative measure of the health of said austenitic steel reformer tube; and  
 including the further step of estimating the remaining service life of said austenitic steel reformer tube as a fraction of the present service life of said austenitic steel reformer tube by the following formulas:
   fractional life remaining L r =|S e −S n |/|S e −S 0 |; and
 
   estimated lifetime remaining T r =(L r /(1−L r ))×T n  where:
 
 
 L r  is the estimated percentage of life remaining; 
 S e  is the third order intermodulation frequencies signal strength converted into decibels dB of an austenitic steel reformer tube at the end of service life; 
 S n  is the third order intermodulation frequencies signal strength converted into decibels dB of the test sample now; 
 S 0  is either the third order intermodulation frequencies signal strength when there is no tube present under the probe, or the third order intermodulation frequencies signal strength of a new tube that has been heated to operating temperature for a few hours, whichever is higher; 
 T r  is the estimated service lifetime remaining for the test sample; and 
 T n  is the present service life of the test sample.