Methods for measuring the flow rate due to a leak in a pressurized pipe system

• US 5,948,969 A
• Filed: 10/20/1997
• Issued: 09/07/1999
• Est. Priority Date: 10/20/1997
• Status: Expired due to Term

• Current Assignee: Vista Precision Solutions, Inc.
• Original Assignee: Vista Research, Inc. (Guidepoint Global LLC)
• Inventors: Maresca, Joseph W. Jr., Fierro, Michael R., Starr, James W.
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): Politzer, Jay L.
• Application Number:
  US08/954,002
• Time in Patent Office: 687 Days
• Field of Search: 73/40.5 R, 73/49.2, 73/49.1, 73/40, 364/509, 364/564, 364/558
• US Class Current: 73/40.50R
• CPC Class Codes: G01M 3/2815 using pressure measurements
• CPC Class Codes: G01M 3/2892 for underground fuel dispensing systems G01M3/30 takes precedence

Abstract

A method and improved apparatus for detecting leaks in pressurized pipeline systems, which compensate for the thermally induced volume changes of the liquid in the line during a test that are produced by both the nonlinear changes in the mean temperature of the liquid and the pressure-induced perturbations of the temperature of the liquid.

Claims

1. A method for measuring the difference in the volumetric flow rate due to a leak between one pressure and at least one other pressure, after compensation has been made for thermally induced changes in the volume of liquid product in a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate) and higher order derivatives of the volume data.

2. The method of claim 1, further comprising the step of comparing the difference in temperature compensated volume rate to a threshold value to determine whether said pipeline system has a leak.

3. The method of claim 1, further comprising the step of computing the temperature compensated volume rate due to a leak at a specific pressure using a model that describes the volume rate due to a leak as a function of pressure.

4. The method of claim 1, wherein the pressure level during one of the measurement periods is approximately equal to atmospheric pressure.

5. The method of claim 4, wherein measurements are made at two pressures, and the difference in the temperature compensated volume rate is the volume rate due to a leak at the pressure which is not atmospheric pressure.

6. The method of claim 1, wherein the duration of the measurement periods is approximately equal.

7. The method of claim 1, wherein the test duration, and the time intervals between measurement periods and between measurement periods and pressure changes, are selected to minimize the combined error due to the ambient thermally induced volume changes and the pressure induced thermal perturbations.

8. The method of claim 1, wherein the time intervals between measurement periods, and the time intervals between measurement periods and pressure changes for a given test duration, are selected to minimize the combined error due to ambient thermally induced volumes changes and pressure induced thermal perturbations.

9. The method of claim 8, wherein the combined error is minimized by maximizing the time intervals between each measurement period and the previous pressure change and by minimizing the time interval between measurement periods.

13. The method of claim 1, wherein the computation of the difference in the temperature compensated volume rate between pressures comprises the steps of:

(a) computing the first derivative of the volume data during a first measurement period at a first pressure, and computing the first derivative of the volume data during a second measurement period at a second pressure;

(b) computing the second derivative of the volume data during the first and second measurement periods;

(c) computing the average of the second derivatives obtained in step (b);

(d) estimating the change in volume rate between the first and second measurement periods by multiplying the time between the mid-point of the measurement periods by the average obtained in step (c);

(e) estimating the first derivative of the volume data during the second measurement period, which does not include any incremental volume change due to a leak between the two pressures, by adding the estimate computed in step (d) to the first derivative of the volume data measured during the first measurement period; and

(f) computing the difference in the temperature compensated volume rate between the first and second pressures by subtracting the estimate obtained in step (e) from the first derivative measured in step (a) during the second measurement period.

14. The method of claim 13, wherein one of the pressures is atmospheric pressure.

16. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures, comprising the method of claim 13 wherein during at least two of the measurement periods the pressure level is the same, and the error is estimated from the difference in the changes of volume measured during the measurement periods when the pressure level is the same.

17. The method of claim 1, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate) and the rate of change of the rate of change of volume during the measurement periods (second derivative of the volume data).

18. The method of claim 1, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate), the rate of change of the rate of change of volume during the measurement periods (second derivative of the volume data), and the third derivative of the volume data collected during the measurement periods.

19. The method of claim 1, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate), the rate of change of the rate of change of volume during the measurement periods (second derivative of the volume data), the third derivative of the volume data collected during the measurement periods, and higher derivatives of the volume data collected during the measurement periods.

20. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures, comprising the method of claim 1 wherein during at least two of the measurement periods the pressure level is the same, and the error is estimated from the difference in the changes of volume measured during the measurement periods when the pressure level is the same.

22. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the method of claim 1 conducted when the pressurized pipeline system is known not to be leaking to obtain the systematic bias error for the pressure, line volume, and liquid at which the measurements are made.

23. A method for adjusting the systematic bias error for differences in pressures, line volume capacities, and liquid, comprising the method of claim 22 and further comprising the steps of:

(a) adjusting the systematic bias error between other pressures by extrapolating from the measured changes, if different pressures are used;

(b) adjusting the systematic bias error if the volume capacity of the pipeline system is increased or reduced by extrapolating for the increase of reduction in capacity of the pipeline system; and

(c) adjusting the systematic bias error obtained for a first liquid to an estimate for a second liquid, when the second liquid is used, by extrapolating for the difference in the thermal coefficients of the expansion for the two liquids.

24. The method of claim 22, in which the time interval between the mid-point of the pressure change and the mid-point of the next measurement period is approximately equal for all measurement periods.

25. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the method of claim 1 conducted on at least one pressurized pipeline system that may be different from the one being tested and is known not to be leaking, to obtain the systematic bias error over the range of and for the pressures, line volumes, and liquids at which the measurements are made.

26. A method for adjusting the systematic bias error for differences in pressures, line volume capacities, and liquid, comprising the method of claim 25 and further comprising the steps of:

(a) adjusting the systematic bias error between other pressures by extrapolating from the measured changes, if different pressures are used;

(b) adjusting the systematic bias error if the volume capacity of the pipeline system is increased or reduced by extrapolating for the increase or reduction in capacity of the pipeline systems; and

(c) adjusting the systematic bias error obtained for a first liquid to an estimate for a second liquid, when the second liquid is used, by extrapolating for the difference in the thermal coefficients of the expansion for the two liquids.

27. The method of claim 25, in which the time interval between the mid-point of the pressure change and the mid-point of the next measurement period is approximately equal for all measurement periods.

28. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the method of claim 1 and using a model to estimate the thermally induced volume changes that would be measured on a pipeline system without a leak to obtain the systematic bias error for the pressure, line volume, and liquid at which the model estimates are made.

29. A method for adjusting the systematic bias error for differences in pressures, line volume capacities, and liquid, comprising the method of claim 28 and further comprising the steps of:

(a) adjusting the systematic bias error between other pressures by extrapolating from the measured changes, if different pressures are used;

(b) adjusting the systematic bias error if the volume capacity of the pipeline system is increased or reduced by extrapolating for the increase or reduction in capacity of the pipeline systems; and

(c) adjusting the systematic bias error obtained for a first liquid to an estimate for a second liquid, when the second liquid is used, by extrapolating for the difference in the thermal coefficients of the expansion for the two liquids.

30. The method of claim 28, in which the time interval between the mid-point of the pressure change and the mid-point of the next measurement period is approximately equal for all measurement periods.

31. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures computed using the method of claim 1, comprising the steps of:

(a) computing the difference between the second derivatives of the volume data during two measurement periods; and

(b) estimating the error as a percentage of the difference obtained in (a).

32. The method of claim 31, further comprising the step of comparing the error to a threshold error value, and repeating the preceding steps if the threshold error value is exceeded.

33. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the method of claim 1, and further comprising the step of estimating the error by computing at least the next higher order derivative of the volume data than is used in the computation of difference in volumetric flow rates, and multiplying said next higher order derivative by the time between the measurement periods.

10. A method for measuring the difference in the volumetric flow rate due to a leak between one pressure and at least one other pressure, after compensation has been made for thermally induced changes in the volume of liquid product in a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between periods, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate) and the rate of change of the rate of change of volume during the measurement periods (second derivative of the volume data).

11. A method for measuring the difference in the volumetric flow rate due to a leak between one pressure and at least one other pressure, after compensation has been made for thermally induced changes in the volume of liquid product in a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between periods, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate), the rate of change of the rate of change of volume during the measurement periods (second derivative of the volume data), and the third derivative of the volume data collected during the measurement periods.

12. A method for measuring the difference in the volumetric flow rate due to a leak between one pressure and at least one other pressure, after compensation has been made for thermally induced changes in the volume of liquid product in a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between periods, wherein the difference in the temperature compensated volume rate between the pressures is computed from the rate of change of volume measured during the measurement periods (first derivative of the volume data or volume rate), the rate of change of the rate of change of volume during the measurement periods (second derivative of the volume data), the third derivative of the volume data collected during the measurement periods, and higher derivatives of the volume data collected during the measurement periods.

15. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period;

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods; and

(d) estimating the error by using at least two of the measurement periods wherein the pressure level is the same, and the error is estimated from the difference in the changes of volume measured during the measurement periods when the pressure level is the same.

21. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods; and

(d) of estimating the error by computing at least the next higher order derivative of the volume data than is used in the computation of difference in volumetric flow rates, and multiplying said next higher order derivative by the time between the measurement periods.

34. A method for measuring the difference in the volumetric flow rate due to a leak between one pressure and at least one other pressure, after compensation has been made for thermally induced changes in the volume of liquid product in a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a second measurement period;

(c) pressurizing said pipeline system to a third pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a third measurement period; and

(d) computing the difference in temperature compensated volume rate between a pressure that is between the second pressure and the first and third pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods, wherein the difference in the temperature compensated volume rate between the pressures is computed by subtracting the time-weighted rate of change of volume measured during the second measurement period from the time-weighted average of the rate of change of volume measured during the first and third measurement periods.

35. The method of claim 34, further comprising the step of comparing the difference in temperature compensated volume rate to a threshold value to determine whether said pipeline system has a leak.

36. The method of claim 34, further comprising a correction for non-linear changes of volume between measurement periods utilizing higher order derivatives of the volume data measured during a plurality of the measurement periods.

37. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures, comprising the method of claim 36 wherein during the three measurement periods the pressure level is the same, and the error is estimated from the difference in the changes of volume measured during the measurement periods.

38. The method of claim 34, further comprising a correction for non-linear changes of volume between measurement periods utilizing the second derivative of the volume data measured during a plurality of the measurement periods.

39. The method of claim 38, wherein the correction includes higher order derivatives of the volume data in addition to the second derivative.

40. The method of claim 34, wherein the pressure level during two sequential measurement periods is approximately equal.

41. The method of claim 34, wherein the pressure level during at least one of the measurement periods is atmospheric pressure.

42. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures, comprising the method of claim 34 wherein during the three measurement periods the pressure level is the same, and the error is estimated from the difference in the changes of volume measured during the measurement periods.

43. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear chances of volume between the measurement periods;

wherein the measurements are conducted when the pressurized pipeline system is known not to be leaking to obtain the systematic bias error for the pressure, line volume, and liquid at which the measurements are made.

44. A method for adjusting the systematic bias error for differences in pressures, line volume capacities, and liquid, comprising the method of claim 43 and further comprising the steps of:

(a) adjusting the systematic bias error between other pressures by extrapolating from the measured changes, if different pressures are used;

(b) adjusting the systematic bias error if the volume capacity of the pipeline system is increased or reduced by extrapolating for the increase of reduction in capacity of the pipeline system; and

(c) adjusting the systematic bias error obtained for a first liquid to an estimate for a second liquid, when the second liquid is used, by extrapolating for the difference in the thermal coefficients of the expansion for the two liquids.

45. The method of claim 43, in which the time interval between the mid-point of the pressure change and the mid-point of the next measurement period is approximately equal for all measurement periods.

46. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period; and

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods;

wherein the measurements are conducted on at least one pressurized pipeline system that may be different from the one being tested and is known not to be leaking, to obtain the systematic bias error over the range of and for the pressures, line volumes, and liquids at which the measurements are made.

47. A method for adjusting the systematic bias error for differences in pressures, line volume capacities, and liquid, comprising the method of claim 46 and further comprising the steps of:

(a) adjusting the systematic bias error between other pressures by extrapolating from the measured changes, if different pressures are used;

(b) adjusting the systematic bias error if the volume capacity of the pipeline system is increased or reduced by extrapolating for the increase or reduction in capacity of the pipeline systems; and

(c) adjusting the systematic bias error obtained for a first liquid to an estimate for a second liquid, when the second liquid is used, by extrapolating for the difference in the thermal coefficients of the expansion for the two liquids.

48. The method of claim 46, in which the time interval between the mid-point of the pressure change and the mid-point of the next measurement period is approximately equal for all measurement periods.

49. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system, comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period;

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods; and

(d) using a model to estimate the thermally induced volume changes that would be measured on a pipeline system without a leak to obtain the systematic bias error for the pressure, line volume, and liquid at which the model estimates are made.

50. A method for adjusting the systematic bias error for differences in pressures, line volume capacities, and liquid, comprising the method of claim 49 and further comprising the steps of:

(a) adjusting the systematic bias error between other pressures by extrapolating from the measured changes, if different pressures are used;

(b) adjusting the systematic bias error if the volume capacity of the pipeline system is increased or reduced by extrapolating for the increase or reduction in capacity of the pipeline systems; and

(c) adjusting the systematic bias error obtained for a first liquid to an estimate for a second liquid, when the second liquid is used, by extrapolating for the difference in the thermal coefficients of the expansion for the two liquids.

51. The method of claim 49, in which the time interval between the mid-point of the pressure change and the mid-point of the next measurement period is approximately equal for all measurement periods.

52. A method for estimating the systematic bias error due to pressure induced temperature perturbations during measurements of the difference in volumetric flow rates between different pressures on a pressurized pipeline system containing liquid product, comprising the steps of:

(a) changing the pressure level in the pipeline system from a first pressure level to a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) changing the pressure level in the pipeline system from the second pressure level to a third pressure level, the difference between the third pressure level and the second pressure level being approximately equal to the difference between the second pressure level and the first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a second measurement period;

(c) changing the pressure level in the pipeline system from the third pressure level to approximately the second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a third measurement period;

(d) computing the difference in temperature compensated volume rate between (i) the second and third pressures during the first and second measurement periods and (ii) the third and second pressures during the second and third measurement periods, including a correction for the thermally induced non-linear changes of volume between the measurement periods; and

(e) subtracting (d)(i) from (d)(ii) to obtain the systematic bias error.

53. The method of claim 52, wherein the measurement periods are located at approximately equal intervals in time from the pressure changes.

54. The method of claim 52, wherein the measurement periods are of approximately equal duration.

55. The method of claim 52, wherein the measurement periods are located at approximately equal intervals in time from the pressure changes, and the measurement periods are of approximately equal duration.

56. A method for estimating the error in a measurement of the difference in volumetric flow rates between different pressures comprising the steps of:

(a) pressurizing said pipeline system to a first pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over a first measurement period;

(b) pressurizing said pipeline system to at least a second pressure level, and measuring those changes in volume of product in the pipeline system that occur when the pressure is maintained at an approximately constant pressure over at least a second measurement period;

(c) computing the difference in the temperature compensated volume rate between one pressure and at least one other pressure from the measured volume data at the pressures, including a correction for the thermally induced non-linear changes of volume between the measurement periods;

(d) computing the difference between the second derivatives of the volume data during two measurement periods; and

(e) estimating the error as a percentage of the difference obtained in (d).

57. The method of claim 56, further comprising the step of comparing the error to a threshold error value, and repeating the preceding steps if the threshold error value is exceeded.