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A method and apparatus for detecting the occurrence of surge or incipient surge in a centrifugal compressor is supplied. The centrifugal compressor has an inlet passage, an inlet passage wall and an impeller. When flowing a fluid through the centrifugal compressor thereby establishing a fluid flow i

A method and apparatus for detecting the occurrence of surge or incipient surge in a centrifugal compressor is supplied. The centrifugal compressor has an inlet passage, an inlet passage wall and an impeller. When flowing a fluid through the centrifugal compressor thereby establishing a fluid flow in the inlet passage, the fluid flow is measured in the inlet passage proximate the inlet passage wall and proximate the impeller. The measurements may include detecting a reverse in the fluid flow direction, measuring a tangential component to the fluid flow, measuring a substantial decrease in the axial fluid flow, and/or measuring the fluid temperature. Fluid flow in the compressor can then be modified or controlled to prevent surge.

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What is claimed is: 1. A method for detecting the occurrence of surge or incipient surge in a centrifugal compressor, the method comprising the steps of: operating the centrifugal compressor having an inlet passage, an inlet passage wall and an impeller, thereby establishing a bulk fluid flow to th

What is claimed is: 1. A method for detecting the occurrence of surge or incipient surge in a centrifugal compressor, the method comprising the steps of: operating the centrifugal compressor having an inlet passage, an inlet passage wall and an impeller, thereby establishing a bulk fluid flow to the compressor and a local axial fluid flow in a recirculation zone in the inlet passage proximate to the inlet passage wall and proximate to the impeller; and detecting a decrease in the local axial fluid flow, wherein the decrease in the local fluid flow is greater than any concurrent decrease in the bulk fluid flow to the compressor. 2. A method as in claim 1 wherein the step of detecting the local fluid flow includes detecting a reversal in the local fluid flow direction in the recirculation zone. 3. A method as in claim 1 wherein the step of detecting the local fluid flow includes detecting a tangential component to the local fluid flow in the recirculation zone. 4. A method as in claim 1 wherein the step of detecting the fluid flow includes detecting a substantial decrease in the local axial fluid flow in the recirculation zone. 5. A method as in claim 1 wherein the step of detecting the fluid flow includes detecting changes in the local fluid flow temperature in the recirculation zone. 6. A method as in claim 2 wherein the step of detecting the fluid flow includes detecting the local fluid flow temperature in the recirculation zone. 7. Method as in claim 1 further comprising the step of controlling the bulk fluid flow through the compressor. 8. A method as in claim 7 wherein the step of controlling the fluid flow includes increasing the bulk fluid flow to the inlet passage. 9. A method as in claim 2 further comprising the step of controlling the bulk fluid flow through the compressor. 10. A method as in claim 3 further comprising the step of controlling the bulk fluid flow through the compressor. 11. A method as in claim 5 further comprising the step of controlling the bulk fluid flow through the compressor. 12. A method as in claim 4 further comprising the step of controlling the bulk fluid flow through the compressor. 13. A method as in claim 1 wherein the step of detecting includes measuring the fluid flow using at least one fluid velocity sensor. 14. A method as in claim 13 wherein the at least one fluid velocity sensor is attached to the inlet passage wall. 15. A method of detecting surge or incipient surge in a centrifugal compressor, the compressor having an impeller and an inlet passage upstream of the impeller, the method comprising the steps of: operating the compressor, thereby establishing a substantially steady state fluid flow through the inlet passage and impeller; and measuring the local fluid flow velocity in a recirculation zone in the inlet passage proximate to the inlet passage wall and proximate to the impeller, wherein the step of measuring the fluid flow includes measuring a tangential component to the fluid flow in the recirculation zone. 16. A method as in claim 15 wherein the step of measuring the fluid flow includes detecting a reversal in the fluid flow direction in the recirculation zone. 17. A method as in claim 15 wherein the step of measuring the fluid flow includes measuring a substantial decrease in the axial fluid flow in the recirculation zone. 18. A method as in claim 15 wherein the step of measuring the fluid flow includes measuring changes in the fluid flow temperature. 19. A method as in claim 16 wherein the step of measuring the fluid flow includes measuring changes in the fluid flow temperature. 20. A method as in claim 15 further comprising the step of controlling the flow through the compressor. 21. A method as in claim 20 wherein the step of controlling the fluid flow includes increasing the fluid flow to the inlet passage. 22. A method as in claim 16 further comprising the step of controlling the flow through the compressor. 23. A method as in claim 19 further comprising the step of controlling the flow through the compressor. 24. A method as in claim 20 further comprising the step of controlling the flow through the compressor. 25. A method as in claim 15 wherein the step of measuring includes measuring the fluid flow using at least one fluid velocity sensor. 26. A method as in claim 25, the inlet passage having an inlet passage wall and wherein the at least one fluid velocity sensor is attached to the inlet passage wall. 27. A method for detecting the occurrence of surge or incipient surge in a fluid flow system, the fluid flow system having a centrifugal compressor in fluid communication with an upstream fluid conduit and a downstream fluid conduit, the centrifugal compressor having an inlet passage and an impeller, the method comprising the steps of: operating the compressor, thereby establishing substantially steady state fluid flow through the inlet passage and impeller; and measuring the fluid flow in a recirculation zone in the inlet passage proximate to the inlet passage wall and proximate to the impeller, wherein the step of measuring the fluid flow includes measuring a tangential component to the fluid flow in the recirculation zone. 28. A method as in claim 27 wherein the step of measuring the fluid flow includes measuring a reverse in the fluid flow direction in the recirculation zone. 29. A method as in claim 27 wherein the step of measuring the fluid flow includes measuring a substantial decrease in the axial fluid flow in the recirculation zone. 30. A method as in claim 27 wherein the step of measuring the fluid flow includes measuring changes in the fluid flow temperature. 31. A method as in claim 27 further comprising the step of controlling the flow through the compressor. 32. A method as in claim 31 wherein the step of controlling the fluid flow includes increasing the fluid flow to the inlet passage. 33. A method as in claim 28 further comprising the step of controlling the flow through the compressor. 34. A method as in claim 29 further comprising the step of controlling the flow through the compressor. 35. A method as in claim 30 further comprising the step of controlling the flow through the compressor. 36. A method as in claim 27 wherein the step of measuring includes measuring the fluid flow using at least one fluid velocity sensor. 37. A method as in claim 36, the inlet passage having an inlet passage wall and wherein the at least one fluid velocity sensor is attached to the inlet passage wall. 38. A method as in claim 27 wherein the fluid flow system comprises a gas pipeline. 39. A method as in claim 28 wherein the step of measuring includes measuring changes in the fluid temperature. 40. An apparatus for detecting the occurrence of surge or incipient surge in a centrifugal compressor, the apparatus comprising: a centrifugal compressor having an inlet passage, with an inlet passage wall, and an impeller, a zone defined proximate the wall of the inlet passage and proximate the impeller, immediately upstream of the impeller; and at least one sensor operable for measuring fluid flow in said zone, wherein the sensor is capable of measuring a tangential component of fluid flow in the zone. 41. An apparatus as in claim 40 wherein at least one sensor is a fluid velocity sensor measuring fluid flow speed and direction. 42. An apparatus as in claim 40 wherein at least one sensor is capable of measuring a reversal in fluid flow direction in the zone. 43. An apparatus as in claim 40 further comprising a temperature sensor. 44. An apparatus as in claim 41 further comprising a temperature sensor. 45. An apparatus as in claim 40 wherein the at least one sensor is attached to the inlet passage wall. 46. An apparatus as in claim 40 further comprising a means of controlling the fluid flow through the centrifugal compressor. 47. An apparatus as in claim 41 further comprising a means of controlling the fluid flow through the centrifugal compressor. 48. An apparatus as in claim 42 further comprising a means of controlling the fluid flow through the centrifugal compressor.