Patent Publication Number: US-6981838-B2

Title: Method and apparatus for detecting the occurrence of surge in a centrifugal compressor

Description:
TECHNICAL FIELD 
   The present invention relates generally to detection of surge conditions in a centrifugal compressor, and more particularly, to methods and apparatus for detecting incipient surge in a centrifugal compressor by measuring temperature and/or flow velocity proximate to the inlet impeller of the compressor and proximate the outer wall of that inlet. 
   BACKGROUND 
   Surge is an unwanted phenomenon in centrifugal compressors which occurs when the fluid flow rate through the compressor is reduced, to an unstable level. When the flow rate is reduced to a point below a required minimum flow rate, fluid resistance increases at the compressor discharge port and as the discharge pressure increases until surge occurs. During the occurrence of surge, the direction of fluid flow is reversed as the higher pressure fluid at the discharge flows backward into the compressor. 
   Surge is undesirable for a number of reasons. Compressor surge produces unstable fluid flow within the compressor, increased thrust loads on compressor components, produces loud noises, and increases the amount of heat generated within the compressor. Frequently, one of the consequences of surge is damage to compressor components. 
   One conventional way of avoiding surge is by increasing the fluid flow rate through the compressor, often by recirculating fluid back through the compressor. Although surge is avoided by increasing the flow rate through the compressor, such recirculation of flow through the compressor adversely affects the compressor efficiency and, therefore, the cost of operation. 
   Surge in centrifugal compressors can be understood to occur at low flow conditions below which the rotating impeller cannot impart sufficient momentum to the flow to overcome the suction to discharge head. During surge, flow through a compressor becomes unstable and momentarily reverses direction, thereby shock loading the compressor, disrupting system operations, and potentially damaging the compressor. Centrifugal compressors that operate over a range of conditions must be protected from inadvertent entry into surge. 
   The approach of a compressor to surge is normally monitored by external measurement of flow rate and pressure to determine the compressor&#39;s condition relative to a line of flow and head that is selected as the surge control limit. This common approach is inferential and is dependent on proper selection of the surge control line. Currently, no widely accepted reliable methods are available to establish an accurate surge margin line. Setting the surge limit at too high a flow rate limits the efficient operation of the compressor and results in wasted energy and an unnecessary loss of efficiency during low flow conditions. Setting the surge limit too low can result in the compressor reaching surge and being damaged. 
   SUMMARY 
   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 operating, a fluid flows through the centrifugal compressor thereby establishing a flow in the inlet passage. When the fluid flow is measured in the inlet passage proximate to the inlet passage outer wall and proximate to the impeller, the measurements will indicate a reversed flow pattern, including a tangential component in the flow, a substantial decrease in the axial flow velocity, and an increase in the fluid temperature. Fluid flow in the compressor can then be modified or controlled to prevent surge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a fluid flow system using a centrifugal compressor; 
       FIG. 2  is a cross-section of a detail portion of a centrifugal compressor; 
       FIG. 3  is a partial elevational view of a centrifugal compressor inlet area; 
       FIG. 4  is a partial cross-sectional view of a centrifugal compressor and housing; 
       FIG. 5  is a graphical representation of an incipient surge test showing inlet wall axial flow velocity as surge is approached; 
       FIG. 6  is a graphical representation of an incipient surge test showing inlet temperature as surge is approached; and 
       FIG. 7  is a graphical representation of an incipient surge test showing axial and tangential flow velocities at several locations. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   Referring now to the drawings, wherein similar reference characters designate corresponding parts throughout the several views,  FIG. 1  is a schematic diagram of a fluid flow system  10 . Fluid flow system  10  includes an upstream and a downstream conduit  12  and  14  fluidly connected to a centrifugal compressor  16 . Fluid flows from the upstream conduit  12 , through the compressor  16  and on through the downstream conduit  14 . A fluid flow control means  18  is also provided and may consist of recirculation pipes and valves, safety and bypass valves, and other control mechanisms as are known in the art. The system  10  may be a gas pipeline system, a gas process system, an air system, or any other fluid flow system known in the art. 
     FIGS. 2 and 3  are detail views of a typical centrifugal compressor  16  with an inlet passage  20 , defined by an inlet passage wall  22 , leading to an impeller  24  encased in a casing  26 . The impeller  24  has multiple impeller passages  28  defined by the impeller shroud  30  and interior wall  31  of the impeller hub  34 . The impeller  24  has blades  32  for directing fluid flow and a hub  33  with a hub center  34 . In use, the centrifugal compressor  16  establishes a fluid flow through inlet passage  20  into the impeller  24  and then out through outlet  36 . When the compressor is operating, a fluid flow  38  is established in the inlet passage  20 . In normal operation, the flow  38  will exist, or be established, in a substantially steady state. 
   Sensors  40  are placed in the inlet passage  20 , in operable communication with the inlet area proximate the inlet passage wall  22  and proximate the impeller  24 , as shown. The sensors  40  are preferably attached to the inlet wall  22 , extending through the casing  26 , but other arrangements can be used. Preferably multiple sensors  40  are employed. The sensors  40  detect changes in magnitude, direction, and character of the fluid flow  38  in a recirculation zone  42  within the inlet passage  20 . 
   The recirculation zone  42 , as shown in  FIGS. 2–4 , is the area within the inlet passage where normal flow conditions will undergo substantial changes prior to the occurrence of surge. The flow pattern changes includes a substantial decrease in axial flow, an increase in tangential flow, an increase in fluid temperature, or a combination of these. The change in flow condition local to the inlet passage wall  22  and near the impeller  24  can be used to determine if the compressor  16  is near the surge condition. The sensors  40  preferably include a temperature sensor, a flow velocity sensor. The flow velocity sensor is preferably capable of detecting axial fluid flow, tangential fluid flow, increases and decreases in these flows, and/or a reversal of flow direction. Optionally, other sensors may be used to detect some or all of these flow characteristics. Any one type of sensor may indicate incipient surge, but preferably a combination of sensors is employed. 
   Controller  18  includes all appropriate electronics, software, hardware, etc., as known in the art, and operates to control operation of the centrifugal compressor. Flow measurements from sensors  40  can be input to the controller. Upon receiving measurements indicative of incipient surge, the controller  18  operates to manipulate the compressor and valve systems to return to a normal flow pattern. Controllers are known in the art and readily available. 
   The invention described herein identifies fundamental changes in the flow patterns within centrifugal compressors that directly signal the approach to surge. This invention provides a means to measure the changes in flow and temperatures internal to compressors that indicate and can measure the approach of a surge condition. It was observed during laboratory testing of a small centrifugal compressor  16  that a recirculation zone  42  develops in the flow immediately upstream of impeller  24 . As a compressor approaches surge, a recirculation develops in the outer annulus of the impeller inlet area and this change in the local flow condition can be used to determine if the compressor is close to a surge condition. 
   Flow and relative temperature measurements made near the outer wall  22  of a centrifugal compressor&#39;s impeller inlet passage  20  indicate changes in magnitude and direction of the flow  38  that are an indication that the compressor is approaching a surge condition. In the normal operating range, the inlet flow  38  to an impeller  24  is uniform in temperature and axial velocity and may be described as steady state. As the flow  38  is reduced toward a surge condition, the flow velocity at the outer wall  22  of the impeller inlet passage  20  decreases considerably more than the mean flow through the inlet and actually reverses in direction before surge occurs, creating a recirculation zone  42 . As a result of this recirculation, the temperature of the gas or other fluid in the outer inlet area increases relative to the bulk inlet gas temperature. A tangential or rotational component is also imparted to the impeller suction flow  38  near the wall  22  immediately upstream of the impeller  24 . All of these changes can be used to indicate that the compressor internal conditions are near the surge condition. 
     FIGS. 2 and 3  show the inlet  20  of a small compressor&#39;s impeller  24  and the locations of temperature and flow velocity sensors  40 . In addition to these measurements, during testing, discharge pressure and other measurements were made to determine the operating condition of the compressor. The temperature sensors used were small rapid response thermocouples. The flow sensors used measured to the fluid velocity, namely the fluid speed and direction. The inlet piping was three inches in diameter. The compressor speed was held constant for each test and the compressor flow was reduced towards surge by partially closing a valve on the discharge side of the compressor. 
   The change in inlet wall flow velocity as the compressor approaches surge is shown in  FIG. 5 . From a normal positive flow, the velocity decreases, becomes negative, and then oscillates between positive and negative when the compressor enters the surge condition. Note that the reverse flow velocity just before surge is the same value as the minimum extreme during the surge cycle. 
   The near outer wall, impeller inlet gas temperature change from a constant 75° F. inlet temperature, is shown in  FIG. 6 . This increase in local temperature is due to the fact that gas returning to the outer wall area during the recirculation has been partially compressed in the impeller before it returns to the impeller inlet. Other inlet temperature measurements near the outer wall and the impeller tips show the same increase in local temperatures as the surge condition approached. 
   A traverse of the inlet flow in front of the impeller was performed to identify the area of reverse flow. At normal operating conditions away from surge the axial flow velocity is quite uniform over the impeller inlet area and there is little to no tangential component in the flow. As surge is approached, the axial velocity near the inside or hub of the impeller inlet is not changed but the velocity near the outer wall decreases and reverses as shown in  FIG. 7 . In  FIG. 7 , the hub of the impeller starts at approximately 0.75 inches on the scale and the outer wall is at approximately 1.35 inches on the velocity verses radial distance plot.  FIG. 7  also shows that at this near surge condition there is a tangential velocity component near the outer wall which decreases to nearly zero near the hub. This type of data was repeated and appears to be a fundamental indication that the compressor is reaching its minimum stable flow-surge limit. 
   This invention shows that a reverse flow and temperature rise in the outer wall area of an impeller inlet is a general attribute of centrifugal compressors as they approach the surge conditions. This invention includes the use of flow sensors and temperature indicators located close to the impeller and close to the inlet wall to detect the fundamental changes that indicate approaching surge. This invention offers an effective method of detecting the approach of surge and of controlling centrifugal compressors operating near surge. 
   These findings are applicable to various size and design of compressors. An exemplary compressor  16  is shown in  FIG. 4  having a housing  50 , a suction chamber  52 , discharge chamber  54 , inlet passage  20 , impeller  24 , casing  26  and other parts as labeled and as known in the art. Sensor  40  is shown extending through casing  26  and into the inlet passage  20  and recirculation zone  42 . 
   Preferably a control means  18  is provided. When surge conditions are present, as indicated by the measurements of sensors  40 , appropriate flow control steps may be taken to prevent surge, such as by increasing flow to the inlet passage, via a recirculation system or by other means known in the art, or otherwise moderating the compressor operation as is known in the art. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the surge detection method and apparatus of the present invention and in construction of this method and apparatus without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.