Compressor thrust bearing surge protection

Abstract

The present invention provides a method and apparatus of inhibiting a thrust bearing capacity of a compression system from being exceeded during a surge event in which a thrust bearing is biased with a biasing force to increase the thrust bearing overload margin between the capacity of thrust bearing to absorb axial forces and the greatest force produced during the surge event. This biasing force can be produced by appropriately sizing the high pressure seal on the side of the impeller opposite to the inlet of a compressor of the compression system so that the back disk force produced in the high pressure region of the high pressure seal and the low pressure region located inwardly of the high pressure region creates the desired bias force value and direction.

We claim:  
     
         1 . A compressor system with thrust bearing surge protection comprising:
 a first compressor having a first compressor shaft and at least one impeller;   a second compressor having a second compressor shaft and at least one impeller;   a motor shaft directly coupled to the first compressor shaft and the second compressor shaft;   an electric motor configured for driving the motor shaft and the at least one impeller of the first compressor and the at least one impeller of the second compressor through rotation of first and second compressor shafts;   a thrust bearing coupled to the motor shaft between the at least one impeller of the first compressor and the at least one impeller of the second compressor, the thrust bearing having a first thrust bearing capacity to absorb axial loads acting upon the thrust bearing in a first axial direction and a second thrust bearing capacity to absorb axial loads acting upon the thrust bearing in a second axial direction opposite to the first axial direction; and   wherein the thrust bearing is biased with a preloaded biasing force acting in the first or second axial direction such that in a two-phase surge event characterized by an initial phase where the sudden loss of discharge pressure causes the flow rate to fall below a minimum flow required at a given speed of the at least one impeller such that a back disk pressure greatly exceeds an eye-side pressure of the at least one impeller to produce a large axial force driving the at least one impeller towards an inlet and a subsequent phase where the back disk pressure is reduced below the eye side pressure of the at least one impeller, resulting in the eye side force rapidly overcoming the back disk force to produce an axial force driving the at least one impeller away from the inlet, and wherein the force associated with the initial phase of the two-phase surge event is partially offset by the preloaded biasing force; and   wherein the first thrust bearing capacity to absorb axial loads acting upon the thrust bearing in the first axial direction relative to the preloaded biasing force is greater than the second thrust bearing capacity to absorb axial loads acting upon the thrust bearing in the second axial direction relative to the preloaded biasing force.   
     
     
         2 . The compressor system of  claim 1  further comprising:
 a first back disk seal coupled to the at least one impeller of the first compressor and disposed between a high pressure outer annular region of the at least one impeller of the first compressor and a low pressure inner annular region of at least one impeller of the first compressor, the first back disk seal sized in a radial direction of the shaft to produce a first portion of the preloaded biasing force resulting from the force difference between the high pressure outer annular region of the first compressor and the low pressure inner annular region of the first compressor during normal operation; 
 a second back disk seal coupled to the at least one impeller of the second compressor and disposed between a high pressure outer annular region of the at least one impeller of the second compressor and a low pressure inner annular region of at least one impeller of the second compressor, the second back disk seal sized in a radial direction of the motor shaft to produce a second portion of the preloaded biasing force resulting from the force difference between the high pressure outer annular region and the low pressure inner annular region during normal operation; and 
 wherein the preloaded biasing force is a sum of the first portion and second portion. 
 
     
     
         3 . The compressor system of  claim 1  wherein the thrust bearing is a magnetic bearing. 
     
     
         4 . The compressor system of  claim 1  wherein the thrust bearing is part of the electric motor. 
     
     
         5 . The compressor system of  claim 1  wherein the first compressor and the second compressor are connected at opposite ends of the motor shaft and in flow communication with each other to provide two successive compression stages. 
     
     
         6 . The compressor system of  claim 3  wherein the thrust bearing further comprises:
 a conductive disk-like thrust runner rotatably coupled to the motor shaft; and 
 an inboard electromagnet and an outboard electromagnet configured to suspend the rotatable disk-like thrust runner between the inboard electromagnet and outboard electromagnet. 
 
     
     
         7 . The compressor system of  claim 6  further comprising a gap sensor assembly operatively coupled to the thrust bearing and configured to maintain the gaps between the disk-like thrust runner and the inboard and outboard electromagnets. 
     
     
         8 . The compressor system of  claim 1  wherein:
 the compressor system is a multi-stage compression system having one or more upstream stages and one or more downstream stages and wherein the first compressor or the second compressor comprise the downstream compression stages; and 
 a back disk pipe in flow communication between the low pressure region of the first compressor or the second compressor and an inlet or an outlet of the upstream compression stage; 
 wherein the preloaded biasing force further comprises a pressurized bleed stream fed to the low pressure region of the first compressor or the second compressor via the back disk pipe.