Abradable seal

Abstract

An abradable seal includes an outer ring arrangement and an energiser for urging said outer ring arrangement against an opposing surface, wherein an outermost surface of said outer ring arrangement defines a sealing surface of said abradable seal; wherein said outer ring arrangement is configured such that after a first period of operation said sealing surface suddenly transitions from having a relatively large surface area to having a relatively small surface area, so as to cause a sudden increase in internal leakage across the seal at said transition.

1 . A method of monitoring a seal, the seal being an abradable seal comprising an outer ring arrangement and an energiser for urging said outer ring arrangement against an opposing surface, wherein an outermost surface of said outer ring arrangement defines a sealing surface of said abradable seal, wherein said outer ring arrangement is configured such that after a first period of operation said sealing surface suddenly transitions from having a relatively large surface area to having a relatively small surface area, so as to cause a sudden increase in internal leakage across the seal at said transition,
 wherein the method comprises:   monitoring internal leakage across said abradable seal; and   detecting a sudden increase in internal leakage due to said sealing surface transitioning from having a relatively large surface area to having a relatively small surface area.   
     
     
         2 . A method as claimed in  claim 1 , wherein said outer ring arrangement comprises an abradable outer ring and an inner ring comprising voids or apertures in its outer circumferential surface, wherein said transition occurs due to said outer ring abrading in use to expose said voids or apertures in said inner ring. 
     
     
         3 . A method as claimed in  claim 2 , wherein said voids or apertures extend between axial ends of said inner ring. 
     
     
         4 . A method as claimed in  claim 2 , wherein said outer ring has a uniform axial cross-section or width throughout its circumference. 
     
     
         5 . A method as claimed in  claim 2 , wherein said inner ring has a non-uniform axial cross-section throughout its circumference. 
     
     
         6 . A method as claimed in  claim 2 , wherein said inner ring has a uniform axial width throughout its circumference. 
     
     
         7 . A method as claimed in  claim 2 , wherein said outer ring comprises a base portion and two circumferentially continuous side portions extending radially inward from respective axial ends of said base portion. 
     
     
         8 . A method as claimed in  claim 7 , wherein said base portion acts as said sealing surface during said first period of operation. 
     
     
         9 . A method as claimed in  claim 7 , wherein said inner ring fits within said side portions of said outer ring, such that once said base portion wears away in use, said side portions remain to provide part of said sealing surface after said transition. 
     
     
         10 . A method as claimed in  claim 1 , wherein said sealing surface has a substantially constant surface area prior to said transition. 
     
     
         11 . A method as claimed in  claim 1 , wherein said first period of time defines a period of normal operation of said seal, and/or said transition defines a point of failure of said seal, or a point at which said seal requires replacement. 
     
     
         12 . A method as claimed in  claim 1 , wherein said relatively small surface area is at least 10%, 20% or 50% smaller than said relatively large surface area. 
     
     
         13 . A method as claimed in  claim 1 , wherein said outer ring arrangement is configured such that said transition does not cause catastrophic failure or damage to said abradable seal. 
     
     
         14 . A method as claimed in  claim 1 , wherein said outer ring arrangement is configured to provide a sealing function after said transition. 
     
     
         15 . A method of monitoring a seal of a seal assembly, the seal assembly comprising:
 a first component;   a second component movable with respect to, and in sealing engagement with said first component; and   an abradable seal comprising an outer ring arrangement and an energiser for urging said outer ring arrangement against an opposing surface, wherein an outermost surface of said outer ring arrangement defines a sealing surface of said abradable seal, wherein said outer ring arrangement is configured such that after a first period of operation said sealing surface suddenly transitions from having a relatively large surface area to having a relatively small surface area, so as to cause a sudden increase in internal leakage across the seal at said transition,   wherein the method comprises:   monitoring internal leakage across said abradable seal; and   detecting a sudden increase in internal leakage due to said sealing surface transitioning from having a relatively large surface area to having a relatively small surface area.