Abradable seal

An abradable seal comprising:

• an outer ring arrangement;
• and
• an energizer 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 seal, wherein the sealing surface is defined as an outer circumferential surface of the outer ring arrangement that is urged against and constantly contacts the opposing surface, and gradually wears away in use;
• wherein said sealing surface of the outer ring arrangement wears away in a radial direction from a first sealing surface having a relatively large surface area to a second sealing surface radially inward of the first sealing surface and having a relatively small surface area, so that the outer ring arrangement is configured such that after a first period of operation said sealing surface suddenly transitions from the first sealing surface having a relatively large surface area to the second sealing surface having a relatively small surface area, so as to cause, in use, a sudden increase in internal leakage across the seal at said transition,
• wherein said outer ring arrangement comprises an abradable outer ring comprising the first sealing surface and an inner ring comprising voids or apertures in its outer circumferential surface,
• wherein the outer circumferential surface of the inner ring corresponds to the second sealing surface,
• wherein said transition occurs due to said outer ring abrading in use to expose said voids or apertures in said inner ring.