The Collaborative International Dictionary of English v.0.48:

Center seal \Center seal\, Centre seal \Centre seal\ (Gas
   Manuf.)
   A compound hydraulic valve for regulating the passage of the
   gas through a set of purifiers so as to cut out each one in
   turn for the renewal of the lime.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Center seal \Cen"ter seal\, or Centre seal \Cen"tre seal\,. (Gas
   Manuf.)
   A compound hydraulic valve for regulating the passage of the
   gas through a set of purifiers so as to cut out each one in
   turn for the renewal of the lime.
   [Webster 1913 Suppl.] centre seal