Abstract: The valve assembly comprises a first ring nut for attachment to the wall delimiting the steam chamber outwardly and a second ring nut for attachment to the partition wall between the steam chamber and steam delivery chamber. Also provided is a shutter rod accessible from the outside and being passed axially through the cited ring nuts. The shutter rod carries first and second seals which movably control the communication of the steam chamber with the steam delivery chamber, respectively, as well as the tightness of the steam chamber toward the outside.