reference numerals in the foregoing and thus will not be explained at this stage in the present modification the foregoing innermost protecting layer 3 has been divided into two mutually tight fit layer elements 3a and 3b although not shown the valve disc is lined with pvdf the first protecting layer elements 3a which is to contact with the elastomer layer 4 is composed of pvdf while the second protecting layer element 3b which is to be exposed to the treating fluid such as gaseous chlorine is composed of ptfe from the foregoing it will be clear that the valve disc and the valve body per se are effectively protected from chemical corrosion and errosion by the provision of respective pvdf protecting layers in addition a smooth sliding performance between the valve disc and the valve body can be well assured by the innermost protecting layer element 3b composed of ptfe and consisting the innermost layer of the separable valve seat ring when seen in the assembled position of the valve elements finally the second embodiment of the invention will be explained with reference to fig4 and 5 respective members and the like which are used in the first embodiment will be denoted the respective same reference numerals in this case yet each being attached with a prime for quicker identification and better understanding in this case a manipulating handle 22 is fixedly attached to the top part of the valve stem 6 39 said handle being provided with a slidable positioning pin 23 which is shown in.