of structure but this is not the major point of the present invention and will not be further described as shown in the drawing this small gap fitting in is to open a small cross section v groove on the piston rod 365 the flush valve is set through inside the pressure water vessel 1 and forms seal with the pressure water vessel 1 the flush valve 4 includes a hollow valve body 41 a big piston 42 an upper cylinder sleeve 43 a small piston 44 and a spring 45 among these above the valve body 41 are set the inlet 411 and outlet 412 the drawing is just a diagrammatical sketch in which the inlet 411 and outlet 412 are shown as one part the valve body 41 is placed inside the pressure water vessel 1 through the opening 11 on its top the bottom of the valve body is fixed on the locating plate 13 in the pressure water vessel 1 the inlet 411 is connected with the outlet 39 of the combination valve 3 the outlet 412 is connected with the inlet 52 of the below described open valve 5 above the valve body 41 is set a normal pressure orifice which is connected with.