The valve of the present invention includes an inflatable seal element located in the housing seat and a gas-pressure-purge arrangement. The inflatable seal element is inflated by means of pressurized fluid to grow in size and close up clearances between gate and seat, thereby establishing a gas-tight seal between the two parts, at will. So this can be done, for example, after the valve has been operated to close (slide gate driven into the valve housing) and a gas-pressure-tight seal is desired. Prior to this event, the gas-pressure-purge arrangement is activated to blast-off solid particles from the gate surfaces where these make contact with the inflatable seal element. It is not mandatory that each and every solid particle be removed from the gate contact surfaces by means of the gas-pressure-purge arrangement. Only a reasonable amount of cleanliness on the gate contact surfaces is desirable. The inflatable seal element is capable of sealing gas-tight even when some solid particles become trapped and embedded on the inflatable seal surface. The purpose of the gas-pressure-purge arrangement is to keep the contact surfaces reasonably free of solid particles only to improve the life of the inflatable seal element. Furthermore, the inflatable seal element can be deflated to shrink in size, thereby providing increased clearance between gate and seat, when desired. So this is done just prior to the moment when the gate is to be operated (driven out of the housing) to open the valve. No scoring can occur on gate or seat on account of the increased clearance.