The sliding parts, the guides and interlocking portions of the interlocking guides and slides of a slide valve are all disposed away from a flow path through the valve. Additionally, the portions of the interlocking guide on the slide or disc of the valve that interlock with the guide on an orifice plate are connected to the slide or disc by welds instead of bolts, greatly increasing durability. As the slide or disc moves in and out to control the flow rate precisely, the guides of the slide or disc move with it. Because the sliding surfaces of the orifice plate and the slide or disc are outside of the flow path, they are consequently not subjected to erosion from the flow through the valve. Accordingly, all critical sliding surfaces are not exposed to the flow path and are greatly increased in durability.