When the application requires a control valve to modulate clean fluid, there are many designs that will provide both tight shut-off and excellent control. However, when the media contains solids (i.e. a slurry), there is no perfect valve. These standard control valves will typically provide shut-off when brand new, however erosion from abrasive fluid often results in the valve leaking shortly after being in service. Relying on a control valve to provide both control and isolation in an abrasive application is not standard industry practice. Control valve manufactures recommend installing a dedicated on/off valve (in line with the control valve) when isolation is required.
Generally, metal-seated slurry control valves allow a certain amount of leakage past the seat when the valve is fully closed. This leakage rate can range from Class I to Class VI (as shown in the below table). Note that Class V and VI shut-off ratings are typically reserved for isolation valves with resilient seats (e.g. rubber, urethane, etc). Slurry control valves that claim to have Class V shut-off often degrade to Class IV (or lower) fairly quickly.