Volume booster is used along with a positioner on a throttling control valve to increase stroking speed.
The booster has fixed deadband, soft seat construction and an integral bypass restriction to eliminate positioner saturation problems that can occur with volume boosters that do not have these features.
They are used to reduce valve stroking time. If precise control is required with volume booster then positioner should also be brought along.
For on-off control integral bypass restriction on the booster must ne closed.
E.g Fisher 2625 and 2625NS