What is the function of a steam throttling valve in a Boiler ?

The function of a throttling valve is to reduce the steam flow rate of the boiler.  This is necessary to prevent steam flashing. 

If a throttling valve is not used, a false temperature reading may reach the boiler blowdown controller.  This will result in wrong operation of the boiler blowdown.  The boiler throttling valve must be installed before the sensor.