FODRV is the abbreviated term for Fixed Orifice Double Regulating Valves and VODRV for Variable Orifice Double Regulating Valves.
Fixed Orifice Double Regulating Valves and Variable Orifice Double Regulating Valves
The term ‘Orifice’ refers to the opening in the valve that the water flows through.
FODRV: With a FODRV the ‘orifice’ that creates the measured pressure drop, is an actual opening of a fixed size, hence the term ‘Fixed orifice’. Fixed Orifice Valves have a single value for the flow coefficient Kv. For the FODRV this value is usually referred to as Kvs.
VODRV: With the VODRV the ‘orifice’ that creates the pressure drop is the seat / disc assembly within the valve. As the gap between the seat and disc is different for different handwheel positions, the flow coefficient Kv is changeable depending on the handwheel position, hence the term ‘Variable Orifice’.
Because the FODRV does not have a changeable Kvs value, the accuracy remains at ±5% irrespective of the handwheel position, whereas, for the VODRV, which has a variable Kv value, the accuracy is ±5% when fully open but reduces to approximately ±18% when the valve is closed to its minimum set position, i.e. 25% open (CIBSE recommendation).
Commissioning Engineers find it easier to work with FODRV because design pressure drops can be calculated before attending site, whereas, with the VODRV the pressure drop must be re-calculated during the commissioning process each time the handwheel position is changed.
Ref - cranefs