Rangeability indicates the extent of flow values that the valve can reliably regulate and is often reported as a ratio of the largest to the smallest flows that can be controlled acceptably.
A control valve with higher rangeability will control flow over wider flow rates. For example, a valve with a rangeability of 50 and having a total flow capacity of 100 GPM, will control flow accurately down to as low as 2 GPM ehen fully open. Rangeability is affected by three factors:
1. Valve geometry – inherent rangeability due to the design of the body and the regulating element.
2. Seat leakage – excessive seat leakage can cause instability as the valve lifts off of the seat.
3. Actuator – diaphragm actuators are seldom accurate at less than 5% of the valve opening, whereas piston-cylinder actuators can provide control within 1% of valve lift due to the presence of air in two chambers.
Valves with high rangeability are sometimes desirable but these will be expensive to manufacture since very close tolerances are involved between the disc and the seat.
A typical commercial valve generally has a rangeability of about 35 to 50.