Control Valve What is Valve Rangeability ?

Rangeability is the ratio of maximum controllable flow to minimum controllable flow

For the control valve selection rangeability is one of the important selection factor.

Minimum and Maximum Cv of Valves are calculated .If the ratio of Cv Maximum to Cv Minimum is within the rangeability of the selected control valve ,the valve should function properly

Cv - Flow Coefficient

Rangeability of Control Valve

 

Rangeability of a control valve is defined as the ratio of maximum to minimum flow that can be acted upon by a control valve after receiving a signal from a controller.

Today’s control valve applications often require a degree of high rangeability, i.e. require a valve to control a flow from small to large flow. The rangeability of any control valve is affected by mainly three factors:

1. Valve Geometry – Valve geometry (i.e. the geometry of the closure element and seat) has an inherent rangeability due to the design and configuration of the body and the regulating element. Sometimes the configuration can be modified, improving rangeability as long as the valve’s sensitivity is not affected.
2. Seat Leakage – Seat leakage also affects rangeability. Excessive seat leakage can cause instability as the valve lifts off the seat.
3. Valve Actuation (or Actuator) – Some actuators are much more stiff at near closure than others.

Usually the valves with highest rangeability are those with the low sensitivity as the regulating element is nearly closed. Because the equal percentage flow characteristic promotes increased sensitivity as the valve opens, it is usually chosen for most throttling applications.

The installed Rangeability of a control valve is a means to express the best working flow range limits of the control valve, say from 5% to 95%. Outside this flow range limits, the installed valve characteristics deviates from the desired characteristics by more than some stated tolerance. It would be mechanically difficult to machine the valve plug and seat to perfection to give the desired characterized flow when the valve plug is just lifting off its seat. True
characterized flow can only begin after the plug contour is sufficiently clear off the seat and orifice, by which time the minimum controllable flow may already be 3% to 5% of its maximum flow.

Thus the Installed Rangeability of a control valve may be defined as the ratio of its maximum controllable flow to its minimum controllable flow.
The Inherent Rangeability of a control valve may be defined as the ratio of its maximum valve coefficient (Cv) to its minimum controllable (Cv)

What is rangeability? Turndown ratio?

Rangeability is the bench-tested ratio of the maximum controllable flow to the minimum controllable flow through a valve; with a constant 1 psi differential pressure across the valve inlet and outlet. For example, if a wide open valve produces 100 gallons per minute (gpm) and when positioned to its minimum opening produces 2 gpm, the rangeability is 100/2 = 50:1

Turndown is similar to rangeability except it is the ratio of full flow to minimum flow produced with the differential pressure available to the valve when installed in a hydronic circuit.

Turndown ratio is typically less than rangeability ratio for pressured dependent valves because the differential pressure across the valve is higher at minimum position than it is at the full open position.  Turndown is typically equal to rangeability for pressure independent valves because the effects of variable differential pressure are negated.