Understanding Valve Flow Characteristics: Inherent vs Installed, and How They Affect Control

Understanding Valve Flow Characteristics: Inherent vs Installed, and How They Affect Control

The flow characteristic of a control valve is one of its most important performance attributes, determining how the flow through the valve changes as the valve travels from closed to fully open. Selecting the correct flow characteristic for each application is essential to achieve stable, responsive process control across the full range of operating conditions. Mismatched flow characteristics result in poor control loop performance, including excessive valve hunting, poor disturbance rejection, and instability at some operating points. Understanding the difference between inherent and installed flow characteristics, and knowing which characteristic is appropriate for each application, is fundamental knowledge for process and instrumentation engineers.

Wofer Valve offers control valves with linear, equal percentage, and quick-opening characteristics, as well as custom-characterized trim for applications requiring non-standard flow relationships. Our application engineers can advise on the optimal characteristic for each service.

Inherent Flow Characteristics

The inherent flow characteristic describes the relationship between valve Cv and travel under constant pressure drop conditions (tested with water at a constant 1 psi pressure drop in a laboratory). Three standard inherent characteristics are defined. Linear characteristic: Cv increases in direct proportion to valve travel, producing equal changes in flow for equal changes in travel at any point in the stroke. This provides consistent gain throughout the valve's travel and good controllability when the pressure drop across the valve is a large, constant fraction of the total system pressure drop. Equal percentage characteristic: equal increments of travel produce equal percentage increases in Cv. This means that the sensitivity (change in flow per unit change in travel) increases dramatically near the fully open position. Quick opening characteristic: a large Cv increase occurs very rapidly at low travel, then the curve flattens as the valve approaches full open. Quick opening is used primarily for on-off service, not for modulating control.

Installed Flow Characteristics

The installed flow characteristic is the actual relationship between valve travel and system flow when the valve is installed in a real piping system with a pump, pipeline resistance, and other pressure drops. It differs from the inherent characteristic because the pressure drop available across the control valve changes as the flow rate changes: at low flow, most of the pump head is available across the valve (high pressure drop), while at high flow the pipeline friction losses consume most of the pump head, leaving less pressure drop across the valve. This pressure drop variation distorts the inherent characteristic: a valve with an equal percentage inherent characteristic installed in a system with variable pressure drop will produce an installed characteristic closer to linear. This is the reason equal percentage characteristics are so widely used in process applications: the installed characteristic resulting from system pressure variation transforms the equal percentage inherent characteristic into approximately linear, providing good process control.

Selecting the Correct Characteristic

The general guidance for characteristic selection is: specify equal percentage when the valve pressure drop varies significantly with flow rate (which is most common in pumped systems), or when the process gain is high at low flow and low at high flow (common in many heat exchanger temperature control loops). Specify linear when the valve is a large fraction of the total system pressure drop at all flow rates (uncommon except in systems with gravity or constant-pressure sources), or when the process gain is constant or increases with flow rate. In practice, equal percentage is specified for approximately 70% of process control valve applications because most piping systems have variable valve pressure drop and because the equal percentage characteristic provides good installed controllability in these systems.

Rangeability and Turndown

Rangeability is the ratio of the maximum controllable Cv to the minimum controllable Cv within which the valve can provide the specified flow characteristic. Below the minimum controllable Cv, the valve may become unstable (the disc may vibrate or chatter) or the flow may not track the control signal reliably. A typical globe control valve has a rangeability of 50:1, meaning it can control flow accurately from 2% to 100% of its maximum Cv. High-performance valves with specially designed trim can achieve rangeability of 100:1 or 200:1. Turndown is a related concept that applies to the actual process: it is the ratio of maximum to minimum process flow that must be controlled. If the required process turndown exceeds the valve's rangeability, a split-range system (two valves controlled from the same controller output) may be required.

Noise and Vibration in Control Valves

Noise and vibration generated by control valves can be a significant operational and maintenance problem, particularly in high-pressure gas and steam applications. The primary mechanisms are aerodynamic noise (from high-velocity gas flow through the throttling restriction, producing broadband turbulence noise and in severe cases screaming noise from vortex shedding), mechanical noise from cavitation in liquid service, and flow-induced vibration of the valve trim and body. Noise levels in high-pressure gas service can exceed 120 dBA, which poses a hearing hazard for personnel and structural risk for piping and instruments. Solutions include low-noise trim designs (tortuous path, multi-hole cage trim that divides the pressure drop into many small stages), acoustic insulation on the valve and downstream piping, and reducing the pressure ratio across the valve by adding downstream backpressure or staging the pressure reduction across multiple valves.