Control-valve selection is not finished when the line size and pressure class are known. The selected flow coefficient, inherent characteristic, rangeability, pressure-drop allocation and actuator response all affect how the valve behaves after installation. A valve that passes the maximum flow can still control poorly at normal or minimum load if its usable operating window is mismatched to the process.
This guide explains the terms engineers should align before requesting a quotation. It complements our broader control-valve selection guide and control-valve instability checklist.
Inherent characteristic versus installed behavior
An inherent characteristic relates valve flow coefficient to travel under a constant differential pressure. SAMSON describes a linear characteristic as equal travel increments producing equal increments of flow coefficient. With an equal-percentage characteristic, equal travel increments produce equal percentage changes in flow coefficient. The actual installed relationship differs because the pressure drop available across the valve changes as the rest of the system responds.
| Characteristic | Inherent behavior | Engineering implication |
|---|---|---|
| Linear | Equal travel increments give approximately equal increments of flow coefficient under the defined test condition. | Evaluate when the pressure drop across the valve remains relatively stable over the operating range. |
| Equal percentage | Equal travel increments give equal percentage changes in flow coefficient. | Often evaluated where system pressure drop varies substantially with flow, but the installed curve must still be calculated. |
| On/off | Designed for switching duty rather than continuous modulation. | Do not substitute for a modulating trim without checking the control objective. |
What rangeability does – and does not – tell you
SAMSON defines rangeability as the ratio of the rated flow coefficient to the smallest coefficient at which the characteristic remains within its permitted gradient tolerance. This is a valve-family property under defined conditions; it is not a guarantee that every installed loop will control over the same process-flow ratio. Seat leakage, friction, dead band, actuator sizing, positioner performance, pressure-drop changes and cavitation or flashing limits can narrow the usable window.
For example, the new SAMSON Type 3254 globe valve data sheet lists a 50:1 rangeability, while the SAMSON Type 3244 three-way valve data sheet lists 50:1 through DN 50/NPS 2 and 30:1 for larger listed sizes. Those published family values must be checked against the selected trim and the actual operating cases.
Minimum data for a credible valve calculation
| Input | Why it matters |
|---|---|
| Fluid composition and state | Establishes density, vapor pressure, viscosity, corrosivity and whether flashing, cavitation or two-phase flow requires special review. |
| Minimum, normal and maximum flow | Prevents selection around a single design point and shows the required control window. |
| Upstream and downstream pressure for each case | Defines the differential pressure available to the valve and the likely installed characteristic. |
| Temperature for each case | Drives body rating, packing, trim, actuator and accessory suitability. |
| Line size and piping arrangement | Supports velocity checks and identifies reducers, elbows and three-way piping constraints. |
| Fail action and air supply | Allows actuator thrust, spring range and fail-open/fail-close behavior to be checked. |
| Control signal and protocol | Defines whether a conventional, HART, PROFIBUS-PA or FOUNDATION Fieldbus positioner is appropriate. |
Positioner and actuator integration
The valve body, actuator and positioner should be treated as one final-control assembly. Compare the SAMSON Type 3271 actuator with the TROVIS 3730-3 positioner, and review our pneumatic versus smart positioner guide. For digital networks, the new Type 3730-4 PROFIBUS-PA and Type 3730-5 FOUNDATION Fieldbus pages summarize verified interface limits.
Practical selection sequence
- Define all operating cases, not only the maximum flow.
- Calculate the required flow coefficient and review noise, velocity, cavitation, flashing and choked-flow risks.
- Select a valve style and characteristic, then calculate installed behavior using the system pressure-drop model.
- Check minimum controllable flow against the proposed trim, actuator and positioner.
- Confirm materials, pressure-temperature rating, leakage requirement, end connections and fail action against the exact order code.
Official references: SAMSON T 8000-1 EN, SAMSON T 8000-3 EN, and the Emerson Control Valve Handbook.