Control Valve Cv Screening

Use this skill for public, educational control-valve sizing screening. It estimates the required flow coefficient (Kv and Cv) and flags choked flow for liquid and gas service using the open IEC 60534-2-1 / ISA-75.01 equations so an agent can scope a valve and decide whether to invoke validated NeqSim valve calculations.

When to Use

• When a user asks roughly what Cv a control valve needs for a duty.
• When an agent needs a quick choked-flow (flashing or critical) flag.
• When examples must run without confidential valve data, vendor curves, or company specs.

Inputs

Common:

• service: "liquid" or "gas".
• inlet_pressure: valve inlet pressure P1 in bar absolute.
• pressure_drop: valve pressure drop dP in bar.
• rated_cv: optional rated valve Cv for the margin check.

Liquid service:

• flow_rate: volumetric flow in m3/h.
• specific_gravity: liquid specific gravity (water = 1).
• vapor_pressure: fluid vapor pressure in bar absolute, default 0.
• critical_pressure: fluid critical pressure in bar absolute (for FF), optional.
• fl: liquid pressure-recovery factor FL, default 0.9.

Gas service:

• mass_flow: gas mass flow in kg/h.
• inlet_density: inlet gas density in kg/m3.
• specific_heat_ratio: ratio of specific heats k, default 1.3.
• xt: pressure-drop-ratio factor xT, default 0.7.

Outputs

• service: the resolved service.
• required_kv: required flow coefficient in metric Kv.
• required_cv: required flow coefficient in US Cv (1.156 * Kv).
• choked: choked-flow flag.
• choke_limit: choked pressure drop in bar (liquid) or choking x (gas).
• cv_margin_ratio: rated_cv / required_cv, or null if no rating.
• valve_warning: ok, watch, under-sized, choked, or no-rating.
• assumptions: public assumptions used by the placeholder model.

Engineering Method

The Python class ControlValveCvModel uses public IEC 60534-2-1 / ISA-75.01 equations:

• liquid Kv uses Kv = Q * sqrt(SG / dP_sizing).
• the liquid liquid-critical-pressure-ratio factor uses FF = 0.96 - 0.28 * sqrt(Pv / Pc).
• liquid choke occurs when dP >= FL^2 * (P1 - FF * Pv); the sizing dP is capped at this limit.
• gas uses x = dP / P1, Fk = k / 1.4, choke x = Fk * xT, expansion factor Y = 1 - x / (3 Fk xT) bounded to [0.667, 1], and the mass-flow form Kv = W / (N6 * Y * sqrt(x * P1 * rho1)) with N6 = 27.3.
• Cv = 1.156 * Kv.

This is educational and screening-only logic. It assumes single-phase, fully turbulent flow with no piping geometry factor (Fp), no Reynolds-number correction, and no two-phase or flashing mixture handling. It is not a replacement for validated control-valve sizing and qualified selection.

Python Usage Pattern

from control_valve_cv_screening import ControlValveCvModel

model = ControlValveCvModel()
result = model.evaluate(
    service="gas",
    inlet_pressure=50.0,
    pressure_drop=12.0,
    mass_flow=5000.0,
    inlet_density=40.0,
    specific_heat_ratio=1.3,
)

print(result.required_cv)
print(result.choked)
print(result.valve_warning)

Related NeqSim Functionality

For validated control-valve behaviour, redirect to existing NeqSim classes:

• neqsim.process.equipment.valve.ThrottlingValve — flow-vs-Cv valve with getCv / setCv and pressure-drop response.
• neqsim.process.equipment.valve.ControlValve — control valve with characteristic and controller coupling.
• neqsim.process.equipment.valve.SafetyValve — relief-valve modelling for the relieving case.

This skill is a public sizing triage layer that decides when to invoke those validated valve classes.

Validation Checklist

• inlet_pressure and pressure_drop are positive and dP < P1.
• Liquid service supplies flow and specific gravity; gas service supplies mass flow and density.
• The choked flag uses FL (liquid) or Fk xT (gas).
• Tests cover liquid sizing, gas sizing, choking, rating margin, and invalid input.
• Real sizing is redirected to validated NeqSim valve classes and qualified review.

Common Mistakes

Limitations

• No piping geometry factor Fp, Reynolds correction, or low-flow laminar handling.
• No two-phase, flashing, or multiphase valve sizing.
• No vendor valve characteristic, noise, or actuator sizing.

References

• NeqSim repository: https://github.com/equinor/neqsim
• NeqSim Skills Guide: https://github.com/equinor/neqsim/blob/master/docs/integration/skills_guide.md