neqsim-control-valve-cv-screening

name	neqsim-control-valve-cv-screening
calculation_basis	screening
version	0.1.0
description	Educational control-valve sizing screening that estimates the required flow coefficient (Kv/Cv) and flags choked flow per public IEC 60534-2-1 / ISA-75.01 equations for liquid and gas service. USE WHEN: a task needs a public, screening-level required Cv and a choked-flow flag before detailed control-valve selection.
last_verified	2026-06-18
requires	{"python_packages":[],"java_packages":[],"env":[],"network":[]}

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

Symptom	Cause	Fix
Cv too small	Used choked dP not realising flow is critical	The model caps dP; check the choked flag
Gas Cv off	Density not at inlet conditions	Pass inlet_density at P1, T1
Liquid choke never flags	vapor_pressure left at 0	Provide vapor and critical pressure
Wrong units	Q in m3/s or W in kg/s	Use m3/h (liquid) and kg/h (gas)

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