neqsim-psv-orifice-screening

name	neqsim-psv-orifice-screening
calculation_basis	screening
version	0.1.0
description	Educational pressure-safety-valve orifice screening using the public API 520 Part I critical gas-flow equation. USE WHEN: a task needs a public, screening-level required PSV orifice area and a mapped API orifice letter before detailed relief-valve sizing and selection.
last_verified	2026-06-18
requires	{"python_packages":[],"java_packages":[],"env":[],"network":[]}

PSV Orifice Screening

Use this skill for public, educational pressure-safety-valve (PSV) orifice screening. It estimates the required effective orifice area for critical gas flow using the open API 520 Part I equation and maps it to a standard API letter designation so an agent can scope a relief study before detailed sizing.

When to Use

• When a user asks roughly what PSV orifice letter a gas relief case needs.
• When an agent needs a quick relief-area triage before detailed API 520 sizing.
• When examples must run without confidential valve data, vendor capacity charts, or company relief specs.

Inputs

• relief_rate: required relief mass flow W in kg/h.
• relieving_pressure: upstream relieving pressure P1 in bar absolute.
• temperature: relieving temperature T in kelvin.
• compressibility: gas compressibility factor Z, default 1.0.
• molecular_weight: gas molecular weight M in g/mol.
• specific_heat_ratio: ratio of specific heats k, default 1.3.
• discharge_coefficient: effective discharge coefficient Kd, default 0.975.
• back_pressure_correction: back-pressure correction Kb, default 1.0.
• combination_correction: rupture-disk combination correction Kc, default 1.0.

Outputs

• coefficient_c: the C coefficient from k.
• required_area_mm2: required effective orifice area.
• selected_orifice: the smallest standard API orifice that meets the area, or none.
• selected_orifice_area_mm2: the area of the selected orifice.
• area_margin_ratio: ratio of selected area to required area.
• psv_warning: ok, watch, or oversize-needed.
• assumptions: public assumptions used by the placeholder model.

Engineering Method

The Python class PsvOrificeModel uses the open API 520 Part I critical gas-flow equation only:

• the coefficient uses C = 0.03948 * sqrt(k (2 / (k + 1))^((k + 1) / (k - 1))) in SI-consistent screening form.
• the required area uses A = (W / (C Kd P1 Kb Kc)) * sqrt(T Z / M).
• the area is mapped to the standard API 526 orifice letters (D through T) using their published effective areas.
• the warning is a simple rule-based label aligned with API 520 intent.

This is educational and screening-only logic. It assumes critical (choked) gas flow and does not check the choked-flow criterion, subcritical flow, liquid or two-phase relief, fire-case sizing, inlet or outlet pressure-drop limits, or vendor certified capacities. It is not a replacement for validated relief-valve sizing and a qualified safety review.

Python Usage Pattern

from psv_orifice_screening import PsvOrificeModel

model = PsvOrificeModel()
result = model.evaluate(
    relief_rate=50000.0,
    relieving_pressure=20.0,
    temperature=323.15,
    molecular_weight=19.0,
)

print(result.selected_orifice)
print(result.required_area_mm2)
print(result.psv_warning)

Related NeqSim Functionality

For validated PSV sizing, redirect to existing NeqSim classes:

• neqsim.process.util.fire.ReliefValveSizing — API 520 gas, liquid, and two-phase relief sizing including fire case.
• neqsim.process.equipment.valve.SafetyValve — safety-valve equipment model for process simulations.

This skill is a public API 520 triage layer that decides when to invoke those validated relief classes.

Validation Checklist

• Relief rate, pressure, temperature, and molecular weight are positive and in the stated units.
• Example inputs are public and synthetic.
• Tests cover an orifice selection, an oversize-needed case, and invalid input.
• Results are described as educational screening indicators.
• Real sizing is redirected to validated methods, API 520, API 526, and qualified review.

Common Mistakes

Symptom	Cause	Fix
Orifice too small	Choked-flow assumption invalid at low overpressure	Check the critical-flow criterion and use subcritical sizing if needed
Area off by orders of magnitude	Mixed mass and molar units	Keep W in kg/h and M in g/mol consistently
Fire case under-sized	Fire-case heat input not included	Use the API 521 fire-case relief rate as input

Limitations

• No proprietary valve data, vendor capacity charts, or company relief specs are included.
• No liquid, two-phase, or fire-case sizing logic is performed.
• No inlet or outlet pressure-drop or certified-capacity checks are included.

References

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