Control Valve Sizing Isa75

Control Valve Sizing (ISA 75.01.01)

Overview

Wrapper around fluids.control_valve.size_control_valve_l (liquid) and size_control_valve_g (gas) per the ISA 75.01.01 / IEC 60534 family of sizing equations.

Prerequisites

1. uv available.
2. On first use, the script writes LICENSE_NOTIFICATION.txt.

When to Use

• Selecting a control valve size for liquid or gas service.
• Checking a vendor-supplied Cv.
• Comparing Cv against the operating envelope (min/normal/max flow).

Don't use for

• Pressure relief / safety valves — use relief-valve-sizing-api520.
• On-off valves, ball-valve trim selection.
• Cryogenic, slurry, abrasive, or high-temperature service with

vendor-specific corrections beyond the ISA standard.

Utility Scripts

• uv run scripts/cv.py liquid --rho 800 --P1 2000000 --P2 1700000 --Q 0.005 --mu 5e-4 --Psat 100000 --Pc 4.2e6 --output /tmp/liq.json
• uv run scripts/cv.py gas --T 300 --MW 30 --mu 1.2e-5 --gamma 1.3 --Z 0.92 --P1 800000 --P2 400000 --Q 0.6 --output /tmp/gas.json

Procedure

1. Determine min / normal / max flow rates for the service.
2. Get fluid properties at upstream conditions (rho, mu, Psat, gamma, Z).
3. Run sizing at each flow rate; the operating Cv should fall in the

20-80% open range at the normal flow.

1. Review cavitation/flashing and choked-flow limits separately against the

valve trim and vendor data.

1. Select the valve from vendor data such that the required Cv is met at

max flow with margin.

Pitfalls

• Sizing only for max flow; the valve may be unstable or hunt at min flow

if it ends up too far closed.

• Using upstream density for downstream conditions when there is a large

pressure drop and significant flashing.

• Treating cavitation onset as the same as choked flow. They are different:

cavitation depends on pressure recovery (FL) and Psat; choked flow depends on pressure ratio and gamma.

• Confusing Kv (m³/hr) with Cv (US gpm); the ratio is ~1.156.
• Using a globe-valve FL for a butterfly-valve service. FL is

trim-specific.

• Sizing with viscosity-corrected FR outside the correlation envelope.

Fallback Strategies

• If fluids.control_valve is unavailable, surface to the user; the

rigorous ISA equation is not reproduced here.

• For two-phase service, the standard ISA 75.01.01 does not apply

directly; consult the vendor or use a specialised correlation.

Verification

• Run the listed script with representative inputs and an --output file when a deterministic calculation is available.
• Confirm the JSON result contains ok: true, expected units, and no unhandled warnings.
• Check result magnitudes against the stated assumptions, references, and a hand calculation or known operating range before reporting them.

References

• references/equation_family.md — pointers to ISA 75.01.01 / IEC 60534.
• fluids.control_valve docs: https://fluids.readthedocs.io/fluids.control_valve.html
• ISA 75.01.01-2012 — Industrial-process control valves — Flow capacity.
• IEC 60534-2-1 — Industrial-process control valves — Part 2-1: Flow capacity.

Anti-Patterns

• Specifying a control valve by line size rather than by Cv requirement.
• Skipping the min-flow check.
• Ignoring cavitation/flashing on liquid service near Psat.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jskherman
• Source: jskherman/engg-skills
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.