Calculator Form
Example Data Table
| Gas | P1 | P2 | Temperature | Diameter | Cd | Gamma | MW |
|---|---|---|---|---|---|---|---|
| Air | 700 kPa abs | 101.325 kPa abs | 20 °C | 10 mm | 0.62 | 1.40 | 28.97 g/mol |
| Nitrogen | 5 bar abs | 2 bar abs | 25 °C | 8 mm | 0.61 | 1.40 | 28.013 g/mol |
| Methane | 80 psi abs | 30 psi abs | 70 °F | 0.25 in | 0.60 | 1.31 | 16.04 g/mol |
Formula Used
Area: A = πd² / 4
Specific gas constant: R = Ru / M
Pressure ratio: r = P2 / P1
Critical pressure ratio: rc = (2 / (γ + 1))^(γ / (γ - 1))
Choked mass flow: m = Cd A P1 √[(γ / RT) × (2 / (γ + 1))^((γ + 1) / (γ - 1))]
Subcritical mass flow: m = Cd A P1 √[(2γ / (RT(γ - 1))) × (r^(2/γ) - r^((γ + 1)/γ))]
The calculator uses absolute pressure. Convert gauge pressure before entering values.
How To Use This Calculator
- Enter upstream and downstream absolute pressures.
- Choose the correct pressure units.
- Enter upstream gas temperature.
- Enter orifice diameter and select its unit.
- Add discharge coefficient, gamma, and molecular weight.
- Set standard pressure and temperature for standard volume flow.
- Press calculate to view the result above the form.
- Use CSV or PDF buttons to save the report.
Gas Orifice Flow Overview
Gas flow through an orifice is controlled by pressure, temperature, area, gas properties, and discharge loss. The calculator uses compressible flow relations, so it handles normal and choked discharge. This is important because gas does not behave like an incompressible liquid when pressure drop becomes high.
Why Choked Flow Matters
Choked flow happens when the downstream pressure is low enough that the gas reaches sonic velocity at the orifice throat. After that point, lowering downstream pressure does not increase mass flow much. The limiting value depends on the ratio of specific heats. Air, nitrogen, methane, propane, and many process gases have different ratios, so the critical pressure ratio changes.
Inputs Used By The Tool
You enter upstream absolute pressure, downstream absolute pressure, gas temperature, orifice diameter, discharge coefficient, molecular weight, and heat capacity ratio. Absolute pressure is required. Gauge pressure can give wrong answers unless it is converted first. The discharge coefficient adjusts the ideal equation for edge shape, plate thickness, and real losses.
What The Results Mean
The main result is mass flow rate. It is shown in kilograms per second, kilograms per hour, grams per second, and pounds per hour. The calculator also reports upstream volumetric flow and standard volumetric flow. Standard flow is useful for comparing gases or equipment ratings at a reference pressure and temperature.
Good Engineering Practice
Use clean units and realistic gas properties. Check whether your orifice diameter is the actual bore size. Confirm that the pressure taps and plate design match your discharge coefficient. For safety work, custody transfer, or regulated systems, compare results with standards and field measurements.
Common Uses
This tool helps estimate purge rates, vent rates, leak flow, regulator sizing, relief paths, and laboratory gas discharge. It is also useful for early design checks before a detailed simulation. The equation gives a practical estimate, but installation effects can change measured flow.
Accuracy Notes
Results improve when temperature is measured near the upstream side. Gas composition should be known when molecular weight changes. Very high pressure, two phase flow, condensation, or strong heat transfer may need specialized methods. Treat the answer as an engineering estimate, not a certified test. Document assumptions before applying results to equipment changes.
FAQs
What is gas flow through an orifice?
It is the gas discharge through a small opening. Flow depends on pressure drop, gas temperature, gas properties, hole size, and discharge coefficient.
What is choked flow?
Choked flow occurs when gas reaches sonic speed at the throat. After that, lowering downstream pressure does not greatly increase mass flow.
Should I use absolute or gauge pressure?
Use absolute pressure. Gauge pressure must be converted by adding local atmospheric pressure before using compressible gas flow equations.
What is discharge coefficient?
It corrects ideal flow for real losses. Edge shape, plate thickness, surface condition, and installation details can change this value.
Why does molecular weight matter?
Molecular weight sets the specific gas constant. It affects density and mass flow for the same pressure, temperature, and orifice size.
What is gamma in gas flow?
Gamma is the ratio of specific heats. It affects expansion behavior, critical pressure ratio, and the choked flow limit.
Can this calculator handle air and natural gas?
Yes. Enter correct molecular weight and gamma. Methane can approximate dry natural gas, but real mixtures need adjusted properties.
Is this calculator suitable for final safety design?
Use it for estimates and early checks. Critical safety, relief, or regulated designs should be verified with standards and qualified engineering review.