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Gas Pressure Regulator Sizing Calculator

Reference: NFPA 58 / IGC

🎛️ GAS

Free gas pressure regulator sizing calculator. Find the required valve flow coefficient (Cv) from inlet and delivery pressure, gas flow and gas type for LPG and natural gas regulators (NFPA 58 / IGC).

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📋 ASSUMPTIONS & BASIS

NFPA 58 / IGC

Design criteria, governing codes and assumptions.

ℹ About This Calculator

A gas pressure regulator must pass the required flow while dropping the supply pressure to the delivery pressure without starving downstream appliances. This calculator estimates the required valve flow coefficient (Cv) from the inlet pressure, the delivery pressure, the gas flow rate and the gas type, handling both sub-critical and critical (choked) flow, so you can select a regulator with adequate capacity for LPG or natural gas service.

Regulator sizing depends on absolute pressures, so add atmospheric pressure to the gauge values. Gas specific gravity differs (natural gas ~0.6, LPG ~1.5), which changes capacity. Choked flow occurs at high pressure drops and caps the flow regardless of further downstream reduction. Always size for the peak simultaneous demand, allow turndown for low fire, and confirm against the manufacturer capacity tables and the applicable gas code (for example NFPA 54/58).

Regulator Cv Method (Critical / Sub-critical Flow)

NFPA 58 / IGC

Step 1 - Check the pressure ratio: if outlet < about half of absolute inlet, flow is critical (choked).
Step 2 - Critical flow: Cv = Q x sqrt(sg) / (constant x P1_abs).
Step 3 - Sub-critical flow: Cv = Q x sqrt(sg / (dP x P2_abs)) / constant.
Step 4 - Select a regulator whose rated Cv (and turndown) exceeds the required value at the design flow.

Worked Example

Delivering 50 Nm³/hr of natural gas (sg ≈ 0.6) from 1.0 bar g to 21 mbar, the large pressure ratio makes the flow choked, so the required Cv is governed by the absolute inlet pressure. The computed Cv sets the minimum regulator size; a model with rated Cv above this and good turndown is selected.

Regulator Selection Notes

Two-stage regulation

Many installations use two-stage regulation, a first-stage regulator near the source drops high supply pressure to an intermediate value, and a second-stage regulator at the appliance sets the burner pressure. Each stage is sized for its own inlet, outlet and flow.

Choked flow and lock-up

At high pressure drops flow is choked, so extra inlet pressure does not increase capacity. Also check lock-up (the small rise above set pressure at no flow) so appliances are not over-pressured at night or low demand.

Confirm with the maker

Use the calculated Cv to shortlist, then confirm against the manufacturer capacity curves at your exact inlet, outlet and gas, and verify relief and over-pressure protection to the gas code.

Frequently Asked Questions

What is Cv for a gas regulator? +

Cv is the flow coefficient, a measure of how much flow a valve passes for a given pressure drop. The regulator you select must have a rated Cv greater than the required Cv at your design flow and pressures.

What is critical or choked flow? +

When the downstream pressure falls below roughly half the absolute inlet pressure, the gas reaches sonic velocity at the throat and flow no longer increases with further downstream pressure reduction. Sizing then depends only on the inlet pressure.

Does gas type change the sizing? +

Yes. Capacity depends on specific gravity, so LPG (heavier) and natural gas (lighter) give different Cv for the same flow and pressures.

Should I use gauge or absolute pressure? +

Use absolute pressure in the equations. Add atmospheric pressure to the gauge readings. Using gauge pressure understates capacity, especially at low pressures.

How much margin should I allow? +

Size for the peak simultaneous demand and pick a regulator with headroom and good turndown so it also controls at low fire. Confirm against the manufacturer capacity data.