Cv Valve Sizing Calculator
Required flow coefficient (Cv) for control valve sizing. Liquid and gas service. ISA-75.01 / ANSI formulas. Enter design flow, differential pressure, and fluid properties.
Required Cv
—
Cv (GPM/√psi)
Recommended valve Cv
—
select ≥ this (1.3× margin)
Kv equivalent
—
Kv (m³/h per √bar)
Velocity at inlet
—
ft/s
// Ad slot — fills after AdSense approval
Cv sizing formulas (ISA-75.01)
LIQUID:
Cv = Q × √(SG / ΔP)
where Q = flow (GPM), SG = specific gravity, ΔP = pressure drop (psi)
GAS (non-choked flow, ΔP < P1/2):
Cv = Q / (816 × P1 × Y) × √(G_g × T / (P1² − P2²))
Simplified for air-like gases:
Cv ≈ Q(SCFH) / (963 × √(ΔP × P_avg / G_g / T_R))
CHOKED FLOW occurs when ΔP > 0.5 × P1 — valve cannot pass more flow
regardless of further pressure drop increase.
Kv ↔ Cv: Kv = 0.865 × Cv | Cv = 1.156 × Kv
Specific gravity reference
| Fluid | SG (liquid) | Notes |
|---|---|---|
| Water @ 60°F | 1.000 | Reference fluid |
| Water @ 212°F | 0.958 | Hot condensate |
| Water @ 300°F | 0.910 | High-temp service |
| Seawater | 1.025 | Marine / offshore |
| Diesel fuel | 0.850 | Petroleum distillate |
| Crude oil (light) | 0.825 | Low API gravity |
| Crude oil (heavy) | 0.920 | High API gravity |
| Ethylene glycol (50%) | 1.065 | Antifreeze solution |
| Sulfuric acid (93%) | 1.840 | Use corrosion-resistant trim |
| Natural gas (typical) | 0.60 (gas SG) | Relative to air |
| Propane (gas) | 1.52 (gas SG) | Relative to air |
| Air | 1.000 (gas SG) | Reference gas |
| Steam (saturated, 100 psig) | Use steam tables | Special case |
Frequently asked questions
How much margin should I leave on Cv selection?
Size the selected valve at 1.3–1.5× your calculated required Cv. This means at design flow the valve is 65–75% open, leaving headroom for increases and ensuring you're not operating wide open. Also check minimum controllable flow — most valves lose controllability below 10% of rated Cv.
What is choked flow?
Choked flow occurs when the pressure drop across a gas valve exceeds approximately 50% of inlet pressure (ΔP > P1/2). At this point, flow velocity at the vena contracta reaches sonic speed and further reducing outlet pressure cannot increase flow. Adding more valve Cv won't help — the system is flow-limited by pressure alone.
What is the difference between a globe valve and a ball valve for control?
Globe valves have inherent throttling characteristics (equal-percentage, linear, or quick-opening trim) and are the traditional control valve. Ball valves can be used for modulating service with characterized balls but have a modified equal-percentage curve. V-notch ball valves work well for dirty fluids or slurries that would plug a globe trim.
What does "equal percentage" trim mean?
Equal-percentage (EP) trim means each incremental opening of the valve increases flow by a constant percentage of the current flow. At 50% open an EP valve passes much less than 50% of rated flow — it's inherently slow at low openings and fast at high openings. This pairs well with processes where ΔP falls as flow increases (most pump systems), linearizing the control loop.