Control Valve Cv Calculator — Saturated Steam (ISA 75.01)
Per ISA 75.01-2012, the saturated steam Cv formula is: Cv = W / (K × P1 × Y × √(x × M × T1 × Z / SG)). W is mass flow in lb/hr; P1 is upstream pressure in psia; Y is the expansion...
Formula
Source: ISA 75.01-2012; IAPWS-IF97 (2012 release); Fisher Control Valve Handbook (5th ed.) | Last reviewed: July 3, 2026
Examples
5000 lb_hr
= 18.5 cv
- p1 = 150
- p2 = 50
- mw = 18
- temp_f = 366
5000 lb/hr steam, 150→50 psia → Cv 18.5
20000 lb_hr
= 50 cv
- p1 = 250
- p2 = 30
- mw = 18
- temp_f = 406
20000 lb/hr steam, 250→30 psia → Cv 50 (choked flow applies since x=0.88 > 0.5)
100000 lb_hr
= 95 cv
- p1 = 600
- p2 = 200
- mw = 18
- temp_f = 486
100000 lb/hr steam, 600→200 psia → Cv 95 (large process steam)
Quick Reference Table
| P (psig) | P (psia) | T_sat (°F) | h_f (BTU/lb) | h_g (BTU/lb) | SG (steam/air) |
|---|---|---|---|---|---|
| 0 | 14.7 | 212 | 180.2 | 1150.6 | 0.62 |
| 50 | 64.7 | 281 | 250.1 | 1174 | 0.61 |
| 100 | 114.7 | 338 | 298.6 | 1187.4 | 0.6 |
| 150 | 164.7 | 366 | 330.6 | 1194 | 0.59 |
| 250 | 264.7 | 406 | 376.1 | 1199.4 | 0.57 |
| 400 | 414.7 | 448 | 424.2 | 1201 | 0.55 |
Where is this used?
For superheated steam, the temperature input is the actual superheated temperature (not the saturation temperature at P1), and the compressibility factor Z may differ from 0.95.
Common mistake: using the water Cv formula for steam — this can over-predict the actual flow by 2-5x.
Always use the steam-specific formula for steam service.
Real-World Usage Scenarios
Sterilizer Steam Control Valve
A hospital sterilizer requires 800 lb/hr of saturated steam at 50 psig (64.7 psia) supplied from a 100 psig (114.7 psia) steam header. The downstream pressure is the sterilizer chamber pressure. P1=114.7 psia, P2=64.7 psia, x=(114.7-64.7)/114.7=0.436, below xc=0.5 so no choking. Y = 1 - 0.436/(3·0.5) = 0.71. Cv = 800/(0.8695 × 114.7 × 0.71 × √(0.436 × 18 × (366+459.67) × 0.95/0.60)) = 800/(0.8695 × 114.7 × 0.71 × 100.6) = 800/7117 = 0.112. Select a 1-inch globe valve with max Cv 0.5 (4× margin for control range).
Common Mistakes to Avoid
Using the water Cv formula for steam
GPM = Cv × √(ΔP/SG) for water does NOT apply to steam. Using it for steam over-predicts the flow by 2-5x, leading to undersized valves. The ISA 75.01 steam formula includes the expansion factor Y and the molecular weight, which the water formula lacks.
Ignoring choked flow when x > 0.5
When x = (P1-P2)/P1 exceeds the critical pressure ratio (xc = 0.5 for steam, 0.45 for air, varies by gas), the flow becomes choked — increasing P2 further does not increase flow. The ISA formula caps x at xc. Failing to detect this gives a valve that limits plant capacity at startup.
Industry Standards Referenced
Frequently Asked Questions
What is the critical pressure ratio for steam?
Per ISA 75.01, xc = 0.5 for saturated steam (varies slightly with superheat). For air and most diatomic gases, xc = 0.45. For heavier gases (CO2, propane), xc = 0.4-0.5. When x > xc, the flow is choked and the formula uses xc.
How is this different from superheated steam?
For superheated steam, the temperature input is the actual superheated temperature (e.g., 600°F at 150 psig, where T_sat = 366°F). The compressibility factor Z is closer to 1.0 (less vapor-liquid interaction). The formula structure is the same; the temperature and Z inputs differ.
Does this work for superheated steam too?
Yes. Use actual superheated temperature (not T_sat), Z≈1.0 (vs≈0.95 saturated), and xc≈0.55. For wet saturated steam, apply ζ correction: Cv_wet = Cv_sat × √ζ.
Reviewed for accuracy
Cross-referenced against ISA 75.01-2012 and IAPWS-IF97 steam tables · Last reviewed: July 3, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.