Water Vapor Pressure vs Temperature – Table & NPSH

Vapor Pressure of Water – Predicting Boiling, Cavitation, and Flash in Pipes

Water doesn’t boil at 100 °C in every system. It flashes at 60 °C in a vacuum line, causes cavitation in a hot pump suction, or stays liquid at 150 °C under pressure. The key? Vapor pressure—the pressure at which liquid and vapor are in equilibrium.

Misjudging vapor pressure leads to pump failure, pipe hammer, flash explosions, undersized condensers, and lost production. This post delivers accurate vapor pressure data, IAPWS-approved equations, step-by-step calculations, NPSH examples, and plant-safe design rules.

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What Is Vapor Pressure?

Vapor pressure (Pᵥ) is the pressure exerted by water molecules escaping from the liquid phase at a given temperature. At Pᵥ = P_system, the liquid boils.

Units:

• kPa, bar, mmHg, psia
• 1 atm = 101.325 kPa = 760 mmHg

For pure water:

• At 20 °C: $P_v \approx 2.34 \, \mathrm{kPa}$
• At 100 °C: $P_v = 101.325 \, \mathrm{kPa}$ (boiling at 1 atm)

Critical point: 374 °C, 221 bar — vapor and liquid become indistinguishable.

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Temperature Dependence: The Clausius-Clapeyron Effect

Vapor pressure rises exponentially with temperature due to increased molecular kinetic energy.

Below is a high-accuracy table from IAPWS-95 (rounded to 2 decimals):

Temp (°C)	Pᵥ (kPa)	Pᵥ (mmHg)	Temp (°C)	Pᵥ (kPa)	Pᵥ (mmHg)
0	0.61	4.58	50	12.35	92.6
5	0.87	6.54	60	19.95	149.6
10	1.23	9.21	70	31.19	233.9
15	1.71	12.8	80	47.39	355.4
20	2.34	17.5	90	70.14	526.0
25	3.17	23.8	100	101.33	760.0
30	4.25	31.8	120	198.5	1489
35	5.63	42.2	150	476.0	3570
40	7.38	55.3	200	1554	11650
45	9.59	71.9	250	3973	29790

Rule of thumb: Every 10 °C rise ≈ doubles Pᵥ below 100 °C.

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Accurate Antoine Equation (1–100 °C, < 0.5 % error)

$\log_{10}(P_v) = A - \frac{B}{T + C}$

where:

• $P_v$ = vapor pressure in mmHg
• $T$ = temperature in °C

Range	A	B	C
1–100 °C	5.11564	1687.537	230.17

To get kPa:
$P_v (\mathrm{kPa}) = \frac{P_v (\mathrm{mmHg})}{7.5006}$

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IAPWS-97 Equation (0–374 °C, < 0.1 %)

For high accuracy (used in REFPROP, Aspen, etc.):

$\ln\left(\frac{P_v}{P_c}\right) = \frac{T_c}{T} \left( a_1 \tau + a_2 \tau^{1.5} + a_3 \tau^3 + a_4 \tau^{3.5} + a_5 \tau^4 + a_6 \tau^{7.5} \right)$

• $\tau = 1 - \frac{T}{T_c}$, $T_c = 647.096 \, \mathrm{K}$, $P_c = 22.064 \, \mathrm{MPa}$
• Coefficients:
  $a_1 = -7.85951783$, $a_2 = 1.84408259$, $a_3 = -11.7866497$,
  $a_4 = 22.6807411$, $a_5 = -15.9618719$, $a_6 = 1.80122502$

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Pressure Effects: Dissolved Gases and Salts

• Pure water only: Use above equations.
• Air-saturated water: Total pressure = Pᵥ + P_air → slightly higher boiling point.
• Brine (5% NaCl): Pᵥ drops ~20 % → higher boiling point.

Use Raoult’s law for dilute solutions.

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Measuring Vapor Pressure

Method	Best For	Accuracy	Cost
Static method (manometer)	Lab reference	±0.1 %	Medium
Dynamic (boiling point)	Quick check	±1 %	Low
Pressure transducer	Inline (vacuum systems)	±0.5 %	High
IAPWS/REFPROP	Design & simulation	±0.05 %	Free

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Why Vapor Pressure Matters in Process Design

1. Pump NPSH and Cavitation

NPSHa must exceed NPSHr + margin:

$\mathrm{NPSHa} = \frac{P_{\text{inlet}} - P_v}{\rho g} + h_{\text{static}} - h_f$

Hot water at 80 °C: $P_v = 47.4 \, \mathrm{kPa}$
→ Only 54 kPa gauge available before cavitation.

2. Flash in Pipes and Valves

Pressure drop → P < Pᵥ → flash steam → shock, erosion, noise.

3. Vacuum Systems and Condensers

Condenser pressure = Pᵥ at cooling water T.
30 °C water → min vacuum ≈ 4.2 kPa abs.

4. Safety in Hot Water Loops

120 °C water at 1 bar → Pᵥ = 198 kPa → instant flash if line breaks.

5. Distillation and Evaporation

Bubble point = Pᵥ → controls column pressure.

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Vapor Pressure Calculation Examples

Example 1: Antoine at 75 °C

$T = 75$

$\log_{10}(P_v) = 5.11564 - \frac{1687.537}{75 + 230.17} = 5.11564 - \frac{1687.537}{305.17} = 5.11564 - 5.529 = -0.41336$

$P_v = 10^{-0.41336} \approx 0.3865 \, \mathrm{bar} = 38.65 \, \mathrm{kPa}$

Table: 70 °C = 31.2 kPa, 80 °C = 47.4 kPa → 75 °C ≈ 38.7 kPa
Error < 0.2 %

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Example 2: Flash temperature in vacuum line

Water at 1 kPa abs → find boiling T.

From table:

• 0.61 kPa → 0 °C
• 1.23 kPa → 10 °C
  Interpolate: ~7 °C

Any water >7 °C will flash.

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Example 3: NPSHa for hot water pump

• Fluid: Water at 90 °C
• Suction: 1.5 m static head, 0.5 m friction loss
• Tank pressure: 1.2 bar gauge = 221.3 kPa abs
• $P_v = 70.1 \, \mathrm{kPa}$
• $\rho = 965 \, \mathrm{kg/m^3}$

$\mathrm{NPSHa} = \frac{221.3 - 70.1}{965 \times 9.81} + 1.5 - 0.5 = \frac{151.2}{9469.65} + 1.0 \approx 0.016 + 1.0 = 1.016 \, \mathrm{m}$

NPSHr = 3 m → Cavitation risk!
Fix: Increase tank pressure or cool suction.

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Example 4: Condenser vacuum limit

Cooling water at 32 °C → $P_v \approx 4.8 \, \mathrm{kPa}$

Minimum condenser pressure ≈ 5–6 kPa abs (with margin).
Lower → impossible without subcooling.

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Takeaway

Vapor pressure controls:

• Boiling point
• Pump cavitation
• Flash risk
• Vacuum limits

Never assume 100 °C boiling. Use:

• Antoine for 1–100 °C
• IAPWS for high T/P
• Tables for daily checks

With the data and examples above, you can now size safe hot water systems, prevent pump damage, and optimize vacuum processes.