Since steam is compressible, valve sizing for steam must be based on other criteria. The most important characteristic of compressible flow is that the speed of flow in the throttling section can only increase up to the speed of sound. When this limit is reached, the speed of flow and thus the volumetric flow, or the steam mass flow, no longer increases, even if the differential pressure ∆p rises. To ensure good controllability and favorably priced valve selection, it is advisable to have the differential pressure in normal operation as close as possible to the critical pressure ratio.
Before starting valve sizing, the plant-related process parameters and the prevailing operating state must be defined:
Absolute steam pressure [kPa abs], [bar abs]
Temperature of saturated or superheated steam [°C]
Differential pressure ∆pmax in normal operation
The dryness of saturated steam at the valve’s inlet must be > 0.98.
During plant startup or shutdown, supercritical pressure conditions can occur:
In terms of potential damage to the valve, a subcritical pressure ratio is far less crucial since the speed of flow lies below the speed of sound, material abrasion is reduced, and the noise level is lower
Sizing procedure
Calculate the steam mass flow m based on the amount of energy required Q100, the steam pressure, and the steam temperature.
Determine whether the pressure ratio is in the sub- or supercritical range.
Determine the kvs value based on the steam mass flow and the steam pressure.
| Steam mass flow |  |
Pressure ratio = |  |
Calculation of kvs value for steam
| Subcritical range | Supercritical range | ||
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| Pressure ratio < 42 % subcritical | Pressure ratio ≥ 42 % supercritical (not recommended) |
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Q100 = rated capacity in kW rp1 = specific heat capacity of steam in kJ/kgK p1 = absolute pressure at the valve inlet in kPa (prepressure) p3 = absolute pressure at the valve outlet in kPa
k = factor for superheating the steam = 1 + 0.0012 x ΔT (for saturated steam, k = 1) ΔT = temperature spread in K of saturated steam and superheated steam |
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= steam mass flow in kg/h
Note
The level of absolute pressure p1 at the valve inlet must be at least such that the absolute pressure p3 at the valve outlet is higher than the atmospheric pressure.
Notes on the supercritical range
When there is a pressure ratio (p1 – p3) / p1 >0.42, the flow passing through the narrowest section of the valve reaches the speed of sound. This can lead to higher noise levels. A throttling system operating at a lower noise level (multistage pressure reduction, damping throttle by the outlet) alleviates the problem.
Subcritical < 42 %
Steam-controlled heat transfer medium without condensation
Shutoff valve on the steam side of condensation-controlled heat transfer media
Supercritical ≥ 42 %
Steam humidifier
Steam-controlled heat transfer medium with condensation in the heat exchanger
Recommendation for differential pressure Δpmax
For saturated and superheated steam, the differential pressure Δpmax across the valve should be as close as possible to the critical pressure ratio.
Abs. operating pressure [bar] |
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Chart example: The chart of the selected valve must be observed X and Y: Suitable actuators, depending on the 2-port valve |
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Medium temperature [°C] |
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Water |
- |
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Wet steam |
To be avoided |
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Saturated steam Superheated steam |
Permissible operating range |
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Water vapor table
Water vapor table for the saturated state (pressure table)
Pressure |
Temperature |
Spec. volume water |
Spec. volume steam |
Density steam |
Enthalpy water |
Enthalpy steam |
Heat of vaporization |
|
|---|---|---|---|---|---|---|---|---|
p |
p |
T |
V' |
V'' |
ρ'' |
h' |
h'' |
r |
[kPa] |
[bar] |
[°C] |
[dm3/kg] |
[m3/kg] |
[kg/m3] |
[kJ/kg] |
[kJ/kg] |
[kJ/kg] |
1 2 3 4 |
0.010 0.020 0.030 0.040 |
6.9808 17.513 24.100 28.983 |
1.0001 1.0012 1.0027 1.0040 |
129.20 67.01 45.67 34.80 |
0.007739 0.01492 0.02190 0.02873 |
29.34 73.46 101.00 121.41 |
2514.1 2533.6 2545.6 2554.5 |
2485.0 2460.2 2444.6 2433.1 |
5 6 7 8 9 |
0.050 0.060 0.070 0.080 0.090 |
32.898 36.183 39.025 41.534 43.787 |
1.0052 1.0064 1.0074 1.0084 1.0094 |
28.19 23.74 20.53 18.10 16.20 |
0.03547 0.04212 0.04871 0.05523 0.06171 |
137.77 151.50 163.38 173.86 183.28 |
2561.6 2567.5 2572.6 2577.1 2581.1 |
2423.8 2416.0 2409.2 2403.2 2397.9 |
10 20 30 40 50 |
0.10 0.20 0.30 0.40 0.50 |
45.833 60.086 69.124 75.886 81.345 |
1.0102 1.0172 1.0223 1.0265 1.0301 |
14.67 7.650 5.229 3.993 3.240 |
0.06814 0.1307 0.1912 0.2504 0.3086 |
191.83 251.45 289.30 317.65 340.56 |
2584.8 2609.9 2625.4 2636.9 2646.0 |
2392.9 2358.4 2336.1 2319.2 2305.4 |
60 70 80 90 100 |
0.60 0.70 0.80 0.90 1.0 |
85.954 89.959 93.512 96.713 99.632 |
1.0333 1.0361 1.0387 1.0412 1.0434 |
2.732 2.365 2.087 1.869 1.694 |
0.3661 0.4229 0.4792 0.5350 0.5904 |
359.93 376.77 391.72 405.21 417.51 |
2653.6 2660.1 2665.8 2670.9 2675.4 |
2293.6 2283.3 2274.0 2265.6 2257.9 |
150 200 250 300 350 |
1.5 2.0 2.5 3.0 3.5 |
111.37 120.23 127.43 133.54 138.87 |
1.0530 1.0608 1.0675 1.0735 1.0789 |
1.159 0.8854 0.7184 0.6056 0.5240 |
0.8628 1.129 1.392 1.651 1.908 |
467.13 504.70 535.34 561.43 584.27 |
2693.4 2706.3 2716.4 2724.7 2731.6 |
2226.2 2201.6 2181.0 2163.2 2147.4 |
400 450 500 600 700 |
4.0 4.5 5.0 6.0 7.0 |
143.62 147.92 151.84 158.84 164.96 |
1.0839 1.0885 1.0928 1.1009 1.1082 |
0.4622 0.4138 0.3747 0.3155 0.2727 |
2.163 2.417 2.669 3.170 3.667 |
604.67 623.16 640.12 670.42 697.06 |
2737.6 2742.9 2747.5 2755.5 2762.0 |
2133.0 2119.7 2107.4 2085.0 2064.9 |
800 900 1'000 1'100 1'200 1'300 1'400 |
8.0 9.0 10 11 12 13 14 |
170.41 175.36 179.88 184.07 187.96 191.61 195.04 |
1.1150 1.1213 1.1274 1.1331 1.1386 1.1438 1.1489 |
0.2403 0.2148 0.1943 0.1774 0.1632 0.1511 0.1407 |
4.162 4.655 5.147 5.637 6.127 6.617 7.106 |
720.94 742.64 762.61 781.13 798.43 814.70 830.08 |
2767.5 2772.1 2776.2 2779.7 2782.7 2785.4 2787.8 |
2046.5 2029.5 2013.6 1998.5 1984.3 1970.7 1957.7 |
1'500 1'600 1'700 1'800 1'900 |
15 16 17 18 19 |
198.29 201.37 204.31 207.11 209.80 |
1.1539 1.1586 1.1633 1.1678 1.1723 |
0.1317 0.1237 0.1166 0.1103 0.1047 |
7.596 8.085 8.575 9.065 9.555 |
844.67 858.56 871.84 884.58 896.81 |
2798.9 2791.7 2793.4 2794.8 2796.1 |
1945.2 1933.2 1921.5 1910.3 1899.3 |
2'000 2'500 3'000 4'000 5'000 |
20 25 30 40 50 |
212.37 223.94 233.84 250.33 263.91 |
1.1766 1.1972 1.2163 1.2521 1.2858 |
0.09954 0.07991 0.06663 0.04975 0.03743 |
10.05 12.51 15.01 10.10 25.36 |
908.59 961.96 1008.4 1087.4 1154.5 |
2797.2 2800.9 2802.3 2800.3 2794.2 |
1888.6 1839.0 1793.9 1712.9 1639.7 |
6'000 7'000 8'000 9'000 |
60 70 80 90 |
275.55 285.79 294.97 303.31 |
1.3187 1.3513 1.3842 1.4179 |
0.03244 0.02737 0.02353 0.02050 |
30.83 36.53 42.51 48.79 |
1213.7 1267.4 1317.1 1363.7 |
2785.0 2773.5 2759.9 2744.6 |
1571.3 1506.0 1442.8 1380.9 |
10'000 11'000 12'000 13'000 14'000 |
100 110 120 130 140 |
310.96 318.05 324.65 330.83 336.64 |
1.4526 1.4887 1.5268 1.5672 1.6106 |
0.01804 0.01601 0.01428 0.01280 0.01150 |
55.43 62.48 70.01 78.14 86.99 |
1408.0 1450.6 1491.8 1532.0 1571.6 |
2727.7 2729.3 2689.2 2667.0 2642.4 |
1319.7 1258.7 1197.4 1135.0 1070.7 |
15'000 20'000 22'000 22'120 |
150 200 220 221.2 |
342.13 365.70 373.69 374.15 |
1.6579 2.0370 2.6714 3.17 |
0.01034 0.005877 0.003728 0.00317 |
96.71 170.2 268.3 315.5 |
1611.0 1826.5 2011.1 2107.4 |
2615.0 2418.4 2195.6 2107.4 |
1004.0 591.9 184.5 0 |