Steam Control Valves - Calculating Kv Values

Design of steam control valves and their Kv values

The procedure below can be used for proper design of steam process control valves and their correct flow factors - Kv.

1. Determine Pressure Coefficient - Pcoef

Start with the absolute pressure before the control valve and the actual pressure drop available over the valve and determine the pressure coefficient - Pcoef - from the table below.

Pcoef = pressure drop / specific volume

Note! The pressure drop can not exceed critical pressure drop which is approximately 42% of the inlet pressure.

Pcoeff
Absolute Pressure
- Pin -
(bar)

Specific Volume
- v -
(m3/kg)

Pressure drop (bar)
0.511.522.533.544.555.566.577.5
1 1.69 0.30
2 0.89 0.56 1.12
3 0.61 0.82 1.64 2.46
4 0.45 1.11 2.22 3.33 4.44
5 0.38 1.32 2.63 3.95 5.26 6.58
6 0.32 1.56 3.13 4.69 6.25 7.81 9.38
7 0.27 1.85 3.70 5.56 7.41 9.26 11.1 13.0
8 0.24 2.08 4.17 6.25 8.33 10.42 12.5 14.6 16.7
9 0.21 2.38 4.76 7.14 9.52 11.90 14.3 16.7 19.1 21.4
10 0.19 2.63 5.26 7.89 10.53 13.16 15.8 18.4 21.1 23.7 26.3
11 0.18 2.78 5.56 8.33 11.11 13.89 16.7 19.4 22.2 25.0 27.8 30.6
12 0.16 3.13 6.25 9.38 12.50 15.63 18.7 21.9 25.0 28.1 31.3 34.4 37.5
13 0.15 3.33 6.67 10.00 13.33 16.67 20.0 23.3 26.7 30.0 33.3 36.7 40.0 43.3
14 0.14 3.57 7.14 10.71 14.29 17.86 21.4 25.0 28.6 32.1 35.7 39.3 42.9 46.4 50.0
15 0.13 3.85 7.69 11.54 15.38 19.23 23.1 26.9 30.8 34.6 38.5 42.3 46.2 50.0 53.9 57.7

Example - Pressure Coefficient

In a steam system with 9 bar gauge pressure the absolute pressure is 10 bar. If a pressure drop of 3.5 bar is acceptable and suitable for the steam consumer, the pressure coefficient Pcoef should be 18.4.

2. Determine Flow Factor - Kv

Select the pressure coefficient - Pcoef - column closest possible to the actual pressure coefficient calculated in step 1.

Follow the column down until reaching the appropriate actual flow value. Interpolate between values if necessary.

Go left in the table to find the actual flow factor Kv.

Steam Flow (kg/h)
KvPressure Coefficient - Pcoeff
11.251.561.952.443.053.814.775.967.45
0.01 0 0 0 0 0 1 1 1 1 1
0.02 0 1 1 1 1 1 1 1 1 1
0.02 1 1 1 1 1 1 1 2 2 2
0.03 1 1 1 1 2 2 2 2 3 3
0.05 2 2 2 2 3 3 3 4 4 4
0.08 2 3 3 3 4 4 5 5 6 7
0.11 4 4 5 5 6 6 7 8 9 10
0.17 5 6 7 8 8 9 11 12 13 15
0.26 8 9 10 11 13 14 16 18 20 22
0.38 12 14 15 17 19 21 24 27 30 33
0.58 18 20 23 26 29 32 36 40 45 50
0.86 27 31 34 38 43 48 54 60 67 75
1.3 41 46 51 57 64 72 80 90 100 112
2.0 62 69 77 86 96 108 120 135 151 168
2.9 93 103 116 129 145 162 181 202 226 253
4.4 139 155 174 194 217 242 271 303 339 379
6.6 208 233 260 291 325 364 407 455 508 568
9.9 312 349 390 436 488 546 610 682 763 853
14.8 468 524 586 655 732 818 915 1023 1144 1279
22.2 703 786 878 982 1098 1228 1373 1535 1716 1918
33.3 1054 1179 1318 1473 1647 1841 2059 2302 2574 2877
49.9 1581 1768 1976 2210 2471 2762 3088 3453 3860 4316
74.8 2372 2652 2965 3315 3706 4143 4632 5179 5790 6474
112 3558 3978 4447 4972 5559 6215 6948 7769 8686 9711
168 5336 5966 6671 7458 8338 9322 10423 11653 13028 14566
253 8005 8949 10006 11187 12507 13983 15634 17479 19543 21849
379 12007 13424 15009 16780 18761 20975 23451 26219 29314 32774
568 18010 20136 22513 25170 28141 31463 35177 39329 43971 49161
852 27016 30204 33769 37755 42212 47194 52765 58993 65956 73741
1278 40523 45307 50654 56633 63318 70791 79147 88489 98934 110612

 

Steam Flow (kg/h)
KvPressure Coefficient - Pcoeff
9.3111.614.518.222.728.435.544.455.569.4
0.01 1 1 1 1 2 2 2 2 2 3
0.02 1 2 2 2 2 3 3 3 4 4
0.02 2 2 3 3 3 4 4 5 5 6
0.03 3 4 4 5 5 6 6 7 8 9
0.05 5 5 6 7 8 9 10 11 12 13
0.08 7 8 9 10 11 13 14 16 18 20
0.11 11 12 14 15 17 19 22 24 27 30
0.17 17 18 21 23 26 29 32 36 40 45
0.26 25 28 31 35 39 43 48 54 61 68
0.38 37 42 46 52 58 65 73 81 91 102
0.58 56 62 70 78 87 97 109 122 136 152
0.86 84 94 105 117 131 146 163 183 204 228
1.3 126 140 157 175 196 219 245 274 306 343
2.0 188 210 235 263 294 329 368 411 460 514
2.9 282 316 353 395 441 493 552 617 689 771
4.4 424 474 530 592 662 740 827 925 1034 1156
6.6 635 710 794 888 993 1110 1241 1388 1551 1734
9.9 953 1066 1191 1332 1489 1665 1862 2081 2327 2602
14.8 1430 1598 1787 1998 2234 2498 2792 3122 3491 3903
22.2 2145 2398 2681 2997 3351 3746 4189 4683 5236 5854
33.3 3217 3597 4021 4496 5026 5620 6283 7025 7854 8781
49.9 4825 5395 6032 6744 7540 8429 9424 10537 11781 13171
74.8 7238 8092 9047 10115 11309 12644 14137 15805 17671 19757
112 10857 12138 13571 15173 16964 18966 21205 23708 26506 29635
168 16285 18208 20357 22760 25446 28449 31808 35562 39759 44452
253 24428 27312 30535 34139 38169 42674 47711 53343 59639 66679
379 36642 40967 45803 51209 57254 64011 71567 80014 89459 100018
568 54963 61451 68704 76814 85880 96017 107350 120021 134188 150027
852 82445 92176 103056 115220 128820 144026 161025 180032 201282 225040
1278 123668 138265 154584 172831 193231 216038 241538 270048 301923 337560

Example - Determine Flow Factor

For a pressure coefficient Pcoef = 18.2 and steam flow = 592 kg/h -  the flow factor Kv is 4.4.

The formula used for the flow factor - Kv

The tables above are based on the formula:

Kv = m / (31.7 (dP / v)1/2)                             (1)

where

m= steam flow (kg/h)

dP = pressure drop (bar)

v = specific volume of steam before control valve (m3/kg)

Related Topics

  • Steam and Condensate - Steam & condensate systems- properties, capacities, pipe sizing, systems configuration and more
  • Control Valves - Control Valve terminology, bodies, trim, flow characteristics, Cv and Kv sizing, noise, actuators and positioners
  • Process Control - Instrumentation and process control systems, design and documentation
  • Control Valves and Equipment - Sizing and dimensions of control valves & equipment in steam and condensate systems

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