Formulas

Formulas - General Usage Formulas

Calculating Condensate Loads

When the normal condensate load is unknown it can be approximately determined by calculating with the appropriate formula.

General Usage Formulas

Specialized Applications

Properties of Saturated Steam

Effect of Back Pressure on Steam Trap Capacity (% reduction in cap)

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Formulas - How Fast Should Steam Mains be Warmed Up?

The table shows linear expansion in inches per 100 feet of 24” pipe from 0 – 1000°F. It can be noted that for heating up the pipe to 1000°F an expansion of 9.4” is encountered. Rapid heating of such lines would destroy the system and it is, therefore, important to reduce the expansion and temperature stresses as much as possible. An extremely important factor is the ability of the steam traps to discharge condensate as it is formed in the relatively cool pipe. Water hammer must be avoided since it can rupture the pipe or cause valve failure, particularly at the end of the line. Closed gate valves are most susceptible to damage at the end of the line because they can crack at the seat ring when struck by a slug of water. Proper calculating of the heat losses and condensate loads will avoid the backing-up of condensate, and most of the difficulties will be eliminated; however, it is recommended, if the header must be ready for service on short notice, to heat the header with a steam flow through a bypass or a 3/4”, 1” valve first, and only after the pipe has been heated to a certain minimum temperature of approximately 250°F should high pressure steam be admitted to the line. Velan traps operate from 0 to maximum pressure and can handle this job, however, bypasses are recommended, and here the Velan Piping King can offer great savings. The stressing of pipes under such conditions are enormous, and it is important to provide proper expansion loops and expansion joins to keep the effective stress forces within allowable limits.

Stresses can be calculated from a formula:

of expansion of pipe material (in/∆°F)
TD = The difference between the initial and final temperature of the pipe

Temperature Expansion of Pipes per 100 Feet (inches)

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Formulas - Calculating Condensate Loads

Warm Up Load ( Heating Loss ) This is the amount of condensate which forms at starting up a power plant to raise the temperature of the metal of pipes, fittings, etc. to the operating temperature without including the normal radiation loss. As far as calculating the condensation load during the warming up period the required time is extremely important for sizing steam traps. Less warm up time increases the necessary capacity per trap. Allowing more time for warm up permits the use of smaller traps in smaller quantity.

           W x (T-t) x Sp. Heat x 60
Q = —————————————
                            L x m

Where Q = Quantity of Condensate ( Lbs/hr )
W = Total weight of pipes in LBS
T = Saturation Steam Temp ( °F ) t = initial temp of pipe ( °F ) usually surrounding air temp
m = minutes to heat up system
L = Latent heat of steam ( BTU’s/Hr )

Normal Condensate Load ( Radiation Loss )

Once the system is heated up steam condenses due to normal radiation losses to the surrounding air. These losses depend of course on the size and length of the pipe, on the pressure of steam and its latent heat and mainly on the type and thickness of insulation. The equation from which a normal regulation load can be calculated is:

               F x HL ( T - t )
Q = ——————————
                         L

Where F = Length of Pipe (ft)
HL ( T – t ) = Heat loss/foot of pipe at the temperature differential between steam and air
L = Latent heat of steam ( BTU’s/Hr. )

Condensation Load for Sizing Steam Traps

The condensation load builds up from 0 to maximum at the point where the warming up load drops to 0. It is assumed, therefore, that the peak is achieved halfway through the warming up period. Therefore, for sizing of steam traps, we take the maximum amount of condensate during the warming up period plus half of the radiation load.

Qt= QW + .5QR

Where Qt = Total condensation load at peak (LBS/Hr)
QW = Condensation load during warm up (LBS/Hr)
QR = Condensation load due to normal radiation loss (LBS/Hr)

Condensate in LBS/Hr created in steam mains 1” to 24” and pressure of 600 to 2500 PSI based on the warm up period of 1 hour and 100 feet of pipe, and based on the above assumption is shown in tables.

For shorter or longer heating up time, multiply by 60/m where m is the warm up time in minutes.

Properties of Saturated Steam

Standard Dimensions for Schedule 40 Pipe

Examples:
Warm up loss
Ambient Temp = 70°F
Working Temp = 366°F (150psig)
Warm up time = 720 minutes
1000 feet of 10 inch Schedule 40 pipe weighs = 40483
Latent heat = 857

        40483 x (366-70) x .12 x 60
Q = —————————————
                        857 x 720

Q = 472 lbs/hr

 

Radiation loss

Ambient Temp = 70°F
Working Temp = 366°F (150psig)
Differential temp = 294°F
Differential multiplier = .98
1000 feet 10 inch Schedule 40 uninsulated pipe
Latent heat = 857

        1000 x 2600 ( .98 [factor for 296° diff] )
Q = ——————————
                    857

Q = 2973 lbs/hr

 

Sizing for steam traps

Qt= QW + .5QR

.5QR = 1486.5 + QW = 472 = 1958.5

 

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Formulas - Calculating Steam Main Warm Up Loads

Warm up load in pounds of steam per 100 feet of steam main --- Ambient temperature 70°F* Main Size

 

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Formulas - Calculating Drip Leg Sizing

Drip legs should be installed roughly every 300 feet and never more than 500 feet. In a supervised warm up drip legs should be minimum of 10 inches long. Drip legs on automatic warm up systems should be a minimum of 28 inches long. As a rule of thumb drip leg length should be 1.5 times the diameter of the steam main. Drip leg diameter should match steam mains up to 4 inches. Above 4 inches the drip leg should be at least half the diameter of the steam main.

NOTE: Drip legs that are too narrow can cause a venturi “piccolo” effect that actually pulls condensate from the trap.

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Formulas - Calculating Condensate Load for Process Air Heaters

            F x Cp x d x 60 min/hr x ∆T
Q = —————————————
                               H

Where:
Q = Condensate Load in lbs/hr
F = Cubic feet of air per minute
Cp = Specific heat of air in btu/lb--°F
d = Density of air--.075 lbs/cu ft
∆T = Temperature rise in °F
H = Latent heat of steam in btu/lb

EXAMPLE:

10 PSIG STEAM
Heating indoor 70° air to 180°F = 110°∆F

F = Cubic feet of air per minute 3000
Cp = Specific heat of air in btu/lb--°F .24
d = Density of air--.075 lbs/cu ft .075
∆T = Temperature rise in °F 110
H = Latent heat of steam in btu/lb 953

        3000CFM x 0.24 x .075 x 60 min/hr x 110
Q = —————————————
                            953

Q = 374 lbs/hr

SAFETY FACTOR:
Using Outdoor Air 3
Using Indoor Air 2

Properties of Saturated Steam

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Formulas - Calculating Condensate for Shell and Tube Heat Exchangers

            L x ∆T x C x 500 x sg
Q = ———————————
                           H

Where:
Q = Condensate load in Lbs/hr
L = Liquid flow in GPM
∆T = Temperature rise in °F
C = Specific heat of liquid in BTU/lb--°F
500 = 60 min/hr x 8.33 lbs/gal
sg = Specific Gravity of liquid
H = Latent heat of Steam in BTU/lb

SAFETY FACTOR: 2

EXAMPLE:

10 PSIG steam system
20 GPM
Heating water from 40° to 120°

          20 x 80 x 1 x 500 x 1
Q = ———————————
                          953

Q= 839 lbs/hr

Properties of Saturated Steam

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Formulas - Calculating Condensate Load for Steam Tracer Lines

         L x U x ∆T x E
Q = ———————
                S x H

Where:
Q = Condensate Load in lbs/hr
L = Length of product pipe between tracer line traps (ft)
U = Heat transfer factor (Btu/sq ft/°F/hr) BELOW
∆T = Temperature differential in °F
E = 1 minus efficiency of insulation
S = Lineal feet of pipe line per sq ft of surface
H = Latent heat of steam in btu/lb

 

Properties of Saturated Steam

 

Standard Dimensions for Schedule 40 Pipe

 

EXAMPLE: SAFETY FACTOR: 2
4 tracer lines
150 PSIG 10” diameter uninsulated pipe
200 feet between traps
50 °F ∆T

L = Length of product pipe between tracer line traps (ft) 200
U = Heat transfer factor (Btu/sq ft/°F/hr) 3.1
∆T = Temperature differential in °F 50
E = 1 minus efficiency of insulation 1-0
S = Lineal feet of pipe line per sq ft of surface .355
H = Latent heat of steam in btu/lb 857

        200 x 3.1 x 50 x (1-0)
Q = ———————               = 1019 lbs/hr ÷ 4 tracer lines = 255 lbs/hr
              .355 x 857

REMEMBER: Because you have 4 tracing lines you need to divide total condensate by 4 in order to calculate load per line/trap

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Properties of Saturated Steam

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Heat in BTU/lb

Gauge Pressure PSIG Temp °F Sensible Latent Total Specific Volume Cu. Ft./lb
25 134 102 1017 1119 142
20 162 129 1001 1130 73.9
15 179 147 990 1137 51.3
10 192 160 982 1142 39.4
5 203 171 976 1147 31.8
0 212 180 970 1150 26.8
1 215 183 968 1151 25.2
2 219 187 966 1153 23.5
3 222 190 964 1154 22.3
4 224 192 962 1154 21.4
5 227 195 960 1155 20.1
6 230 198 959 1157 19.4
7 232 200 957 1157 18.7
8 233 201 956 1157 18.4
9 237 205 954 1159 17.1
10 239 207 953 1160 16.5
12 244 212 949 1161 15.3
14 248 216 947 1163 14.3
16 252 220 944 1164 13.4
18 256 224 941 1165 12.6
20 259 227 939 1166 11.9
22 262 230 937 1167 11.3
24 265 233 934 1167 10.8
26 268 236 933 1169 10.3
28 271 239 930 1169 9.85
30 274 343 929 1272 9.46
32 277 346 927 1273 9.10
34 279 248 925 1173 8.75
36 282 251 923 1174 8.42
38 284 253 922 1175 8.08
40 286 256 920 1176 7.82
42 289 258 918 1176 7.57
44 291 260 917 1177 7.31
46 293 262 915 1177 7.14
48 295 264 914 1178 6.94
50 298 267 912 1179 6.68
55 300 271 909 1180 6.27
60 307 277 903 1180 5.84
65 312 282 901 1183 5.49
70 316 286 898 1184 5.18
75 320 290 895 1185 4.91
80 324 294 891 1185 4.67
85 328 298 889 1187 4.44
90 331 302 886 1188 4.24
95 335 305 883 1188 4.05
100 338 309 880 1189 3.89
105 341 312 878 1190 3.74
110 344 316 875 1191 3.59
115 347 319 873 1192 3.46
120 350 322 871 1193 3.34
125 353 325 868 1193 3.23
130 356 328 866 1194 3.12
140 361 333 861 1194 2.92
145 363 336 859 1195 2.98
150 366 339 857 1196 2.74
155 368 341 855 1196 2.68
160 371 344 853 1197 2.60
165 373 346 851 1197 2.54
170 375 348 849 1197 2.47
175 377 351 847 1198 2.41
180 380 353 845 1198 2.34
185 382 355 843 1198 2.29
190 384 358 841 1199 2.29
195 386 360 839 1199 2.19
200 388 362 837 1199 2.14
205 390 364 836 1200 2.09
210 392 366 834 1200 2.05
215 394 368 832 1200 2.00
220 396 370 830 1200 1.96
225 397 372 828 1200 1.92
230 399 374 827 1201 1.89
235 401 376 825 1201 1.85
240 403 378 823 1201 1.81
245 404 380 822 1202 1.78
250 406 382 820 1202 1.75
255 408 383 819 1202 1.72
260 409 385 817 1202 1.69
265 411 387 815 1202 1.66
270 413 389 814 1203 1.63
275 414 391 812 1203 1.60
280 416 392 811 1203 1.57
285 417 394 809 1203 1.55
290 418 395 808 1203 1.53
295 420 397 806 1203 1.49
300 421 398 805 1203 1.47
305 423 400 803 1203 1.45
310 425 402 802 1204 1.43
315 426 404 800 1204 1.41
320 427 405 799 1204 1.38
325 429 407 797 1204 1.36
330 430 408 796 1204 1.34
335 432 410 794 1204 1.33
340 433 411 793 1204 1.31
345 434 413 791 1204 1.29
350 435 414 790 1204 1.28
355 437 416 789 1205 1.26
360 438 417 788 1205 1.24
365 440 419 786 1205 1.22
370 441 420 785 1205 1.20
375 442 421 784 1205 1.19
380 443 422 783 1205 1.18
385 445 424 781 1205 1.16
390 446 425 780 1205 1.14
395 447 427 778 1205 1.13
400 448 428 777 1205 1.12
450 460 439 766 1205 1.00
500 470 453 751 1204 .89
550 479 464 740 1204 .82
600 489 475 728 1203 .74

Red denotes vacuum