Steam, Its Generation and Use Part 44

[Ill.u.s.tration: Bankers Trust Building, New York City, Operation 900 Horse Power of Babc.o.c.k & Wilc.o.x Boilers]

FLOW OF STEAM THROUGH PIPES AND ORIFICES

Various formulae for the flow of steam through pipes have been advanced, all having their basis upon Bernoulli's theorem of the flow of water through circular pipes with the proper modifications made for the variation in constants between steam and water. The loss of energy due to friction in a pipe is given by Unwin (based upon Weisbach) as

f 2 v W L E_{f} = ---------- (37) gd

where E is the energy loss in foot pounds due to the friction of W units of weight of steam pa.s.sing with a velocity of v feet per second through a pipe d feet in diameter and L feet long; g represents the acceleration due to gravity (32.2) and f the coefficient of friction.

Numerous values have been given for this coefficient of friction, f, which, from experiment, apparently varies with both the diameter of pipe and the velocity of the pa.s.sing steam. There is no authentic data on the rate of this variation with velocity and, as in all experiments, the effect of change of velocity has seemed less than the unavoidable errors of observation, the coefficient is a.s.sumed to vary only with the size of the pipe.

Unwin established a relation for this coefficient for steam at a velocity of 100 feet per second,

/ 3 f = K| 1 + --- | (38) 10d /

where K is a constant experimentally determined, and d the internal diameter of the pipe in feet.

If h represents the loss of head in feet, then

f 2 v W L E_{f} = Wh = ---------- (39) gd

f 2 v L and h = -------- (40) gd

If D represents the density of the steam or weight per cubic foot, and p the loss of pressure due to friction in pounds per square inch, then

hD p = --- (41) 144

and from equations (38), (40) and (41),

D v L / 3 p = -------- K | 1 + --- | (42) 72 g d 10d /

To convert the velocity term and to reduce to units ordinarily used, let d_{1} the diameter of pipe in inches = 12d, and w = the flow in pounds per minute; then

[pi] / d_{1} w = 60v --- | ---- |^{2} D 4 12 /

9.6 w and v = -------------- [pi] d_{1}^2 D

Subst.i.tuting this value and that of d in formula (42)

/ 3.6 w^{2} L p = 0.04839 K | 1 + ----- | ----------- (43) d_{1} / D d_{1}^{5}

Some of the experimental determinations for the value of K are: K = .005 for water (Unwin).

K = .005 for air (Arson).

K = .0028 for air (St. Gothard tunnel experiments).

K = .0026 for steam (Carpenter at Oriskany).

K = .0027 for steam (G. H. Babc.o.c.k).

The value .0027 is apparently the most nearly correct, and subst.i.tuting in formula (43) gives,

/ 3.6 w^{2} L p = 0.000131 | 1 + ---- | ----------- (44) d_{1}/ D d_{1}^{5}

/ pDd_{1}^{5} w = 87 | -------------- |^{} (45) | / 3.6 | | | 1 + ---- | L | d_{1}/ /

Where w = the weight of steam pa.s.sing in pounds per minute, p = the difference in pressure between the two ends of the pipe in pounds per square inch, D = density of steam or weight per cubic foot,[80]

d_{1} = internal diameter of pipe in inches, L = length of pipe in feet.

TABLE 66

FLOW OF STEAM THROUGH PIPES +---------------------------------------------------------------------------------------+ |Initl|Diameter[81] of Pipe in Inches, Length of Pipe = 240 Diameters | |Gauge|---------------------------------------------------------------------------------+ |Press| | 1 | 1 | 2 | 2 | 3 | 4 | 5 | 6 | 8 | 10 | 12 | 15 | 18 | |Pound|---------------------------------------------------------------------------------+ |/SqIn| Weight of Steam per Minute, in Pounds, With One Pound Loss of Pressure | +-----+---------------------------------------------------------------------------------+ | 1 |1.16|2.07| 5.7|10.27|15.45|25.38| 46.85| 77.3|115.9|211.4| 341.1| 502.4| 804|1177| | 10 |1.44|2.57| 7.1|12.72|19.15|31.45| 58.05| 95.8|143.6|262.0| 422.7| 622.5| 996|1458| | 20 |1.70|3.02| 8.3|14.94|22.49|36.94| 68.20|112.6|168.7|307.8| 496.5| 731.3|1170|1713| | 30 |1.91|3.40| 9.4|16.84|25.35|41.63| 76.84|126.9|190.1|346.8| 559.5| 824.1|1318|1930| | 40 |2.10|3.74|10.3|18.51|27.87|45.77| 84.49|139.5|209.0|381.3| 615.3| 906.0|1450|2122| | 50 |2.27|4.04|11.2|20.01|30.13|49.48| 91.34|150.8|226.0|412.2| 665.0| 979.5|1567|2294| | 60 |2.43|4.32|11.9|21.38|32.19|52.87| 97.60|161.1|241.5|440.5| 710.6|1046.7|1675|2451| | 70 |2.57|4.58|12.6|22.65|34.10|56.00|103.37|170.7|255.8|466.5| 752.7|1108.5|1774|2596| | 80 |2.71|4.82|13.3|23.82|35.87|58.91|108.74|179.5|269.0|490.7| 791.7|1166.1|1866|2731| | 90 |2.83|5.04|13.9|24.92|37.52|61.62|113.74|187.8|281.4|513.3| 828.1|1219.8|1951|2856| | 100 |2.95|5.25|14.5|25.96|39.07|64.18|118.47|195.6|293.1|534.6| 862.6|1270.1|2032|2975| | 120 |3.16|5.63|15.5|27.85|41.93|68.87|127.12|209.9|314.5|573.7| 925.6|1363.3|2181|3193| | 150 |3.45|6.14|17.0|30.37|45.72|75.09|138.61|228.8|343.0|625.5|1009.2|1486.5|2378|3481| +---------------------------------------------------------------------------------------+

This formula is the most generally accepted for the flow of steam in pipes. Table 66 is calculated from this formula and gives the amount of steam pa.s.sing per minute that will flow through straight smooth pipes having a length of 240 diameters from various initial pressures with one pound difference between the initial and final pressures.

To apply this table for other lengths of pipe and pressure losses other than those a.s.sumed, let L = the length and d the diameter of the pipe, both in inches; l, the loss in pounds; Q, the weight under the conditions a.s.sumed in the table, and Q_{1}, the weight for the changed conditions.

For any length of pipe, if the weight of steam pa.s.sing is the same as given in the table, the loss will be,

L l = ---- (46) 240d

If the pipe length is the same as a.s.sumed in the table but the loss is different, the quant.i.ty of steam pa.s.sing per minute will be,

Q_{1} = Ql^{} (47)

For any a.s.sumed pipe length and loss of pressure, the weight will be,

/240dl Q_{1} = Q|-----|^{} (48) L /

TABLE 67

FLOW OF STEAM THROUGH PIPES LENGTH OF PIPE 1000 FEET

+--------------------------------------------------++----------------------------------------+ | Discharge in Pounds per Minute corresponding to || Drop in Pressure in | | Drop in Pressure on Right for Pipe Diameters || Pounds per Square Inch corresponding | | in Inches in Top Line || to Discharge on Left: Densities | | || and corresponding Absolute Pressures | | || per Square Inch in First Two Lines | +--------------------------------------------------++----------------------------------------+ | Diameter[82]--Discharge || Density--Pressure--Drop | +--------------------------------------------------++----------------------------------------+ | 12 | 10 | 8 | 6 | 4 | 3 | 2| 2 | 1| 1 ||.208 |.230|.284|.328|.401|.443|.506|.548| | In | In | In | In | In | In | In | In | In | In || 90 | 100| 125| 150| 180| 200| 230| 250| +--------------------------------------------------++-------+--------------------------------+ |2328|1443| 799| 371|123. |55.9|28.8|18.1|6.81|2.52||18.10|16.4|13.3|11.1|9.39|8.50|7.44|6.87| |2165|1341| 742| 344|114.6|51.9|27.6|16.8|6.52|2.34||15.60|14.1|11.4|9.60|8.09|7.33|6.41|5.92| |1996|1237| 685| 318|106.0|47.9|26.4|15.5|6.24|2.16||13.3 |12.0|9.74|8.18|6.90|6.24|5.47|5.05| |1830|1134| 628| 292| 97.0|43.9|25.2|14.2|5.95|1.98||11.1 |10.0|8.13|6.83|5.76|5.21|4.56|4.21| |1663|1031| 571| 265| 88.2|39.9|24.0|12.9|5.67|1.80|| 9.25|8.36|6.78|5.69|4.80|4.34|3.80|3.51| |1580| 979| 542| 252| 83.8|37.9|22.8|12.3|5.29|1.71|| 8.33|7.53|6.10|5.13|4.32|3.91|3.42|3.16| |1497| 928| 514| 239| 79.4|35.9|21.6|11.6|5.00|1.62|| 7.48|6.76|5.48|4.60|3.88|3.51|3.07|2.84| |1414| 876| 485| 226| 75.0|33.9|20.4|10.9|4.72|1.53|| 6.67|6.03|4.88|4.10|3.46|3.13|2.74|2.53| |1331| 825| 457| 212| 70.6|31.9|19.2|10.3|4.43|1.44|| 5.91|5.35|4.33|3.64|3.07|2.78|2.43|2.24| |1248| 873| 428| 199| 66.2|23.9|18.0|9.68|4.15|1.35|| 5.19|4.69|3.80|3.19|2.69|2.44|2.13|1.97| |1164| 722| 400| 186| 61.7|27.9|16.8|9.03|3.86|1.26|| 4.52|4.09|3.31|2.78|2.34|2.12|1.86|1.72| |1081| 670| 371| 172| 57.3|25.9|15.6|8.38|3.68|1.17|| 3.90|3.53|2.86|2.40|2.02|1.83|1.60|1.48| | 998| 619| 343| 159| 52.9|23.9|14.4|7.74|3.40|1.08|| 3.32|3.00|2.43|2.04|1.72|1.56|1.36|1.26| | 915| 567| 314| 146| 48.5|21.9|13.2|7.10|3.11|0.99|| 2.79|2.52|2.04|1.72|1.45|1.31|1.15|1.06| | 832| 516| 286| 132| 44.1|20.0|12.0|6.45|2.83|0.90|| 2.31|2.09|1.69|1.42|1.20|1.08|.949|.877| | 748| 464| 257| 119| 39.7|18.0|10.8|5.81|2.55|0.81|| 1.87|1.69|1.37|1.15| .97|.878|.769|.710| | 665| 412| 228| 106| 35.3|16.0| 9.6|5.16|2.26|0.72|| 1.47|1.33|1.08|.905|.762|.690|.604|.558| | 582| 361| 200|92.8| 30.9|14.0| 8.4|4.52|1.98|0.63|| 1.13|1.02|.828|.695|.586|.531|.456|.429| +--------------------------------------------------++----------------------------------------+

To get the pressure drop for lengths other than 1000 feet, multiply by lengths in feet 1000.

Example: Find the weight of steam at 100 pounds initial gauge pressure, which will pa.s.s through a 6-inch pipe 720 feet long with a pressure drop of 4 pounds. Under the conditions a.s.sumed in the table, 293.1 pounds would flow per minute; hence, Q = 293.1, and

_ _ | 24064 | Q_{1} = 293.1 | ------- |^{} = 239.9 pounds |_ 72012_|

Table 67 may be frequently found to be of service in problems involving the flow of steam. This table was calculated by Mr. E. C. Sickles for a pipe 1000 feet long from formula (45), except that from the use of a value of the constant K = .0026 instead of .0027, the constant in the formula becomes 87.45 instead of 87.

In using this table, the pressures and densities to be considered, as given at the top of the right-hand portion, are the mean of the initial and final pressures and densities. Its use is as follows: a.s.sume an allowable drop of pressure through a given length of pipe. From the value as found in the right-hand column under the column of mean pressure, as determined by the initial and final pressures, pa.s.s to the left-hand portion of the table along the same line until the quant.i.ty is found corresponding to the flow required. The size of the pipe at the head of this column is that which will carry the required amount of steam with the a.s.sumed pressure drop.