|
| 215. | 919 | 1493 | 181.58 | 363.2 |
|____________|_____________|____________|____________|_____________|

Elbows, globe valves and a square-ended entrance to pipes all offer
resistance to the passage of steam. It is customary to measure the
resistance offered by such construction in terms of the diameter of the
pipe. Many formulae have been advanced for computing the length of pipe
in diameters equivalent to such fittings or valves which offer
resistance. These formulae, however vary widely and for ordinary
purposes it will be sufficiently accurate to allow for resistance at the
entrance of a pipe a length equal to 60 times the diameter; for a right
angle elbow, a length equal to 40 diameters, and for a globe valve a
length equal to 60 diameters.

The flow of steam of a higher toward a lower pressure increases as the
difference in pressure increases to a point where the external pressure
becomes 58 per cent of the absolute initial pressure. Below this point
the flow is neither increased nor decreased by a reduction of the
external pressure, even to the extent of a perfect vacuum. The lowest
pressure for which this statement holds when steam is discharged into
the atmosphere is 25.37 pounds. For any pressure below this figure, the
atmospheric pressure, 14.7 pounds, is greater than 58 per cent of the
initial pressure. Table 68, by D. K. Clark, gives the velocity of
outflow at constant density, the actual velocity of outflow expanded
(the atmospheric pressure being taken as 14.7 pounds absolute, and the
ratio of expansion in the nozzle being 1.624), and the corresponding
discharge per square inch of orifice per minute.

Napier deduced an approximate formula for the outflow of steam into the
atmosphere which checks closely with the figures just given. This
formula is:

pa
W = ---- (49)
70

Where W = the pounds of steam flowing per second,
p = the absolute pressure in pounds per square inch,
and a = the area of the orifice in square inches.

In some experiments made by Professor C. H. Peabody, in the flow of
steam through pipes from 1/4 inch to 1-1/2 inches long and 1/4 inch in
diameter, with rounded entrances, the greatest difference from Napier's
formula was 3.2 per cent excess of the experimental over the calculated
results.

For steam flowing through an orifice from a higher to a lower pressure
where the lower pressure is greater than 58 per cent of t