2000 Hook-up Book

Sizing Steam Lines On Velocity

Formula For Velocity Of Steam In Pipes

The appropriate size of pipe to carry the required amount of steam at the local pressure must be chosen, since an undersized pipe means high pressure drops and velocities, noise and erosion, while a generously sized pipe is unnecessarily expensive to install and heat losses from it will also be greater than they need be. Steam pipes may be sized either so that the pressure drop along them is below an accept able limit, or so that velocities along them are not too high. It is convenient and quick to size short mains and branches on velocity, but longer runs of pipe should also be checked to see that pressure drops are not too high. Steam Line Velocities In saturated steam lines, reason able maximum for velocities are often taken at 80/120 ft. per sec ond or 4800/7200 fpm. In the past, many process plants have used higher velocities up to 200 ft. per second or 12,000 fpm, on the basis that the increased pipe noise is not a problem within a process plant. This ignores the other problems which accompany high velocities, and especially the erosion of the pipework and fit tings by water droplets moving at high speed. Only where apprecia ble superheat is present, with the pipes carrying only a dry gas, should the velocities mentioned be exceeded. Velocity of saturat ed steam in any pipe may be obtained from either Table 1, Fig. 1 or calculated in ft. per minute using the formula:

Line velocity is also important in discharge piping from steam traps where two-phase steam/ condensate mixtures must be slowed to allow some gravity sep aration and reduce carryover of condensate from flash vent lines. Here line velocities of the flash steam should not exceed 50/66 ft. per second. A much lower veloci ty must be provided for separation inside the flash vessel by expanding its size. The flash load is the total released by hot condensate from all traps drain ing into the receiver. For condensate line sizing example, see page 46 and see page 43 for vent line sizing example.

SYSTEM DESIGN

V = 2.4Q Vs A Where: V - Velocity in feet per minute Q - Flow lbs./hr. steam Vs - Sp. Vol. in cu. ft./lb. at the flowing pressure A - Internal area of the pipe— sq. in. Steam Piping For PRV’s and Flash Vents Velocity in piping other than steam distribution lines must be correctly chosen, including pres sure reducing valve and flash steam vent applications. A look at Steam Properties (Table 3) illustrates how the spe cific volume of steam increases as pressure is reduced. To keep reducing valve high and low pres sure pipe velocity constant, the downstream piping cross-sec tional area must be larger by the same ratio as the change in vol ume. When downstream pipe size is not increased, low pres sure steam velocity increases proportionally. For best PRV operation, without excessive noise, long straight pipe runs must be provided on both sides, with piping reduced to the valve then expanded downstream gradually to limit approach and exit steam velocities to 4000/ 6000 fpm. A sizing example is given in Fig. 1.

3