Page 149 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Fluid Flow 135

Example 2-18: Calculation of Ste
Flashing

There are 79,500 lbs/hr of 450 psig condensate flowing
into a flash tank. The tank is to be held at 250 psig, gen-
erating steam at this pressure. Determine the quantity of
steam produced.

Enthalpy of liquid at 450 psig = 441.1 Btu/lb
Enthalpy of liquid at 250 psig = 381.6 Btu/lb
Latent heat of vaporization at 250 psig = 820.1 Btu/lb
VD .Typical deportion critical velocity
VT. Typical viscous tnndion uitiil rdocity
% flash into steam = 441.1 - 381.6 (100) = 7.25%
820.1
LOG V Steam formed = (0.0725) (79,500) = 5,763 lbs/hr
Condensate formed = 79,500 - 5,763 = 73,737 Ibs/hr
HETEROGENEOUS SLURRY IAl MOMOGENEOUS SLURRY 16)
Sizing Condensate Return Lines
FULL SUSPENSION
Steam condensate lines usually present a two-phase
flow condition, with hot condensate flowing to a lower
pressure through short and long lines. As the flow pro-
FULL MOVEMENT FULL TURBULENCE
gresses down the pipe, the pressure falls and flashing of
condensate into steam takes place continuously. For small
lengths with low pressure drops, and the outlet end being
within a few pounds per square inch of the inlet, the flash
will be such a small percent that the line can often be
BED BUILDING -Vg LAMINAR FLOW - VT sized as an all liquid line. However, caution must be exer-
cised as even 5% flashing can develop an important
Figure 2-48. Critical velocity characteristics depend on whether impact on the pressure drop of the system.
slurry is heterogeneous or homogeneous. By permission, Deramme-
laere, R. #. and Wasp, E. J., ‘‘Fluid How, Slurry Systems and calculation of condensate piping by two-phase flow
Pipelines,” Encyclopedia of Chemical Processing and Design, J. techniques is recommended; however, the tedious work per
McMetta, Ed., M. Dekker, wol. 22, 1985 [2!5]. line can often be reduced by using empirical methods
and charts. Some of the best are proprietary and not avail-
able for publication; however, the Sarco method [42] has
pounds as well as mixtures. Although this presentation is been used and found to be acceptable, provided no line
limited to stearn, the principles apply to other materials.
less than 1?4“ is used regardless of the chart reading.
Under some circumstances, which are too random to
Steam condensate systems often are used to generate
lower pressure steam by flashing to a lower pressure. When properly describe, the Sarco method may give results too
small by possibly a half pipe size. Therefore, latitude is
this occurs, some steam is formed and some condensate recommended in selecting either the flow rates or the
remains, with the relative quantities depending upon the pipe size.
pressure conditions. Figure 2-53 is a typical situation.

Percent incoming condensate flashed to steam: Design Procedure Using Sarco Chart 1421
1. Establish upstream or steam pressure from which
(h, - 1-1, )IO0 condensate is being produced and discharged into a
%flash = ___- (2 - 132)
L” return line through steam traps, or equivalent, psig.
2. Establish the steam condensate load or rate in
lbs/hr flow.
where hi = enthalpy of liquid at higher pressure, Btu/lb 3. Establish the pressure of the condensate return
h, = enthalpy of liquid at lower or flash pressure,
Btu/lb line, psig.
L,, = Latent heat of evaporation of steam at flash pres- 4. The method is based on an allowable 5,000 ft/min
sure, Btu,/lb velocity in the return line (mixture).