Page 151 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Fluid Flow 135

Example 2-18: Calculation of Steam Condensate
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-
"'
z
0 erating steam at this pressure. Determine the quantity of
... steam produced .
�
"'
...
0
Enthalpy of liquid at 150 psig = 441. l Btu/lb
Enthalpy of liquid at 250 psig = 381.6 Btu/lb
Latent heat of vaporization at 250 psig = 820.1 Btu/lb
Vo· Typicol dtpostion criticol vttocity
Vy· Typicol viau1 tnniitian critiul vllocity 44 381 6
% flash into steam = 1.1 - · (100) = 7.25%
R20.1
LOG V Steam formed = (0.0725) (79,500) = 5,763 lbs/hr
Condensate formed== 79,500 - 5,763 = 73,737 lbs/hr
HETEROGENEOUSSLURRY(A) HOMOGENEOUS SLURRY (Bl
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
AT CRITICAL VELOCITY lengths with low pressure drops, and the outlet end being
-<l
µfiit¥'t\¥i#H41\lW -·� .. ,;: 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 - Vo LAMINAR FLOW - Vy 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. H. and Wasp, E. J., "Fluid Flow, 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
McKetta, Ed., M. Dekker, vol. 22, 1985 [25]. 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 steam, the principles apply to other materials.
less than lW 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.

Design Procedure Using Sarco Chart [42]
Percent incoming condensate flashed to steam:
1. Establish upstream or steam pressure from which
1
(h -h )100 condensate is being produced and discharged into a
2
% 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 h 1 = enthalpy of liquid at higher pressure, Btu/lb
h2 = enthalpy of liquid at lower or flash pressure, 3. Establish the pressure of the condensate return
Btu/lb line, psig.
L,. = latent heal 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).