Page 115 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Fluid Flow 101

Flow a piping or process system, there may be (1) adiabatic
Orifice Gate Control Gate
+ heat into or from the pipe. This is expressed by:
Plate Volve Valve Valve flow where for practical purposes there is no exchange of

P’ V,k = constant (adiabatic) (2-73)

Crude Product Storage or, (2) isothermal flow, which is flow at constant tempera-
Tank at Atmospheric ture (often close to practical experience) and:
Pressure

igure 2-29. Liquid flow system, Example 2-7. P’ V, = constant (isothermal) (2-74)

9. Total Pressure Drop Often for a large variety of process gases, some relation-
ship in between expresses the pressure-volume relation-
The control valve must be sized to take the residual ship by:
pressure drop, as long as it is an acceptable minimurn.
Pressure drop accounted for:
P’ V,” = constant (potytropic) (2-75)
Total psi drop = (245 + 73) (1.52/100) + 3 = 10 psi
For gases/vapors Bowing in a pipe system from point 1
Drop required across control valve: with pressure PI and point 2 with pressure P2, the PI - P2
is the pressure drop, AP, between the points [3].
Reactor = 350 pig
§torage = 0 psig Velocity of Compressible Fluids e
Differential = 350 psi
AP = 10 psi (sys. friction)
Control Valve AP = 340 psi
(2-76)
Note that this control valve loss exceeds 60 per-
cent of this system loss, since the valve must take the
-
difference. For other systems where this is not the sit- where v, = mean velocity in pipe, at conditions stated for V,
uation, the system loss must be so adjusted as to ft/min.
assign a value (see earlier section on control valves)
of approximately 10 to 20 psi or 25 to 60 percent of Mi = flow rate, lbs/hr
the system other than friction losses through the v = fluid specific volume, cu ft/lb, at T and P
valve. For very Bow pressure systems, this minimum
value of control valve drop may be lowered at the sac- d = inside pipe diameter, in.
rifice of sensitive control. p = fluid density, lbs/cu ft, at T and P

P‘ = pressure, pounds per 59 foot absolute
Friction Pressure ressible Fluid Flow
k = ratio of specific heats, cp/c,
Vapors and Gases
Note that determining the velocity at the inlet condi-
The flow of compressible fluids such as gas, vapor, tions to a pipe may create significant error when results
steam, etc., is considered in general the same as for liq- are concerned with the outlet conditions, particularly if
uids or non-compressible fluids. Specific semi-empirical the pressure drop is high. Even the average of inlet and
formulas have been developed which fit particular sys- outlet conditions is not sufficiently accurate for some sys-
tems and have been shown to be acceptable within engi- tems; therefore conditions influenced by pressure drop
neering accuracy.
can produce more accurate results when calculations are
Because of the importance of the relationship between prepared for successive sections of the pipe system (long
pressure and volume for gases and vapors as they flow in or high pressure).