Page 84 - Applied Process Design for Chemical and Petrochemical Plants Volume I

Fluid Flow 71

Note that the E/D factor from Figure 2-11 is used uid and cavitation with erosion will occur. Then the cal-
directly in Figure 2-3. As an example that is only applica- culated flow rates or pressure or pressure drops are not
ble in the range of the charts used, a 10% increase in E/D accurate or reliable.
:o account for increased roughness, yields from Figure 2-
3, an f of only 1.2% greater than a commercial condition
pipe. Generally the accuracy of reading the charts does Pressure Drop in Fittings, Valves, Connections:
not account for large fluctuations in f values. Of course, f, Incompressible Fluid
can be calculated as discussed earlier, and a more precise
number can be achieved, but this may not mean a signifi- The resistance to flow through the various “piping”
cantly greater accuracy of the calculated pressure drop. components that make up the system (except vessels,
Generally, for industrial process design, experience tanks, pumps-items which do not necessarily provide
should be used where available in adjusting the roughness frictional resistance to flow) such as valves, fittings, and
and effects on the friction factor. Some designers increase connections into or out of equipment (not the loss
the friction factor by 10% to 15% over standard commer- through the equipment) are established by test and pre-
cial pipe values. sented in the published literature, but do vary depending
on the investigator.
Pressure Drop in Straight Pipe: Incompressible Fluid
Resistance to fluid flow through pipe and piping
The fictional resistance or pressure drop due to the components is brought about by (1) pipe component
flow of the fluid, he, is expressed by the Darcy equation: internal surface roughness along with the density and
viscosity of the flowing fluid, (2) directional changes in
the system through the piping components, (3)
*
h, =- a’’ ft of fluid, resistance obstructions in the path to flow, and (4) changes in sys-
,
(2g)
tem component cross-section and shape, whether grad-
pfv ZL ual or sudden.
or, 6P = ~- , resistance loss, lbs / sq in. (2-1)
3443 (2g)
Note: these values for hf and AP are differentials from hf = K (v2/2g), ft of the fluid flowing (2-23)
point (1) upstream to point (2) downstream, separated by
a length, L. These are not absolute pressures, and cannot be Velocity and Velocity Head
meaningfully converted to such units. Feet of fluid, h, can
be converted to pounds per square inch by: The average or mean velocity is determined by the flow
rate divided by the cross section area for flow in feet per
second, v. The velocity in a pipe is related to the decrease
h, =-- - ft, for any fluid (2- 20) in static head due to the velocio only by:
P
h, = h, = $/2g, termed velocity head, ft (2-24)
Referenced to water, convert psi to feet of water:

Note the static reduction (loss) due to fluid flowing
[(I lb/sq in.)] (144)
h, (ft)=- = 2.31 ft (2- 21) through a system component (valve, fitting, etc.) is
62.3 Ib/cu ft
expressed in terms of velocity head, using the resis-
tance coefficient, K, in the equation above. This K rep-
For conversion, 1 psi h 2.31 ft of water head resents the number of velocity heads lost due to flow
This represents a column of water at 6OoF, 2.31 feet through the respective system component. It is always
high. The bottom pressure is one pound per square inch associated with diameter for flow, hence, velocity
(psi) on a gauge. The pressure att the bottom as psi will through the component. Actually, for most system com-
vary with the density of the fluid. For fluids other than ponents, the static losses due to pipe friction due to
watler, the relationship is: internal roughness and the actual length of flow path
are minor when compared to one or more of the other
I psi = Z.Sl/(Sp (Gr rel. to water), ft fluid (2-22) losses listed in the previous paragraph [3]. The resis-
tance coefficient, K, is considered independent of fric-
With extreme velocities of liquid in a pipe, the down- tion factor or Reynolds number and is treated as a con-
stream pressure m,ay fall to the vapor pressure of the liq- stant for any component obstruction (valve or fitting)