Page 83 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Fluid Flow 67

RH = hydraulic radius, ft welded pipe-2 in. and larger
Situations may dictate deviations.
cross - section for fluid flow, sq ft
R = ------------- (2-9) Never use cast iron fittings or pipe in process situations
1-I wetted perimeter for fluid flow, fl
unless there is only gravity pressure head ( or not over 10
psig) or the fluid is nonhazardous. One exception is in
DH= hydraulic diameter, (equivalent diameter), ft some concentrated sulfuric acid applications; however,
extreme caution must be used in the design of the safety
DH= 4 R1-i, ft (2-10) of the system area. Never use in pulsing or shock service.
Never use malleable iron fittings or pipe unless the
d1-1 = hydraulic diameter, (equivalent diameter), in. fluid is nonhazardous and the pressure not greater than
25 psig. Always use a pressure rating at least four times
d1-1 = 48 R, in. (2-11) that of the maximum system pressure. Also, never use cast
iron or malleable iron fittings or valves in pressure pulsat-
4 ( cross - section area for flow), sq in. ing systems or systems subject to physical shock.
d = �---------------- (2-12) Use forged steel fittings for process applications as
H (wetted perimeter for fluid flow), in.
long as the fluid does not create a serious corrosion prob-
For the narrow shapes with width small relative Lo lem. These fittings are attached to steel pipe and/ or each
length, the hydraulic radius is approximately [3]: other by threading, socket welding, or direct welding to
steel pipe. For couplings attached by welding to pipe, Fig-
R11 h 1/2 (width of passage) (2-13) ure 2-4B, use either 2,000 psi or 6,000 psi rating to give
adequate area for welding without distortion, even
For those non-standard or full circular configurations though the process system may be significantly lower
of flow, use d equivalent to actual flow area diameter, and ( even atmospheric). Branch connections are often
D equivalent to 4RH. attached to steel pipe using forged Weldolets® or Thread-
olets® (Figure 2-10).
[ cross - section available for fluid flow, of duct ] ® = Bonney Forge, Allentown, Pa.
d = 4
wetted perimeter of duct
Mean pressure in a gas line [57].
This also applies to circular pipes or ducts and oval and
rectangular ducts not flowing full. The equivalent diame-
ter is used in determining the Reynolds number for these P (mean or average)= 1[ (P 1 + P 2) (2-14)
cases, but does not apply to very narrow or slotted or
annular flow cross-sections.
Minimum size of pipe is sometimes dictated by structural This applies particularly to long flow lines.
considerations, i.e., 1J1-inch Schedule 40 steel pipe is con- The usual economic range for pressure loss due to liq-
sidered the smallest size to span a 15' to 20' pipe rack with- uid flow; (a) Suction piping-� tc 1X psi per 100 equiva-
out intermediate support. lent feet or pipe.
Gravity flow iines are often set at lX inch to 2 inch min- (b) Discharge piping-1 to 5 psi per 100 equivalent
imum, disregarding any smaller calculated size as a poten- feet of pipe.
The Appendix presents useful carbon steel and stain-
tial source of trouble. less steel pipe data.
Overflow pump suction lines are designed for about a
one foot/second velocity, unless a higher velocity is nec- Reynolds Number, R., (Sometimes used NRE)
essary to keep small solids or precipitates in suspension.
Suction line sizes should be larger than discharge sizes. This is the basis for establishing the condition or type
Flooded suction lines to pumps must be designed so of fluid flow in a pipe. Reynolds numbers below 2000 to
that pressure drop in the pipe is safely less than head 2100 are usually considered to define laminar or viscous
available. flow; numbers from 2000 to 3000-4000 to define a transi-
tion region of peculiar Ilow, and numbers above 4000 to
As a general guide, for pipe sizes use: define a state of turbulent flow. Reference to Figure 2-3
threaded pipe--up to and including Bi in. or 2 111. and Figure 2-11 will identify these regions, and the fric-
nominal tion factors associated with them [2].