Page 69 - Applied Process Design for Chemical and Petrochemical Plants Volume I

56 Applied Process Design for Chemical and Petrochemical Plants

2. Flow rate and length known, determine pressure Important Pressure Level References
drop and line size.

Usually either of these conditions requires a trial Figure 2-1 presents a diagrammatic analysis of the
approach based upon assumed pipe sizes to meet the stat- important relationships between absolute pressure, gauge
ed conditions. Some design problems may require deter- pressures, and vacuum. These are key to the proper solu-
mination of maximum flow for a fixed line size and tion of fluid flow, fluid pumping, and compression prob-
length: however, this just becomes the reverse of the con- lems. Most formulas use absolute pressures in calcula-
ditions above. tions; however, there are a few isolated situations where
Optimum economic line size is seldom realized in the gage pressures are used. Care must be exercised in fol-
average process plant. Unknown factors such as future lowing the proper terminology as well as in interpreting
flow rate allowances, actual pressure drops through cer- the meaning of data and results.
tain process equipment, etc., can easily over-balance any
design predicated on selecting the optimum. Certain
guides as to order of magnitude of costs and sizes can be Pipe, Fittings, and Valves
established either by one of several correlations or by con-
ventional cost estimating methods. The latter is usually
more realistic for a given set of conditions, since general- To ensure proper understanding of terminology, a
ized equations often do not fit a plant system. brief discussion of the “piping” components of most
There are many computer programs for sizing fluid process systems is appropriate.
flow through pipe lines. An example can be found in Ref- The fluids considered in this chapter consist primarily
erence [32]. However, before “blindly” jumping to use of liquids, vapors, gases, and slurries. These are transport-
such programs, the designer should examine the bases ed usually under pressure through circular ducts, tubes, or
and sources of such programs. Otherwise, significant pipes (except for low pressure air), and these lengths of
errors could destroy the validity of the program for its pipe are connected by fittings (screwed or threaded, butt
intended purpose. welded, socket welded, or flanged) and the flow is con-
trolled (stopped, started, or throttled) by means of valves
Factors of “Safety” for Design Basis
fixed in these line systems. The components of these syp
tems will be briefly identified in this chapter, because the
Unless noted otherwise the methods suggested here do
not contain any built-in safety factors. These should be calculation methods presented are for flows through these
components in a system. These flows always create some
included, but only to the extentjustified by the problem at degree of pressure drop (or loss of pressure head), which
hand. Although most designers place this factor on the then dictates the power required to move the fluids
flow rate, care must be given in analyzing the actual con- through the piping components (Figure 2-2).
ditions at operating rates below this value. In some situa-
tions a large factor imposed at this point may lead to unac-
ceptable conditions causing erroneous decisions and Pipe
serious effects on the sizing of automatic control valves
internal trim.
As a general guide, factors of safety of 20 percent to 30 Process plants use round pipe of varying diameters
percent on the friction factor will accommodate the (see pipe dimensions in Tables A-14, A-15, and A-16 in
change in roughness conditions for steel pipe with aver- Appendix). Connections for smaller pipe below about 1%
age service of 5 to 10 years, but will not necessarily com- in. to 2 in. (Figures 244, 2-4B) are threaded or socket
pensate for severe corrosive conditions. Corrosion condi- welded, while nominal pipe sizes 2 in. and larger are gen-
tions should dictate the selection of the materials of erally butt-welded or socket welded (Figure 2-4C) with
construction for the system as a part of establishing the valves and other connections flanged into the line.
design criteria. Beyond this the condition often remains Steam power plants are a notable exception. This chapter,
static, but could deteriorate further. This still does not however, does not deal with power plant design, although
allow for increased pressure drop due to increased flow steam lines are included in the sizing techniques. Pipe is
rates. Such factors are about 10 percent to 20 percent generally designated by nominal size, whereas calcula-
additional. Therefore for many applications the conserv- tions for flow considerations must use the actual standard
ative Cameron Tables [4] give good direct-reading results inside diameter (I.D.) of the pipe. For example: (Note:
for long-term service. See Table 2-22. O.D. refers to outside diameter of pipe.)