Page 157 - Chemical Process Equipment - Selection and Design

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ID TRANSPORT EQUIPMENT

lthough liquids particularly can be transported by gleaned from manufacturers’ catalogs. Special problems such
operatlgrs carrying buckets, the usual mode of as mechanical flexibility of piping at elevated temperatures
transport of fhids is through pipelines with pumps, are beyond the scope here, and special problems associated
blowers, compressors, or ejectors. Those categories with sizing of piping for thermosyphon reboilers and the
of equipment will be considered in this chapter. A few suction side of pumps for handling volatile liquids are deferred
statements wi/l be made at the start about piping, fittings, and to elsewhere in this book.
valves, although for the most part this is information best

pass plug cocks, butterfly valves, slide valves, check valves, various
quick-opening arrangements, etc. have limited and often indispens-
Standard pipe is made in a discrete number of sizes that are able applications, but will not be described here.
designated by nominal diameters in inches, as “inches IPS (iron The spring in the relief valve of Figure 7.1(c) is adjusted to
pipe size).” Table A5 lists some of these sizes with dimensions in open when the pressure in the line exceeds a certain value, at which
inches. Depending on the size, up to 14 different wall thicknesses time the plug is raised and overpressure is relieved; the design
are made With the same outside diameter. They are identified by shown is suitable for pressures of several hundred psig.
schedule numbers, of which the most common is Schedule 40. More than 100 manufacturers in the United States make valves
Approximately,
that may differ substantially from each other even for the same line
size and pressure rating. There are, however, independent
Schedule number = 1000 PIS,
publications that list essentially equivalent valves of the several
manufacturers, for example the books of Zappe (1981) and Lyons
where
(1975).
P = internal pressure, psig
S = allowable working stress in psi. CONTROL VALVES
Control valves have orifices that can be adjusted to regulate the
Tubing for hecat exchangers, refrigeration, and general service is flow of fluids through them. Four features important to their use are
made with odside diameters measured in increments of 1/16 or capacity, characteristic, rangeability and recovery.
1/$ in. Standard size pipe is made of various metals, ceramics, glass, Capacity is represented by a coefficient
and plastics.
Dimensional standards, materials of construction, and pressure Cd = C,/d2,
ratings of pipiing for chemical plants and petroleum refineries are
covered by ANSI Piping Code B31.3 which is published by the where d is the diameter of the valve and C, is the orifice coefficient
ASME, latest issue 1980. Many details also are given in such in equations such as the following
sources as C~ocker and King, Piping Handbook (McGraw-Hill,
New York, 19\67)> Perxy’s Chemical Engineers Handbook (1984), Q = C,d(P, - P2)/p,, gallmin of liquid,
and Marks Standard Handbook for Mechanical Engineers (1987). Q = 22.7C,,l/(P1 - P2)P2/p,T, SCFM of gas when PJP1 > 0.5,
In sizes 2in. and less screwed fittings may be used. Larger
joints commonly are welded. Connections to equipment and in lines Q = 11.3CvPl/~T, SCFM of gas when PJP, <0.5,
whenever need for disassembly is anticipated utilize flanges. Steel
flanges, flanged fittings, and valves are made in pressure ratings of where Pi is pressure in psi, p, is specific gravity relative to water, pa
150, 300, 600, 900, 1500, and 2500 psig. Valves also are made in 125 is specific gravity relative to air, and Tis temperature “R. Values of
and 25Opsig cast iron. Pressure and temperature ratings of this cd of commercial valves range from 12 for double-seated globe
equipment in various materials of construction are specified in the valves to 32 for open butterflies, and vary somewhat from
piping code, and are shown in Chem. Eng. Handbook 1984, pp. manufacturer to manufacturer. Chalfin (1980) has a list.
6.75-6.78. Characteristic is the relation between the valve opening and the
flow rate. Figure 7.1(h) represents the three most common forms.
VALVES The shapes of plugs and ports can be designed to obtain any desired
mathematical relation between the pressure on the diaphragm, the
Control of flow in lines and provision for isolation of equipment travel of the valve stem, and the rate of flow through the port.
when needed are accomplished with valves. The basic types are Linear behavior is represented mathematically by Q = kx and equal
relatively few, some of which are illustrated in Figure 7.1. In gate percentage by Q = k, exp(k,x), where x is the valve opening.
valves the flolw is straight through and is regulated by raising or Quick-opening is a characteristic of a bevel-seated or Bat disk type
lowering the gate. The majority of valves in the plant are of this of plug; over a limited range of 10-25% of the maximum stem
type. In the wide open position they cause little pressure drop. In travel is approximately linear.
globe valves the flow changes direction and results in appreci- Over a threefold load change, the performances of linear and
able friction even in the wide open position. This kind of valve is equal percentage valves are almost identical. When the pressure
essential when, tight shutoff is needed, particularly of gas flow. Multi- drop across the valve is less than 25% of the system drop, the equal

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