Page 30 - Chemical Process Equipment - Selection and Design

1.9. SAFETY OF PLANT AND ENVlFiQNMENT 7
TABLE 1.4. Safety Factors in Equipment Design: Results of a Questionnaire

Range of Safety
Equipment Design Variable Factor (%)
Compressors, reciprocating piston displacement 11-21
Conveyors, screw diameter 8-21
Hammer mills power input 15-21 a
Filters, plate-and-frame area 11-21”
Filters, rotary area 14-20a
Heat exchangers, shell and tube for area 11-18
liquids
Pumps, centrifugal impeller diameter 7-14
Separators, cyclone diameter 7-1 1
Towers, packed diameter 11-18
Towers, tray diameter 10-16
Water cooling towers volume 12-20
aBased on pilot plant tests.
[Michelle, Beattie, and Goodgame, Chem. Eng. Prog. 50,332 (1954)l.
only to a certain accuracy. AT may be uncertain because of possible anticipated ranges of operating conditions. In addition, the design
fluctuations in regulated steam and tower pressures. A, the effective of equipment and plant must minimize potential harm to personnel
area, may be uncertain lbecause the submergence is affected by the and the public in case of accidents, of which the main causes are
liquid level controller at .the bottom of the column. Accordingly,
a. human failure,
- _- +A+- b. failure of equipment or control instruments,
dA d(A.T)
dq-dU
4 AT ’ c. failure of supply of utilities or key process streams,
d. environmental events (wind, water, and so on).
that is, the fractional uncertainty of q is the sum of the fractional
uncertainties of the quantities on which it is dependent. In practical
cases, of course, some uncertainties may be positive and others A more nearly complete list of potential hazards is in Table 1.5, and
negative, so that they may cancel out in part; but the only safe a checklist referring particularly to chemical reactions is in Table
viewpoint is to take the sum of the absolute values. Some further 1.6.
discussion of such cases is by Sherwood and Reed, in Applied Examples of common safe practices are pressure relief valves,
Mathematics in Chemical Engineering (McGraw-Hill, New York, vent systems, flare stacks, snuffing steam and fire water, escape
1939). hatches in explosive areas, dikes around tanks storing hazardous
It is not often that proper estimates can be made of materials, turbine drives as spares for electrical motors in case of
uncertainties of all the parameters that influence the performance or power failure, and others. Safety considerations are paramount in
required size of particular equipment, but sometimes one particular the layout of the plant, particularly isolation of especially hazardous
parameter is dominant All experimental data scatter to some operations and accessibility for corrective action when necessary.
extent, for example, heat transfer coefficients; and various cor- Continual monitoring of equipment and plant is standard
relations of particular phenomena disagree, for example, equations practice in chemical process plants. Equipment deteriorates and
of state of liquids and gases. The sensitivity of equipment sizing to operating conditions may change. Repairs sometimes are made with
uncertainties in such data has been the subject of some published ‘‘improvements” whose ultimate effects on the operation may not
information, of which a review article is by Zudkevich [Encycl. be taken into account. During start-up and shut-down, stream
Chem. Proc. Des. 14, 431-483 (?982)]; some of his cases are: compositions and operating conditions are much different from
those under normal operation, and their possible effect on safety
1. Sizing of isopentane/]pentane and propylene/propane splitters. must be taken into account. Sample checklists of safety questions
2. Effect of volumetric properties on sizing of an ethylene for these periods are in Table 1.7.
compressor. Because of the importance of safety and its complexity, safety
3. Effect of liquid density on metering of LNG. engineering is a speciality in itself. In chemical processing plants of
4. Effect of vaporization equilibrium ratios, K, and enthalpies on any significant size, loss prevention reviews are held periodically by
cryogenic separations. groups that always include a representative of the safety depart-
5. EEects of VLE and enthalpy data on design of plants for ment. Other personnel, as needed by the particular situation, are
coal-derived liquids. from manufacturing, maintenance, technical service, and possibly
research, engineering, and medical groups. The review considers
Examination of such studies may lead to the conclusion that some any changes made since the last review in equipment, repairs,
of the safety factors of Table 1.4 may be optimistic. But long feedstocks and products, and operating conditions.
experience in certain areas does suggest to what extent various Detailed safety checklists appear in books by Fawcett and
uncertainties do cancel cut, and overall uncertainties often do fall in Wood (Chap. 32, Bibliography 1.1, Part E) and Wells (pp.
the range of 10-20% as stated there. Still, in major cases the 239-257, Bibliography 1.1, Part E). These books and the large one
uncertainty analysis should be made whenever possible. by Lees (Bibliography 1.1, Part E) also provide entry into the vast
literature of chemical process plant safety. Lees has particularly
1.91. SAFETY OF PUNT AND ENVIRONMENT complete bibliographies. A standard reference on the properties of
dangerous materials is the book by Sax (1984) (References, Section
The safe practices described in the previous section are primarily for 1.1, Part E). The handbook by Lund (1971) (References, Section
assurance that the equipment have adequate performance over 1.1, Part E) on industrial pollution control also may be consulted.