Allowance design

The pinch design method developed earlier followed several rules and guidelines to allow design for minimum utility (or maximum energy recovery) in the minimum number of units. Occasionally, it appears not to be possible to create the appropriate matches because one or other of the design criteria cannot be satisfied. 

Modularity. Modularity requires that designs be divided into physically and functionally distinct units to faciUtate removal and replacement. Modularity allows design of components as removable and replaceable units for minimum downtime. 

Porous bron2e and iron, a variety of plastics, carbon—graphite, wood, and mbber are widely used in dry sliding or under conditions of sparse lubrication. These materials have commonly allowed design simplifications, freedom from regular maintenance, reduced sensitivity to contamination, and good performance at low speeds and with intermittent lubrication. Although these materials are often used dry or with sparse lubrication, performance normally improves the closer the approach to full-film lubrication. 

Impact testing is not required if the design temperature is helow —29 C (—20 F) hut at or above —46 C (—50 F) and the maximum operating pressure of the fabricated or assembled components will not exceed 25 percent of the maximum allowable design pressure at ambient temperature and the combined longitudinal stress due to pressure, deadweight, and displacement strain (see Par. 319.2.1) does not exceed 41 MPa (6000 Ibfiin ). 

There are obvious benefits to be derived from consensus standards which define the chemistry and properties of specific materials. Such standards allow designers and users of materi s to work with confidence that the materials supplied will have the expected minimum properties. Designers and users can also be confident that comparable materials can be purchased from several suppliers. Producers are confident that materials produced to an accepted standard will find a ready market and therefore can be produced efficiently in large factories. 

Don t allow designers to change approved designs without prior approval. 

Very low velocities U ill allow particles to drift through the mesh and be carried out with the lea dng vapor. Also, very high velocities will carry liquid to the top of the mesh, establish a flooding condition, and then reentrain the lit]uid from the surface of the mesh. For most situations very good performance can be expected for all velocities from 30% to 100% of the optimum allowable design velocity. The minimum allowable safe design velocity is 10 percent of the value calculated by the equation. The flooding velocity of the mesh is usually about 120 percent to 140 percent of the maximum allowable velocity. 

This is 97% of maximum allowable design, too high. Second Try ... 

Ref. [27] presents a thorough discussion of limits to structural components strengths, and these should be observed. Ductile design practices should be used. The maximum allowable design stress should not exceed 25% of the ultimate strength. The strength of the enclosure should exceed the vent relief pressure by at least 0.35 psi. 

Corrosion allowance = Design life x Expected annual corrosion rate... 

Comprehensive application data should be made available for all marketed packaged units, to allow designers or sales engineers to make the correct selections for their purposes. However, it should be borne in mind that manufacturers or sales outlets are frequently not aware of all the parameters of an installation, and the interpretation of catalogue data has many pitfalls. 

With new plastics and processing techniques always becoming available, the design challenge becomes easier, even when taking today s solid-waste problem into account. Today s plastics and processes allow designers to incorporate and interrelate all the aspects of success. In products such as electronics, medical devices, transportation controls, and many others where user-friendly design is required, it has to be obvious to all that plastics play an important role. 

For simplicity, the condition considers the conservative case where the pipe acts simply as a support. The normal practice is to solve all these equations simultaneously, then determine the minimum wall thickness that has strains equal to or less than the allowable design strain. Thus, the minimum structural wall thickness is dictated by the longitudinal tensile load. 

Future computer aids will allow design and control engineers to examine many more alternatives much more thoroughly and thus produce better solutions to problems within the known technology. 

There are obvious benefits to be derived from consensus standards that define the chemistry and properties of specific materials. Such standards allow designers and users of materials to work with confidence that the materials supplied will have the expected minimum... 

The strength of metals decreases with increasing temperature (see Chapter 7) so the maximum allowable design stress will depend on the material temperature. The design temperature at which the design stress is evaluated should be taken as the maximum working temperature of the material, with due allowance for any uncertainty involved in predicting vessel wall temperatures. 

In the British Standard, PD 5500, the nominal design strengths (allowable design stresses), for use with the design methods given, are listed in the standard, for the range... 

The maximum compressive stress in a vessel wall should not exceed that given by equation 13.74 or the maximum allowable design stress for the material, whichever is the least. 

For exchangers with fixed tube-plates the longitudinal stresses in the tubes and shell must be checked to ensure that the maximum allowable design stresses for the materials are not exceeded. Methods for calculating these stresses are given in the standards. 

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