Design life

The design life is distinguished by the AASHTO methodology into analysis period and 

The performance period refers to how long a new pavement structure will last before it needs rehabilitation. The performance period is distinguished into minimum and maximum periods. 

The minimum performance period is the shortest amount of time an initial pavement structure, or stage construction, lasts before some intervention is performed. 

The maximum performance period is the maximum practical amount of time expected from a given pavement structure or stage construction. Theoretically, the maximum performance period should be equal to the analysis period, but in practice, this rarely happens (taking into consideration the effect of environmental factors, surface deterioration, etc.). 

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To the process designer, life-cycle analysis is useful because focusing exclusively on waste minimization at some point in the life cycle sometimes creates problems elsewhere in the cycle. The designer can often obtain useful insights by changing the boundaries of the system under consideration so that they are wider than those of the process being designed. 

The computerised ultrasonic P-scan system (FORCE Institute, Denmark) has been in operation in Ukraine since 1992. Over this period rather extensive new technological experience has been accumulated of solving the complicated tasks of reliability of the constructions the design life of which is over. 

The typical amounts of sodium and vanadium in the fuel should be less than 1 ppm. Figure 29-42 shows the effect of sodium and vanadium on the life of the blade and on the combustor life. Figure 29-43 shows the reduction in firing temperature required to maintain design life (hrs) of a typical turbine (IN718) blade due to sodium and vanadium in the fuel. 

In design against creep, we seek the material and the shape which will carry the design loads, without failure, for the design life at the design temperature. The meaning of failure depends on the application. We distinguish four types of failure, illustrated in Fig. 17.3. 

The material properties of window glass are summarised in Table 18.1. To use these data to calculate a safe design load, we must assign an acceptable failure probability to the window, and decide on its design life. Failure could cause injury, so the window is a critical component we choose a failure probability of 10The vacuum system is designed for intermittent use and is seldom under vacuum for more than 1 hour, so the design life under load is 1000 hours. 

Those seals work w ell within theii" designed life. Theii designed life is about 2,000 hours. An automotive drive shaft spinning at about 1,800 rpni w onld move the ear at approximately 50 miles per hour. 2,000 hours would be ee]uivalent to about 100,000 miles on a ear. 2,000 hours (at 1,800 rpm) on a pump w ould be equivalent to about 83 days at 24/7 operation. Many meehanies have questioned the logie of installing a 3-month seal to proteet a 5-year bearing. 

Figure 4-117. Temperature and stress limits related to design life of Waspaloy buoket material.

The principal economic implications of corrosion of a plant are the initial cost of construction, the cost of maintenance and replacement, and the loss of production through unplanned shutdowns. The initial cost of the plant is influenced by material selection, and a choice of material that is more corrosion resistant than is necessary for the safe operation of the plant over its design life is a very expensive error. This cost involves initial outlay of money, and plants have been built which could never be profitable because of the inappropriate materials selection. 

In order for rolling element bearings to achieve their design life and perform with no abnormal noise, temperature rise, or shaft excursions, the following precautions should be taken ... 

The design life of the plant has to be stated so that corrosion allowances may be calculated ... 

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

In practice the loss for an iron anode is approximately 9 kg/Ay. Thus consumable anodes must be replaced at intervals or be of sufficient size to remain as a current source for the design life of the protected structure. This poses some problems in design because, as the anode dissolves, the resistance it presents to the circuit increases. More important, it is difficult to ensure continuous electrical connection to the dissolving anode. 

The anode material must stay firmly attached to the steel insert, which is necessary to conduct the current from the anode to the structure, throughout its design life to remain effective. Consequently surface preparation (by dry blast cleaning ) of the insert prior to casting, to ensure a sound bond with the anode material, is essential. Voids at the insert/anode material interface are undesirable as these will also affect the bond integrity. 

It is important that the correct current density requirement is assigned for design purposes. If too high a value is used the structure may be waste-fully overprotected, whereas a value too small will mean that the protection system will underprotect and not achieve its design life. 

Obviously, the total weight of the anode material must equal or be greater than the total weight, IF, calculated above. Similarly each anode must be of sufficient size to supply current for the design life of the cathodic protection system. The anodes must also deliver sufficient current to meet the requirements of the structure at the beginning and end of the system life. That is, if current demand increases (as a result of coating breakdown, for example) the output from the anodes should meet the current demands of the structure. 

A check to ensure that the anodes will deliver sufficient current to protect the structure at the end of the design life should be conducted. This entails calculating the expected anode output at the end of its life and checking that it meets the demands of the structure. Generally the output is calculated using a modified resistance based on an anode that is 90% consumed. 

It is evident that a greater number of anodes distributed over the structure will improve current distribution. However, aside from the unacceptable cost incurred by attaching excessive numbers of anodes, an anode must continue to function throughout the life of the structure and must, therefore, be of sufficient size to meet the design life. A very large number of heavy anodes is clearly impracticable and uneconomic. 

Note I Present day economics usually favour a protective scheme adequate for the full design life of the structure. 

There are two criteria to use as the basis for evaluation. The design life of the shelf is determined by deciding what the product will tolerate in deflection and still be useful. This is combined with the cost effectiveness value the product must meet. For example, we can say that if it costs A" the life must be A months, if it costs Y it must last B months, and if it costs Z, it must last C months. This can be presented as a table or it can be graphed as the criteria range that it must meet. 

The various designs and costs can be tabulated and the ones that are the most economical can be determined. At this point, it may become evident that the design life can be long and the cost of increasing the design life small, or, alternatively, it may be that the cost of small increments in the design life are... 

A different design approach is used in this case. Instead of assuming an apparent modulus of elasticity using a constant creep situation covering the life of the chair, it is better to determine the actual creep deflection over a typical stress cycle, the creep recovery over a non-use cycle, and so on until the creep is determined after a series of what might be considered typical hard usage cycles for the chair. The accumulated creep after a period of two weeks can be assumed to represent the base line for an apparent modulus of elasticity to determine the design life of the chair. 

In each case the section is designed to keep the deflection to less than 2 in. in 16 in. for a design life of 5 years and the extreme fiber stress is kept to a value less than the yield strength of the material. The first step in the analysis is to determine the necessary section to resist the bending load using the short-term tensile and compressive strength and modulus values. The extreme fiber stress is calculated for these sections to determine that the chair will not break when deflected. 

A time dependent modulus is then calculated using the extreme fiber stress level for each of the materials at the initial stress value level using the loading-time curve developed. If the deflection at the desired life is excessive, the section is increased in size and the deflection recalculated. By iteration the second can be made such that the creep and load deflection is equal to the maximum allowed at the design life of the chair. This calculation can be programmed for a computer solution. 

The ultimate requirement of any product is that it performs the function for which it is designed. With many materials the design life of the product is usually not as important as it is with plastics because of the behaviors such as creep of plastics (Chapter 2). In all cases the useful life is an important consideration whether the item is a pan for the kitchen or a bridge to handle traffic in a city. 

Determination of the hazard potential and designing to eliminate the hazard is one element of the solution. The use of carefully designed accelerated and continuous prototype testing is another element. The instruction in use and proper maintenance procedure is the third element. By exercising judgment as to the appropriate combination of these elements consistent with the economic factors involved, the designer can have a product that will perform for its projected design life with... 

Motor windings and Insulation systems are specially designed, developed and applied as an Integral part of the pump so that design life is at least as great as for conventional air cooled motors. Winding temperature is primarily influenced by pumped fluid temperature and secondarily by use of cooling jacket. Fluid temperature is considered In pump... 

The efficiency of power generation is significantly reduced by any deposits formed on the turbine blades by BW carryover and severe turbine damage may also result. Tiirbine efficiency also is reduced by demands for output that exceed the rated maximum and by extended operation beyond the maintenance period or design life. Additionally, errors in steam flow meters, thermometers, and pressure gauges, and so forth cause the control system to regulate the generation of electricity at some further reduced level. 

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