Design assumptions

In concert with selection of design conditions, some design assumptions are needed before design can proceed. Design assumptions encompass the human factor, equipment failures, and emergency operation scenarios. Some examples of design assumptions are  

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As the cost of testing vast quantities of equipment would be too great and take too long, qualification tests, particularly on hardware, are usually performed on a small sample. The test levels are varied to take account of design assumptions, variations in production processes and the operating environment. 

C. F. Braun stated that the data bases were such that design assumptions for the commercial concepts were not all confirmed and capital estimates might be optimistically low. 

A municipal wastewater having a BOD of 250 g/m3 is to be treated by a two-stage trickling filter. The desired effluent quality is 25 g/m3 of BOD. If both of the filter depths, De, are to be 1.83 m and the recirculation ratio, R, is 2 1, find the required filter diameters. Assume the following design assumptions apply. 

In view of its practical importance, it is surprising that there has been relatively little detailed study of this form of transfer, even to the extent of defining the rate of formation of a transfer film, or the effects of such factors as counterface material, hardness and surface finish. The general design assumptions tend to be based on the requirements for transfer from composites, namely a surface roughness of 0.2 fjm C.L.A., and the possible use of a soft plating or a chemical conversion coating on the counterface. 

Having gone through the design process, the Technical Manager will be intimately familiar with the design assumptions and why certain decisions were made. As mentioned before, this continuity of design into the startup phase is very important. 

D. Criteria such as safety, operability, reliability, and flexibility are of vital importance in process design. These features make the design more robust to variations in the design assumptions and operating requirements. A safe, operable, and reliable plant will often require more expense above the cost of the optimal design. This extra expense is difficult to trade off against the non-financial benefits of having a process that is easier to run. 

An important design assumption was that refuse would be collected 5 days a week for 52 weeks a year (260 days a year) but because of maintenance and downtime, refuse could be burned only 230 days a year. It was also assumed that units could only burn refuse at 90 o of their rated design capacity. Three basic cases were considered plants with rated capacities of 18.2, 45.5, and 90.9 metric tons/day, respectively. Table II details the actual daily and annual capacities of the installations. The small unit was evaluated for one- and two-shift per day operation with periodic ash removal whereas the larger plants were assumed to operate three shifts per day for five days a week with continuous ash removal. 

This conceptual design and economic evaluation is based on theoretical considerations only and many of the assumptions used in this design need to be verified in an actual pilot-plant operation. These various design assumptions include (1) MgO/ash separation, (2) HC1 removal and purge, and (3) spray dryer design. 

The initial and perhaps most critical design assumption is that both the MgO and the fly ash, which are carried out of the calciner and into the MgO product cyclone, have the same particle size distribution and density. This design (i.e., no possibility of physical separation of MgO and fly ash) is a conservative design assumption and adds complexity to the FGD process. It results in the need to recirculate large quantities of the MgO/fly ash mixture through the spray dryer and the calciner. In order to keep the MgO/ash recycle streams to a reasonable size, the mechanical collectors in the main flue gas ducts upstream from the SO2 absorber, which were used in the initial design because they are relatively inexpensive but yet remove only 80% of the fly ash, had to be replaced with the 95% efficient ESP mentioned earlier. 

Once again, audits and performance assessments should start from the safety constraints and design assumptions and rationale. The goal should be to determine whether the safety constraints are being enforced in the operation of the system and whether the assumptions underlying the safety design and rationale are still true. Audits and performance assessments provide a chance to detect whether the behavior of the system and the system components still satisfies the safety constraints and whether the way the controllers think the system is working—as reflected in their process models—is accurate. 

The design premise defines the operational envelope for the system (i.e., the allowable limits on various parameters). In general, condition control is focused on identifying aspects of the operation or system that are outside the defined envelope. This means that the design assumptions are compared with the observations and data provided by monitoring and inspection activities. 

Maintenance, surveillance and in-service inspection have a common objective, which is to ensure that the plant is operated in accordance with the design assumptions and intent, and within the operational limits and conditions. Maintenance, for example, should always be followed by a series of tests. Results of surveillance or in-service inspection should be compared with the acceptance criteria. If the results fall outside the acceptance criteria, corrective actions should be initiated. Such actions should include corrective maintenance measures such as adjustment, repair or replacanent of defective items to prevent recurrence. These activities should be planned and co-ordinated effectively. A conunon database should be established in order to share relevant data and evaluations of results among the organizations that are involved in the planning and implementation of MS I activities. 

The operating organization should ensure that MS I for SSCs important to safety are of such a standard and frequency as to ensure that the level of reliability and functionality of the SSCs remains in accordance with the design assumptions and intent throughout the plant s operating lifetime. 

A reflux ratio of 0.5 is assumed for the case study. Normally, this value must be determined experimentally. Design Assumptions... 

P(4) An adequate quality control system is required to guarantee that the Joint is manufactured according to the design assumptions. 

A proper surface treatment is probably the most important single factor in the process of ensuring the reliability and durability of a bond. Therefore, the importance of the surface treatment prior to bonding cannot be over emphasised. One of the basic design assumptions is that no adhesive failure is encountered. To guarantee that this assumption will be valid on all occasions, all bond surfaces shall be treated according to the EUROCOMP Design Code. 

There are a number of reasons why it may be necessary to test a complete structure or part of a structure. These include situations in which the design assumptions and the analysis methods used do not accurately represent the complete structure. In addition tests may be required when there are doubts about the materials or the workmanship. 

The test programme was divided into two parts testing of material properties and verification testing of the design assumptions and procedures. Material property tests were conducted to measure ... 

In practice, a combination of these methods is normally needed. Too often there is a tendency to under-resource the site queries resolution function, especially on fast-track projects where many design assumptions are made, and a high number of discrepancies have to be resolved on site (a fairly predictable outcome). There may also be an attitude by the plant owner or project manager that this concentration of resources should not be necessary if the engineering work is performed properly in the first place, even though half the problems are usually generated by manufacturing or construction error. Whatever the cause, false economies should be avoided. 

A number of different plant layouts integrating hydrogen membrane reformers have been proposed in the literature. The main characteristics of these plants are reported in Table 10.2. Due to the wide variety of proposed layouts and design assumptions, it is difficult to obtain a clear indication of their relative thermodynamic performance. Efficiencies between 43% and 53% are reported, corresponding to penalties of 5-10% points with respect to the reference technologies without CO2 capture note that the extent of CO2 capture is -100% in cases where high selectivity dense membranes are used and no natural gas external firing is adopted. 

There is a comprehensive research and development effort going on. This effort is oriented to validate the design assumptions made for completing the basic engineering. The main areas and installations involved in the development are ... 

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