The Secondary Elements

The majority of heat-resisting alloys contain at least one element which is selectively oxidised (e.g. Cr, Si, Al). The resulting scale is highly protective, but depletion of the secondary element may occur if the diffusion rate of this element in the alloy is low. Then repetitious loss of the scale can be profoundly detrimental to long-term performance, since depletion of the alloying element near the surface may occur to such an extent that its concentration falls below the critical level for the exclusive reformation of the highly protective scale... 

The mass separated, pulsed, and focused primary ions with the energy of 1 -25 keV, typically liquid metal ions such as Ga, Cs, and O", are used to bombard the sample surface, causing the secondary elemental or cluster ions to emit from the surface. The secondary ions are then electrostatically accelerated into a field-free drift region with a nominal kinetic energy of ... 

The analysis of the cytochrome crystal structures reveals extensive differences among the mammalian and the human CYPs that may reflect the structural flexibility of these enzymes and therefore the broad substrate specificity observed. In general, the secondary elements and the overall structure of the CYP are conserved. Nevertheless, it seems that in most enzymes, the most flexible regions are located between the F-G helixes and the B-C loop. For example in the CYP2B4 apoprotein (lpo5) the... 

Recently, several CYP crystal structures have been deposited in the P D B and could be used to understand the different metabolic properties. From the analysis of the different protein structures, it could be concluded that in general CYPs maintain the secondary elements across the different subfamilies, but they have a quite high flexibility, accommodating the protein structures to the ligand bound in the enzyme cavity. 

There are two elements in a head flow meter the primary element is the restriction in the line, and the secondary element is the differential pressure measuring device. Figure 1 shows the basic operating characteristics of a head flow meter. 

The primary elements provide the input to the secondary element which provides for indications, alarms, and controls. 

A loss of differential pressure integrity of the secondary element, the DP transmitter, will introduce an error into the indicated flow. This loss of integrity implies an impaired or degraded pressure boundary between the high-pressure and low-pressure sides of the transmitter. A loss of differential pressure boundary is caused by anything that results in the high- and low-pressure sides of the DP transmitter being allowed to equalize pressure. 

Another approach is to map the arrangement of secondary structural elements onto the known tertiary structures of other proteins. Currently, approximately one hundred unique protein folds have been identified. There is some question as to if this is an upper limit. If this is indeed the case, then the protein of unknown structure must adopt a known topological fold. The secondary structural elements are mapped onto the template of the different known protein structures. The best fits, as judged by the environmental factors (solvent accessibility) of the individual amino acids, are then further analyzed as probable folds. This procedure is referred to as threading the secondary elements into three-dimensional structures [28],... 

The three-dimensional structures of three non-plant sesquiterpene synthases reveals a single domain composed entirely of a-hehces and loops despite the low homology on amino acid sequence level [14,16,17]. The secondary elements of 5-epi-aristolochene synthase, a plant sesquiterpene synthase, conform to this pattern with the exception of two domains solely composed of a-helices and loops. It is reasonable, but still a matter of debate, to extrapolate these data to the case of amorpha-4,11-diene synthase, which will probably only display a-hehces and loops once the crystal structme has been solved. 

Fig. 14.14. top Schematic diagram of the structure of 3a,20yS-HSD and the nomenclature of the secondary elements... 

Figure 17-4 Amino acid sequence alignment of rat calcineurin a with kidney bean purple acid phosphatase within the P-a- -a-p motif. The calcineurin Aa sequence [92] corresponds to residues 85-158 while the sequence of purple acid phosphatase [93] from residues 130-209 is represented. The secondary elements comprising the P-tx-P-

FIG. 24. Schematic structuie of If 4Fe Fd generated by simply extending the a helix B by four residues and sheet A by 2 residues on the N terminus, and by 1 residue at the C terminus, relative to the 77 Fd structuie. Solid lines illustrate the relative positions of the secondary elements, the cluster, helices A and B, p sheets A and B, and four turns A-C, D. The position of the disulfide link involving Cys-21(V) and Cys-48(VI) is also shown. Ihe portion between p sheet B and helix B retained in the other hypertheimostable Tm and Tl Fds is shown as a dashed line. The portion of the stnicture between P sheet B and helix B retained in the Fd from the Dg mesophile is shown as a dotted line. The shaded portion of the structure represents the parallel modular extension of helix B and P sheet A that results from the 6,7 extra residue in If relative to Tm or Tl Fd. [Reprinted from P.-L. Wang, A. Donaire, Z. H. Zhou, M. W. W. Adams, and G. N. La Mar, Biochemistry 35,11319 (1996), with permission.]... 

The secondary elements interact as one would predict from their net charges. Helix-helix interactions are strongly repulsive, helix-Beta slightly attractive and Beta-Beta interactions clearly attractive, as one would expect from their H-bonding alignment. 

Control problems of principal interest are those involving two dynamic elements. Loops comprised of only one element are nothing more than limits of two-element loops. The diffieulty of each of these processes is found to be proportional to the ratio of the secondary to the primary element. In addition, the period of the closed loop is a function of the secondary element alone. A performanee index can be envisioned which would combine the sensitivity of the loop to disturbances with the time required to recover from them. This index would vary as the square of the secondary element. The significance of secondary elements is paramount. 

Settings of reset and derivative time are also directly related to the value of the secondary element. This rule seems as illogical as that governing the period of the pendulum, which varies with length, not with mass. Visualize length as the secondary element and mass as the primary, as a memory aid. 

It happens that a reactor of this description will oscillate at a period of about 35 min in a closed loop. Even if all the secondary elements consisted of pure dead time, they could not cause the period to exceed 29 min. Therefore, some secondary element remains hidden, and the only place it could hide is in the reaction mass. The assumption has been made, in calculating its time eonstant, that the reaetion mass was perfeetly mixed -that it was all at the same temperature. This, of eourse, is a false premise, beeause it is impossible to transport fluid, henee heat, from the wall of the vessel to the temperature bulb in zero time. Heat is transferred both by eonvection and by conduetion-eonduetion would be the meehanism if the fluid were motionless. It has been pointed out that... 

Starting from the classification of the secondary elements in NSEs (attachments) and SEs, the following can be asserted (1) given a constmc-tion, it includes a principal (P) and a secondary (S) structure (2) if the S structure has a negligible influence on the P structure, the whole constrac-tion (PS) structural behavior can be decoupled ... 

A further feature of the structure of the functional domain arises from the central role of the irreducible element in the description of functionality and the definition of that element, as depicted in Fig. C4.4. Due to this view of an engineering project as an optimisation of the balance between cost and revenue, the secondary elements fall into two completely separate categories the elements describing aspects of the cost, and the elements describing aspects of the performance, as already mentioned at the end of Sec. C4.3 and illustrated in Fig. C4.6. 

Following on from the structure of the functional domain, real functional elements, as a system of imaginary elements, wiU also have certain structural features. To see how this comes abouL consider a particular project and develop, at firsL the plant s capability, i.e. what it must do, as a system of primary functional elements. The elements and structure of that system will, of course, depend both on the required capability and on our choice of functions to achieve it. However, once that system has been created, and we turn to describing the cost of providing the functions and the additional requirements on such aspects of the performance as reliability, maintainability, and safety, the corresponding elements refer to the primary elements. That is, the primary elements form what we might think of as a skeleton structure, and the secondary elements dress up this skeleton to form a real elemenL thereby inheriting some of the same structure. 

When we say some of , it is because the extent to which the secondary elements inherit the structure of the primary elements depends on the type of element. The cost elements will inherit the subdivision into functions, but will not... 

The secondary elements describe aspects of the functionality that are additional to the actions described by the primary elements, and they arise out of two very different groups of requirements. Some secondary elements are required in direct response to corresponding requirements in the stakeholder requirements a typical example is a stakeholder requirement for dependability of the service, which then needs to be reflected in a requirement on the reliability of the system of primary elements. Other secondary elements are required as inputs to the primary elements, and again, reliability (or availability) might be required as input to a revenue element in order to be able to optimise the ROI, even if there is no explicit stakeholder requirement for reliability. Another example of this is cost there may not be any direct requirement on the cost, but a requirement for optimising the ROI will demand that cost is included in the description of the functionality. 

The three most extensively used types of flow-metering devices are the thin-plate square-edged oriflce, the flow nozzle, and the venturi tube. They are differential-head instruments and require secondaiy elements for measimement of the differential pressure produced by the primary element. The Supplement to ASME Power Test Codes Instruments and Apparatus, describes construction of the above primary flow-measuring elements and their installation as well as installation of the secondary elements. The method of flow measimement, the equations for flow computation, and the limitations and accimacy of measurements are discussed. Diagrams and tables showing the necessary flow coefficients as a function of Reynolds number and diameter ratio are included in the standards. Diagrams of the expansion factor for compressible fluids are given. 

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