Fail position

Check fail position, e.g., fail open, fail closed, fail as is. 

Check fail position at loss of power (simulate). 

All control valves have a fail position. In the event of a power failure and/or loss of instrument air, the valve s spring operator will cause the valve to fail open, fully closed, or remain in its current position. A useful task for the team is to check the fail position of each control valve, and to ask What-If questions as to whether the designers got it right. An analysis of this type is particularly valuable if more than one accident scenario has to be considered, and if the different scenarios call for different valve positions. 

The ordinary BO approximate equations failed to predict the proper symmetry allowed transitions in the quasi-JT model whereas the extended BO equation either by including a vector potential in the system Hamiltonian or by multiplying a phase factor onto the basis set can reproduce the so-called exact results obtained by the two-surface diabatic calculation. Thus, the calculated hansition probabilities in the quasi-JT model using the extended BO equations clearly demonshate the GP effect. The multiplication of a phase factor with the adiabatic nuclear wave function is an approximate treatment when the position of the conical intersection does not coincide with the origin of the coordinate axis, as shown by the results of [60]. Moreover, even if the total energy of the system is far below the conical intersection point, transition probabilities in the JT model clearly indicate the importance of the extended BO equation and its necessity. 

Place 2 ml. of the periodic acid reagent in a small test tube, add one drop (no more—otherwise the silver iodate, if formed, will fail to precipitate) of concentrated nitric acid, and shake well. Add one drop or a small crystal of the compound to be tested, shake the mixture for 15-20 seconds, and then add 1-2 drops of 3 per cent, silver nitrate solution. The instantaneous formation of a white precipitate of silver iodate is a positive test. Failure to form a precipitate, or the appearance of a brown precipitate which redissolves on shaking, constitutes a negative test. 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Hiiekel m.o. ealenlations fail badly with benzimidazole. Loealisation energies for the free base and the eation indicated C(4) to be the most reactive position towards electrophilic attack, - and led to the false conclusion that substitution involved the free base, the orientation being controlled by charge densities. 

Bromination of 2-dialkylaminothiazoles has been reported to be successful by one author (415) and to fail by others (375. 385). If the mechanism of direct electrophilic substitution is accepted for these compounds, it is difficult to understand why alkyl substitution on such a remote position as exocyclic nitrogen may inhibit this reaction in the C-5 position. 

When unsubstituted, C-5 reacts with electrophilic reagents. Thus phosphorus pentachloride chlorinates the ring (36, 235). A hydroxy group in the 2-position activates the ring towards this reaction. 4-Methylthiazole does not react with bromine in chloroform (201, 236), whereas under the same conditions the 2-hydroxy analog reacts (55. 237-239. 557). Activation of C-5 works also for sulfonation (201. 236), nitration (201. 236. 237), Friede 1-Crafts reactions (201, 236, 237, 240-242), and acylation (243). However, iodination fails (201. 236). and the Gatterman or Reimer-Tieman reactions yield only small amounts of 4-methyl-5-carboxy-A-4-thiazoline-2-one. Recent kinetic investigations show that 2-thiazolones are nitrated via a free base mechanism. A 2-oxo substituent increases the rate of nitration at the 5-position by a factor of 9 log... 

All attempts to prepare selenazole derivatives by the Gatterman (for-mylation) or Friedel-Crafts (alkylation) methods failed (19, 26). indicating that the electrophilic reactivity of the 5-position is less than that of benzene or toluene. 

This reaction leads to thiazoles variously substituted in the 2-position, depending on the experimental conditions. Although this method has been studied rather extensively, it has limited scope and frequently fails to give thiazoles. 

The method has not been studied extensively and is restricted to the preparation of alkyl-, aryl-, or alkoxy-substituted thiazoles mostly in 2-, 5-, or 2,5-positions. Yields ranged from 45 to 80%. Sometimes this method gives good results when the usual Hantzsch s synthesis fails. There has been very little speculation about the mechanism of this reaction. 

Note that a negative determinate error introduced by failing to recover all the analyte is partially offset by a positive determinate error due to a failure to remove all the interferent. 

Methylation of hydrocortisone/prednisolone ia positions C-4, C-7, C-12, and C-21 failed to give useful products. Methylation at C-16, ia contrast, led to 16a- and 16P-methyl-9a- uoroprednisolones which were exceptionally useful. Both series were prepared usiag 3a-acetoxy-5a-pregn-16-ene-ll,20-dione derived from desoxychoHc acid (80). A much shorter route was subsequentiy developed from... 

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