Conservative assumptions, application

It is important that the exposure estimate be realistic yet protective of human health and that it takes into account the frequency and duration of exposure. Uncertainties in the exposure assessment, including the fact that exposure studies are carried out under controlled circumstances, may be compensated for by using conservative assumptions such as maximum application rate, upper bound values and 100 % dermal penetration. These conservatisms can have a negative impact on the regulatory decision and conld be avoided with access to more complete data. 

This assessment included the estimate of the residues of PBO following application at the recommended application rate and at 10 times the single application rate- This incorporates two highly conservative assumptions. First, the I OX rate simulates the maximum number of applications allowed on the label and incorporates the unrealistic assumption that no degradation or dissipation occurs in the field (or all 10 applications are made at the same lime). This is a worst-case scenario considering the degradation predicted by the environmental fate studies and the actual levels of PBO observed in the field, as well as the low likelihood that 1U applications of PBO would ever be made prior to harvest. The second extremely conservative assumption used in this exposure assessment is that I009f- of the avian diet is derived from treated crop,... 

Releases of the most representative chemical species shall be assessed by the designer in general with realistic/best estimate assumptions, with the exception of the conservative assumptions listed in Section 2-1-2-4. In the methodology applicable to DBA release targets, the same limitations and the same warnings are applicable as those given for DEC (see Section 2-1-B-1). 

Markov methods are also applicable for general MooN methods, and is in fact mathematically more accurate since none of these make the conservative assumptions when deriving the formulas for the 8-factor model and the PDS method (Rausand Hoyland 2004). 

Some dispersion studies are responses to regulations. It is becoming more conunon to require inclusion of a worst-case scenario for toxic releases in licensing applications and other plant reports. One definition of worst case is the release of the largest inventory of toxic material over a 10-min period with a l.S m s wind and stability F. The use of F stability is the most conservative assumption. Some regulations allow the use of a more unstable Pasquill class when F stability is a very rare phenomenon. Each class has built into the dispersion model a set of values for horizontal and vertical... 

Fire analysis can be performed to a different level of details. The approaches on this vary from study to study. In many fire analysis conservative assumptions are made, e g. when fire occurs in a certain room all components in the room are lost. These assumptions may not necessarily be applicable for shutdown operating mode. During plant outage there are a lot more people in the plant that could either detect or extinguish the fire, though the automatic fire detection system may be out-of-service at that time. The fire analysis for plant outage should also considered that the fire zones during power operation could be different than in shutdown. 

Simplified mathematical models These models typically begin with the basic conservation equations of the first principle models but make simplifying assumptions (typically related to similarity theory) to reduce the problem to the solution of (simultaneous) ordinary differential equations. In the verification process, such models must also address the relevant physical phenomenon as well as be validated for the application being considered. Such models are typically easily solved on a computer with typically less user interaction than required for the solution of PDEs. Simplified mathematical models may also be used as screening tools to identify the most important release scenarios however, other modeling approaches should be considered only if they address and have been validated for the important aspects of the scenario under consideration. 

The equations describing the concentration and temperature within the catalyst particles and the reactor are usually non-linear coupled ordinary differential equations and have to be solved numerically. However, it is unusual for experimental data to be of sufficient precision and extent to justify the application of such sophisticated reactor models. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the catalyst bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on reaction rate. A useful approach to the preliminary design of a non-isothermal fixed bed catalytic reactor is to assume that all the resistance to heat transfer is in a thin layer of gas near the tube wall. This is a fair approximation because radial temperature profiles in packed beds are parabolic with most of the resistance to heat transfer near the tube wall. With this assumption, a one-dimensional model, which becomes quite accurate for small diameter tubes, is satisfactory for the preliminary design of reactors. Provided the ratio of the catlayst particle radius to tube length is small, dispersion of mass in the longitudinal direction may also be neglected. Finally, if heat transfer between solid cmd gas phases is accounted for implicitly by the catalyst effectiveness factor, the mass and heat conservation equations for the reactor reduce to [eqn. (62)]... 

Underwood s equations were designed for systems with constant relative volatility and with constant molal heats of vaporization. As discussed, these assumptions for most systems prove conservative and therefore indeed merit the application herein of practical process engineering. The Underwood equations for RM determination are ... 

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