Gas screening value

CIRIA Report C659/665 has two methods of characterising the level of risk associated with a site, one that has heen specifically developed for the NHBC that applies for low rise housing and the other for any other type of development. Both methods use the gas screening value (GSV) to characterise a site. The GSV is given by  

It should be noted that the gas concentration is the mathematical form (e.g. 50% is 0.5 in the equation). 

Given that the most onerous concentration and borehole flow values may never be idenhfied directly from a set of borehole gas monitoring readings (see Chapter 5) it is far better to derive a set of design parameters using a appropriate factor of safety. Thus the GSV can be estimated using two approaches that are discussed below. 

An alternative, very conservative, approach adopted by some landfill gas practitioners is to determine the GSV using a worst possible approach combined with a factor of safety of one. In this approach the worst combination of parameters from the data set, irrespechve of whether they can physically 

The gas screening value is the maximum value obtained from all the wells 

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These results gave a gas screening value of 0.072 which indicated characteristic situation 2 (see Chapter 6). However, the results only just exceeded the threshold for this characteristic situation and given the nature of the source it was considered unlikely that further results would increase the characteristic situation (either the concentration or flow rate would need to increase by a factor of 10) and so further monitoring was not required. 

Typical maximum concentration (%v/v) Gas screening value (l/h) Typical maximum concentration (%v/v) Gas screening value " (l/h)... 

The Gas Screening Value thresholds should not generally be exceeded without the completion of a detailed gas risk assessment taking into account site-specific conditions. 

Characteristic Situation (CIRIA 149)2+6 Risk classifi- cation Gas screening value (CH.orCOp (l/h) Additional factors Typical source of generation ... 

Gas screening value (litres of gas/hour) is calculated by multiplying the maximum gas concentration (%) by the maximum measured borehole flow rate Site characterisation should be based on gas monitoring of concentrations and borehole flow rates for the minimum periods defined in Section 5... 

Gas screening value l/h Gas screening value l/h Gas screening value l/h Housing... 

No exposure guidelines for Agent GA degradation products were found. The estimated oral reference dose for GA for humans is 0.04 qg/kg/day (Opresko et al., 1998). Estimated health-based soil screening values for residential soil range from 1.2 to 3.1 mg/kg of soil (US Army CHPPM, 1999). 

Based on USEPA Method 21, all applicable components / equipment at the Qatargas 1, Qatargas 2 and Qatargas 3 4 LNG Trains (Trains 1 to 7) are monitored and VOC emissions rates estimated using the EPA Correlation Approach found in USEPA s Protocol for Equipment Leak Emission Estimates (USEPA, 1995) and is applicable to refineries, marketing terminals and oil and gas production operations. The method provides an equation to predict mass emission rate as a funetion of screening value for a particular equipment / component type. 

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

Start the vapour generator cycle so that the absorption cell is flushed with argon gas and the pre-set volume of NaBH4 (1 mL) is pumped into the sample vessel. After the pre-selected reaction time (0.5 minute), AsH3 vapour is flushed into the absorption tube. Record the value of each arsenic signal as a peak height measurement. Read off the arsenic concentration of the sample, which is displayed on the instrument video screen. 

The molal diamagnetic susceptibilities of rare gas atoms and a number of monatomic ions obtained by the use of equation (34) are given in Table IV. The values for the hydrogen-like atoms and ions are accurate, since here the screening constant is zero. It was found necessary to take into consideration in all cases except the neon (and helium) structure not only the outermost electron shell but also the next inner shell, whose contribution is for argon 5 per cent., for krypton 12 per cent., and for xenon 20 per cent, of the total. 

Werpy, T. Peterson, G. (eds) (2004) Top Value-Added Chemicals from Biomass, Vol. 1. Results of Screening of Potential Candidates from Sugars and Synthesis Gas. Pacific Northwest National Laboratory and National Renewable Energy Laboratory, US, August 2004. 

In the current era neutral compounds are important simply because there are so many of them especially in screening collections made in automated chemistry for HTS. Other things being equal, a compound with an ionizable moiety is preferred to a neutral compound. By definition a neutral compound will not give an acid or basic pJCj value. Some essentially neutral compounds can form aqueous unstable salts. This most commonly occurs when an extremely poorly basic compound is dissolved in organic solvent and the salt is precipitatated, e.g. by bubbling in HCl gas. 

Molecules in real solvents can exist in one or more tautomeric forms. The use of different tautomers in calculations can lead to significant variation in the estimated log P values (Pig. 15.1). Accurate prediction of the dominant tautomer requires ah initio calculahons. Due to speed limitations such calculations are not feasible for virtual screening and prediction of large compound collections. Moreover, the interpretation of the results can also be difficult, for example, the lacton-lactim (Pig. 15. IB) is the stable form of maleic hydrazide in the gas phase but the difference between this and the dilacton form (Pig. 15.1C and D) disappears in solution... 

The equipment is quite adequate for screening purposes. In its simplest form (i.e., a glass tube in an oven), it is a relatively low cost technique that can be assembled with standard laboratory equipment. However, the simple test set-up provides no quantitative thermal data for scale-up purposes, but only T0 values. The more advanced instruments like the SEDEX and SIKAREX, which are also isoperibolic calorimetry equipment, acquire specific thermal stability data that can be used for scale-up. Furthermore, the small autoclave tests provide gas evolution data. 

The true gas phase basicity order is observed in the montmorillonite (43), whereas in solution the well known amine anomaly exists, i.e. the expected inductive effects of the organic carbon chain are screened off by the solvent. For example, identical AG values of protonation are found in solution for methyl- and butylammonium. 

Top Value Added Chemicals From Biomass—Results of Screening for Potential Candidates from Sugars and Synthesis Gas, eds. T. Werpy and G. Petersen, US Department of Energy, Oak Ridge, TN, August 2004, vol. 1 available at WWW.eere.energy.gov/biomass/pdfs/3 5 523. pdf. 

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