Development of a Calculation Methodology for the Ventilation on a Besis of a Mobile Unit

An algorithm for the analysis of safety and efficiency of the processes, which are located inside the mobile unit are developed. It follows from the calculations that the safe concentration of combustible material in the space of industrial premises is about 3.69%. Automation systems must be focused on this value. The time of occurrence of the maximum permissible concentration of pollutant was determined and amounted to 160 seconds. It is shown that the ventilation rate of 0.5 would be sufficient for functioning of the object.


Analysis of the facility safety
Installation, that is located in a mobile unit, is under overpressure of 0.04 atmospheres. Based on the fact that the gas is promptly removed from the gas tank -maximum pressure in the system does not exceed 1.04 atmospheres.
Part of methane obtained after purification is used for the gas electric generator disposed in the container, providing the work of all electrical systems and serving as a source of heat. The power of generator is 1000 watts. Burning of 4 m 3 of methane in power generator provides 12 kW of power and 24 kW of heat.
The main hazard in the operation of the facility is methane gas. Table 1 summarizes the main characteristics of methane [3]. It follows from the analysis of the results that inflammation area in air is 5,28-14,1% vol., concentration limit of inflammation (reduced to 25 °C): lowest 5.28, top. 15% vol.
We are interested in the lowest concentration limit of inflammation -about 5.28%. Measures to ensure fire and explosion safety processes were developed according to the requirements of State Standard 12.1.004 and State Standard 12.1.010.
According to these documents to ensure fire and explosion safety of production processes, processing, storage and transportation of substances and materials it is necessary to use data rates of fire and explosion hazard of substances and materials with a safety factor φg no ≤ 0.9 (φn -0.7 R) where φg safety -a safe concentration of the flammable substances,% vol. (gm-3); φn -the lower flammability limit of a mixture of a fuel with air,% vol. (gm-3); R -reproducibility of the method of determining the fire danger rating at 95% confidence level.
Calculations show that a safe concentration of the flammable substance is about 3.69%.

Definition of the time to reach maximum permissible concentration (MPC) or explosive concentrations of pollutant from the material balance
When production area has no forced ventilation or ventilation is not reliable (there are no redundant fans, electric power is not carried out by two independent feeders), the most favorable conditions for the formation of explosive concentrations in case of damages and accidents at production facilities and pipelines are created. In this case, the amount of flammable material coming out of the equipment during the time interval dτ, must be equal to the increment of the amount of flammable material in the air volume over the same period of time dτ, or  (2) Figure 1 shows a graph of a linear relationship -the result of the calculation using the equation (2), which clearly shows that the increase in the concentration of the pollutant is uniform over time. The graph shows that the time to reach a dangerous situation, if the exhaust velocity of methane in the volume space is 160 seconds. The rate of inflow of methane in the areas was taken from statistically average datas -0.00072 g / s. The time of hazard situation was determined based on the fact that the lowest flammable limit was taken (5.28 vol.%), which was converted based on the safety factor (0.32%). Finally we got the concentration (3.69% vol.), which was correlated with the dependence graph, thereby the occurrence time of a dangerous situation was determined. The obtained time plays an important role in the choice of technological equipment of ventilation systems, which the operation of automation systems will focus on. During the calm weather, general ventilation, which is carried by the deflector will not be effective. In this case, when the concentration of the pollutant reaches its limit, the start of emergency ventilation must be operated automatically. Figure 2 shows the results of further calculations, taking into account the ventilation rate in the room. Analyzing the chart we see that the ventilation rate of 0.5 would be sufficient for the safe operation of the facility. This value is important in the selection of ventilation systems, because it shows the required volume of air to be disposed from the room.
In case of guaranteed ventilation rate a part of an outgoing vapors or gases in the accident are continuously withdrawn from the production area to the outside, as a result the increase in the concentration to dangerous limits will relatively slow down. In significant damages and accidents devices or pipelines with flammable vapors and gases in the premises an explosive concentration may form even when the operation of ventilation (including emergency), and so quickly that it would be impossible to implement the necessary emergency actions manually [1,2,[4][5][6].
The inclusion of the ventilation system is performed when a safe concentration of a fuel (methane) is exceeded, which is about 3.69%. Figure 3 provides an algorithm for the research of the object on the basis of the mobile unit.

Conclusion
In the result of the research the algorithm to analyze the safety and efficiency of the processes, which are located inside the mobile unit was developed.
It follows from the calculations that the safe concentration of combustible material in the space of technological premises is about 3.69%. Automation systems must focus on this data. The time of occurrence of the maximum permissible concentration of pollutant was determined and amounted 160 sec. It was shown that the ventilation rate of 0.5 would be sufficient for the investigated object.