Impact of Workplace Ventilation Systems on Occupational Asthma

Summary

Assessment of explosion risk in industrial facilities, identification of hazardous areas where explosive atmospheres may occur and determination of zone classifications is conducted in accordance with TS EN 60079-10-1 and TS EN 60079-10-2 standards. Facilities where flammable substances are loaded, unloaded or stored must be designed, constructed, operated and maintained in such a manner that during normal or abnormal operation, all flammable substance releases and the resulting hazardous areas are kept at the lowest level in terms of size, frequency, duration and quantity of release. The size of the explosive atmosphere is determined by the quantity of release and the duration that the released substance remains in the environment. When taking explosion prevention measures, reduction of release quantity and use of Atex equipment in the area are paramount. Another measure is to reduce the time the released substance remains in the environment and to provide an environment below the Lower Explosion Limit that creates explosive properties. In processes, air effects are utilized in the release of gas and vapor to the work environment. Gas and vapor concentration can be reduced to some extent; in such a case, the diffusion-dispersion mechanism is important in concentration change. Proper ventilation reduces the persistence of the hazardous area created in an explosive gas environment. Gas and vapor released to the atmosphere are diluted in air through diffusion, bringing them below the lower explosion limit value. Ventilation and air movement have two fundamental functions. These can be described as: increasing dilution to reduce the hazardous area coverage limit and reducing the persistence of the explosive gas environment to lower the hazardous area type. Appropriate ventilation rates can reduce the persistence time of the explosive gas environment. This condition affects the zone type. Ventilation is achieved by moving air through wind or temperature difference or by artificial means such as fans. Therefore, there are two basic types of ventilation. Natural ventilation and Mechanical (Forced) Ventilation.

Explosive Gas Atmosphere

An explosive gas and dust atmosphere is a mixture formed between air and combustible substances in the form of gas, vapor or dust under normal atmospheric conditions. After ignition, the explosion spreads to the entire mixture that has not yet burned (IEC 60079-10-1). 1. Situations where the concentration of combustible gas and vapor or dust is low and air concentration is high are called very weak mixtures and in such a case, combustion does not occur, therefore an ideal safety environment is created. 2. When the concentration of combustible gas and vapor or dust increases and reaches a certain concentration, when the substance reaches the "Lower Explosion" and "Upper Explosion" limit values, explosion and burning occur at any value between these two values. To prevent this situation, air concentration must be increased to bring it below the lower explosion limit value of the chemical substance. 3. Situations where air concentration is low and combustible gas and vapor concentration is high are very rich mixtures.

Assessment of Explosive Atmosphere

Combustible and flammable chemical substances used in chemical processes and workplaces; • In order not to cause harm to workers, the work environment and surroundings, * prevention of explosive atmosphere formation or, * in cases where explosive atmosphere formation is inevitable due to the nature of the process, measures must be taken to prevent ignition of the explosive atmosphere and simultaneously reduce the harmful effects of explosion.

Figure 1. Distribution of gas concentration in atmosphere

Regarding the presence of explosive gas atmosphere, hazardous areas are Zone 0, Zone 1, Zone 20, Zone 21. Regarding the presence of explosive gas atmosphere, non-hazardous areas are Zone 2, Zone 22.

General Safety Principles

Facilities where flammable and explosive substances are loaded, unloaded or stored must be designed, constructed, operated and maintained in such a manner that during normal or abnormal operation, all flammable substance releases and the resulting hazardous areas are kept at the lowest level in terms of size, frequency, duration and quantity of release.

Reduction of Hazardous Environment Effects

Control measures to be applied in priority order to eliminate risk; • The quantity of hazardous substances should be minimized, • Prevent or minimize the spread of hazardous substances, • Remove hazardous substances from the environment through ventilation or dilute the hazardous atmosphere in the environment to prevent dangerous atmosphere formation, • Avoid ignition sources, • Keep under control the risks of temperature, pressure and liquid overflow that could cause danger. The most basic principle for document preparation is as follows: After completion of zone classification, it is to determine whether the consequences of ignition of the explosive atmosphere require the use of equipment with a higher protection level or whether equipment with a lower protection level than normally required is being used.

Importance of Ventilation in Explosive Atmosphere

Gas and vapor dispersion commonly occurs at low rates in industrial processes. Open surfaces, drainage, printing machines and situations such as evaporation of flammable liquids like paint manufacturing are situations where dispersion occurs at low rates. Small leaks and seepage in pressurized equipment result in gas and vapor discharge into the environment.

Release Forms

• A gas that may be at elevated temperature or pressure, • A gas liquefied by application of pressure (for example LPG), • A gas liquefied by cooling alone (for example methane), • A liquid and the flammable vapor release associated with this liquid. Due to release, gas, vapor, mist and dust in the zone where they are located create an explosive atmosphere. The size of the explosive atmosphere is determined by the quantity of release and the duration that the released substance remains in the environment. When taking explosion prevention measures, reduction of release quantity and use of Atex equipment in the area are paramount. Another measure is to reduce the time the released substance remains in the environment and to provide an environment below the Lower Explosion Limit that creates explosive properties.

Ventilation and Dilution

In processes, air movements are effective in the release of gas and vapor or dust to the work environment. Ventilation, gas and dust concentration can be reduced to some extent; in such a case, ventilation characteristics are important in concentration change. Proper ventilation reduces the persistence in the environment of the hazardous area created in an explosive gas and dust atmosphere. Gas and vapor released to the atmosphere are diluted in air through diffusion, bringing them below the lower explosion limit value.

Comparison of Ventilation and Dilution

Ventilation mechanically provides the entry and exit of air in an enclosed space. Dilution, on the other hand, is the mechanical dispersion of a gas and vapor cloud. In enclosed spaces, danger associated with work involving hazardous gases, vapors and liquids is inversely proportional to ventilation. Whether the density and spread rate of gas emitted from the dispersion source creates danger depends on the sensitivity of the ventilation.

Assessment of Ventilation Rate

Removal of the explosive atmosphere from the area is provided by air flow, natural ventilation and mechanical ventilation systems. The best scenario is air being supplied to the environment homogeneously and at low speed. The ventilation system designed in enclosed spaces should also not affect the comfort of workers. In Natural Ventilation, wind speed is taken as 0.5 m/s. The annual average wind speed on world's land areas is 5.1 m/s. Air speed lower than 0.1 m/s is not perceived and creates a feeling of stuffiness, Speed between 0.25 m/s and 0.50 m/s is insufficient. Air speed of 1.5 m/s and above creates air currents and is effective.

Ventilation Types

Ventilation is achieved by moving air through wind or temperature difference or by artificial means such as fans. Therefore, there are two basic types of ventilation. • Natural ventilation, • Mechanical (Forced) Ventilation (General and Local Ventilation).

Mechanical (Forced) Ventilation

a) With mechanical ventilation; the explosive atmosphere zone type and zone scope can be reduced, b) Reduction of the persistence time of the explosive gas atmosphere can be achieved, c) Formation of the explosive gas environment can be prevented.

Recommended Ventilation Examples

Not Recommended Ventilation Example

 

Basic Components of Local Exhaust System

In a Good Ventilation System

1. Required fresh air should be supplied to the environment, 2. There should be no disruptive air flow (draft) in the environment, 3. The ventilation system should distribute and collect the ambient air in a uniform manner, 4. If present, fans should operate quietly. Most capture speeds are approximately around 0.5 m/s.

Lower and Upper Explosion Limits of Some Chemical Substances

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İlker Erdoğan Chemical Engineer İSGDER-Technical Protection