4.4　Emergency Ventilation

4.4.1　Emergency exhaust systems shall be provided for the buildings where sudden huge amounts of harmful gas or explosive hazardous gas may be possibly generated.

4.4.2　Emergency exhaust air rate shall be determined by calculation based on the properties and emission rate of harmful gas or explosive hazardous gas.If related data is unavailable，the total of normal exhaust air rate and emergency exhaust air rate may be calculated on a basis that the minimum air change rate shall be not less than 12 AC/h，but for CategoryⅠor Ⅱ pump houses and compressor buildings，the total exhaust air rate shall be the normal exhaust air rate plus an additional emergency exhaust air rate not less than 8 AC/h.Where emergency exhaust air rate is determined by air change rate，the volume of rooms shall be calculated in accordance with Appendix B of this Specification.

Note:The fire hazards for production buildings are classified into 5 categories in accordance with SH/T 3017，i.e.，Category Ⅰ，Ⅱ，Ⅲ，Ⅳ and Ⅴ.

4.4.3　Settlement of the emergency exhaust system shall comply with the following requirements:

a）If the normal exhaust system is sized to have an additional capacity for emergency exhaust，emergency exhaust system may not be provided.

b）If the normal exhaust system cannot meet the emergency exhaust requirements，individual emergency exhaust system shall be provided and sized based on the shortage of exhaust air rate.

c）If exhaust systems for frequent service are unavailable，emergency exhaust systems shall be provided.

4.4.4　When it is not possible to use ventilator in the emergency exhaust system to exhaust explosion hazardous gases or vapor，the induced exhaust system or forced blower type emergency ventilation system may be provided.

4.4.5　Arrangement for exhaust inlets of emergency exhaust systems shall comply with the following requirements:

a）The exhaust inlets shall be arranged at locations with potential largest emissions of explosive hazardous gas or harmful gas or vapor.

b）For cases where flammable gas or vapor is lighter than air at emission temperature，exhust inlets shall be arranged immediately adjacent to the ceilings，and the distance from the upper edge of the exhaust opening to the ceiling should not be larger than 0.4m.

c）Where normal exhaust systems are also served for emergency exhaust，a specified number of exhaust inlets should be arranged near the equipment where emergency cases may probably occur.

4.4.6　Arrangement for exhaust outlets of emergency exhaust systems shall comply with the following requirements:

a）Exhaust outlets of emergency exhaust systems shall not be arranged at the locations with frequent occupancy or access.

b）When the horizontal distance between the exhaust outlets and the air inlets of the mechanical air supply system is less than 20m，the outlets shall be 6m higher than the air inlets；

c）If the emitted air containing flammable gases or vapor，the distance of the exhaust outlets shall be more than 20m away from the ignition source；

d）Exhaust outlets of emergency exhaust systems shall not be settled at the locations where emissions are difficult for dispersion.

4.4.7　In designing emergency exhaust systems，if the requirements specified in Article 4.4.6 of this Specification are met，axial-flow fans may be installed on exterior walls（or exterior windows）or roof fans may be installed on building roof to dispel the air out of the rooms，however，short-circuit shall be avoided for air flow path.

4.4.8　Emergency exhaust systems shall not be provided in production buildings containing extremely toxic substances，when it is necessary，air from emergency exhaust systems shall be purified before venting.

4.4.9　Emergency fans provided in production buildings containing highly toxic or explosive hazardous substances shall be interlocked with toxic or flammable gas detectors and alarms if provided.

4.4.10　The make-up air system need not be arranged for the normal building emergency exhaust.However，the enclosed rooms without windows shall be provided with a mechanical make-up air system，and the emergency exhaust shall be interlocked with the make-up air system.

4.4.11　The electrical switches of the ventilators for emergency ventilation shall be set up respectively outdoors and indoors at the points accessible for operation，and the power supply reliability step class shall be the same as that for the process.