3 General Requirements
3.0.1 In detection of possible combustible gas and toxic gas leakage in area for process units producing or consuming gases as well as for storage and transportation facilities，gases detector shall be equipped according to the following requirements:
1 Upon leakage of combustible gas or combustible gas containing toxic gas，if concentration of combustible gas may reach 25% of its lower explosion limit while toxic gas could not reach its maximum allowable concentration，combustible gas detectors shall be equipped；
2 Upon leakage of toxic gas or toxic gas containing combustible gas，if concentration of the toxic gas may reach its maximum allowable value while combustible gas could not reach 25% of its lower explosion limit，toxic gas detectors shall be equipped；
3 In the place where both combustible gas and toxic gas are present，if concentration of combustible gas may reach 25%of its lower explosion limit and that of toxic gas may also reach its maximum allowable value，combustible gas and toxic gas detectors shall be equipped respectively；
4 When a gas is both combustible and toxic，only toxic gas detectors shall be equipped.
3.0.2 Two levels of alarms shall be set for the detection system of combustible gas and toxic gas.When the toxic gas and combustible gas detectors in the same detection area alarm simultaneously，the following principles shall be complied with:
1 For alarms of the same level，alarm for toxic gas shall be priority；
2 Level Ⅱ alarm shall be priority over level Ⅰ alarm.
3.0.3 Combustible gas and toxic gas detection system with monitoring，recording，alarming and printing functions shall be provided for continual detection in hazardous areas with special process requirements or personnel forbidden zone during operation.
3.0.4 Alarm signals shall be sent to local alarm devices with audible and visual alarms in control room or field operation room with personal on duty.
3.0.5 Layout of local alarming devices in process areas shall be determined according to area of the plant，arrangement of equipment，structures and buildings，physical-chemical properties of release sources and field air flow characteristics.Local alarms may be horns and beacon.
3.0.6 All combustible gas detectors shall be qualified products with metering device manufacturing certification，explosion proof certification and fire fighting certification by national authorized organizations.
3.0.7 Toxic gas detectors specified by national laws and regulations shall be qualified products with metering device manufacturing certification by national authorized organization，in addition explosion proof type toxic gas detectors shall be products with explosion proof certification by national authorized organization.
3.0.8 Detectors in locations where there is combustible gas or toxic gas shall be of fixed types.
3.0.9 Combustible gas and toxic gas detection and alarm system should be designed as independent system.
3.0.10 Portable combustible gas or toxic gas detectors shall be provided according to field conditions of process plants，combustible，explosive and toxic properties of the process materials and number of operators on site.
3.0.11 Fixed detection and alarm systems for combustible gas and toxic gas in process units and storage and transportation facilities should be powered by UPS.Detection and alarm systems for combustible gas and toxic gas at gas stations，LNG/LPG station and scattered or independent sale facilities of toxic and combustible and explosive materials may be powered by normal power supply.
3.0.12 Properties of common combustible gases and vapors are shown in Appendix A.Properties of common toxic gases and vapors are shown in Appendix B.