6.4 Compressed Air System
6.4.1 Intermediate pressure and low pressure compressed air systems may be installed in the powerhouse of the hydropower station，and their scales shall be defined according to air volume，working pressure and relative humidity as required in the design.
6.4.2 The pressure in an intermediate compressed air system used to charge air for oil pressure units shall be defined according to the rated working pressures of the oil pressure units.There shall be two air compressors，one for working and another as a standby，and a compressed air tank shall be provided.
The capacity of air compressor may be defined based on the requirement that air pressure of one pressure oil tank is charged from normal pressure to rated pressure in 60-120min if all compressors work simultaneously.
The volume of the compressed air tank may be defined according to the supplementary air volume of the pressure oil tank during operation.The rated working pressure of the compressed air tank should be 0.2-0.3MPa higher than the rated working pressure of the pressure oil tank.The compressed air tank shall be equipped with safety valve，pressure gauge and drain valve.
6.4.3 The pressure of a low pressure compressed air system used for unit braking，overhaul and maintenance，and sealing of circular band of turbine main shafts shall be 0.7MPa-0.8MPa.Air used for unit braking shall meet the following requirements:
1 Exclusive air tank and exclusive air supply pipe should be equipped for air used for unit braking.
2 The total volume of compressed air tank for storing air for unit braking shall be defined according to the total air consumption of a number of units that are braking simultaneously.(https://www.daowen.com)
3 The capacity of air compressor shall be defined according to the air consumption of units that are braking simultaneously and the time of restoring working pressure in the compressed air tank.The time of restoring working pressure in the compressed air tank may be 10-15min.
4 A standby air compressor or other standby air source should be prepared for air used for unit braking.
6.4.4 In case the units are to be used for phase modulation by the method of aeration and pressurized water，the following requirements shall be met:
1 The air compressor used for aeration may be used as the air compressor for unit braking，its capacity shall be defined according to the air consumption of bearing on water for phase modulation，but the air supply pipe for phase modulation and the compressed air tank shall be separated from the unit braking air system.
2 The volume of compressed air tank used for phase modulation shall be defined according to the air volume as required by initial depression of the water level in the runner chamber during phase modulation of one unit.
3 The total capacity of air compressors used for phase modulation shall be finalized according to the time of restoring working pressure of compressed air tank after the first water pressurization of one unit and the total air leakage of units that have been put into operation for phase modulation.The time to restore the working pressure of the compressed air tank may be 15-45min.
