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Fig. 9.5: Safety valve on combined compressed air-oil receiver
of an oil-injection cooled screw compressor | The installation of a safety valve on compressed air receivers is prescribed by law.
If the internal pressure of the receiver p N (network pressure) rises to the maximum operating pressure of the tank (e.g., the maximum compressor pressure 10 bar, tank operating pressure 11 bar), the safety valve must slowly open.
If the system pressure rises to 1.1 times the nominal pressure ( e.g., tank pressure 11 bar, safety valve 12.1 bar ), the safety valve must open fully and blow off the excess pressure. Care must be taken that the cross-section of the outlet aperture of the safety valve is of a size that allows the entire output of all connected compressors to be blown off without the pressure in the receiver rising.
When an existing pneumatic system is extended at a later date the number of compressors increases. An appropriate upgrading of the safety valve can easily be overlooked when this happens. If the safety valve is no longer able to blow off the entire output of the compressors the operating pressure in the receiver will rise. This can cause the receiver to explode in extreme cases.
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Fig. 9.6: Diagram symbol for a safety valve | Safety inspection
The safety valve must be checked every time a compressor station is extended so as not to have a valve with too low a capacity.
The mains connection to the receiver must be shut off. The press switches must then be bridged, so that the compressors can no longer switch off automatically.
The pressure in the receiver rises until the safety valve switches. The receiver pressure must not exceed 1.1 times the limit (e.g., receiver pressure 11 bar, safety valve 12.1 bar ). If this does happen, the safety valve is below par and must be replaced. |