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What are they?
Airline valves are used to control the airflow to each aeration device when an air pump supplies more than one device. Usually made of plastic, they are inexpensive and are typically sold as two or more in one package.
When supplying more than one airstone or other devices, each airline valve installed by cutting the tubing after any tees inline with each device. By partially opening each valve, the desired amount of air is delivered to each device. If a device needs to be removed from service for cleaning, maintenance, etc. close the valve delivering air to that device. Otherwise the open tubing will cause most if not all of the air to be delivered to the open tube and the other devices may receive no or insufficient air to operate those devices.
The air pump must be operated so as to allow it's total output continues to flow. If the valves are over-throttled and the air pump is not able to deliver it's total capacity, damage to the pump diaphragms may occur. Increase in the noise level is an indication of reduced total airflow. If this occurs, open each of the valves as desired slightly to allow the pump to deliver it's total capacity until the noise level is reduced to normal.