Bypass connections are required for pneumatic components that are in continuous use applications, so that the flow of air is not interrupted when the component is maintained or taken out of service.
Typically, shut-off valves are installed before and after the component and a piping loop with a shut-off valve routed around the component for bypass purposes.
The photograph at upper right shows a bypass installed in a food processing plant. The plant uses lubricated compressors that introduce a small amount of lubricant into the air steam. The plant is finding, despite coalescing filters installed on the dryer, that a small amount of compressor lubricant remains in the air and this passed downstream.
Because the bypass line is lower than the component (a precision pressure regulator), the lubricant forms droplets at flow into the section of bypass line both before and after. There is no drain installed from which to drain any contamination. Drop by drop, over time the small amount of lubricant eventually fills up the bypass lines.
Nothing really happens until the bypass line is opened — then, a large slug of compressor lubricant, collected through years of operation, flows into the plant, contaminating downstream piping and mechanisms, and in extreme cases, food products.
The photograph at lower left shows a similar installation involving two bypass valves around inlet and outlet filters to an air dryer. The wet airstream before the dryer fills the first bypass mostly with water, the second, filtering dry air, fills mostly with compressor lubricant. Again, no drains are installed, when these filters are serviced, and the bypass valve opened, slugs of liquid pass into the plant.
Bypass piping should be installed so there is no way to collect liquid in the bypass piping. The air flow should be straight through to the component being bypassed, with the bypass loop going up and over, or to the side of the component. Make sure that you follow this sort of setup in order to avoid problems in the future.