A bypass loop is a slightly more complicated configuration that not only isolates the fluid system line under maintenance, but also reroutes the flow so the process can continue to function during maintenance.
Bypass Loop
For example, the first block valve in bypass configuration may be a three-way valve — diverting flow around the section requiring maintenance. The system filter can now be changed out without requiring downtime.
Another reason for a bypass is to avoid hydraulic shock or “water hammer” that results when there is a sudden system flow shutoff.
Ref: Swagelok