A bypass loop is a series of pipe, valves and fittings that allow for a valve, instrument or piece of equipment to be bypassed. Most bypass loops are installed to allow for continuious operation of a facility while maintenance is being performed. Some examples of bypass loops are as follows:
Control Valves - A bypass loop is installed to allow for removing the valve from service for maintenence or for calibrating a positioner or actuator to ensure the valve continues to operate as intended. A control valve has a couple different options for the bypass valve. If the process is at steady state, the designer may opt to include a globe valve or some other type of valve that can help control pressure or flow across the valve.
Instrumentation - Some types of flow meters require periodic maintence to ensure they are working properly Inline meters, such as coriolis, magnetic flow meters and orifice plates may need to be taken offline for troubleshooting. If the meter is a master meter, it may need to be removed from service to be calibrated.
Equipment - Bypass loops for equipment may not always depend on downtime for maintenance. Some heat exchangers may be bypassed when not in use to prevent fouling. As temperatures begin to go out of specification, the exchangers may be brought online. Sometimes upstream conditions are such that a water softener may not be necessary. In-line gas filters and liquid-gas separators will eventually need to have their elements changed or replaced. A bypass loop will allow for the filter to be bypassed while the elements are replaced.
Bypass loop Considerations
A challenge for any operator is how to design a bypass loop without creating a deadleg in the system. For services that are corrosive, having zero flow through a portion of the pipe will increase the rate at which corrosion forms. As shown in the figure below, when the bypass valve is closed, there is a portion of the pipe that remains stagnent. While the loop can be reconfigured several different ways, it is difficult to design a loop which does not create a dead leg.