As Mr Claus said, is not normal found DCU units, that have recirculation line for heater pumps, I have been in two units, that have that line; one of them worked through a Restriction orifice and the other have a control valve (Flowmeter with minimum flow set point ), that unit additionally to the control valve had a switch (low pressure discharge) that starts the spare pump (automatically), also the recirculation line had a steam injection downstream and up to the control valve to ensure that the line was available (in order to avoid plugging)
These systems had to be designed in this way, motivate the high capacity of the unit and low turndown and where the customer is NOT allowed INSTALLATION OF 3 pumps (two in operation and one in spare, only allowed the installation of two pumps)
Points to keep in mind are: the type of control valve, vibration of the line due to differential pressure that is generated between the discharge pressure of the pump and the point of return (bottom of the fraction), and plugging for bad warming of the line
Best Regards
