Problems with heat exchangers

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Ladies and gentlemen, our plant is a hydrogenation complex. The diesel produced at the bottom of the diesel distillation tower is used as the heating medium for the reboiler at the bottom of the jet fuel distillation tower. The process is as follows: the jet fuel enters the reboiler directly and then returns to the distillation tower, with no bypass route； For the diesel stream, there is staged control: one path goes to the reboiler to exchange heat with jet fuel, while another path serves as a bypass that does not pass through the reboiler. In this diesel stream, there is also a large cross-connected pipe that goes directly around the reboiler, controlled by on-site manual valves. In actual production, when all the main pathways for diesel are closed and all the bypass pathways are open, the bottom temperature of the kerosene tower remains very high; therefore, the experienced workers advised opening some large cross-connections on site, which is easy to understand. I went to the site and checked the process again; for the diesel route, that is, the route that passes through the reboiler, the bypass valve was open. I asked the experienced technician for the reason, and he said that the purpose of keeping the bypass line for the control valve open is to prevent leakage from the heat exchanger. When the control valve that regulates the flow of diesel passes through the heat exchanger closes, the heat exchanger may leak due to uneven heating on one side; therefore, keeping the bypass line open ensures that a certain amount of flow continues to pass through the main path. What I would like to ask here is whether, in the systems you are working with, the bypass lines of the main control valves are all open. If so, is such a design reasonable? As everyone knows, opening the bypass line of a control valve affects its regulating performance; I hope experts can offer some guidance.