Does a hot oil pump cause the oil temperature to rise?

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Recently, a similar problem arose in a previous project, a sludge thermal drying project in which three lines operated in parallel. There were open connecting valves in the hot oil pipeline system; the main hot oil flow path was boiler – pump – thin-layer evaporator – boiler, with the thin-layer evaporator using the heat from the oil temperature to dry the wet sludge； It was recently observed that after shutting down the boiler for one of the lines (at this point, no sludge is fed into the thin layer, so no heat is consumed; however, the hot oil circulation pump continues to operate), the oil temperature within the closed hot oil circulation system dropped from 176 degrees Celsius to 100 degrees Celsius, after which it started to rise again, increasing by about 1.5 degrees Celsius per hour, until it reached 160 degrees Celsius at which point the increase stopped and the temperature remained constant. Is this behavior normal? The designer suspects that oil leakage was caused by a faulty or improperly sealed connecting valve; however, on-site inspection showed that the connecting valve was closed (it is unknown whether it is damaged internally, as the system has been in operation for 5 years). Does the general contractor think it is caused by the conversion of mechanical energy in the hot oil pump into heat energy? What do you all think, experts? The circulation pump has a flow rate of 450 cubic meters per hour, and its power is 130 KW ； The oil density is 0.8, and the weight of the oil in the pipeline is approximately 20 tons. According to the operation manual, the pump can be stopped when the oil temperature reaches 100 degrees; however, the operator failed to turn it off in order to reduce the frequency of starting and stopping the pump.