I remember the similar instance during early days of my career. I went for commissioning of fired heater / burner system. Normally a fuel gas is used for firing and incase fuel gas is not available, as a back up fuel oil system provided. It was dual fired burner system. Since, this fuel oil system is normally remained idle, the similar issue we faced during start-up. Although heat tracing was provided, since the system remains idle, most of the times, no operator usually pays attention. And when it has to actually work, heat tracing was out most of the times due to either thyrester trip or any other reason. It resulted one small accident also, despite of low temperature, operator started the pump which was positive displacement pump and keep pushing the fluid only unable to do it (due to very high viscosity), resulted in gasket failure of one the discharge filter and oil spilled out. Fortunately, no one was harmed. What we had to do is, take steam hose and heat the entire system from outside manually. This worked for timebeing but for sure this is not the safe way to do it and not an SOP. Not recommended as designed system.
I think, your system is designed to operate continuously. So you may not face big issues. I dont have full picture of your system design, hence can give you some general design points as below.
1) Heat tracing failure should alarm in the control room and shall be designated as "high priority" alarm
2) Provide one recirculation line downstream of heat exchanger back to suction vessel to establish heated oil profile. your pump and heat exchanger should be near to suction vessel. Moreover, as you rightly said, try to reduce your system volume as much as you can. have your suction vessel, electric heater and pump near to discharge point as much as you can.
3) provide back-up electric heater inside your suction vessel from which pump is taking suction.
4) Provide heat conservative insulation. Failure of heat tracing will not cool down the inventory in a short time. Heat conservative insulation will help you to get sometime to solve the issue. Also, if your tracing is working fine, failure of pump / tripping of pump, may not lead any operation issue as oil temperature is still maintained.
5) Better to consider steam tracing. Make sure standard steam trap arrangements have bypass. so that, you can manually ensure the condensate is drained and steam is available inside the tracing. If your system is so critical, consider providing electrical tracing in parallel with steam tracing, as a back-up.
6) make sure all dead legs and drain points are adequately heat traced. if possible, provide additional tracing points.
7) your heat exchanger design is also critical. Design the heat exchanger taking conservative oil viscosity and at low oil velociy. Although, actual velocity during normal operation is higher, it will result in improved HTCoefficient in operation. This will give you some margin for upsets.
Hope this helps.
Edited by Jiten_process, 16 September 2020 - 02:20 AM.