Hi again Pengze,
You appear to have a lot of problems on your ship, exploding generators, worn out fuel pumps and now Thermal oil plant without an instruction manual!
1)The expansion tank should not be lagged because it is open to the air, if the oil inside became hot it would oxidise, it is purely an expansion tank. There should be no ciculation through it, so it should not get hot.
For safety you could fit wire mesh around it, to prevent personal injury if the tank became hot because the system was being incorrectly operated.
2) There is no need to fit safety valves because it is not a steam system, and the system is free to expand to the expansion tank. However, it would always be safer to fit a relief valve on the boiler in cause someone tried to fire it up with the valves closed!.
3) If the circulating pumps are positive displacement pumps they would be fitted with internal relief valves, just as Lubricating oil or Fuel transfer pumps would be, because if they were run against a closed discharge valve it would damage the motor and pump.
4) Not sure if you mean the thermal oil leaks inside, say a heated fuel Tank or inside the boiler. However, the great thing about Thermal oil is that the system is full of oil, the pipes do not corrode and you dont have all the problems with pipe failures, gaskets blowing out, leaking valve glands etc, that you have with steam systems. The first ship I sailed on with Thermal oil was 5 years old when I joined and I sailed on her for 3 years, in all that time we never had an oil leak or had to add any oil.
The circulating pumps had never been touched in 8 years, and were still running perfectly. The only work we ever did on the system was to service the burner. If you had a thermal oil leak inside a tank or other heating coil you would notice a drop in the level of the oil expansion tank. Then by isolating all the heating coils in turn you would determine where the leak was. You would then have to open up the tank/heater and flush the leaking line through to make sure it was not contaminated, gas free it and repair it. Then carry out a pressure test, if that is OK circulate cold oil through the system with the air vent valves open, to make sure no air was trapped in the system. Then close the air vent valves, and fire up the plant.
The pipework in the ER will be insulated to protect the crew and I have seen drip trays fitted under every pipe flange on some ships. For safety the oils are always used well below their flash point, several different types of oil are available with different Flash Points.
If there is any leakage inside the boiler, the oil will burn off and you will start making smoke, when you open up the burner to service it you will find the leak.
I have said that you must always make sure that the air vents, from the high points in the system to the top of the expansion tank, should always be kept closed when the Burner is firing, however, it is equally important that the run down valve from the expansion tank to the circulating pump suction, is always kept open, otherwise the expansion of the oil with temperature could lead to serious over pressure in the system.
It is always better to ask a stupid question than to do a stupid thing.