Type of pump?

The temperature between 95 and 120 is quite huge for an oil and its viscosity is likely to drop sharply.

This drop ins viscosity is like to reduce all the line resistances and that will cause the pressure to drop and that may be compounded by quite a bit drop in the pump output flow.

Check the flow rate at the said temperatures and then you may have some idea of what is happening.

Appraise for your respond,

Type of Pump, Etanorm SY40-250, suitable for Thermal oil up to 300⁰C and pressure of max. 16 Bar.

This is Thermal Oil Boiler/heater (Hot oil loop system)

This might be a case of cavitation at the pump suction. Is there a suction pressure/vacuum gauge in addition to the discharge pressure gauge? If so, what does it read? Are there any partially closed valves or other restrictions between the tank and the pump suction?

Appraise for your respond,

Unfortunately no gauge at the suction line. I wl provide gauge to see the reading.

This is for Thermal oil Boiler /heater system. we cheked the system line and valves were in order.

You have a centrifugal pump. The viscosity change of any decent Thermal oil between 95 and 120 deg C is relatively small as these are usually high viscosity index oils so this should not affect the pump or the system too much. (Not enough to explain a 6.4 bar pressure difference anyway).

I would strongly suggest that your system is contaminated with water and that you are having problems pumping steam. Increase the temperature to around 110 deg C while continuously venting the system and see if you get any vapour coming out. Keep venting until all moisture is removed from your system.

At 120 c you are more than likely cooking off hydro carbons try using diffusers in the high points of your system. At some point as the oil cooks more and more hydro carbons off there will be less gases in your system and you will expierence less cavitation. Be aware when adding new oil as your problems will start all over again.

Without using trade names, a good "thermal oil" (heat transfer fluid) is good at bulk temperature up to around 300 degC. There will be no "cooking off" of hydrocarbons at 120 deC.

I do not think that in Thermal oil is cooking the oil.

The oil here has job to receive the heat given by burner in Boiler and transfer to others media intended to be heat up.