The hot oil system expansion vessel or surge drum should be connected to the system return line on the pump suction side.
The vessel should be elevated so that the normal operating level of the heat transfer fluid (HTF) in the vessel is higher than the highest possible HTF level in the system. (This will facilitate proper venting and provide sufficient NPSH for the pump.)
If this requirement is difficult to meet, a lower elevation may be selected but additional design measures are required to prevent vapor being locked in the higher parts of the circuit; to evacuate such a vapor lock if it occurs; to cater for HTF draining into the expansion vessel during a shut-down to avoid HTF overflow into the flare system; and to ensure that the HTF system pressure remains positive at the highest point to avoid boiling.
In addition there should be height considerations to ensure there is sufficient NPSH available.
As the HTF will degrade or may absorb material from the flare, the vapor pressure of the HTF in the system will be higher than that of the pure HTF as supplied by the Manufacturer. This lowers the NPSH.
To determine the NPSH available to the pump, it should be assumed that the vapor pressure of the HTF is equal to the pressure in the expansion vessel at normal operating temperature. If necessary, the height of the drum should be raised to ensure that there is sufficient NPSH available.