(3) For a Tanker fitted with heating coils
This tanker is fitted with heating coils in all the cargo tanks and, subject to the limitations of the cargo heating system, can carry cargoes which require heating for either pumpability or sludge deposition/control.
General Guidance
Attention is drawn to the difficulties which may be encountered with certain crude oils. During the discharge of crude oils that exhibit the necessary criteria that would create either pumpabiiity problems or sludge deposition, crude oil washing of each tank scheduled for such an operation should be carried out concurrently with the discharging of the particular tank in order to minimise the affect on the crude oil residues for cooling. Cooling will increase both the Kinematic and Dynamic Viscosities1 of the tank residues and therefore affect the efficiency of the Crude Oil Washing programme.
As a general guidance to the suitability of an oil for crude oil washing on board this tanker, the following criteria should be used:
For Aromatic crude oils whose Kinematic viscosity is the temperature controlling characteristic, the Kinematic viscosity of the oil used for crude oil washing should not exceed 60 centistokes at the oil wash medium temperature;
OR
For Paraffinic crude oils whose pour point temperature is the controlling characteristic, the temperature of the cargo to be used for crude oil washing should exceed its Cloud Point temperature by at least 10℃ if excessive sludging is present and should only be used once in a "closed cycle" washing programme.
The approximate Cloud Point temperature of an oil may be calculated by use of the following formula where the Pour Point temperature(x) of the crude oil is known
The Cloud Point Temperature ℃ = 20.2 (100.00708 x - 0.1157714) + 8
Worked example:
Assume the Pour Point Temperature is + 1℃.
Then:
= 20.2 x(10(0.00708x1)-0.1157714)+8
= 20.2 x(100.00708-0.1157714)+8
= 20.2 x(10-0.1086914)+8
= (20.2 x0.7785896)+8
= 15.72+8
Answer: Approx. Cloud Point ℃ = 24 ℃
1Definitions: Kinematic viscosity is the measure of resistance to flow with gravitation under its own mass force. This is normally reported in the units of centistokes(cst) or mm2 /sec.
Dynamic viscosity is the measure of resistance to now with an induced shear stress or at a known rate of shear. This is determined from the equation of Shear Stress (Pascals - Pa) divided by Rate of Shear (inverse seconds - s-1). The unit of Dynamic viscosity is either Centipose (cps) or mPa s.
