We have different fired heaters with fuel oil and gas burners in our refinery. It is seen that the metal tube surface is coated with fine layer of powder (slight yellow colour) externally. This is loosely held to its surface (as seen through inspection door) and reduces the heat transfer leading to high skin, arch and stack temperature. My suspicion on the Fuel oil quality became stronger. So I found that Clarified oil from FCC is not going to the Fuel oil tank. Upon testing the IFO for ash content, it came out to be 0.075%. Is this okay? Any other testing is required?
What probably might be the reason for this powder coating and measures to solve the problem?