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Hot oil Flashpoint

Hot oil Flashpoint

Hot oil Flashpoint

Good day Gents,
We are using a thermal oil as heating source to heat up our HC products and Glycol system. A flash fire occurred while removing an insulation material to investigate source of smoke in the flange area. As part of investigation, oil sample was collected, analyzed and the result shows a reduction in the flashpoint of hot the oil (186 & original flashpoint was 215 - as per MSDS) whereas our normal operating temperature is 240 C. Thoroughly checks carried out to check possible of degradation / contamination in the oil but all parameters found to be normal.

Can you share with me your thought to figure out the possible other root cause of reduction in flashpoint and what could be the impact to continue running the system with the same status (operating temperature higher than flashpoint)? and also possible root cause of flash fire considering that an oil sweep observed in the flange area and coke formation found to be around he flange!


RE: Hot oil Flashpoint

thanks for your reply and apologize for being late in my reply.

RE: Hot oil Flashpoint

What was the source of ignition? It couldn't be auto-ignition?

RE: Hot oil Flashpoint

An insulated flange was smoking and while removing the insulation material to investigate a flash fire occurred as the insulation material found to be soaked with hot oil due to leaking flange.

RE: Hot oil Flashpoint

But your system operates nowhere near the autoignition temperature so there must have been a spark? Was a grinder being used to remove the insulation?

I am asking these questions because I believe there needs to be an evaluation of safety practices when such a leak occurs.

You should also consider using an oil with a flash point that exceeds your normal operating temperature.

RE: Hot oil Flashpoint

"Spontaneous combustion" in porous , oil-soaked materials (oily rags, e.g.) is a well known phenomenon. Oxidation generates heat, and the heat is trapped by the insulation.

RE: Hot oil Flashpoint

Thanks Gentlemen for your feed back; well appreciated.

RE: Hot oil Flashpoint

Possible causes for thermal degradation of hot oil:
a)Leak of glycol or hydrocarbons through leaking tubes or tube-tubesheet joints at heat exchangers into hot oil
b)Inadequate heating surface area provided at hot oil fired heater, resulting in high tube surface and film temp. Thermal design tube surface temp at the hot end of the radiant section should be kept below max permissible continous operating temp of hot oil - see technical spec sheets for hot oil.
c)Hot spots in fired heater radiant section
d)Inert gas or N2 not used as blanketing gas at hot oil expansion drum.

RE: Hot oil Flashpoint

Thanks for your input, appreciated!

RE: Hot oil Flashpoint

LPS for CompositePro. That is an important item for OP to consider. Oil soaked insulation / rags are a unique hazard. As I understand it one big problem is thermal insulating effects which trap the heat released by oxidation reactions (slow aging oxidation reactions initially below flash point but still exothermic), which can in turn push local temperatures higher than surrounding temperatures and higher than flashpoint.


result shows a reduction in the flashpoint of hot the oil (186 & original flashpoint was 215 - as per MSDS) whereas our normal operating temperature is 240 C.
I don't quite follow the logic of the sentence. Flashpoint was already below operating temperature, and degradation just moved it further below? Are there estimates of temperature in the area where the oil might end up?

(2B)+(2B)' ?

RE: Hot oil Flashpoint

I believe your interpretation is correct. Looking at the PDS for a few heat transfer oils, they all had maximum rated continuous operating temperatures that are above their flashpoint. Whether it is wise to operate them in that condition is another question. There are oils available with flashpoints that exceed OP's operating conditions. Viscosity may be an issue, however.

RE: Hot oil Flashpoint

The operation of heating oils above flash point is normal practice. It will not cause a problem, as long as there is no flame that can contact the evaporated oil.

That the flash point goes down during operation in a system is normal. It is the result of cracking of oil constituents in the thermal fluid in the system due to local hot spots. Cracking leads to both high viscosity components and lower viscosity components. The latter are the reason for a lower flash point.

That a thermal fluid can be used in a system with temperatures above the flash point is due to the fact that normally the "overheated" thermal fluid does not come into contact in a appropriately designed installation.

Hot oil leaked into porous insulation material poses a danger. As soon as the insulation material is tampered with, oxygen may leak into it, which can lead to spontaneous combustion.

RE: Hot oil Flashpoint

Piping for hot oil lines should be B31.3 class 300 minimum, and preferably RTJ.

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