PetroStreet

Any arbitrary value of temperature of a service material above the critical / allowable limit is considered as 'Hot Spot' and the affected area needs some remedial steps to save it from deterioration / damage.

If the definition is correct, what are the criteria for declaring Critical Limit for hot spot temperature?

Lets take a practical example !

We have a Waste Heat Boiler (refractory lined exchanger at downstream of secondary reformer) whose design temperature is 650C and having construction material as Carbon Steel & Low Alloy Steel (1-1/4Cr 1/2Mo) at different sections. If case of refractory failure inside, the temperatures at CS & Low Alloy Steels come around 800C. What would be critical service temperatures for both materials in this scenario? Again what would be the general criteria to assess the hot spot temperature limit?

Regards

The basic idea behind any hot spot is to identify the locations on refractory lined vessel / piping where the temperatures are more than calculated while performing refractory calculations. Now these temperatures sometimes are well within the range to operate safely without any issue. What's that acceptable range? The metal design temperature must be the cut-off limit ideally as all the designing has been done with a specific design temperature.

Commenting on your case, 800 C is much much higher for carbon steel and even low alloy steel. If you are doing that, I do not know what the basis you have.

Following the design codes where allowable stresses have been tabulated for different materials, you can find maximum temperature limit. For plain carbon steels, the maximum temperature is 427 C however, you have to specifically see which low alloy steel you are using; there are many.

Nice explanation by mechcolor.
I would add that the critical temperature normally defined by the manufacturer / designer is any temperature exceeding the design limit of material mentioned in design specification sheet.
Here we always monitor anything which is abnormal (abnormal means any spot where the temperature is higher than the remaining area) and if this abnormality continues and temperature increases near to the design value, hot spot is declared and proper controls are taken to minimize the skin temperature.

Thank you for quality inputs Mechcolor & Arcpro.

So the final cut off limit for declaring Hot Spot is the Max. Allowable Stresses. What is the impact of internal operating pressure to hot spot material?
Like CS, can we operate material upto 427 C safely at any internal operaing pressure?

Abbas,
What do you mean by final cut off limit of hot spot is the maximum allowable stresses? Hot spot is an area where the temperature is higher than the design temperature of that specific material.
And at any temperature the stresses in the material must not breach the allowable stress limit at that temperature, given by applicable code.

You are bit confused with your questions, I believe.