Q & A > Question Details
In our Once Through Hydrocracker, the Fractionator Feed Furnace has options for both Fuel Oil and Fuel Gas Firing. Currently due to some problem in the electrical heater in the Fuel Oil Circuit we are using only fuel gas. Some days back inspection department reported a much higher skin temperature in the radiation section of the Furnace. The same report was also upheld during various cross-checks by other departments. Could this be due to the reason as we are not using Fuel Oil? If so, then could somebody explain? Another thing to consider, we are running at 70% T'Put and design conversion so in general the burners are supposed to operate at the given Heat Duty.
19/05/2016 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
Changeover from fuel oil to fuel gas often see this effect. A possible reason can be shifting of heat duty. Fuel oil firing is associated with higher flue gas quantities and thus higher convective heat transfer. Whereas flue gas mass is far lesser in fuel gas firing which increases the radiant load. Thus, flux increases and correspondingly, skin temp shoots up.
09/06/2010 A: Vishnu Ram A S, RIL, vishnu.selvaraj@ril.com
Flame height can be around 50%. The higher the flame, the higher the efficiency, but up to the bridge wall temperature raise limitations.
You might be running the FO burners with max air flow, but the fuel gas burners require less air than the Fuel oil. So the pattern might have changed wider towards the outer side of the burner which reduces the distance between the tube surface and the flame. Moreover the flame height would reduce on the high air flow. This causes for the flame pattern towards the tubes. If you have O2 meter in the arch zone it can be well noted.
Reduce the air and straighten the flame pattern on first priority. If required pinch the individual air damper for the burner, to optimise the air flow.
30/05/2010 A: Hubert Mueller, Bayernoil, guhumueller@web.de
High tube skin temperatures may have various reasons, following are a few points you might consider while analyzing the problem.
-check the heater duty/heatflux by comparing the actual heater inlet/outlet temperatures with the design case (this is a simple way) and find out if you have a possible preheat problem.
-coke layers inside the tubes tend to be a excellent insulator and lead to high skin temperatures
-gasflames normally are shorter than oil flames and that corresponds with a shorter range of the peak temperatures about 1 meter above the highest visible point of the flame and the danger of high skintemperatures in that area.
-is there any flame impingement visible in the furnace?
-You can try to achieve longer flames by reducing the excess air rate to each individual burner. But this tends also to produce more unstable flames with a tendency to flame impingement.
26/05/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Skin temperature of furnace wall? Or process tube skin temperature? If furnace shell skin temp, it indicated some refractory/ high temperature insulation has fallen out. Whether or not it is a safety issue depends on how much and also hot hot the skin is. One can 'insert' a wire basket with high temperature ceramic blanket into the hot area while operating if the firing rate is reduced a bit and induced draft fans can pull the internal pressure at the hot spot negative. Then one can plasma-cut out the hot shell plate, drop the new , hole sized basket of ceramic blanket in a basket (made of high temp wire of course) into the opening (new plate a little larger to lap over old plate, and seal the edge seam with insulating rope, tack weld as required to hold it all in place. This ASSUMES the remaining refractory is good and not ready to fall out.
IF the hot 'skin temperature' is on a tube surface, a plugged tube is the cause and the tube should be 'pinched off' top and bottom to isolate it for it will shortly fail from creep rupture.