Temperature Overshooting in Fired Heater

[Alaaeldin55]

Dear Colleagues,

We are currently investigating an incident involving one of our multiple convection fired heaters.

During this event, the process outlet temperature exceeded the design limits for both the heater coil and the outlet process associated piping.

While we initially considered potential causes such as low process flow, excessive heat input, or malfunctioning fuel control valves, a sharp increase in process flow during the overshoot event makes these less likely.

A plausible explanation is that one of the two tube coil heating passes within the heater may have become partially blocked. This blockage could have led to localized overheating, and once the blockage was cleared, the increased flow through the previously restricted pass caused a sudden spike in outlet temperature.

Unfortunately, the absence of temperature and flow sensors on individual heating passes limits our ability to directly confirm this hypothesis.

We seek your expert advice on the credibility of this scenario and potential strategies to ensure the safe operation of the heater.

Thank you for your valuable insights.

 

[LittleInch]

Well only you have knowledge of what happened in terms of temperature, temp rise rate, flow, heat input (gas?) flow rate, did the lining overheat or supply radiant heat at low flow which added to input heat? Fluid being heated? Why would it block under heat?

If you don't supply useful information you won't get good responses.

 

[Alaaeldin55]

Well only you have knowledge of what happened in terms of temperature, temp rise rate, flow, heat input (gas?) flow rate, did the lining overheat or supply radiant heat at low flow which added to input heat? Fluid being heated? Why would it block under heat?

If you don't supply useful information you won't get good responses.

- The fluid being heated is Hot Oil, which is used as a heating medium at other areas of the plant.

- Fuel type used in the heater is Fuel Gas.

- During the event , the temp increased from 230 to 330 C in 3 minutes and the flow of hot oil increases from 100 to 152 m3/hr during the same time interval. During the overshoot event , the heater tripped on HH process temp, 250 C and fuel gas valves closed but temp continued to increase sharply

- I have no idea, why would blockage could develop inside heater coil, may be coking or fouling, especially that this heater was shutdown for two months. we have never experienced a similar event, or may it happened before but was not severe and therefore gone unnoticed.

 

[LittleInch]

That's a bit more like it.

How much fluid is inside the heater in the tubes?

Is certainly very odd.

Was the inlet temperature steady during this event?
Is there any bypass or return loop from hot side to cold side?

How the heater taken out of service? Drained or left full of oil?
What is the layout of tubes inside? Any drawings of the internals or P&ID?

 

[georgeverghese]

Your theory about one of the 2 heater passes getting unblocked does sound credible given the limited info provided.
Maybe there are one or more users of hot oil that have slow acting hot oil feed control valves. This would be the case if, at these users, the heating control loop is a very basic TIC - TCV setup - in such cases, the sluggish TIC does not "see" the sudden increase in hot oil flow when pressure upstream of the TCV has suddenly risen.

 

[LittleInch]

It is credible if the amount of hot oil within one tube (assuming it gets blocked right at the end, is greater than 6 m3 as otherwise cooler oil would enter the blocked tube and start reducing the temperature.

But without data on incoming oil temp and volumes it is just a possibility.

 

[georgeverghese]

Think the temp spike on hot oil supply can be corrected if you operate the min flow bypass as a max DP bypass, assuming you have a generous inventory of hot oil in the expansion drum. This change may result in additional power draw at the hot oil pump, if it is operating on a fixed speed drive.