I see red, I see red, I see red!
I see red, I see red!
(Red)
I see red!

("I See Red," by T. Finn of the Spit Enz, 1978)

 

SOMETIMES SEEING RED IS A GOOD THING

The bright yellow streaks in the firebox shown above signal mega trouble.

The Outside Process Operator would much prefer to observe a uniform shade of orangish-red in each of the tube passes on both sides of the firebox.

The temperature indicator scale on the left reveals that the glowing orangish-red color of the firebox tubes means they are operating in the desirable 1170-1190°F range.

The tubes with bright yellow streaks and yellow patches are operating in the elevated temperature range of 1230-1280 °F.

Even more alarming, white patches are viewable on the 3rd and 4th tubes counting up from the firebox floor.  The white patches indicate temperatures are exceeding 1300 °F!

What happened?

Why are some of the tubes the desired orangish-red color while others are bright yellow and even white?

The three heat transfer methods are not working as intended in this firebox and poor process operations are at fault!  

 

POOR HEAT TRANSFER = HOT SPOTS AND COKE DEPOSITS

The areas where the temperature has exceeded the desired operating range are called hot spots.

Left unattended, tubes that are chronically exposed to high temperature can fatigue the metal that they are made of and eventually rupture.

The much hotter yellow tubes are warning that the radiant heat from the burners is not being conducted at a fast enough rate (Q/t) through the heater tubes.

Since the Conduction Heat Transfer rate is too slow in these tubes, the Convection Heat Transferred into the process fluid is also diminished.

Otherwise stated:

The intended scheme of Radiant Heat Transfer from flames → Conduction Heat Transfer through heater tubes→ Convection Heat Transfer into process fluid is log-jammed; the flux of heat through the heater is not a-happening as intended.

The backed up radiant heat makes the tubes glow in the spectrum that warns the Outside Operator performing his/her rounds:

Beware! This firebox is operating way too hot!

The culprit that caused all these problems is coke laydown.

Had the Outside Process Operator understood how heat transfer in a firebox works, the first observed hot spots would have been dealt with proactively as discussed at the end of this PTOA Segment.

 

COKE DEPOSITS: THE ROOT OF ALL EVIL

What is Coke and What are Coke Deposits?

 

 

Coke is ash made of carbon fragments that lays down and coats the interior of the tubes.

Coke deposits drop out of the process stream if/when the stream gets too hot and starts to thermally degrade

 

How Does Coke Buildup Inhibit Conduction Heat Transfer?

PTOA Readers and Students are already experts in understanding the components that define Conduction Heat transfer (Q/t, in BTUs or Joules per hour):

Conduction Heat Transfer Rate is defined:

Q/t = [k* A* (Delta T)] / d

Increasing the thickness of the barrier (d) that separates the hot side of a heater tube from the inside of that same heater tube where the process fluid is flowing will decrease the rate of Heat Transfer via Conduction.

The laydown of coke has the same effect of increasing the thickness of the heater tube; the coke effectively creates a wider thickness in the barrier through which Heat Transfer via Conduction is supposed to take place.

And that's not all!

The conductivity factor "k" of carbon steel pipes is 54. The conductivity factor for carbon (which is what coke is made of) is 1.2.

Coke laydown on the interior of the tube is like adding a liner of insulating material to the interior of the tube!

 

How Does Coke Buildup Inhibit Convection Heat Transfer?

PTOA Readers and Students also proficiently understand how the below definition for  Heat Transfer via Convection (q/t, in Joules/hr or BTU/hr) is impacted by several variables, one of which is the mass flow rate of the process fluid (m/t) through the heater tubes:

q/t (in Joules/hr or BTU/hr ) =

                       C_p          *   m/t        * (Delta T)

"Mass per unit of time,"...  m/t ... is called a "mass flow rate."

A decrease in mass flow rate (m/t) also decreases Convection Heat Transfer (q/t).

The mass flow rate (m/t, measured in pounds or kilograms per hour) is permanently decreased when the inside of the tube becomes caked with coke because not as much process fluid can flow through the tube as when it was clean.

 

How Does Coke Start Depositing in Heater Pipes?

Coke deposition starts when the process fluid that contacts the inside heater tube wall gets too hot and starts degrading.

The Outside Process Operator and Board Operator are responsible for the original coke buildup.

During the startup of the process unit and during other prolonged periods of low flow rates, the firing of the burners must be adjusted downward to accommodate the lower process stream flow rates.

Inattentive Process Operators do not diligently adjust firing rates for lower process flow rates.

The combined effect of low process flow rates and normal radiant heat rates will result in cooking the process fluid that flows closest to the tubes.

This outcome is predicted by the mathematical expression that defines Convection Heat Transfer, conveniently repeated below:

A slower mass flow rate (m/t) will result in less convection heat transfer into the flowing process stream.

q/t (in Joules/hr or BTU/hr ) =

                       C_p          *   m/t        * (Delta T)

Since there is not enough mass flow rate to transfer the conducted heat on the inside of the tube, the process fluid touching the interior of the hot tube starts to thermally degrade ... forming coke deposits.

 

THE ENDLESS LOOP OF HEAT TRANSFER PROBLEMS

The above discussion should clarify that when hot spots caused by coke buildup are left unchecked, the  Outside Process Operator and Board Operator will have a super conundrum to deal with.

The PTOA Department of Redundancy Department has identified this avoidable and potentially dangerous process operating outcome worthy of summarizing one mo' time:

The steps that lead up to the catastrophe are as follows:

The coke buildup originates with low process flow rates that are too low to sufficiently transfer the rate of conducted heat transfer because the firing rates of the burners have not been adjusted downward.

As the buildup of coke continues, the rate of Heat Transfer via Conduction is inhibited due to the additional tube wall thickness made of an insulating material.

The buildup of coke also decreases the diameter of the heater pipe; a permanent reduction in mass flow rate results and inhibits the rate of Heat Transfer via Convection.

Eventually the heater will not be able to reach the desired process temperatures.

The inability to attain desired process temperature results in the inability to upgrade feedstocks into products that customers want to buy.

The Process Operators and Board Operators end up running the fired heater too hot in a futile attempt to attain the required process temperatures. That is precisely the condition shown in the photo to the left.

The coke buildup problem spreads to other tubes; the Conduction and Convection Heat Transfer problems spread to other tubes that have been overloaded by Process Operators and Board Operators in their attempt to make up for the lost heat transfer through the first few inhibited heater passes.

The final solution to the conundrum will be a shutdown (hopefully planned and not emergency due to tube failure).

The best case scenario would be the cleaning of heater tubes via a controlled burn for light hydrocarbons or perhaps cleaning with a device known called a 'pig' for harder scale and buildup.

If the damage caused by chronic thermal fatigue is sufficiently severe, entire sections of piping passes will need to be replaced.

The new tubes will require special welding expertise, integrity testing of the welds, and possibly refractory repair.

 

HOW TO PREVENT THE CONUNDRUM

Process Operators and Board Operators that understand the fundamentals of Heat Transfer via Radiation, Conduction, and Convection will not allow the firebox dynamics to spiral into the abysmal state of the heater box featured at the top of this PTOA Segment.

Board Operators and Outside Operators must communicate when process stream flow rates have been decreased; the firing rate for the burners must be adjusted accordingly.

During normal rounds, the Outside Operator must be constantly vigilant regarding detection of hot spots.

The Outside Operator can use hand-held devices that incorporate modern technology that detect and measure heater tube temperatures.

In the event a hot spot is detected, process flow rates through heater passes should be adjusted to enhance the flux of heat transfer through the firebox.

The uninformed Process Operator might actually decrease flow through a heater tube when the correct remedial action would be to increase process flow through the pass to increase the convective heat transfer into the flowing process stream.

 

TAKE HOME MESSAGES: Proper operation of fireboxes requires Process Operators to understand how Heat Transfer via Radiation, Conduction, and Convection are supposed to work together.

The flux of heat transfer through a firebox can be inhibited by the formation of coke deposits which will be indicated by hot spots on the heater tubes.

Outside Process Operators must be constantly vigilant regarding detection of coke deposits. Modern pyrometers with infrared cameras can detect hot spots.

Coke buildup starts when burner firing rates exceed that needed for the process flow rate through the heater tubes. Outside Operators and Board Operators must constantly communicate when process flow rates have been decreased temporarily.

Coke inhibits Heat Transfer via Conduction by increasing the thickness of the heater tube with a layer of insulating carbon.

Coke inhibits Heat Transfer via Convection by decreasing the mass flow rate that can flow through the heater tube.

Process Operators that do not understand the mechanics of heat transfer typically make exactly the wrong adjustments regarding proactively dealing with the first indication of coke deposits.

 

©2015 PTOA Segment 00071
Process Industry Equipment Troubleshooting Operations

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