Oh you've given us a story that forever we will use
Because this is how we choose to remember
To remember, To remember
To remember, Just where we were.

("To Remember," by Josh Kelley, 2008)

 

PTOA Segment 70: FLAME MANAGEMENT 101

PTOA Readers and Students learned about Flameout and Flame Impingement, two potential operating problems that are associated with using the combustion reaction to create radiant heat.

PTOA Readers and Students learned that Flameout is a dangerous situation that occurs when the dynamics of burning no longer support the formation of a flame.

The study of Flameout in PTOA Segment 70 was focussed on fired heaters; however, flameout conditions can occur in any equipment that uses the combustion reaction to create radiant heat.

During the interval that flameout occurs, fuel continues to be loaded into the firebox;  ergo, the firebox will detonate like a bomb if it contacts an ignition source.

PTOA Readers and Students already know that new Process Operators must pay heed to the specific start up and shut down procedures for each piece of combustion equipment; however the Flameout condition universally will mandate immediate heater shutdown and purging of unburned fuel with an inert gas like nitrogen.

PTOA Readers and Students learned that the two causes of Flameout are:

• Loss of Draft.
• Insufficient Oxygen to support combustion.

This PTOA Segment introduced PTOA Readers and Students to the concept of measuring a pressure that is below the atmospheric pressure which everybody walks about in daily.

A pressure that is below atmospheric pressure is called a "vacuum pressure."

There are several uses of vacuum pressure in the processing industries. Draft through a chimney stack is one example of the industrial use of vacuum pressure.

Creating and maintaining vacuum pressures takes vigilance on the part of Process Operators.

PTOA Readers and Students learned that draft is controlled by the adjustments that the Outside Process Operator makes on a damper that is located in the stack.

Draft is measured in units of "inches water column," which is an effective measurement range when the pressure that is being measured is just under the atmospheric pressure (meaning a very slight vacuum pressure).

PTOA Readers and Students learned in the previous PTOA Segment 69 that the oxygen (from air) needed to complete the combustion reaction increases when the amount of carbon in the fuel gas increases.

In this PTOA Segment 70, PTOA Readers and Students learned that insufficient oxygen to support combustion occurs when the composition of fuel gas changes and air supply is not likewise adjusted.

PTOA Readers and Students learned that the amount of excess oxygen measured in the stack is the parameter Control Board Operators use to insure sufficient oxygen (aka air) is available for combustion.

PTOA Readers and Students learned that long flames that extend from the burners to the heater tubes are also caused by too little oxygen to successfully combust a carbon rich fuel.

Flame Impingement that is not corrected will cause the tubes and refractory to weaken and fatigue.

PTOA Readers and Students learned that astute Outside Operators detect Flame Impingement while doing their rounds and fix the problem by reducing the fuel flow rate to achieve a more optimal oxygen-to-fuel ratio.

 

PTOA Segment 71: YOUR MISSION: FIND HOT SPOTS AND COKE

PTOA Readers and Students had recently learned in PTOA Segment 68 how the three modes of heat transfer work together to heat up process fluids, boiler feed water, and reaction gases in fired heaters, package boilers, and reaction furnaces.

This PTOA Segment 71 showed what happens when the heat flux through a fired heater is impeded by the buildup of coke deposits on tube interiors.

PTOA Readers and Students learned that coke deposits impede conduction heat transfer by effectively lining the interior of the tube with an insulating layer of carbon ash (aka coke).

Otherwise stated in the jargon of conductive heat transfer, the effective thickness of the tube wall (d) is increased with a material that has a much lower conductivity factor (k).

PTOA Readers and Students learned that coke deposits impede convection heat transfer by decreasing the mass flow rate (m/t) of the flowing fluid which impairs the ability to remove heat from temperature-increasing equipment.

PTOA Readers and Students learned that coke deposition begins when firing rates are excessive compared to the mass flow rate of the flowing fluid.

The slower mass flow rate of a fluid provides enough time for the fluid at the interior of tube walls to be cooked (aka thermally degrade) by the excessive heat, thus forming deposits of coke ash.

PTOA Readers and Students learned that Outside Process Operators are constantly vigilant with regard to detecting coke deposits by watching for the appearance of hot spots on tube exteriors.

In the USA, Outside Process Operators  often use pyrometer technology in hand held devices to detect hot spots.

Once detected, the correct response is for the Outside and Control Board Operators to work together with good communication in an effort to remove coke deposition by temporarily increasing the mass flow rate through the pass of concern in the firebox.

Process Operators who are uninformed about heat transfer mechanics typically decrease mass flow rates and thereby unwittingly increase coke buildup.

 

PTOA Segment 72: WHY IS THIS DUDE SMILING?

This admittedly long PTOA Segment introduced PTOA Readers and Students to yet one more problem caused by insufficient combustion air: soot.

Flameout and flame impingement were presented in PTOA Segment 70 as problems caused by insufficient combustion air. Unlike flameout and flame impingement, soot build up is a chronic type of problem that inhibits heat transfer and  creates a confirmed airborne health hazard.

PTOA Segment 72 delineated the difference between the radiant and convection sections of fired heaters and the associated hardware in each section.

Unlike flameout and flame impingement, soot creates heat transfer problems in the convection section of the combustion equipment as opposed to the radiant section of the firebox.

Thirty percent of total heat transferred into a flowing fluid occurs in the preheater tubes that run through the convection section of a fired heater.

Soot inhibits convection section heat transfer by reducing the amount of conduction heat transferred through the preheater tubes that run through the convection section. Conduction heat transfer is reduced through the tubes because:

• Soot reduces the surface area (A) by clogging the fins on the tubes.
• Soot has a low conductivity factor (k). A layer of soot effectively adds a layer of insulation on the exterior on the preheater tubes.
• A layer of soot on the exterior of the finned preheater tubes effectively increases the pipe thickness (d)  that separates the hot fireside of the tube from the cooler interior through which the process fluid flows.

Soot also accumulates on preheaters, economizers, and superheaters that are piped through the convection section of package boilers and the ducts of steam reformers.

Fired heaters that use heavy hydrocarbons for combustion fuel must have soot blowers installed to periodically steam away soot that impedes heat transfer.

Almost 500 words of PTOA Segment 72 were dedicated to introducing PTOA Readers and Students to the Clean Air Act regulations that apply to processing industries and then advocating the economic and environmental need for such regulations.

Your Mentor has worked in all phases of process industries and can assure PTOA Readers and Students that without regulations in place the energy sector would freely pollute air and water; confirmation of this statement simply requires review of industrial performance prior to 1970.

Process Operators are on the front line with regard to mitigating the environmental degradation which is a natural byproduct of industrial processing. Adopting an attitude of environmental stewardship will result in cleaner air and water and the processing industries will still be adequately profitable.

 

©2015 PTOA Segment 00084
PTOA Deja Vu Review 3-5

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