And it burns, burns, burns,
The ring of fire, the ring of fire.

("Ring of Fire," by J. Carter Cash and M. Kilgore, 1963)

 

RISKY BUSINESS

PTOA Readers and Students already know that every increase in process temperature originates from Heat Transfer via Radiation taking place in a heater/furnace somewhere in the processing facility.

Intentionally creating a fireball does not happen risk-free. However, astute operations minimizes the risk.

Process Operators and Board Operators must be vigilant for abnormalities in heater operation and the first place to look is at the flames ... and even more important ... the lack of flames where there are supposed to be flames.

 

FLAMEOUT

A flameout condition is present when the flame on a burner goes out while fuel is still being charged into the firebox.

Flameout is a dangerous situation. When not noticed and left unattended, flameout will result in the explosion of the fired heater.

Once a flameout situation is noticed, the shutdown protocol should begin immediately.

In real life, the Outside Process Operator is put under enormous pressure to return process temperatures to the minimal required to make products that can be sold.

Regardless of the pressure, the fired heater must be shutdown and purged free of hydrocarbon fuel.

Purging the firebox with inert gas will both decrease the firebox temperature and dilute the fuel source required for the combustion reaction to occur.

Once a safe environment has been reestablished, the ignition process can be tried again.

Flameout can be caused by:

A. Loss of Draft

Draft is the upward flow of combustion products through the heater that simultaneously creates a negative pressure (aka vacuum).

Huh?

Say what?

 

PTOA Readers and Students who are reading the PTOA in the intended sequential order already know The Universe mandates that a fluid can only flow when a pressure differential exists; furthermore, the fluid shall always flow from the area of high pressure to low pressure.

Combustion gases are made of water vapor, carbon dioxide gas, carbon monoxide gas (an incomplete combustion byproduct) and non-reacted air compounds ... definitely a fluid!

The pressure at the bottom of the fired heater is the same pressure PTOA Readers and Students walk around in everyday .... aka atmospheric pressure. In fact, atmospheric air provides the air used for combustion.

When atmospheric pressure is the starting point for a "high" pressure and a low pressure is needed to make a fluid flow, the only way to satisfy the Universe is to "create a Vacuum " .... an area of lower-than-atmospheric pressure that will allow flow to happen.

Hot combustion products ... even in a campfire ... have the natural tendency to flow upward.

When the combustion takes place in a structure that has a decreasing diameter connected to a stack,  the flow of hot combustion products increases in velocity while flowing upward.

Draft is controlled by a movable plate that is called a damper.

The damper is installed at the lower end of the stack.

The stack is located above the breeching section of the heater.

The breeching section of the heater is the slanted area above the convection tubes that funnels the combustion gases/flue gases into the stack.

The control cables shown in the diagram above extend to a level that the Outside Operator can reach to adjust the damper position.

Draft is gauged by measuring the difference in pressure below the damper and above the damper.

Draft is gauged by measuring the difference in pressure below the damper and above the damper.

Draft is measured in "inches of water column" (in WC), which sounds strange but works well when measuring a vacuum.

Draft gauges are located below and above the damper.

The draft measured above the damper is subtracted from the draft measured below the damper and the result is a difference in pressure (DP), also represented in inches of water column (in WC).

The draft can be monitored by an Outside Operator on a local indicating and recording instrument.

Modern technology is now available to bring the draft reading into the board where its trend can be monitored and alarm points set as shown in the illustration below.

In either case, an erratic change in measured draft indicates an unstable flame and warrants immediate attention.

 

B. Insufficient Oxygen  for Combustion.

PTOA Readers and Students who are reading the PTOA Segments in the sequential order as intended recently learned that larger hydrocarbons require more oxygen (from air) to combust.

Complex fuel and petrochemical complexes purchase natural gas as a backup fuel; burning hydrocarbon fragments that are leftover from physical separations and chemical reactions is more cost effective for Plant Owners.

Ergo, the composition of the fuel gas in the Fuel Gas Drum changes with the ability to profitably sell each commodity that the processing complex makes (for example: fuels, or soaps, or the building blocks that make plastics).

There are dozens of process conditions that can cause a significant and rapid change in fuel gas composition.

The graph to the right compares the fuel gas flow to the burners of a heater and the excess oxygen measured at the stack of the same heater over a seven hour period, from 8 am to 3 pm.

The fuel gas volumetric flow rate is the black line (measured in MMscfd, the scale to the right). Variance in the fuel gas flow rate required for combustion is evident.

The amount of oxygen required for combustion can be inferred from the amount of excess oxygen that is detected and measured in the heater stack. In the above graphic, excess oxygen is labelled "Stack, O2 %" and is the maroon line and scale on the left.

The graph illustrates that the amount of oxygen needed for combustion varies abruptly depending upon the fuel gas composition and process flow rates through the heater.

Alert Board Operators must be vigilant to notice abrupt changes in fuel gas flowrate like the one that occurs at 8:45 in the above graph.

The Board Operator must constantly monitor and verify that sufficient combustion oxygen is supplied to the burner while fuel gas composition changes are ongoing.

Too little oxygen can cause flame instability and flameout.

 

FLAME IMPINGEMENT

Long flames that make contact with the  heater tubes can also be caused by too little oxygen to support combustion.

If left uncorrected and sustained, the flames can cause the tubes and refractory to overheat and fatigue.

Eventually the tube will break.

Although this is a totally avoidable outcome in the first place and scary to deal with while ongoing, a firebox is a preferable place to have an unwanted process fire because of the relative ease of containing the fire and eventually snuffing it out.

The astute Outside Process Operator will not allow impingement to proceed to the point of tube fatigue.

Decreasing the fuel to the burner will rebalance the oxygen-to-fuel ratio.

 

TAKE HOME MESSAGES:  Radiant heat transfer requires flame monitoring and management.

Two potential operating problems related to Radiant Heat Transfer and the need to create combustion are Flameout and Flame Impingement.

Draft is the upward flow of combustion/flue gases through a heater which simultaneously creates a negative pressure(aka Vacuum) that can be measured in inches of water column.

Monitoring and controlling draft with the stack damper reduces the occurrence of flameout.

Anticipating the need for sufficient combustion oxygen that accommodates changes in fuel gas composition also reduces the occurrence of flameout.

Flame Impingement is the direct contact of flames on the exterior walls of tubes caused by too little oxygen to support combustion.

Flame Impingement can be eliminated by adjusting the oxygen-to-fuel ratio by reducing the fuel flowrate.

 

©2015 PTOA Segment 00070
Process Industry Equipment Troubleshooting Operations

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