short-term high temperature failures.pdf

7/28/2019 Short-Term High Temperature Failures.pdf

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Forgotten Element In Boiler Rooms

Creep and Creep Failures

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How to Destroy a Boiler -- Part 1



Identifying Pressure Vessel Nozzle

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Inspection, Repair, and Alteration of

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Inspection, What Better Place to

Begin

Laminations Led to Incident

Liquid Penetrant Examination

Low Voltage Short Circuiting-GMAW

Low Water Cut-Off Technology

Low-Water Cutoff: A Maintenance

Short-Term High Temperature Failures

For all materials used in boiler construction, the strength decreases as temperature increases. Table Ilists the short-term tensile strength for SA192 and SA213 TP321H that illustrates this point.

The simplest explanation for all "short-term" overheating failures is: when the tube metal temperaturerises so that the hoop stress from the internal steam pressure equals the tensile strength at elevatedtemperature, rupture occurs. For example, in a super-heater of SA192 tubes, with a designed metal

temperature of 800oF, the ASME Boiler and Pressure Vessel Code gives the allowable stress at800oF as 9,000 psi. If the tube-metal temperature should rise to a temperature of around 1300oF, the

hoop stress would be equal to or slightly greater than the tensile strength at 1300oF, and failurewould occur in a few minutes.

The balance between heat flow and fluid flow can be upset from either side; too much heat flow ortoo little fluid flow. In a waterwall tube, steam forms as discrete bubbles, nucleate boiling. When thebubble is large enough, the bubble is swept away by the moving fluid, and the cycle repeats. At toohigh a heat flux or too low a fluid flow, steam-bubble formation is too fast for removal by the movingfluid. Several bubbles join to form a steam blanket, a departure from nucleate boiling, DNB. Heattransfer through the steam blanket is poor, steam is an excellent insulator, and tube-metaltemperatures rapidly rise and failure occurs quickly.

In a superheater or reheater, DNB cannot occur as only steam super heating takes place, no boiling.However, short-term overheating failures do occur but usually during start-up. Boiler operationalproblems that can lead to these short-term high-temperature failures include, among others:

David N. French President, David N. F rench Inc, Mettalurgists

Category: Operations

Summary: The following article is a part of the National Board Technical Series. This article was originallypublished in the April 1991 National Board BULLETIN. (4 printed pages)

The operation of a boiler is a dynamic balance between heat flow from the combustion of a suitable fuel andeither steam formation within the furnace or steam super heating within the superheater or reheater. Ineffect, the steel tube is "heated" by the flame or hot flue gas and simultaneously "cooled" by the fluid (steam,water or a mixture of steam and water) flow. When this balance is maintained within the design limits, metaltemperatures are also maintained within design parameters.

TABLE I -- SHORT TIME ELEVATED TEMPERATURE TENSILE STRENGTH

TEST TEMPERATURE, oF TENSILE STRENGTH, PSI

SA192 SA213 TP321H80 55,000 84,000

300 59,000 68,000

500 59,500 62,500700 52,600 60,000

900 41,000 56,000

1100 20,000 49,300

1300 9,900 38,0001500 5,600 23,000

The expected life under these conditions can be measured in a score or more years. However, whenthe balance is upset, metal temperatures rise and failures occur sooner that expected. Depending onthe relative temperature rise, failures can occur either very quickly, that is, in a matter of minutes; orover a much longer time period, that is, in matter of many months. For convenience these tworegimes are defined as "long term" and "short-term" overheating. This article will discuss "short-term"high-temperature failures.


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Magnetic Particle Examination

Maintaining Proper Boiler

Inspections Through Proper

Relationships

Microstructural Degradation

Miracle Fluid?

Organizing A Vessel, Tank, and

Piping Inspection Program

Paper Machine Failure Investigation:

Inspection Requirements Should Be

Changed For Dryer Can

Pipe Support Performance as It

Applies to Power Plant Safety and

Reliability

Polymer Use for Boilers and

Pressure Vessels

Pressure Vessels: Analyzing

Change

Preventing Corrosion Under

Insulation

Preventing Steam/Condensate

System Accidents

Proper Boiler Care Makes Good

Business Sense:Safety Precautionsfor Drycleaning Businesses

Putting a Stop to Steam Kettle

Failure

Quick Actuating Closures

Quick-Actuating Door Failures

Real-Time Radioscopic Examination

Recommendations For A Safe Boiler

Room

Recovering Boiler Systems After A

Flood

Rendering Plants Require Safety

Residential Water Heater Safety

School Boiler Maintenance

Programs: How Safe Are The

Children?

Secondary Low-Water Fuel Cutoff

Probe: Is It as Safe as You Think?

Short-Term High Temperature

Failures

Specification of Rupture Disk Burst

Pressure

Steam Traps Affect Boiler Plant

Efficiency

Stress Corrosion Cracking of Steel in

Liquefied Ammonia Service - A

Recapitulation

Suggested Daily Boiler Log Program

Suggested Maintenance Log

Program

System Design, Specifications,

Operation, and Inspection of

Deaerators

Tack Welding

Temperature And Pressure Relief

Valves Often Overlooked

Temperature Considerations for

Pressure Relief Valve Application

'

Flame impingement from misaligned or worn burners that leads to the formation of a steamblanket, as the local heat flux is too great for the fluid flow through the tube.

Blockage of a superheater tube with condensate or foreign material that prevents steam flow. These problems are more frequent during start-up.

Reduced flow in either a water or steam circuit that leads to inadequate cooling. Pinhole leaks,especially at poor welds or slag falls, severe dents from slag falls or ruptured tubes, and partialblockage from debris or other foreign matter are some of the more obvious causes.

Foreign objects, broken attemperation- spray nozzles, for example, in headers that partiallyblock a superheater or reheater tube.

Regardless of the location within the boiler that these failures occur, the appearance is similar. Thereis a wide-open burst with the failure edge drawn to a near knife-edge condition, and the length of theopening four or five tube diameters. These failures display considerable ductility; the thinning at thefailure lip may be more than 90% of the original wall at the instant of rupture. The microstructuresthroughout the failure will usually indicate, in the case of ferritic steel, the peak temperature at thetime of failure. For ferritic steels there is a transformation from ferrite and iron carbide or pearlite, toferrite and austenite. This temperature is referred to as the lower-critical transformation temperatureand occurs at 1340oF or higher, depending on the exact alloy composition.

For failures that occur below the lower critical transformation temperature, the microstructure throughthe failure lip shows considerable distortion and elongation of the ferrite and pearlite. If the peaktemperature at the instant of failure is in the austenite and ferrite temperature regime when failureoccurs, the escaping steam will rapidly cool the metal to the steam temperature. The resultingmicrostructure will contain ferrite which undergoes no transformation on rapid cooling, and bainitewhich is the transformation product of austenite on rapid cooling. The failure lip may also showconsiderable distortion to the ferrite and bainite, again, indicative of a rapid metal deformation at thetime of failure. At the end of the failure where there has been much less distortion and swelling of the

tube prior to failure, the microstructure is ferrite and bainite, but without the elongation to the grains,indicative of tube swelling.

Under rare conditions, and usually in low-pressure boilers, the peak temperature at the moment of

failure can be in excess of 1600oF, a microstructure at the time of failure that is all austenite. Whenthese failures occur, the microstructure does not show the distortion to the austenite, as thetransformation to bainite effectively wipes out the evidence. When the cooling rate is slightly slower,ferrite will form preferentially at the austenite grain boundaries and along certain planes of atoms inthe austenite. The remaining austenite then bans forms to bainite. The resultant microstructure is amixture of ferrite and bainite and is sometimes referred to as a "Widmanstatten structure."

One final comment, ductile failures can also occur at normal operating conditions but are not, strictlyspeaking, high-temperature failures. Wastage of a tube from corrosion or erosion can reduce the wallthickness, which, in turn, raises the hoop stress. Such failures occur in waterwall tubes, for example,where sootblower erosion has reduced the wall thickness, or in the convection pass from fly-asherosion. These failures can occur at normal operating temperatures if the wall thickness reduction is

sufficient.

While these microstructures and the estimated peak temperature at the time of failure cannot predictthe sole cause of the failure, the metallurgical analysis can suggest the kind of boiler-operationalproblem that is likely to be the cause of the rupture.

Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changedbecause of advances in material technology and/or actual experience. The reader is cautioned torefer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for currentrequirements.


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e u orze nspec ors

Responsibility for Dimensional

Inspection

The Effects of Erosion-Corrosion on

Power Plant Piping

The Forgotten Boiler That Suddenly

Isn't

The Trend of Boiler/Pressure Vessel

Incidents: On the Decline?

The Use of Pressure Vessels for

Human Occupancy in Clinical

Hyberbaric Medicine

Thermally Induced Stress Cycling

(Thermal Shock) in Firetube Boilers

Typical Improper Repairs of Safety

Valves

Wasted Superheat Converted to Hot,

Sanitary Water

Water Maintenance Essential to

Prevent Boiler Scaling

Water Still Flashes to Steam at 212

Welding Consideration for Pressure

Relief Valves

Welding Symbols: A Useful System

or Undecipherable Hieroglyphics?

What Should You Do Before Starting

Boilers After Summer Lay-Up?

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