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A Boiler: The Explosive Potential of a Bomb
Acoustic Emission Examination of Metal Pressure Vessels
Anatomy of a Catastrophic Boiler Accident
Austenitic Stainless Steel
Auto-Refrigeration
Basic Weld Inspection - Part 1
Basic Weld Inspection - Part 2
Black Liquor Recovery Boilers - An Introduction
Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies
Boiler Logs Can Reduce Accidents
Boiler/Burner Combustion Air Supply Requirements and Maintenance
Carbon Monoxide Poisoning Preventable With Complete Inspection
Combustion Air Requirements:The Forgotten Element In Boiler Rooms
Creep and Creep Failures
Description of Construction and Inspection Procedure for Steam Locomotive and Fire Tube Boilers
Ensuring Safe Operation Of Vessels With Quick-Opening Closures
Environmental Heat Exchangers
Factors Affecting Inservice Cracking of Weld Zone in Corrosive Service
Failure Avoidance in Welded Fabrication
Finite Element Analysis of Pressure Vessels
Fuel Ash Corrosion
Fuel Firing Apparatus - Natural Gas
Grain Boundaries
Heat Treatment - What Is It?
How to Destroy a Boiler -- Part 1
How to Destroy a Boiler -- Part 2
How to Destroy a Boiler -- Part 3
Identifying Pressure Vessel Nozzle Problems
Inspection, Repair, and Alteration of Yankee Dryers
Inspection, What Better Place to Begin
Laminations Led to Incident
Lay-up of Heating Boilers
Liquid Penetrant Examination
Low Voltage Short Circuiting-GMAW
Low Water Cut-Off Technology
Low-Water Cutoff: A Maintenance Must
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 Vessel Fatigue
Pressure Vessels: Analyzing Change
Preventing Corrosion Under Insulation
Preventing Steam/Condensate System Accidents
Proper Boiler Care Makes Good Business Sense:Safety Precautions for 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
Repair or Alteration of Pressure Vessels
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
Steps to Safety: Guide for Restarting Boilers after Summer Lay-Up
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
The Authorized Inspector's 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
Top Ten Boiler and Combustion Safety Issues to Avoid
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?
Why? A Question for All Inspectors


Water Still Flashes to Steam at 212


 

Category: Incidents 

 

Summary: The following article is a part of the National Board Technical Series. This article was originally published in the October 1984 National Board BULLETIN. (3 printed pages)

 


 

Certain laws of physics are taken for granted at times. One of the most basic laws we learn, and one that should never be forgotten, is that water will flash to steam when it reaches 212°F.

This was the case recently when a repair crew in a large powerhouse was attempting to replace the piping between the main steam stop and the non-return valves on a high pressure boiler, which was in battery with three other boilers. The workers noted the main steam valve next to the boiler was leaking steam, however, they did not consider it a problem at the time. It was believed that because the members of the company-employed repair crew were experienced maintenance workers, it wasn't necessary that they be performance qualified-tested in accordance with the ASME Boiler and Pressure Vessel Code , Section IX, or use welding procedures similarly qualified. They, therefore, began the job of replacing the spool piece.

The first circumferential root pass was made on both ends of the spool on a Friday afternoon. The root pass, while only welding approximately one fifth of the total through thickness, effectively created a pressure containment vessel, and at this point the repair crew quit for the weekend. The leaky steam valve continued to leak steam into the new spool piece. The spool piece had a short drain valve which was closed. Over the long weekend the steam condensed and the hot water accumulated in the spool piece and the drain valve line.

On Monday, it didn't occur to the workers to open the short drain valve to make certain the area inside, where they were about to weld, was free of water. The welder completed the circumferential weld adjacent to the non-return valve with four additional passes. During the welding of these four passes, the condensed water was reheated and flashed into steam. Since the steam was confined within the spool piece, it could not expand to 1700 times its original volume and, therefore, compressed and built up great pressure. This is in conformance with another basic law of physics of importance to this trade; when water flashes to steam, it will always attempt to increase to 1700 times its original volume.

The workers, unaware of this tremendous pressure build-up, began to weld the second pass at the other end of the spool piece.

To explain how this flashing of steam and pressure build-up occurred; the welding arc was in excess of 10,000o F, the molten weld puddle was nearly 2800°F, and the heat affected zone ranged from approximately 1200°F to just below the temperature of the molten steel. Since the root pass was approximately 3/32 inch- thick, the second pass had molten metal within 1/16 inch of the inside of the pipe. Experience tells us, therefore, that the inside of the pipe was cherry red to hot white. Steels are very weak at these elevated temperatures and this contributed to the failure.

The welder was about two-thirds of the way around the pipe with this second pass when tragedy struck. As it always does, the weld and adjacent metal blew outwardly with frightening speed and surprise. The weld root pass and the section of the molten second pass exploded, striking the welder in the neck and chest with molten metal mixed with steam, causing serious injuries. The welder's helper was hit directly in the face and eyes. At this writing, he is blinded and it is doubtful that he will regain his sight. These injuries are a high price to pay for the errors made during a seemingly minor repair job.

Tragedies such as these can be averted. Awareness of the danger is the best defense. Keep these guidelines in mind:

 

  • Never weld on any pressure retaining surface which contains unvented liquids that can be pressurized.

     

  • When welding on a vessel containing either a liquid or gas vapor, always remember that stress values of the metal in the weld and heat affected zones are low due to the high temperatures, therefore, the weld strength will contain virtually no internal pressures.

     

  • Before beginning any welding, always check to see if anything might be contained by or on the opposite surfaces. Welding on containers holding unknown substances can be extremely dangerous.

     

  • Always use proper welding procedures qualified in accordance with ASME Code Section IX.

     

  • Always use welders that have demonstrated their proficiency and have passed a performance qualification test in accordance with ASME Code Section IX.

It appears that most of these guidelines were ignored in the incident described above. The results of this carelessness too often encompasses death, injury, and destruction of property.

Safety is no accident. We have to work at it at all times!

 


 

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

 







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