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Black Liquor Recovery Boilers - An Introduction
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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
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How to Destroy a Boiler -- Part 1
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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 is the Best Welding Process?
What Should You Do Before Starting Boilers After Summer Lay-Up?
Why? A Question for All Inspectors

The Use of Pressure Vessels for Human Occupancy in Clinical Hyberbaric Medicine

W.T. Workman
NFPA Technical Committee for Hyperbaric and Hypobaric Facilities

Jack Maison, Ph.D., P.E.
ASME Committee for Pressure Vessels for Human Occupancy

67th General Meeting 1998  

Category: Operations


Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (6 printed pages)




What is a PVHO?

Although pressure vessels have been in use many years in a wide range of applications, it was not until the development of commercial diving and underwater construction (or caisson)-related activities that there was a defined need to design and fabricate these devices specifically for man-rating. Today, the use of pressure vessels for human occupancy (PVHOs) is becoming more widespread. Examples include diving recompression or decompression chambers, medical hyperbaric treatment chambers, hypobaric (altitude simulation) chambers, and tourist submarines.

Historically, the use of PVHOs dates to the early 1600s when Drebbel developed the one-atmosphere diving bell. It was not until Cochrane patented the use of compressed air in caissons that their use became more widespread. The first documented use of a pressure vessel for potential medical purposes dates to 1662 when the English clergyman Henshaw attempted to use compressed air devices for "afflictions of the lung." Regulation of PVHOs was not an issue until the late 1960s when commercial diving in support of offshore oil production became the responsibility of the U.S. Coast Guard.

Why Regulate PVHOs?

Previously, these vessels were built to ASME Code Section VIII rules. However, it was soon learned that these rules were not specific enough to ensure the safety of human occupants. One problem was the range of steels allowed under Section VIII; many were not fracture-tough for cold weather application typical of offshore diving. Another problem area was that Section VIII did not recognize the use of acrylic plastic for viewports.

In the 1960s, viewports were fabricated of tempered glass. Glass gave no precursor of failure, often resulting in sudden catastrophic loss of pressure and serious injury to occupants. Unlike glass, acrylic gave visible and early signs of stress-induced crazing, discoloration, and distortion. This added a significant safety margin in that material failure was neither sudden nor catastrophic.

In the 1970s, the U.S. Navy sponsored extensive testing of acrylic plastic for use as PVHO windows. In spite of the data, Section VIII did not consider adoption of acrylic plastic. Many PVHOs were built for windows, tested with metal blanks, stamped, and then the acrylic plastic windows were installed. This was not reassuring to the buyer of a PVHO or to the insurer.

A third problem arose when applying Section VIII rules to the external pressure design of submersibles. Section VIII PVHOs would not float! A fundamental principle of submersible safety is vessel recovery. This is best achieved with a buoyant PVHO. If disaster strikes, ballast can be dropped and the vessel rises to the surface. Section VIII vessel designs for all but shallow operations were negatively buoyant and therefore inherently unsafe.

Another real hazard to vessel occupants was the safety relief valve that could kill the occupants if activated. More specifically, safety valve activation can result in ear and/or sinus rupture, decompression sickness, or air embolism from ruptured lungs. (Lung alveoli can rupture from exposure to a pressure change of as little as 3 to 5 feet or 1.34 to 2.23 pounds per square inch.)

In response to these needs, ASME created the Committee for Pressure Vessels for Human Occupancy in 1974. The first PVHO safety standard was issued in 1977.

In addition to required guidance for the design and fabrication of PVHOs, it became apparent that operational safety standards were also needed, primarily in the area of fire prevention. In the late 1960s, due to increased operational safety concerns as well as some highly publicized accidents, the National Fire Protection Association was asked to develop fire safety requirements. Two highly visible fatal accidents that helped put tentative standards into place were the Apollo space capsule fire, and then two days later, a 100 percent oxygen-filled hypobaric chamber fire that killed two young airmen. In 1970, NFPA adopted NFPA 56D, Hyperbaric Facilities. This standard was based on the fire prevention requirements in place at the time for administration of flammable anesthetic gases and was so strict that most manufacturers and operators of PVHOs simply ignored the code. In response to this untenable situation, the standard was revised and relocated to NFPA 99, Standard for Healthcare Facilities, in the 1984 edition, where it resides today as Chapter 19. Fire safety requirements for hypobaric chambers are now contained in NFPA 99B, Hypobaric Facilities .

A recent survey concerning hyperbaric chamber fires occurring from 1923 to 1996 revealed there were 35 chamber fires in Asia, Europe, and North America with a total of 77 fatalities. None of these fatalities occurred in North American clinical hyperbaric treatment chambers. Further analysis revealed that prior to 1980, the majority of the hyperbaric fires were electrical in nature. After 1980, the use of prohibited materials and/or operator error were the primary causes.

How Are PVHOs Used Today?

Even though there is still activity within the commercial diving industry, there has been little growth in recent years due to the development of remotely operated vehicles (ROVs). The tourist submarine industry is even more limited with only one viable tourist submarine company. The primary growth internationally for PVHOs has been in medical application of hyperbaric oxygen therapy.

Hyperbaric oxygen therapy is defined by the Undersea and Hyperbaric Medical Society (UHMS) as a therapy in which a patient breathes 100 percent oxygen intermittently while the pressure of the treatment chamber is increased to a point higher than sea level pressure (i.e., greater than 1 atmosphere absolute, or ATA). Typical pressures applied are up to 29.4 psig (3 ATA) for routine clinical applications and up to 73.5 psig (6 ATA) for the treatment of decompression sickness or air embolism.

Specific medical indications recognized by UHMS are:


    • Problem wounds
    • Air or gas embolism
    • Carbon monoxide poisoning and smoke inhalation
    • Gas gangrene
    • Crush injury, compartment syndrome, and other acute traumatic ischemias
    • Decompression sickness
    • Exceptional blood loss anemia
    • Necrotizing soft tissue infections
    • Osteomyelitis
    • Compromised skin grafts and flaps
    • Thermal burns
    • Intracranial abscess

It is difficult to define the exact number of hyperbaric chambers in operation throughout the United States since the majority of information comes from voluntary industry surveys. However, according to UHMS, there are a total of 270 facilities operating a combined total of 539 hyperbaric chambers ranging from single-patient (monoplace) to multiple-patient (multiplace) chambers. Based on industry input, the growth of hyperbaric facilities is accelerating at approximately 30 percent per year. Much of this growth is occurring in nontraditional facilities such as free-standing clinics, health clubs, mobile units, and other nonhospital-based locations not currently covered by NFPA 99.

Who Invokes PVHO Rules?

The U.S. Coast Guard develops rules that govern the operational use of PVHOs used in the commercial diving industry and for tourist submarines. The U.S. Navy mandates ASME-PVHO for military diving operations. PVHO is adopted by legislation in only 10 states and three cities within the United States. There are three Canadian provinces that also require PVHO. (National Board Synopsis on CD-ROM, 2004 Edition).

Even though PVHO has only been adopted legislatively by 10 states and three cities, the loop is closed via enforcement of NFPA 99 by state and local fire marshals. NFPA 99, Standard for Healthcare Facilities, states in 19-2.2.1, "Chambers for human occupancy and their supporting systems shall be designed and fabricated to meet ANSI/ASME PVHO-1a, Safety Standard for Pressure Vessels for Human Occupancy ."

To eliminate any ambiguity, NFPA established a special task group to redefine what a "healthcare facility" actually is. As healthcare has changed, so has the location in which care is provided. Finding healthcare only in traditional hospital settings is a thing of the past. Instead of centralization as the theme, the current trend is to decentralize. This is no different in the hyperbaric medicine community. Outpatient hyperbaric treatment centers are now often found in strip shopping malls, business parks, health clubs, and even private homes. The NFPA task group has emphasized the type of procedure conducted in the facility, rather than the classical definition of where the facility is located. Also, some jurisdictions are not using the current edition of the code as the primary enforcement tool. All jurisdictional agencies are urged to use the current edition of the standard, which is revised every three years. The latest edition was issued in 2002.

An agency with growing importance in the safe use of hyperbaric chambers as medical devices is the Food and Drug Administration (FDA). According to the FDA, a hyperbaric chamber is a Class II prescriptive medical device. A prescriptive medical device is one that is purchased under a physician's direction and its use is prescribed by a physician. As Class II devices, all hyperbaric chambers require a 510(k) pre-market clearance before the manufacturer can enter into the commercial market and actually accept orders for delivery and installation. The 510(k) process helps establish substantial equivalency of a manufacturer's device with those currently on the market for sale. A more detailed review is required for those devices that have no substantially equivalent device in the marketplace. Even though the majority of hyperbaric chamber manufacturers are in compliance with public law, there are a few who are actively marketing their products without proper clearance to do so. Known abuses to this requirement should be reported to the FDA Compliance Division for action.

The Changing Face of PVHO

Currently, the scope of PVHO defines the requirements for the design, fabrication, testing, inspection, marking, and stamping of PVHOs and related piping. It covers pressure vessels that enclose human beings while under internal or external pressure regardless of the magnitude of the pressure. There is a common misconception that PVHO only covers pressure vessels designed for pressures greater than 15 psig. This incorrect notion stems from exemptions in Section VIII, which PVHO-1 relies upon. Recall that serious injury can occur from a 1-2 psig pressure drop. The scope of PVHO specifically excludes nuclear containment vessels, pressurized aircraft, and caissons.

There is growing interest at PVHO to incorporate new materials and concepts to help the rules keep pace with technology. Among new interest areas are the use of post-tensioned concrete, fiberglass and other composites, fabrics, and modular-expandable configurations for pressure vessels.

Another area of focus is the ASME-PVHO special task group for post construction and inservice inspection rules, which is called PVHO-2. The current PVHO-1 deals with new construction. In response to industry needs for rules to guide repair and replacement of system components, this post construction task group was established. PVHO-2, Safety Standards for PVHO, Inservice Guidelines for PVHO Acrylic Windows , has been approved and will soon be published, according to ASME's Web site.

Regulatory Needs

The rapid growth of medical applications for hyperbaric chambers and the lack of a coordinated national regulatory consensus should cause great concern to safety professionals. During periods of rapid growth in just about any venture, entrepreneurial spirit abounds. The field of hyperbaric medicine is no different. Examples of this "spirit" are flexible fabric chambers touted for routine clinical care and fabric chambers with plastic windows and zippered closures in use in health clubs. There are others. The concern is that these systems are not designed with accepted safety factors or rules proven by the test of time. And the fact that these pressurized systems are being operated by people with little or no training, much less a working knowledge of the hazards of pressure, causes more alarm.

More traditional hyperbaric chambers have also been designed recently that do not comply with the existing code requirements. There are hyperbaric chamber installations in private homes, operated by family members. Many of these installations include pressure vessels that were purchased on the used chamber market and fabricated long before the appropriate safety codes were developed. Operators of these types of facilities usually will not pay to have the vessel inspected prior to purchase, therefore, vessel integrity cannot be assured. There are even cases on record of hyperbaric chamber operators having their chambers modified by noncode fabrication shops with catastrophic results.

Presently, there are no nationally recognized medical "standards of practice" to establish minimum training or staffing guidelines for hyperbaric facilities to adhere to. The Operations Committee of UHMS has been charged with the task of developing such standards. Once complete, standards must be enforced if they are to be effective.


To assist in maintaining the current safety record of the use of these systems in the United States, there should be a coordinated effort to educate all agencies with jurisdiction interest about the current uses of these systems and their rates of growth. Current rules and requirements from organizations such as the National Fire Protection Association, the Food and Drug Administration, the ASME-PVHO committee, and the National Board must be understood, implemented, and enforced. In addition, there should be an aggressive pursuit of legislative action to formally adopt the rules of PVHO on a state-by-state basis.

As regulatory requirements are further refined, there should be a third-party certification program. The FDA Modernization Act of 1997 includes provisions for third-party independent certification of medical devices. However, the overall certification program for medical hyperbaric facilities should be focused on validation of facility and system safety. Efforts to achieve international baseline safety standards are also needed.

In order to keep the operation of hyperbaric facilities safe and cost-effective, the coordinated efforts of all regulatory agencies are needed to enforce existing requirements.




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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