The evolution of medical gas systems design
Medical gas systems design has changed, and is outlined in NFPA 99: Health Care Facilities Code
- Understand how NFPA 99: Health Care Facilities Code categorizes medical gas system failure and associated risk level.
- Learn about the scope and coverage of NFPA 99, and updates to the 2021 code regarding medical gas systems.
- Follow the impacts of NFPA 99 on medical gas system design and where and when to use valves in medical gas and vacuum systems.
NFPA 99: Health Care Facilities Code is the foundation for the design of medical gas systems and adopted as code in most states. The NFPA 99: Health Care Facilities Code Handbook provides supplemental information to aid designers in interpreting the intent of the code.
Medical gas systems (which includes oxygen, nitrous oxide, medical air, medical vacuum and others) serve patients in hospitals and clinical settings. Patient safety is paramount and the requirements of these systems vary based the application and on the risk to the patient or staff should those systems fail. To this end, NFPA 99 established four risk categories to define the level of risk to the patient and staff in the event of a systems failure as follows:
- Category 1 (high risk): Systems in which failure is likely to cause major injury or death of patients or staff.
- Category 2: Systems in which failure is likely to cause minor injury to patients or staff.
- Category 3: Systems in which failure is not likely to cause injury to patients or staff.
- Category 4 (no impact): Systems in which failure will have no impact on patient care or staff.
The NFPA 99 Code Handbook further explains that it is the health care facility’s responsibility to perform the risk assessment and that the health care facility and authority having jurisdiction are to collaborate with each other in making this determination. If the health care facility is providing a Category 1 medical gas system, then a risk assessment is not required as the most stringent level of system design is already being provided.
Some characteristics of the risk categories and their application to medical gas systems are as follows:
- Category 1: General anesthesia and deep sedation are implemented. All medical gas system components shall be designed for full redundancy with the ability to provide continuous service in the event of a single source component failure. All medical gas system valves and outlets are required to meet the Category 1 requirements.
- Category 2: General anesthesia and deep sedation are not planned and are not allowed. Minimal to moderate sedation, if any, is performed. Redundancy is not required for the medical gas system. All medical gas system valves and outlets shall still meet the Category 1 requirements.
- Category 3 (and Category 4): No or minimal sedation is implemented. Redundancy is not required for the medical gas system. Often any gases will be provided using portable cylinders. When piped gases are provided, all medical gas system valves and outlets shall meet the Category 1 requirements.
The 2021 version of NFPA 99 further clarifies that medical gas systems are permitted to be designed to higher risk categories even though they may be serving spaces with lower risk categories. Previous versions of NFPA 99 referenced Facilities Guidelines Institute but did not define FGI in the definition section. The 2021 version of NFPA 99 added a definition for FGI guidelines, describing it as a collection of documents that includes guidelines for the design and construction of hospitals, outpatient facilities and residential health, care and support facilities. In most states, FGI is adopted or referenced as code for health care facilities.
With this basic understanding of risk categories and their implications to medical gas system design, we now want to focus specifically on the major medical gas system source types and requirements per NFPA 99.
Medical air source systems
While NFPA 99 defines the different requirements relative to the quality and characteristics of the medical air system, the basis for these is further explained in the NFPA 99 Code Handbook. Because medical air is administered to patients, the U.S. Food and Drug Administration and the United States Pharmacopeia considers medical air as a manufactured drug.
This categorization means that the entire medical air system and all components must be carefully considered to ensure the delivery of safe, quality air while minimizing any chance of contamination or system breakdown. Medical air is limited for use with patient’s respiration and the calibration of medical devices.
NFPA 99 prohibits any other connections to equipment or general use to limit the potential for cross-contamination. There are three types of medical air systems identified within the code, which include compressor source systems, cylinder source systems and proportioning source systems. The most common of these is the compressor source systems.
Category 1 medical air compressor source systems are those in which a system failure is likely to cause major injury or death of patients or staff and as a result are designed for full redundancy with the ability to provide continuous service in the event of a single source component failure. To ensure the integrity of these systems, NFPA 99 defines requirements for a Category 1 medical air compressor source systems as:
- Compressor source systems shall have a minimum of two air compressors, each sized for full redundancy (N+1).
- Air receivers shall be sized with enough capacity to prevent short-cycling or the excessive starting and stopping of air compressors. This greatly extends the life of the air compressors.
- Air dryers shall be capable of producing a maximum dew point of 32°F at 50 to 55 pounds per square inch.
- Air filters shall be sized for 100% of the peak demand at a minimum 98% efficiency for particles 1 micron or larger.
- Air intakes must be carefully located away from areas where contaminated air may be drawn in. NFPA 99 lists distance requirements from potential dirty sources such as exhaust, vents, doors and window openings.
NFPA 99 requires that the delivery and sizing of the compressor source system be sufficient to maintain 50 to 55 psi throughout the entire medical air system. While other air compressor systems such as in laboratories may allow for higher system pressures with downstream regulators to reduce system pipe sizes, this is not permitted per NFPA 99 in a medical air application due to potential risks to patients in the event of a regulator failure.
Medical-surgical vacuum systems
Like medical air systems that are administered directly to patients, medical-surgical vacuum systems are also considered part of a Category 1 life-support system and as such has specific requirements under NFPA 99. All the system components shall be designed for full redundancy with the ability to provide continuous service in the event of a single source component failure and include additional provisions:
- A minimum of two vacuum pumps each sized at 100% (N+1 redundancy). Alternation of pumps is desirable to ensure even wear and help ensure that the backup pump is operational in the event of primary pump failure.
- Receiver tanks designed to accommodate sudden or unusually high system demands and protect the vacuum pumps from overloading.
- Because vacuum exhaust can carry potentially infectious materials, NFPA 99 requires that the exhaust terminate outdoors at a minimum of 25 feet from any intake or building opening such as doors and windows, outside air intakes or medical air intakes. Understanding the intent of the code, the exhaust termination point should also consider prevailing winds, building orientation and other factors to protect against exhaust entrainment.
Medical-surgical vacuum systems also pose risks of exposure to staff and maintenance personnel to potentially infectious material. Vacuum filtration was added in more recent versions (2018 and later) of NFPA 99 and requires that a minimum 99.97% HEPA filter be installed between the vacuum pump and pipeline to limit contamination of vacuum equipment. In the 2021, NFPA 99 further clarified that liquid ring vacuum pumps are exempted from the HEPA filter requirement as the liquid ring provides an improved seal. While working with any potentially infectious equipment, proper personal protective equipment and handling precautions are recommended.
A waste anesthetic gas disposal system is a medical-surgical vacuum system serving areas where nitrous oxide or other halogenated anesthetic gas are used. NFPA 99 does not require that WAGD systems be completely independent of central medical-surgical vacuum systems, but instead permit WAGD systems to be integrated into a common medical-surgical vacuum source with a dual-use piping configuration. For dual-use piping systems, NFPA 99 requires either that the total concentration of oxidizers remains below 23.6% or the vacuum pump is made of materials that are inert in the presence of oxygen and halogenated anesthetics.
Each area where nitrous oxide or halogenated anesthetic gas are used must have at least one designated WAGD terminal and the WAGD terminals should not be labelled as suctions. In 2021, NFPA 99 further added that if surgical-medical vacuum systems also serve WAGD systems, the valves on the source side connection to the WAGD systems shall be labelled to indicate use to protect against cross-connections to WAGD piping.
Changes in NFPA 99-2021
Most articles about NFPA 99 medical gas systems are organized to follow the order subjects are covered in the code and often begin with bulk oxygen systems. The 2015 and earlier versions of NFPA 99 used bulk oxygen systems terminology when referring to medical oxygen tank sources. In 2018, NFPA 99 then replaced bulk oxygen systems terminology with cryogenic fluid supply systems. With the 2021 version, the entire cryogenic fluid supply system section was moved to after the medical vacuum section, presumably for better flow.
NFPA 99-2021 references the NFPA 55: Compressed Gases and Cryogenic Fluids Code and makes a clear delineation as to when NFPA 55 and NFPA 99 codes apply. Cryogenic fluid supply systems and components before the source valve shall follow NFPA 55 while NFPA 99 governs the design and installation of systems after the source valve, including all wiring and alarms.
A definition added to NFPA 99 in 2021 is compressed medical gases, or CMG. NFPA 99 describes CMG as a liquified or vaporized gas alone or in combination with other gases that is classified as a drug, matching exactly the definition used in the NFPA 55-2020. For further clarity on CMG, reference the 2020 NFPA 55 Section A.3.22:
“Classifications are defined in 201 (g)(1) of the Federal Food, Drug and Cosmetic Act, 21 USC 321 (g)(1). This includes gases recognized in the current United States Pharmacopeia and National Formulary (USP-NF) or supplements and gases intended for direct use or as a component of gases in the diagnosis, cure, mitigation, treatment or prevention of disease in man or in animals that achieves its intended purpose through chemical rather than physical means.”
Cryogenic fluid central supply system is the common oxygen source for hospital buildings and is normally in a remote location outside the building. This central supply system may also be extended to multiple buildings on a campus. The emergency oxygen supply connection is located outside of the exterior wall of the building with proximity to emergency supply vehicle access and is locked to prevent tampering. NFPA 99-2021 just added the requirement for adding four alarm points to both master alarms panels for monitoring the temporary supply while in use.
Medical gas and vacuum valves
As this section applies to both medical gas and vacuum systems and to avoid confusion, commonly used terms of “downstream and upstream” will not be used. Instead, simple terms such as “before and after” will be used to describe the placement of the valves in relation to other devices. NFPA 99 requirements for medical gas and vacuum valves have gone through several iterations over the years:
- 2012. Specific construction and types were defined for medical gas and vacuum valves.
- 2015. Medical gas and vacuum valve construction was clarified to note materials suitable for service, but then omitted the requirement for the specific type mentioned. This version also noted that new materials may be aligned with the current available technologies, though it later identified that this revision requires additional requirements to preserve the original intent of the code and eliminated any potential cutting corners in manufacturing of the valves, such as providing less internal parts.
- 2018. Valve flow coefficient requirements were added to medical gas and vacuum valves, ensuring that the required flow characteristics through the valve were not compromised.
- 2021. Valve types are clearly defined as dual port. Earlier NFPA 99 versions noted that the port on the valve is required but did not indicate single or dual port and the relative placement of the valve.
NFPA 99 identifies different types of valves as source, main line, shut-off, riser, service and zone valves. Each valve type corresponds to a different level of access and security to maintain the integrity of these systems. Source valves are to be provided in the immediate vicinity of the central supply system when located inside the same building. When a source system is located remotely away from the building being served, NFPA 99 requires a main line valve also be installed in addition to the building source valve. The closing of either the building source and/or the main line valves will completely shut off the system from the entire building.
NFPA 99 includes a detailed diagram showing the intent for placement of the different types of valve. To add clarity to the system configuration, this article takes the diagram as the base source and adds commentary on options for the main line valve.
Shut-off valves, which include riser, service and zone valves, isolate areas of the piped distribution system for maintenance and testing. Except for the zone valve box assemblies, all shut-off valves must be in secured areas such as equipment rooms or above the ceiling and be locked (or latched) in their operating position. When located above the ceiling, shut-off valves are to remain accessible and shall not be blocked by ducts or other obstructions. To accommodate incremental shutdowns without taking down the entire system to the building, NFPA 99 requires the installation of riser valves at the base of pipe risers throughout the building.
Service valves are provided in the branch piping from the main. Only one service valve for each branch is required regardless of the number of zone valves installed after it. Good practice, however, also places a service valve before each zone valve box to provide added flexibility and serviceability. If a room modification requires the removal or relocation of a zone valve box, the service valve before the zone valve allows for the shutdown to be limited with only one zone impacted instead of multiple zones connected to the main branch piping.
Of all the valves mentioned in this article, the zone valve boxes are the only ones readily accessible to staff. Having the zone valve boxes accessible gives staff the capability to isolate the rooms that the zone valve for fire control purposes. Due to the important function of the zone valve box in an event of emergency, NFPA 99 strictly outlines specific accessibility, visibility and safety requirements for locating the zone valve boxes.
Though rarely provided, In-line shut-off valves may be added after of the zone valve box. Where the zone valve box isolates a group of rooms within the zone, the in-line shut-off valves further isolate individual rooms or spaces within the zone. If provided, the requirements previously outlined for service valves shall be followed.
The future of medical gas systems
Medical gas systems design and NFPA 99 have evolved significantly over the past decade together with our understanding of the systems, processes and components that impact the safe and reliable operation of these critical life safety systems. The changes in the code over the years at times are subtle and it is only when you are able to take a step back that you can begin to see how the design of these essential systems comes into focus.
While we expect there will continue to be refinements to NFPA 99 in the future, the work done to this point has created a solid foundation of intent from which medical gas systems design will continue to evolve and further improve the quality of these life-saving systems.