OXYGEN STORAGE IS A CRITICAL FEATURE OF MEDICAL GAS SYSTEMS SERVING HOSPITALS.
By Steven Dannaway, PE, DBIA – Medical gas systems are key to delivering patient care in hospital facilities. Bulk oxygen storage systems are common elements of the medical gas systems that serve hospitals. Two of the primary codes that regulate bulk oxygen storage systems are National Fire Protection Association (NFPA) 55, Compressed Gas and Cryogenic Fluids Code, and NFPA 99, Health Care Facilities Code.
NFPA 55 (2020 edition), Section 126.96.36.199 defines a Bulk Oxygen System as, “an assembly of equipment, such as oxygen storage containers, pressure regulators, pressure relief devices, vaporizers, manifolds, and interconnected piping, that has a storage capacity of more than 20,000 SCF of oxygen and that terminates at the source valve.” A volume of 20,000 SCF of oxygen is equivalent to 173 gallons of liquid oxygen. It does not take a significant quantity of liquid oxygen as part of an oxygen supply system to qualify as a bulk oxygen system in accordance with NFPA 55.
Separation from Exposure Hazards
A primary fire/life safety consideration for outdoor bulk oxygen systems is the separation from exposure hazards. NFPA 55, Table 9.3.2 lists 18 exposure hazards that require varying levels of physical separation distance from the bulk oxygen system. Commonly encountered exposure hazards and the required separations may include:
- Item 1 – Buildings of Type I and II construction: 1 ft
- Item 2 – Buildings of Type III, IV, or V construction: 50 ft
- Item 3 – Wall openings, measured from regulators, pressure relief devices, vaporizers, manifolds, and interconnected piping: 10 ft
- Item 4 – Property Lines: 5 ft
- Item 5 – Public Sidewalks: 10 ft
- Item 7 – Areas occupied by nonambulatory patients as measured from the primary pressure relief device discharge vent and from filling and vent connections: 50 ft
- Item 8 – Parked Vehicles: 10 ft
In lieu of several physical separation distances, NFPA 55, 188.8.131.52 permits a 2-hour fire barrier wall to interrupt the line of sight between uninsulated portions of the bulk storage system and the exposure. This exception only applies to Items 2, 4, 5, 8, and 10 through 14 of NFPA 55, Table 9.3.2. Of particular note, this exception does not exempt the 50 ft separation distance from the bulk oxygen storage system and areas occupied by nonambulatory patients.
In cases where the bounding walls of the bulk oxygen storage yard create a court with three or more sides, NFPA 55, Section 184.108.40.206.2.1 requires the bulk oxygen tanks to be located a distance away from two bounding court walls, equal to at least the height of the wall. For example, where a bulk oxygen system yard is bounded by 12 ft high walls, the yard must be arranged such that the oxygen storage tanks are located at least 12 ft horizontally away from at least two of the bounding walls. This requirement intends to avoid creating a confined space around the oxygen storage system, allowing for free air movement and ventilation within the equipment yard.
Separation between Combustible Surfaces
A final consideration we will highlight is NFPA 55, Table 9.3.2., Item 16. The spill of liquid oxygen on organic materials such as asphalt or wood can create an explosion hazard, as the oxygen will soak into these materials and will create an oxygen enriched environment. Vehicles driving over or staff walking over the oxygen-enriched organic material, such as wood or asphalt, can result in an explosion. Item 16 of Table 9.3.2 prescribes a physical separation of a minimum of 3 ft between combustible surfaces, such as asphalt or bitumastic paving, from areas below connections where liquid oxygen can fall during loading or unloading operations and system operation. NFPA 55, 220.127.116.11 addresses the required dimensions of noncombustible paving around the liquid mobile supply equipment for refill of the system. Liquid oxygen connections, including the emergency oxygen supply connection required by NFPA 99, should not be located over asphalt and the bulk oxygen storage systems should not be installed upon asphalt or similar paving materials.