Wall oxygen supply and pressure regulators

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At one stage, in Question 18.1 from the first paper of 2008, the college wanted to know a little about the wall oxygen regulator. At another stage, in Question 10 from the second paper of 2016 the college required a very detailed answer about gas storage systems. Also, they always ask about the little regulator knobs and wall outlet safety features, i.e. "what is usually done to prevent people from accidentally connecting the nasal prongs to suction".

This fascinating topic can certainly throw an easily distracted person into a deep weird rabbit hole of gas physics and industrial engineering. For the time-poor candidate, Christ Thompson's lecture from the University of Sydney is suggested as the single most important source: on its own, this is enough to pass the SAQs. For the person with nothing better to do, one might recommend Chapter 2, "Medical Gas Pipeline Systems" from Understanding Anaesthesia Equipment by Dorsch and Dorsch (p.26 of the fifth edition). This can be accessed by Google Books. And if one were completely mad, one might wish to submerge into the nightmarish depths of a 187-page Department of Health Technical Memorandum (2006), where one may find specific directions as to how thick a pipe one might require to supply one's hyperbaric oxygen chamber.

Medical gas supply sources

In Dorsch and Dorsch a helpful diagram is available. An unhelpful version of it is reproduced below. It describes a typical oxygen storage system for a large hospital.

At a basic level, the system consists of:

• A central source
• A vapouriser
• Pipelines
• Pressure regulators (decreasing pressure along the system)
• Manual and service shut-off valves (to isolate whole sections of the system)
• Pressure release valves upstream of any regulators and shut-off valves, to prevent the whole thing from exploding.
• Pressure monitors and alarms to detect the fact that you're out of oxygen.

Liquid oxygen gas

After a 2-stage fractional distillation the gas you get from the wall is supposed to be 99.5% oxygen and 0.4% argon. You can not get rid of that argon very easily because its boiling point is fairly close to oxygen, and therefore separating the two is quite expensive. Fortunately, argon is quite benign, being wholly disinterested in reacting with the molecules of your body. It is unlikely to do any harm even if its was present in substantial quantities (eg. as an 80:20 argon:oxygen mixture) and one would only ever get into trouble if the concentration of argon in the air gas mixture completely replaced oxygen, suffocating you.

That digression aside, liquid oxygen is a form of bulk supply which is more economical than cylinders if your need for oxygen is constant and high in volume. One litre of liquid oxygen yields 842 litres of gaseous oxygen at atmospheric pressure and 15°C, according to Das et al (2013).

Depending on what the size of your hospital, the size and nature of your oxygen storage may differ. Clearly, it would not be economical to install a liquid oxygen storage facility out on the back porch of a little rural dentistry clinic. The general rule of thumb is to judge economic effectiveness  according to the expected rate of oxygen consumption. The liquid oxygen storage tank continuously vapourises (even with the best insulation) and unless that gas is being used, it ends up being vented into the atmosphere. This means that unless you are using around 300L/min of oxygen during your peak time, the liquid oxygen system is going to be wasteful and you might be better off with a manifold setup of multiple oxygen cylinders.

Anyway. Pressurised gas cylinders are boring, and are dealt with elsewhere. Let us discuss the oxygen storage facilities of a large tertiary university-affiliated hospital.

The central storage tank

In the central reservoir, a pool of liquid oxygen sits at close to 1000 kPa (10 atmospheres).  This reservoir is called a "Vacuum Insulated Evaporator" (VIE).  The VIE is designed in a manner resembling a thermos, usually with an evacuated cavity (0.3 kPa) between two pressure-durable metal jackets.  It is expected to keep the gas at  around -150° C. The critical temperature is  -118º C; above this temperature the oxygen would become gaseous no matter the pressure, and the VIE would explode. Generally, if the hot weather brings the internal pressure to above 1500kPa, the blowoff valve opens and releases some oxygen, cooling the rest of the contents (according to the gas laws, as the gas loses pressure it also loses temperature).

At the bottom of the container, the liquid gas is sloshing around; at the top evaporated gas collects at a pressure of around 1000kPa. The difference in pressure between the top and the bottom of the tank can be used to estimate the remaining content (as it evaporates, the pressure at the bottom of the tank decreases).

How big a tank do you need? Big VIEs can hold up to 1,500L. David Highley (2012, UK) mentions that a hospital should have enough oxygen stores for 14 days of continuous supply. However, it is unclear how much of this supply should be in liquid form versus tanks, and whether this recommendation is made on the basis of some sort of research, engineering guidelines, health care directives, or simply pulled out of the arse. Surely, one could extend the duration of supply by decreasing the rate of oxygen delivery. In any case the 2006 UK DH guideline does not contain any specific size recommendations, instead suggesting that the estimates be based on "historical consumption data".

The heater and vapouriser

Even though the tank is insulated, exposure to the normal temperature of an Australian summer's day will result in some gradual heating of the tanks' contents, and some of the gas will return to vapour state. This gradual evaporation is uncontrolled, and is generally not enough to supply the hospital. Thus, another system of heating is required.

 In order to supply a reliable amount of gas to the hospital, on its way out of the cylinder the oxygen passes through a heat exchanger. This thing is usually a U-shaped length of pipe which features a carefully controlled "loss" of insulation. This piece of pipe is usually encrusted with ice. Inside it, the liquid oxygen reaches the state of vapour, and is then released.

Beyond this point, the characteristics of the supplied oxygen remain largely unchanged. It gets pressure-regulated down to about 415 kPa, but otherwise it remains unchanged. The gas coming at your face out of the wall is very cold (lower than ambient room temperature) and totally dry (humidity is zero).

The pipeline

The gas which comes from the wall travels from a central reservoir, along seamless copper pipes. These pipes are degreased, sealed and steam cleaned; as a part of commissioning the system, they are tested to 1400 kPa. The standard (mandatory) wall gas pressure is about  415 kPa (about 4 atmospheres), but for the pressurised air which powers surgical tools, the pressure is around 1400 kPa.

That seamless copper is not quite the same as the pipes you get from the hardware store. Specific high quality phosphorus-containing, de-oxidized, non-arsenical copper alloy is required. This prevents the degradation of medical gases (i.e. it does not react with them in any measurable way). It has the additional benefit of being antibacterial, which is helpful. Pipes leaving the central VIE are usually quite thick (~40mm), and they are downsized to 15mm for the 415 kPa gas.

Safety features

The following safety features are incorporated into the system:

• Liquid gas content indicator (i.e. to indicated when the tank is empty)
• Pressure gauge of central tank
• Pressure release valve (set to 150% of normal expected tank pressure)
• A reserve gas supply tank
• A "reserve in use" alarm indicator
• An isolation valve
• A pressure regulator (from tank side to wall side) with a pressure release valve
• A supply-side shut-off valve

Wall outlets

"Each terminal unit shall include a gas-specific connection point which shall accept the appropriate gas-specific probe only", says the British Standard (1998). I could not fine a more recent standard, nor is there an easily avaialble Australian alternative, but one might expect this recommendation remains in force.

Anyway. Question 10 from the second paper of 2016 asked specifically for a list of "safety features that are in place to prevent incorrect connection of hoses and regulators to gas outlets". A study of such safety features would be well informed by an exploration of horrible gas mistakes which we have historically made. In that spirit, one's reading on this subject should begin with Michael Richard Cohen's Medication Errors (2006), where on page 322 one finds the chapter on "Fatal Gas Line Mix-up". Recommendations for safety made in this book are summarised below:

• Non-interchangeable connectors (i.e. the air regulator cannot be connected to the oxygen outlet: it is physically impossible). This is called the Diameter Index Safety System (DISS).
• Standardised flow meters, regulators and connectors (i.e. they all look the same everywhere in the hospital)
• Easily identifiable gas connectors (obvious colour, as well as outlet shape and texture)
• Observable connections (i.e. not hidden under a shelf or table)
• Regular testing and  preventative maintenance of gas supply system (i.e. even if a gas outlet is not in routine us, the engineers need to regularly test it to make sure it still supplies the specified gas)

Wall oxygen supply rotameter

• Analogue measurement of gas flow
• Height of elevated float is dependent on the equilibrium between gravity and the upward acceleration due to flowing gas.
• The tube with the float inside is conical, allowing different points of equilibrium at different elevations.