Medical Gas Supply

by Frank Weith÷ner


Beside a functioning supply of water and electricity also the gas supply is important for a hospital. Medical gases are used in the operating room for anaesthesia and in the wards (oxygen) as an additive to the breathing air.
The gas supply includes the procurement of gas cylinders, the transport, the maintenance of the tubing, the repair of regulators and the understanding of dangers while dealing with gases and high pressure cylinders.


The gas cylinder
The gas cylinder is a tank used to store gas under high pressure. The cylinder is long and narrow and stands upright on a flattened bottom. Such a pressure cylinder is usually made out of steel. Aluminium cylinders and cylinders made from carbon fibre also exist but are not common.



The height in mm for the commonly used standard cylinders sizes B to G

The size of a gas cylinder is quoted in litres, which means the volume of a cylinder. Gas cylinders are available from 1.2 to 50álitres. Unlike liquids, gases can be compressed. Their density increases under pressure. That is why a gas cylinder of 50álitres volume under pressure can contain gas that expands to 10á000álitres of volume when released to normal atmospheric pressure.
The typical gas pressure for a full 50ál cylinder is 200ábar or 2,900ápsi.
The pressure of smaller cylinders (3ál-16ál) is usually lower at 140ábar or 2,015ápsi.
Gas cylinders are found in the operation theatre or if the hospital has a central gas supply in a special gas supply room. Here the gas cylinders are connected through a pipe system with the anaesthesia machines in the operation theatre.
Oxygen cylinders are also found in the wards where the oxygen is given to patients with breathing or lung problems in addition to the normal air. As an alternative to oxygen cylinders, often oxygen concentrators are often used which produce the oxygen by extraction from the surrounding air.
On top of the gas cylinders is the outlet connector and a stop valve. To prevent mixing up the different gases, medical gas cylinders also use a pin-index system. Only the correct pressure regulator (with the correct pin) fits to the correct cylinder.
When a gas cylinder is to be transported, the metal protection cap should always be mounted. It protects the valve from tearing off in case the cylinder falls over.


Pressure regulator
A pressure regulator is first of all a pressure reducer. It reduces the high cylinder pressure to a low, usable pressure for our applications. Furthermore, this outlet pressure is regulated and kept stable regardless of the filling level of the cylinder and how much gas (flow) is demanded.
The pressure regulator usually has two gauges, one showing the cylinder pressure (which corresponds to the amount of gas in the cylinder), the other the reduced outlet pressure. Often this outlet pressure is adjustable with a knob or screw. For a typical anaesthesia machine a gas pressure of 3á-á6ábar (45á-á85áPSI) is required.



Standard adjustable pressure regulator with gauges for cylinder pressure and outlet pressure

Note!   Pressure regulators are always used in combinations with other equipment
          (e.g. anaesthesia machine, welding torch) and never with a patient.

The main components of a pressure regulator are a diaphragm (sometimes a piston), a spring which is located on one side of the diaphragm and a valve on the other side.

image
In standby state with no inlet pressure the spring presses against the diaphragm and keeps the valve open. When the pressure increases the gas pressure works on the diaphragm, against the spring pressure and slowly closes the valve. The higher the inlet pressure the more the valve closes. Is the pre-set outlet pressure reached the valve closes completely.
The spring force and thus the outlet pressure can be adjusted by preloading the spring by a hand wheel.
When the valve does not close completely due to a defective gasket, the outlet pressure would slowly increase and the safety valve would open.
By the way, regulators for gas and for water operate on the same physical principle but in practice they must not be interchanged. Furthermore, not all gas regulators must be used in conjunction with oxygen.

Pressure regulators should correctly fit to the gas cylinder. Unfortunately there is large number of different types of connectors depending on the country of manufacture and the type of gas. Sometimes it is difficult to find the right regulator.

When purchasing a new regulator, make sure that

  the outlet pressure is within the needed range
  the connection thread fits to your cylinder
  it is suitable for oxygen (when an oxygen cylinder has to be connected)

Tip!  Oxygen regulators designed for gas welding can be perfectly used for medical applications. These regulators are much cheaper than 'medical' regulators and are locally available.


Flow regulator
Flow regulators are only used together with oxygen and in connection with a patient. They are the interface between patient and machine.
A flow regulator might also have two gauges, one showing again the cylinder pressure and the other to indicate the amount of gas coming out of the cylinder, the flow. This flow is always adjustable. A typical flow regulator allows adjustment of the flow between 1 and 5ál/min).
Flow regulator are only used together with oxygen.
Often flow regulators do not have a round gauge for the flow but a rotameter. This is a transparent plastic tube with a scale. Inside this cylinder floats a little ball. The hight of the ball indicates the flow of the oxygen. With a knob under this tube the flow of oxygen can be adjusted.
More about flow regulators and humidifiers under The Oxygen Concentrator.



Pressure regulator or flow regulator
A flow regulator attached to a gas cylinder is only used in combination with oxygen.
Also, a flow regulator always delivers oxygen directly to a patient, never to a machine.
This also applies the other way round: Pressure regulators are only used in combination with a machine, never directly to a patient.
This is very important. Sometimes you may find anaesthesia machines are connected to an oxygen cylinder via a flow regulator, but this is absolutely wrong and will damage the machine.


Gas pressure
Gases can be stored in a gas cylinder as a gas or as a liquid, depending on the type of gas. Propane for example, which is used as a cooking gas, is liquid under high pressure. You can feel the liquid when shaking the cylinder. The disadvantage of 'liquid' gases is, that the pressure in the cylinder is in no relation with the amount of gas inside. The pressure does not fall until the cylinder is nearly exhausted. You would need to measure the weight of the cylinder to get an indication of the amount of gas left.
The medical gases used in the hospital are gaseous in the cylinder. A connected pressure gauge shows the actual pressure which depends on the amount of gas. Half the pressure means the cylinder is half full.

The SI unit for gas pressure is pascal (Pa), bar or psi (pounds per square inch)
200ábar = 20áMPa = 2,900ápsi

Conversion: www.google.com and type in:   200bar in psi

All cylinders, no matter if big or small are under a pressure of 200ábar when they are completely filled.
An empty and open gas cylinder of 20ál contains 20ál of air. The surrounding atmospheric air pressure is 1ábar (1,013ámbar). This is also called the absolute pressure.
When a compressor is connected to the cylinder the pressure inside the cylinder increases and the cylinder gets filled with more air. The amount (litres) of gas gets more. At 200ábar absolute pressure the cylinder contains 4,000ál of gas.
Pressure (bar) x Cylinder volume (l) = Gas volume (l)
A 20-litre-cylinder contents:

Absolute pressure Gas amount
1 bar 20 l
10 bar 200 l
100 bar 2,000 l
200 bar 4,000 l


Gauge pressure and absolute pressure
As per definition, the surrounding air pressure on sea level is 1ábar (1,013ámbar). This is the true or absolute pressure.
But a pressure gauge shows 0ábar. That means 0ábar gauge pressure means 1ábar absolute pressure. Or in other words: The gauge pressure is 1ábar less, than the absolute pressure.
As a result, we have to subtract 1ábar from the result in the table to get the pressure shown on the pressure gauge.
Now we know that a cylinder with 100ábar is only half full and contains 2,000ál of gas. But how long will the gas last? This depends on the flow.


Flow
For applying oxygen to a patient it is important to know how much gas the patient should receive in a certain time. This is the flow, which can be measured with a flow-meter. The unit is litres per minute (l/min).

Example: A doctor prescribes for a patient 4ál/min of oxygen and the patient is connected to a full 10ál-cylinder. How long does the cylinder last?

A 10ál cylinder contains 2,000ál of oxygen under a pressure of 200ábar (200ábar x 10ál).
At a flow of 4ál per minute the patient can get oxygen for 500áminutes.
(2,000ál / 4ál/min = 500ámin)
500áminutes are 8 hours and 20 minutes.


Different gases
The most common gas which is needed in hospitals is oxygen. Other gases are nitrous oxide, nitrogen, carbon dioxide and medical air. They are all filled in cylinders and have to be purchased at a local gas supplier. The gases are available in two different standards: Industrial gases and medical gases.
In some areas medical gases are often difficult to get and very expensive. It is said that medical gases are specially filtered but usually it is the same gas as sold as industrial gas. The only difference is the connecting system of the cylinder. For medical gases a special pin-index connector is used. This system makes it impossible to interchange the different gases when connecting.



Pin-index system. Each gas has a different connection

In order to distinguish the different gases, all cylinders are marked by different colours. The cylinder itself is mostly black (industrial), green (food) and white (medical) but the shoulders have different colours depending on the gases they contain. The US has its own standard.


Oxygen (O2)
Oxygen is needed in the operation theatre during anaesthesia and for patients with breathing or lung problems as an additive to the breathing air.
Medical oxygen and industrial oxygen are produced by the same process. The gases are practically the same. Industrial oxygen is perfectly fine for medical use.
The oxygen concentration in a gas mixture is measured with an oxygen meter. Normal breathing air contains 21% of oxygen. More under The Oxygen Concentrator.
Oxygen cylinders are indicated by a white shoulder. In the US the oxygen cylinders are completely green.
Oxygen supports combustion and in high oxygen concentration almost everything burns. That is why connectors, valves and regulators have to be clean and oil free. Grease or oil will burn in combination with pure oxygen. Therefore all pipes, fittings and threads which are exposed to oxygen and have been touched with oily hands must be cleaned.


Nitrous oxide (N2O)
Nitrous oxide is an anaesthesia gas. It is commonly known as laughing gas, nitrous, nitro or NOS.
The shoulder of the cylinder is dark blue. American cylinders are completely blue.


Compressed Air
Compressed air is required in hospitals to operate machines such as some ventilators for anaesthesia machines and some surgical instruments such as pneumatic drills and saws. It is not used for patient care.
Cylinders with compressed air have black and white shoulders, in the US they are completely yellow.
Along the coasts, compressed air cylinders can be also filled up cheaply by scuba dive shops.
Instead of bottled compressed air, normal air compressors (with tanks) can also be used when a reliable power supply exists.


Entonox
Entonox is a medical gas mix of 50% nitrous oxide (N2O) and 50% oxygen (O2). It is frequently used in childbirth and emergency medicine situations.
The shoulder of the cylinder is white and blue.


Changing a cylinder
Changing a gas cylinder is not difficult. But there are some things to consider:
  Gas cylinders should only be transported with their protective metal cap in place.
  Be sure that the cylinder contains the right gas.
  The valve must be closed and the pressure has to be released completely before
    disconnecting the regulator.
  The valve must not be damaged and must be free of dirt, dust and grease.
  When the regulator has an O-ring, check if it is correctly in place and not damaged.
  Do not use Teflon tape on the high pressure side.
  Use the right spanner key or spanner for tighten or loosen the pressure regulator.
  Do not use too much force. Never use a longer lever or a hammer to tighten a regulator.
    The regulator will tighten itself under pressure.
  Open the cylinder valve slowly.
  Check for leaks around the regulator. Listen to hissing sounds. After closing the cylinder,
    valve the pressure shown on the pressure gauge should remain stable. In case of a doubt
    use a leak detection liquid (see below).


Safety
Gas cylinders in the operation theatre or wards have to be secured against falling over. The easiest way to do so is to secure the cylinder with a chain against the wall. Keep also the following in mind:
  Large cylinders should be carried only by two people. Never carry a large cylinder by
    yourself.
  Open flames should be kept away from gas cylinders. Smoking close to gas cylinders is
    dangerous and therefore prohibited.
  Do not use oil or grease in combination with pure oxygen.


Storage
Gas cylinders have to be stored in a dry and well ventilated room. The room must not contain inflammable materials like fuel or paints. The room has to be locked all the time. Smoking and open flames are prohibited in and around this room. Warning signs in the local language should indicate this.

Large cylinders should stand upright and against a wall fixed with a chain or stored in a metal rack. Small cylinders should lie horizontal in a shelf.
The different gases should be stored separately. When the storage room is big enough, empty cylinders should also be separated from full cylinders. It is a good idea to remove the plastic caps from empty cylinders.


Typical repairs and common problems
When buying oxygen cylinders from a doubtful source, check the purity of the oxygen with an oxygen meter. An oxygen meter should be present in any workshop especially when also oxygen concentrators have to be maintained or repaired.
Oxygen concentrators can be an alternative for oxygen gas cylinders. In theory. They produce the needed oxygen but at some cost, as these machines need intensive maintenance and a reliable power supply.
More about it under The Oxygen Concentrator.

Often anaesthesia machines require medical oxygen. But in fact it is not the gas itself that is required but just the special pin-index connection. In this case it is a good idea to replace the 'medical' pressure regulator by an industrial one. Industrial oxygen is not only much cheaper than medical oxygen, it is also available in every little town. You can buy a suitable oxygen pressure regulator at the gas supplier or in hardware shops.



'Welding regulator' in the operating theatre


Here are more common problems:

  Leakages
    Leakages make a hissing sounds. To locate the source of the leakage, use a soap water
    solution for detection on the fitting. Apply the soap water with a brush to the suspected
    connection. The escaping gas will create bubbles.
  Damaged O-rings and Bodok washer
    A Bodok washer is a metal ring with an embedded neoprene washer. It is only used in
    regulators with pin-index gas connections. Both, Bodok and normal rubber washers are
    inexpensive and should be present as spare parts in every workshop.


  Wrong pressure regulator
    Flow regulators or often seen in connection with anaesthesia machines. But machines
    need a pressure regulator. You should also note the required pressure range of a
    connected machine and adjust the pressure regulator accordingly.
  Broken gauges
    Broken or damaged gauges of pressure and flow regulators are very common. Sometimes
    they can be fixed but often not. It is anyway a good idea to keep all broken gauges or
    complete regulators in your workshop, as it is often possible to make one functioning out
    of two damaged ones.
  Leaking hoses
    Low pressure hoses from the pressure reducer to the anaesthesia machine are often
    leaking. Rubber or plastic tubes do age and become brittle and develop cracks. Often it
    happens at the ends of the hose where the adapter and the anaesthesia machine is
    mounted. Then the broken part can be cut off. Otherwise exchange the hose completely.
    Buy only best quality, reinforced with fabric for example. Hoses are available at
    welding accessories shops.


Links and sources
Here are some Wikipedia Medical Equipment articles:

     Medical gas supply
     Gas cylinder
     Pressure
     Pressure regulator
     Rotameter
     Breathing gas
     Oxygen
     Nitrous oxide
     Nitrogen
     Carbon dioxide
     Entonox
     Pin index system
     Bodok seal