Gas Cylinders
Description
Anaesthetists use gas cylinders every day as part of their
clinical practice. This session examines the physical
components of the gas cylinders, analyses the pressure
changes that exist in oxygen and nitrous oxide cylinders,
and explores Entonox (BOC Healthcare) cylinders.
Session introduction
Learning objectives: by the end of this session
you will be able to:
Identify the physical composition of cylinders,
cylinder valves and their storage.
Interpret the key aspects of Pin Index System.
Recognize and manage pressure changes in Oxygen
and Nitrous Oxide cylinders.
Define the features of Entonox cylinders.
introduction
•
Medical gases and vapours used in
hospitals are often stored in cylinders
under pressure. The cylinders which
hold such gases are made of thin
walled steel which is designed and
fabricated to withstand considerable
internal pressure.
•
However to reduce weight, a recently
introduced design means that
aluminum alloy rather than steel is
now commonly used for cylinder
construction.
•
On the steel cylinder shown, note
components such as the valves,
plastic disc and neck. The year the
cylinder was last examined can be
identified from the shape and colour
of the disc
oxygen
• Oxygen has been used in clinical
practice for more than 200 years. It
is probably the most widely
prescribed medication in
prehospital and hospital
environments.
• If appropriately used is life saving
and part of the first line treatment
in many critical conditions. It is
produced by the separation of air
using cryogenic distillation and
stored in cylinders at a pressure of
13700 KPa.
• because oxygen is a gas, there is a
linear and proportional reduction in
oxygen cylinder pressure as the
cylinder empties at a constant
temperature.
Nitrous oxide
• Nitrous oxide is used as an adjunct to other anaesthetic drugs and also as
an analgesic. It is stored in a liquid phase with its vapor at a pressure of
4400 KPa.
• The nitrous oxide cylinder is only partially filled because in liquid form, it is
less compressible than as a gas, the amount of filling is known as the filling
ratio.
• As the cylinder contains liquid and vapor. Initially the pressure remain
constant as more vapor is produced to replaced what has been used. Once
all the liquid has been evaporated, the pressure in the cylinder decreases.
• The cylinder pressure does not accurately indicate the cylinder contents.
The temperature in such a cylinder can decreases due to the loss of the
latent heat of vaporization which lead to the formation of ice on the out
side of the cylinder.
Nitrous oxide
• Nitrous oxide cylinders are only partially filled
with liquid in order to reduce the risk of a
dangerous increase in pressure, an increase in
the ambient temperature can lead to an
explosion.
• In UK, the filling ratio for nitrous oxide and
carbon dioxide is 0.75.
• In hotter climates, the filling ratio is reduced
to 0.67.
Cylinder size and color coding
•
For patient safety and from a
practical point of view, different
medical gas cylinders types have
various distinguishing features, such
as their size and color.
•
Cylinders are manufactured in
different sizes depending on the gas
they contain.
•
These cylinder sizes are labelled from
A to J. however, sizes A and H are not
used for medical gases.
•
Those attached to the anaesthetic
machine are usually size E, while size
J cylinders are commonly used for
cylinder manifold.
Color coding
Cylinder markings and engravings
•
Several important distinguishing marks are engraved by the manufacturers on the
exterior of all medical cylinders. These include:
•
Test pressure: is employed to test the integrity of the cylinder and its ability to
withstand extremely high pressures – the cylinder is subjected to pressures of
approximately 22000 KPa.
•
Dates of tests performed: every five years, cylinders in use are checked and
tested by manufacturer.
•
Chemical formula of cylinder contents: in addition to colour coding the cylinders-
the chemical formula confirms the contents of the cylinder.
•
Tare weight: this is the weight of nitrous oxide cylinder when empty.
Cylinder valves
•
A cylinder valve is mounted on the top of the
cylinder to seal its contents. The chemical
formula of a particular gas is engraved on the
side of the valve, which is made of brass.
•
An on/off spindle is used to open and close
the valve. It should be opened slowly when
attached to the anaesthetic machine or
regulator to prevent a rapid rise in pressure
and temperature of the gas in the machines
pipelines.
•
When the cylinder is in use, the valve should
be fully open- this requires two full anti
clockwise revolutions. In case of over
pressure, a safety relieve device allows the
discharge of the cylinder contents into the
atmosphere.
•
Bodok seal: is a compressible yoke – sealing
washer placed between the valve outlet and
the apparatus to make a gas – tight joint.
•
It should be carefully inspected for damage
prior to use. It is also advisable to have a
spare seal readily available.
The pin index system
•
In the interest of patient safety, the
process of setting up any gas cylinder for
use within a hospital environment must
receive due care and attention. Certain
safety features have been put in place to
minimize the likelihood of the wrong gas
being used on any occasion.
•
One good example of this is that the gas
exit port will not seal against the washer
of yoke unless the pins and holes are
correctly aligned.
•
This non interchangeable safety system is
known as the pin index system. There is a
specific pin configuration for each medical
gas on the yoke of the anaesthetic
machine or regulator. The matching
configuration of holes on the valve block
allows only the correct gas cylinder to be
fitted in the yoke.
Entonox
•
Entonox, a gas mixture of 50% oxygen and 50% nitrous oxide by volume, can be
used to provide analgesia. It is compressed into cylinders to a pressure of
13700KPa.
•
A two stage pressure demand regulator is attached to the cylinder. As the patient
inhales through the mask or mouthpiece, gas flow is allowed to occur. Gas flow
ceases at the end of an inspiratory effort.
Entonox
The Poynting Effect:
The liquefaction and separation of the two component gases of entonox can occur if a
cylinders temperature is decreased to below -5.5 C ( the poynting effect). This result
in:
At the bottom of the cylinder, a liquid mixture containing mostly nitrous oxide with
about 20% oxygen dissolved in it.
Above the liquid, a gas mixture of high oxygen concentration.
When used at a constant flow rate, a gas with a high concentration of oxygen is
supplied first. This is followed by a gas of decreasing oxygen concentration as the
liquid evaporates. This may lead to the supply of hypoxic mixtures with less than 20%
oxygen as the cylinder empties.
This separation and liquefaction can be reversed by re – warming the cylinder and
mixing its contents by inverting it repeatedly.
Entonox
Avoiding entonox separation:
Cylinders are stored horizontally for
about 24 hours at temperature of
about 5 C or above before use, to
increase the area available for
diffusion.
If the contents are well mixed by
repeated inversion, cylinders can be
used sooner than 24 hours.
Large cylinders have a dip tube with
it ending in the liquid phase. this
result in the liquid being used first,
preventing the delivery of an oxygen
concentration of less than 20 %.
Cylinder storage and maintenance
•
Because of the inherit dangers associated
with working in the vicinity of pressurized
cylinders, further safety procedures must
be rigorously adhered to for the
maintenance and storage of gas cylinders.
•
Full cylinders should be separated from
empty ones to avoid accidents. This is to
avoid the risk of disrupting patient gas
supply by accidentally connecting an
empty cylinder.
•
Size F,G and J cylinders should be stored
upright to avoid damage to the valves.
•
Size C,D and E cylinders can be stored
horizontally on shelves made of a
material that does not damage the surface
of the cylinders, usually tember.
Session key points
•
Gas cylinders are made of thin walled molybdenum steel to withstand high
pressures.
•
They are made in different sizes.
•
Oxygen cylinders contain gas whereas nitrous oxide cylinder contain a mixture of
liquid and vapour. In the UK, they are 75% filled with liquid nitrous oxide ( filling
ratio).
•
At a constant temperature, the pressure in a gas cylinder decreases linearly and
proportionally as the cylinder empties. This is not true in a cylinder containing
liquid/ vapour.
•
Gas cylinder are colour coded and display labelling and marking.
•
They undergo regular testing and checking.
•
A cylinder valve is mounted on the neck of the cylinder which acts as on/off device
for the discharge of the contents. The pin index system prevent cylinder
identification errors.
•
Entonox is a gas mixture of 50% oxygen and 50% nitrous oxide by volume.