Anesthesiologist's perspectives for knowledge of medical gas cylinders

1. Medical Gas Cylinder
R.Srihari

2. Topics for Discussion
• Definitions
• Medical Gas Cylinder
– Components
– Sizes
– Contents and Pressure
– Testing and Filling
– Color
– Marking, Labelling and Tags
– Rules for safe use of cylinders
– Hazards

3. Defintions
• Psi: Pounds per square inch
• Psig: Pounds per square inch gauge
GAUGE PRESSURE
It is the difference between the measured
pressure and the atmospheric pressure
Most gauges are measured to read zero
at atmospheric pressure

4. • Psia: Pounds per square inch absolute
ABSOLUTE PRESSURE
It is based on reference point of zero in a
perfect vacuum
Psia = Psig + atmospheric pressure
Eg: at sea level- atmospheric pressure is
0 but psia is 14.7psia

5. • Units of Pressure:
– 1 atmosphere = 14.7 psi
760 mmHg
1030 cm H20
1000 mbar
100 KPa

6. • Critical temperature:
– It is defined as the temperature below which no
gas can be liquefied irrespective of which pressure
is applied
• Critical Pressure:
– Defined as the minimum pressure required to
liquefy a gas at its critical temperature

7. • Compressed Gas:
– It is defined as any mixture having in a container
an absolute pressure exceeding 40 psi at 70 F
OR
– Regardless of the pressure at 70 F having a
absolute pressure exceeding 104 psi at 130 F
OR
– Any liquid having a vapour pressure exceeding 40
psia at 100 F

8. • Non-Liquified Gas:
– These are gases that do not liquefy at ambient
temperatures regardless of the pressures applied
– These gases do become liquids at lower
temperatures at which point – called as
CRYOGENIC LIQUIDS
– Eg: Oxygen, Nitrogen, Air, Helium

9. • Liquified Compressed Gases:
– It is one in which gas becomes liquid at ambient
temperatures at pressures varying from 25-100
psig (172 -10340 Kpa)
– Eg: N20, CO2

10. Standards for Cylinders in India
• Gas Cylinders Rules(1981)
• Static and Mobile Pressure Vessels(unfired)
rules (1981)
• Bureau of Indian Standards Act(1986)

11. • All those who produce, supply and transport or
use medical gases must comply with variety of
safety regulations
 put forth and enforced by agencies at
state and central government
• Purity of gases specified by USP and enforced
by FDA

12. Components
• Consists of:
– Body
– Valve
– Port
– Stem
– Pressure relief devices

13. • Body:
– Most medical gas cylinders are made of steel with
various alloys added
– In recent years, manufacturers have moved from
traditional steel cylinders towards steel-carbon
fibres cylinders
 Can hold more gas and light in weight

14. – MRI compatible cylinders are made of aluminium
– Modern cylinders are made of alloy of
MOLYBDENUM + STEEL +/- CHROMIUM
Alloy containing Molybdenum (0.15-0.25%)
Chromium (If +  0.8-1.1%)
Is used to increase strength and to minimise weight
and wall thickness

15. • Walls of the cylinder vary from 5/64 to ¼ inch
thickness on an average
• Cylinders that have a marking 3AA are made
from steel. The marking 3AL or 3ALM indicates
that the cylinder is made from aluminium
• Cylinder have a flat or a concave base. The other
end may taper into a neck that is fitted with
tapered screw threads that attach to the cylinder
valve

17. • Valve:
– Cylinders are filled and discharged through a valve
(spindle shaped) attached to the neck
– It is made of bronze or brass which is heavily plated
with nickel or chromium so as to allow rapid
dissipation of heat of compression
– The end which enters the neck of the cylinder is
threaded to fit a corresponding screw thread inside the
neck itself

18. • A sleeve or washer of soft alloy ( containing high
proportions of lead) completes the gas tight seal
as the valve is screwed into the neck of the
cylinder
• Cylinder valves are of various types.
– Those used on anesthesia machines are ‘flush’ types
which fits with the pin index system on the machine
– For medium and large capacity cylinders – bull nose
valves are used

21. • Cylinder valves are of 2 types –packed type and
diaphragm type
– Packed type:
• Capable of withstanding high pressures
• A.k.a direct acting valve
• Stem is sealed by resilient packing such as TEFLON which
prevents leaks around the threads
• In large cylinders, the force is transmitted by means of driver
square
• It is opened by 2-3 turns
• Used in most of the cylinders

23. – Diaphragm type:
• Stem is separated from the seat
• Closure between the cylinder interior and atmosphere is
accomplished by using a seal and a bonnet nut that clamps one or
more circular discs in place
• These discs separate upper and lower stems which may be
permanently attached to the diaphragms
• Upper stem is actuated by manual/automatic means and Lower
stem shuts/permits flow through the valve
• Can be opened fully by ½ to ¾ turns
• Seat does not turn-so less likely to leak
• Generally preferred when pressures are relatively low and no leaks
can be allowed
• However - expensive

25. • Port:
– It is the point of exit for the gas
It fits into the nipple on the hanger yoke of the
anesthesia machine
– It should be protected in transit by a covering
– When installing a cylinder on anesthesia machine, it is
important for the user not to mistake the port for the
conical depression

27. – Conical depression is situated on the opposite side of
the port on the cylinder valve and is situated above the
safety relief device
It is present on those cylinders which are designed to
fit on anesthesia machine
– Conical depression is designed to receive the retaining
screw on the yoke of the anesthesia machine
 Screwing the retaining screw into the port may
damage the port

29. • Stem:
– Each valve contains a stem (spindle/screw-pin) or shaft
that is rotated to open or close the cylinder valve
– It is made up of very hard steel
– To close the valve the stem seals against the seat that is
part of the valve
 when the valve is opened –stem is moved
upwards and allows the gas to flow to the port

30. • Pressure Relief Devices:
– Every cylinder is fitted with pressure relief devices
whose purpose is to vent the cylinder’s contents to the
atmosphere
 if the pressure of enclosed gas increases to
dangerous levels
– Types:
• Rupture Disc
• Fusible Plug
• Combination of Both
• Pressure Relief valve (spring loaded)

31. – Rupture Disc:
• When pre-determined pressure is reached the disc
ruptures and allows the gas contents to be discharged
• It is a non- reclosing device held against an orifice
• It protects against excess pressure as a result of high
temperature/overfilling

33. – Fusible Plug:
• It is thermally operated
• It is a non-reclosing pressure relief device where the plug is
held against the discharge channel
• It provides protection against excess pressure due to high
temperature but not overfilling
• YIELD TEMPERATURE: Temperature at which fusible
material becomes sufficiently soft to extrude from its holder-
so that cylinder contents are discharged

34. – Spring loaded pressure relief device:
• It is a reclosing device
• When set pressure is exceeded, the pressure in the
cylinder forced the spring to open the channel for letting
out the gases
Gas flows around the safety valve seat to discharge
channel till excess pressure is relieved

35. • Handle/ Handwheel:
– It is used to open or close a cylinder valve
– It is turned counter-clockwise to open and clock-wise
to close
 this causes the stem to turn
– A good practice is to attach a handle to each anesthesia
machine or other apparatus for which it may be needed
– Each large cylinder has a permanent attached
handwheel that uses a spring and nut  to hold it
firmly in place

38. Non-Interchangeable Safety Systems
• With withspread use of cylinders containing
different gases , a potential hazard is
connection of a cylinder to equipment intended
for a different gas
• For safety purposes
– Color coding for each gas
– Pin index safety system

39. Color coding
GAS SHOULDER BODY
OXYGEN WHITE BLACK
NITROUS OXIDE BLUE BLUE
CYCLOPROPANE ORANGE ORANGE
CARBON DI OXIDE GREY GREY
AIR WHITE GREY
NITROGEN BLACK BLACK
ENTONOX WHITE BLUE

40. Pin Index Safety System
• The use of PISS began in 1952
• In order to ensure that the correct cylinder is
attached to the appropriate hanger yoke of the
anesthesia machine/workstation
A series of pins on the hanger yoke is made to fit
into the corresponding indentations(pits/holes)
drilled into cylinder valve

41. • It consists of holes on the cylinder valve positioned in an
arc below the outlet port
• Positions of the cylinder valve are on the circumference of a
circle of 9/16 inch (14.3mm) radius centered on the port
• The port has a diameter of 7mm
• The distance between the centre and lower part of the yoke
is 20.6mm

43. • Unless the pins and holes are aligned, the port
will not seat
• The indentations on the cylinder valve/yoke block
are counted 1-6 from left to right
• The distance between the centre of the 1st pin and
6th pin should be 16mm
• There are 7 positions for pins and holes

44. • The pins are 4.75mm in diameter and 6mm
long except for pin number 7 which is slightly
thicker and placed at the centre (between port
3 and 4)

45. GAS PIN INDEX
AIR 1,5
OXYGEN 2,5
NITROUS OXIDE 3,5
NITROGEN 1,4
O2-CO2 (CO2 <7.5%) 2,6
O2-CO2 (CO2 >7.5%) 1,6
ENTONOX 7

46. Sizes
• Gas suppliers classify cylinders by using a letter code
with A being the smallest
• Volume and Pressure of gas in a particular size cylinder
vary
• O2 and Air – similar ; CO2 and N20- similar
• Size E is the most commonly used in anesthesia
machine and for patient transport and resuscitation
• Size D cylinders are used for limited supplies of gases
where size and weight considerations are important

47. CYLINDER SIZE DIMENSIONS
(OUTER
DIAMETER X
LENGTH) IN
INCH
EMPTY
CYLINDER
VOLUME
KG
INTERNAL
VOLUME
L
D 4.5 X 17 5 3
E 4.25 X 26 6.3 5
H 9.25 X 51 53 46-50

48. O2/AIR N20/CO2
SIZE D – CONTENT IN
LITRES
400 940
SIZE D- PRESSURE IN
PSIG
1900 745/838
SIZE E- CONTENT IN
LITRES
660 1590
SIZE E- PRESSURE IN
PSIG
1900 745/838
SIZE H –CONTENT IN
LITRES
6900 15800
SIZE H- PRESSURE IN
PSIG
2200 745

49. Contents and Pressure
• In a cylinder containing a non-liquefied gas
Pressure declines as the contents are
withdrawn
Hence pressure can be used to measure
cylinder volume(approximately)

51. • In a cylinder containing a liquified gas, the pressure
depends on VAPOUR PRESSURE of the liquid and is
not an indication of the amount of gas remaining in the
cylinder as long as the contents are partly in the liquid
state
Pressure remains nearly constant till all liquid has
evaporated
After which pressure declines till cylinder is exhausted

53. Testing
• A cylinder must be inspected and tested atleast
every 5 years or with special permit upto every 10
years
• The test date must be permanently stamped on the
cylinder
• Each cylinder must pass an internal and external
visual check for corrosion and evidence of
physical impact or distortion

54. • Cylinders are checked for leaks and retention
of structural strength by testing to minimum of
1.66 times their service pressure
• SERVICE PRESSURE IS DEFINED AS THE
MAXIMUM PRESSURE TO WHICH THE
CYLINDER MAY BE FILLED AT 70 F

55. • Other tests that are done:
– Tensile test
– Flattening test
– Bend test
– Impact test
– These are carried on atleast one out of every 100
cylinders

56. Filling
• If a cylinder containing a gas under a safe
pressure at normal temperature is subjected to
higher temperature the pressure may
increase to dangerous levels
• To prevent this, regulations have been drawn
limiting the amount of gas a cylinder may
contain

57. • Non- liquefied gases – may be allowed an
additional 10% filling
• Liquefied gas containing cylinders:
– Pressures will remain constant as long as there is
liquid in cylinder
 to prevent cylinder being overfilled
Maximum amount of gas allowed is defined by the
filling density(filling ratio) for each gas

58. • Filling density- percent of ratio of weight of
the gas in a cylinder to the weight of water that
the cylinder would hold at 60 F
– N20 – 68%
– CO2- 68%

59. Marking/Labelling/Tags
• Important for identification
• To check test date
• In case of flammable gases- Caution/ Danger/
Warning label is needed
• Tags should contain either full/ in use/ empty

61. Hazards
• Incorrect cylinder
• Incorrect valve
• Incorrect color/labelling
• Inoperable valve/ Damaged valve
• Suffocation/fire explosion
• Overfilled contents in cylinder
• Nitrous oxide theft
• Contaminated contents in cylinders
• Thermal injury

62. Rules for safe use of cylinders
1. To be handled by trained staff
2. Store cylinders in a cool, clean room with adequate
ventilation
3. Do not drape cylinder with any material during
storage
4. Cylinders are best stored upright in a cylinder stand
5. Keep the valve closed when not in use

63. 6. Identify contents by label
7. Remove wrappings( protective covering) before
use
8. Remove dust/ foreign bodies before connecting
9. ‘Cracking’ to be done to reduce to possibility of
flash fire

64. 10. A sealing washer in good condition should be used
11. Flow control valves should be closed before cylinders
opened
12. Quick opening to be avoided as it can generate heat
leading to flame
13. Valve should be fully opened when in use
14. To be kept away from oil, rubber and combustible
substances
15. Do not expose cylinder to heat or higher temperatures

65. Thank You