Medical Gas Cylinder

R.Srihari

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

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

• 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

• Units of Pressure:– 1 atmosphere = 14.7 psi

760 mmHg

1030 cm H20

1000 mbar

100 KPa

• 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

• 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

• 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

• 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

Standards for Cylinders in India

• Gas Cylinders Rules(1981)• Static and Mobile Pressure Vessels(unfired)

rules (1981)• Bureau of Indian Standards Act(1986)

• 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

Components

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

• 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

– 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

• 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

• 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

• 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

• 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

– 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

• 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

– 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

• 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

• 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)

– 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

– 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

– 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

• 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

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

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

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

• 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

• 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

• 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)

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

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

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

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

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)

• 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

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

• 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

• Other tests that are done:– Tensile test– Flattening test– Bend test– Impact test

– These are carried on atleast one out of every 100 cylinders

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

• 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

• 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%

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

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

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

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

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

Thank You