7. Pressure Regulators

Question 1

What are the pressures at which the common anaesthetic gases are stored and delivered at?

Answer 1

Table 56.1 Storage and delivery pressures for commonly used medical gases

Site Oxygen Air Nitrous oxide

Cylinder pressure 137 bar 137 bar 52 bar (room temp.)

Pipeline pressure 4 bar 4 bar 4 bar

Common gas outlet p 2 cm H2O 2 2

Question 2

What are Pressure regulators

Answer 2

(or pressure-reducing valves)

devices that reduce a higher variable

inlet pressure to a

constant lower outlet pressure.

Question 3

What different types of pressure regulators do you know?

Answer 3

The different types of pressure regulators

may be classified as follows:

> Direct –

> Indirect –

> Two-stage –

> Slave –

Question 4

Direct –

Answer 4

where the cylinder pressure opens the valve

Question 5

> Indirect –

Answer 5

where a spring opens the valve

in response to falling outlet pressure

Question 6

> Two-stage –

Answer 6

where the input of one
is the output of the other,

this reduces wear on the diaphragm
and reduces pressure fluctuations
in high gas flows

(e.g. demand valves used on entonox cylinders)

Question 7

> Slave –

Answer 7

where the output of one valve

is dependent on the output of another

(e.g. nitrous oxide valve will not open
unless there is output from the O2 valve).

Question 8

Why is the gas cylinder on an anaesthetic machine not

connected directly to the rotameter block?

Answer 8

Gas (and vapour) cylinders are used to

store gases and vapours at high pressures.

These pressures vary according to the

type of gas or vapour used,

the cylinder content
and
temperature.

The temperature and cylinder content

will in turn vary as the gas or vapour is used.

If unregulated this would

lead to a variable flow of gas or vapour
to the patient.

In order to provide a
safe and
constant mixture
of gases during anaesthesia,

it is important to protect both
the patient
and
anaesthetic machine

from exposure to higher pressures,

surges in pressure when cylinders are opened

and changes in flow.

This is achieved by using

pressure regulators,
flow restrictors and
pressure relief valves.

It is important to note that the
pressure regulators are specific
to particular gases and are set during servicing.

Question 9

How does a single-stage regulator work?

Answer 9

> Pressure regulators consist of two chambers,

a high-pressure chamber
and a low or control pressure chamber,

separated by a conical valve whose
orifice is controlled by a spring
connected to a diaphragm in the
control chamber.

> They work by balancing
the force from the spring
against the force generated by
pressure against the diaphragm.

> This shows that the performance
of the valve is

related to the ratio of the areas
of the diaphragm and the valve:

the bigger the difference,

the greater the drop in pressure
for the same force applied by the spring.

From the diagram it can be seen that

if the inlet pressure (P) increases,

the diaphragm will lift 
and 
the conical valve will shut
and 
vice versa. 

The pressure in the control chamber
can be fixed
or
varied

by altering the
tension in the spring.

Question 10

How does a two-stage pressure regulator work?

Answer 10

> A demand valve on
an entonox cylinder
or
diving cylinder

is an example
of a two-stage pressure regulator.

> The first stage of the regulator is identical

to that already described,

but now the flow of gas
from the low-pressure chamber
enters the second stage chamber.

> The second-stage chamber contains

a larger diaphragm that operates a valve,

which connects it to the
first-stage low-pressure chamber.

> The demand valve detects
when the patient inspires
and
supplies a breath of gas at ambient pressure.

> As the patient inspires,
the pressure within the
second-stage chamber reduces,

moving the diaphragm,

which opens the valve 
and 
allows gas to enter 
the second-stage chamber 
from the first-stage low-pressure
chamber.

Question 11

How does the Ritchie whistle work?

Answer 11

> The Ritchie whistle is an
oxygen failure alarm which is
powered solely by
falling oxygen pressures.

> It was invented by John Ritchie,
a New Zealand anaesthetist, and was
introduced into practice in the mid-1960s.

> Again, its design was tailored around a
single-stage pressure regulator.

> Its working principle is simple:

as the oxygen pressure falls,

the force acting on the diaphragm reduces

and a point is reached

when the opposing force applied

by the spring is greater,

allowing the valve to open and
oxygen to leak past
and activate the whistle.

> Current oxygen failure devices

must have an auditory sound of at least
60 dB lasting 
for 7 seconds 
and 
should be activated 
when oxygen pressure falls to 200 kPa. 

It should be linked to a system
that shuts off the supply of
all other gases
apart from oxygen and air.

> Modern devices now use electronic sensors