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IHEA Professional Development Seminar No. 2 / 2006 Developments in the Design And Installation of Medical Gases

DESIGN STANDARDS

Presented by Geoff Hislop

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DESIGN STANDARDS

How many, how much and how safe. 1. INTRODUCTION

The main requirements for medical gas systems are performance, reliability and safety.

In Australia we have a comprehensive set of Australian Standards to use to achieve these ends.

The main information missing from these Australian Standards is sizing data for medical gas piping.

2. AUSTRALIAN STANDARDS

Some medical gas standards applicable are: -

AS 2896 – 1998 Medical Gas Systems – Installation and Testing of Non-flammable Medical Gas Pipeline Systems.

AS 1894 – 1997 The Storage and Handling of Non-flammable Cryogenic and Refrigerated Liquids.

AS 4332 – 2004 Storage and Handling of Gases in Cylinders.

There are other standards referred to in these standards.

3. NUMBER, TYPE AND LOCATION OF OUTLETS

The number and type of outlets is often set by the users to meet particular procedure requirements.

Guideline requirements for many room types have been set out by Department of Human Services.

These show information on the number of medical gas outlets and type of outlets required.

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For those locations and applications where no diversity factor is nominated, AS 2896 gives a formula for calculating a diversity factor to be used for design.

Diversity factor

Where n is the total remaining number of terminal units on the branch or main

Graphing the diversity factor against the number of outlets gives the following relationship

Notice that the diversity factor tends to be asymptotic towards 0.4 or 40%.

All hospital systems need to have spare capacity in their design to allow for future increase in demand.

This is likely to be conservative in practice, particularly for large systems. Use of a higher diversity factor does, however, provide a safety factor to allow for future increase in demand and connection of additional outlets.

4. PIPE SIZING

Pipeline sizing is based on the flow rate required and a design maximum pressure loss from the source of supply to the last outlet.

( )nF

10log3.1

=

1 1.32 0.9992083235 0.765169483

10 0.6520 0.56496438730 0.52480273140 0.49960416150 0.48166522760 0.46793708670 0.45692625880 0.44779872790 0.44004512

100 0.433333333

DIVERSITY FACTOR

00.20.40.6

0.81

1.21.4

0 20 40 60 80 100 120

Number of outlets

Fact

or

Series1

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Typical figures for use in pipe sizing for various medical gases are: -

Medical gas and medical compressed air 5%- 10%

Theatre medical compressed air 7.5%

Medical suction 10%

It is also necessary to know the design flow rate for outlets and the pressure loss through outlets. The following is extracted from AS 2896.

Gas Nominal pressure at terminal

Flow rate Pressure loss across terminal

Medical air, oxygen,

entonox

375 kPa 250 l/min 100 kPa

Nitrous oxide, oxygen + carbon dioxide, carbon dioxode

375 kPa 40 l/min 10kPa

Surgical tool gas 1200 kPa 250 l/min 50 kPa

Suction, active gas scavenge

-55 kPa 40 l/min 15 kPa

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Reticulation pipe sizing information for various gases flowing in pipes is not readily available and is not included in AS2896. One source of pipe pressure loss chart information is British Health Technical Memorandum No.22 “Piped Medical Gases, Medical Compressed Air and Medical Vacuum Installations”.

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6. SUMARY OF DESIGN STANDARDS

• The basis of design for Medical gases should be AS 2896.

• The sizing of piping needs to be based on flow charts suitable for the gas involved.

• The total pressure drop of the piping system must be used as the basis for pipe sizing taking into account diversity and pressure loss of the outlet.

• Pipe sizing should have an allowance in it for future capacity and installation of additional outlets for flexibility.