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Guideline: Gas sensor

Purpose

To set out the guidelines for the choice, installation, and operation of gas sensors in University workplaces.

Guideline

Scope

  1. This guideline provides information on the selection, installation and operation of fixed gas sensors and monitoring systems in University workplaces.
  2. To ensure an effective gas monitoring system, it is a requirement to involve the Occupational Health and Safety Branch during the system design stage.

Exemption

  1. An exemption to this guideline is only available in writing after appropriate discussions and agreement with the Occupational Health and Safety Branch.
  2. Portable gas monitors are not covered by this guideline. However, portable gas monitors/detectors must be operated by trained and competent people. A portable monitor must be maintained in-line with the manufacturer's requirements.

Gas system design

  1. A gas monitoring system aims to provide:
  • (Early) Detection and warning of a gas leak,
  • Actions in the event of process or gas system failure, and/or
  • Warning of a hazardous work environment that may present a risk to health or safety.
  1. The gas supply system and supporting gas monitoring system must be discussed and designed with input from the Occupational Health and Safety Branch (contact ohs.officer@anu.edu.au). Shared processes, gas specificity, cross-sensitivities, sensor poisoning and contamination must be considered to ensure reliable gas monitoring and to minimise false alarms.
  2. The presence of a gas monitoring system does not remove the requirement to have appropriate gas system shutdown devices and emergency procedures.

Gas monitoring options

  1. Gas sensors are available for a variety of gases. Where a specific gas sensor is not available, a surrogate gas sensor (e.g. oxygen for inert gases) may be used.
  2. Gas sensors shall be located and positioned such that they can quickly detect an abnormal gas or process situation. Possible sensor locations are:
  • Associated with the gas supply network (e.g. gas cage and protective ducting/housing),
  • Within the process equipment (e.g. gas control panel, point of consumption),
  • Within the habitable work environment,
  • After a gas cleaning device (scrubber), and
  • Within the exhaust ventilation system[1].
  1. One or several of these locations may be required for monitoring the gas. The work environment must be monitored when a leak would pose a significant risk to life and health.
  2. The height of the gas sensor will be dependent on the gas's relative density. Heavy or cold gases will be monitored at a lower level than lighter or hot gases. Additional detail can be found in the table below.
  3. A gas monitoring display/panel should be provided for laboratory gas monitoring systems. Any gas display panel must be located outside the (potentially) hazardous zone/room.
  4. A battery backup for the gas monitoring system must be considered, especially where the process may continue without power or ramp-up quickly after power is restored. This ensures that the gas sensor requires little or no time to stabilise and the system reliably operates.

Setting gas detector responses

  1. The concentration of gas that triggers an alarm must be discussed and determined in consultation with the OHS Branch. Some common examples are given in the table below.
  2. The actions in the event of a high or dangerous gas concentration/situation must be pre-determined and agreed with the OHS Branch. Some options include:
  • A link to the fire/emergency evacuation alarm system to initiate a building evacuation,
  • Interaction with the building management system to shut down the building's ventilation system or increase exhaust ventilation flow rates,
  • Activation of gas shut-off valves,
  • Restricting Facility, room or door access or operation, and
  • Simple audible and/or visible alarm signals.

Calibration

  1. Calibration of gas sensors must occur -
  • prior to the University accepting the monitoring system,
  • after replacement of the gas sensor/detector cell, and
  • annually, as a minimum, or
  • at a calibration frequency determined by the manufacturer/supplier.
  1. In some monitoring systems (e.g. for oxygen), a simple air challenge can be used to determine proper function. Monthly detector challenges and 6-monthly service intervals should also be considered.

Maintenance

  1. A gas monitoring system must be maintained in line with the manufacturer's requirements.

Documentation

  1. The installation of a gas monitoring system must be accompanied by:
  • Design specifications and objectives,
  • Operating instructions,
  • Maintenance instructions and schedule,
  • Standard gas system alarm response procedure(s),
  • Documented actions in the event of another building emergency (e.g. nearby fire),
  • A list of local contact personnel,
  • Details of people/companies able to conduct calibration, and
  • Signage near/on the gas sensor units, display panel, audible/visual alarms (etc. as appropriate).
  1. Information and awareness training about the monitoring system is required for all users of the gas/process system.

Table 1: Typical Gas Sensor Requirements

Gas type
(Dangerous Goods Class)

Gas system example

Monitoring

Sensor location

Alarm set points

(LA - low alarm,
UA - upper alarm)

Alarm action

2.1 - Flammable (or explosive) gases

Acetylene

LPG

Process

Work Environment

Exhaust stream

1.5 m of floor

LA - 10% LEL

UA - 30% LEL

· Warning

· Increase exhaust ventilation

· Shut-off supply gas

· Evacuate area at UA

Hydrogen

Process

Work Environment

Exhaust stream

At least 1.5m off the floor to ceiling

LA - 10% LEL

UA - 20% LEL

· Warning

· Increase exhaust ventilation

· Shut-off supply gas

· Evacuate area at UA

LPG

Work Environment - gas water or gas heaters

0.4 m off floor for LPG detection. Alternate locations are possible when hot gases may be involved.

LA - 10% LEL

UA - 30% LEL

· Warning

· Increase exhaust ventilation

· Shut-off supply gas

· Evacuate area at UA

2.2 - Non-toxic, non-flammable gases

Nitrogen, Argon

Work Environment - Oxygen depletion in the work environment.

Oxygen monitoring.

1.5 m off floor

LA - 19.5% O2

UA - 23% O2

· Warning

· Increase exhaust ventilation

· Shut-off supply gas

· Evacuate area at LA or UA

liquid nitrogen, cryogenic liquids

Work Environment - Oxygen depletion in the work environment.

Oxygen monitoring.

0.4 m off floor

LA - 19.5% O2

UA - 23% O2

· Warning

· Increase exhaust ventilation

· Evacuate area at LA or UA

2.3 - Toxic gases

Arsine, ammonia

Process

Work Environment

Gas storage cabinet

Exhaust stream 1

Workspace

Depends on the gas, exposure limits (TLV) and biological responses.

LA - 10-30% of TLV, 8hr TLV or 10% peak TLV

UA - 50-80% of TLV, or STEL, 20% peak

· Warning

· Increase exhaust ventilation

· Shut-off supply gas

· Evacuate area at UA

· Evacuate the building

Also consider -

· Links to fire/evacuation system

· Gas shut-off valves

· Dual level warnings/alarms

· Restricting room/door access

LEL = Lower Explosive Limit

TLV = Threshold Limit Value (an exposure limit)

STEL = Short Term Exposure Limit,

Note: This table provides possible gas system structures. These must be confirmed with the OHS Branch for each individual gas monitoring installation.

[1] Some gas monitors may not function correctly in high velocity exhaust systems.

Information

Printable version (PDF)
Title Gas sensor
Document Type Guideline
Document Number ANUP_000743
Version 5
Purpose To set out the guidelines for the choice, installation, and operation of gas sensors in University workplaces
Audience Staff
Category Administrative
Topic/ SubTopic Health, Safety & Environment - Occupational Health & Safety
 
Effective Date 1 Aug 2011
Review Date 2 Aug 2014
 
Responsible Officer Director, Human Resources
Approved By: Director, Human Resources
Contact Area Human Resources Division
Authority