Many different instruments Many different operating principles including: May sample many different gases & vapour From relatively simple to complex

Transcription

1 Part 2 Direct Reading Instrumentation

2 Direct Reading Instruments Many different instruments Many different operating principles including: Electrochemical Photoionisation Flame ionisation Chemiluminescence Colorimetric Heat of combustion Gas chromatography May sample many different gases & vapour From relatively simple to complex

3 Uses of Direct Reading Instruments Where immediate data is needed For personal exposure monitoring Sample air in confined spaces Help develop comprehensive evaluation programs Evaluate effectiveness of controls Prepare for emergency response

4 Passive/Diffusion sampling: x mol wt Calculation of results Conc (mg/m 3) = W (µg) x A r x t where W = contaminant weight (µg) A calculation constant = 1000 / Sampling rate r = recovery coefficient t = sampling time in minutes Conc (ppm) = W (µg) x B r x t where W = contaminant weight (µg) B = calculation constant = 1000 x / Sampling rate r = recovery coefficient t = sampling time in minutes

5 Uses of Direct Reading Instruments (cont) For difficult-to-sample chemicals Multi sensors Multi alarms For stationary installations Use for fit testing of respirators Video monitoring

6 Limitations Often costly to purchase Need for frequent and regular calibration Lack of specificity too generic Effect of interferences Cross sensitivity Need for intrinsically safe instruments in many places Battery life Sensors Finite life, poisoning, lack of range

7 Advantages Direct reading Continuous operation TWA, STEL & Peaks Data logging (recording)

8 Other Limitations E.g. Catalytic combustion detectors React with other flammable gases Poisoned by Silicones Phosphate esters Fluorocarbons

9 Single Gas Monitor Interchangeable sensors including: O 2, CO, H 2 S, H 2, SO 2, NO 2, HCN Cl 2, ClO 2, PH 3 STEL, TWA, peak Alarm if exceeding limits Data logging Source: Industrial Scientific Inc reproduced with permission

10 Multigas Monitor 1 6 gases Interchangeable sensors: LEL, CH 4, CO, H 2 S, O 2, SO 2, Cl 2, NO, ClO 2, NH 3, H 2, HCl, PH 3 STEL, TWA, peak Alarm Data logging

11 Gas Badges Two year maintenance free For single gas monitor Sensors include CO, H 2 S, O 2 SO 2 Turn them on & let them run out Alarms Some data logging ability Source: Industrial Scientific Inc reproduced with permission

12 Photo Ionisation Detectors (PID) Dependent on lamp ionisation potential Typically for non specific VOCs or total hydrocarbons Some specific eg benzene, NH 3, Cl 2 Not for CH 4 or ethane Affected by humidity, dust etc. Source: Airmet Scientific-reproduced with permission

13 Flame Ionisation Monitor Similar to PID but using flame Non specific, broad range Less sensitive to humidity & other contaminants Poor response to some gases Needs hydrogen (hazard) Source: Airmet Scientific-reproduced with permission

14 Portable Gas Chromatograph Highly selective for certain chemicals Range depends on type of detector used Complex instrument requiring extensive operator training Non continuous monitoring Source: Airmet Scientific-reproduced with permission

15 Infra-redred Analyser For organic vapours Specific gases Portable Expensive

16 Two principles: Mercury Vapour Detectors UV Interferences: Ozone Some organic solvents Gold Film High cost Gold film needs regular cleaning

17 Maintenance & Calibration Source: Industrial Scientific Inc reproduced with permission

18 Guidelines for Using Gas Detection Equipment Bump or challenge test Daily before use, known concentration of test gas to ensure sensors working correctly Calibration Full instrument calibration, certified concentration of gas(es), regularly to ensure accuracy & documented Maintenance Regular services provides reassurance instruments repaired professionally & calibrated & documented

19 Typical Basic Instrument Checks Physical appearance Ensure instrument is within calibration period Turn instrument on and check battery level Zero the instrument recorded data Bump test (functionality test) instrument Clear the peaks

20 Standard Gas Atmospheres Primary Gas Standards Are prepared from high purity 5.0 (dec pt.) gases ( %) or 6.0 gases ( %) by weighing them into a gas cylinder of known size Secondary Gas Standards Are prepared volumetrically from these (primary gases) using gas mixing pumps or mass flow controllers (mixture of gases) Source: University of Wollongong

21 IECEx Standards Intrinsic Safety Equipment for use in explosive areas e.g. Underground coal mines Oil refineries Petrol stations Chemical processing plants Gas pipelines Grain handling Sewerage treatment plants

22 Intrinsic Safety (cont) Classification of zones Gases, vapours, mists Dusts Explosive atmosphere is present Zone 0 Zone 20 Most of the time Zone 1 Zone 21 Some time Zone 2 Zone 22 Seldom or short term Source: TestSafe reproduced with permission

23 Group 1 Group II Intrinsic Safety (cont) Gas or Explosive Groups Equipment used underground eg methane & coal dust Equipment used in other (above ground) hazardous areas IIA - least readily ignited gases eg propane & benzene IIB more readily ignited gases eg ethylene & diethyl ether IIC most readily ignited gases eg hydrogen and acetylene

24 Temperature classes Intrinsic Safety (cont) Group I Surfaces exposed to less than 150 C Group II Temp Class Max permissible surface temp C (auto ignition) T1 450 T2 300 T3 200 T4 135 T5 100 T6 85 Source: TestSafe reproduced with permission

25 Intrinsic Safety (cont) Levels of Protection & Zones Levels of protection ia ib ic Suitable for use in Zones 0, 20 (safe with up to 2 faults) Zones 1, 21 (safe with up to 1 fault) Zones 2, 22 ( safe under normal operation) Source: TestSafe reproduced with permission

26 Intrinsic Safety Markings Example ia IIC T4 Smith Electronics Model TRE Ex ia IIC T4 Cert 098X Serial No equipment suitable for zone 0 application equipment is suitable for Gas Groups IIA,IIB, IIC equipment is suitable for gases with auto ignition temp greater than 135 C

27 Detector Tubes - Colorimetric Tubes Change in colour of a specific reactant when in contact with a particular gas or vapour Source: Dräger Safety Reproduced with permission

28 Advantages Relatively inexpensive & cheap Wide range of gases and vapours approx 300 Immediate results No expensive laboratory costs Can be used for spot checks No need for calibration No need for power or charging

29 Limitations Interferences from other contaminants Need to select correct tube & correct range Results should NOT be compared to TWA Correct storage Limited shelf life

30 Colour Tubes / Badges Available For Instantaneous short term measurement Long term measurements pump Long term measurements diffusion

31 End of Part 2

32 Part 3 Personal Air Sampler

33 Air Sampling There are various locations at which one may wish to take an integrated sample of a chemical in the plant air. A general plant air sample is useful to give an overall measure of plant contamination. One might also be concerned with escape of chemical at a known or suspected point source, such as an open vat, a spraying operation, or a valve. Measurements made at a source of contaminant escape should not be used as values representing overall contamination of plant air. Air collected at a point source will later be diluted by plant air or may be removed effectively by the ventilation system. Such a reading indicated hazard to a worker at the location and estimates the effectiveness of systems that clear the air.

34 Air Sampling A variety of stationary devices are available that either collect a sample for later analysis of give a direct reading of the contamination of the air at that location. Such devices may depend on appearance of a specific absorption of infrared light, change in the transparency of a filter, change in the pressure drop across a filter, scattering of light by airborne particulate of variety of other techniques. Devices are available to take samples automatically at timed intervals.

35 PERSONAL AIR SAMPLERS The most important air to sample is the air inhaled by the individual worker. Such air must be collected near the face. Unless we wish to attach the worker by a tube to a large stationary device, which would restrict the free movement of the worker and thereby distort the results of the study, the entire apparatus must be small and lightweight enough to be carried about conveniently by the worker. Such personal air samplers are available and are in common use. They consist of a small, battery-powered air pump that can be worn on the belt, to which a trapping device is attached. A tube pinned to the clothing near the face carries the air to the trapping device.

36 End of Part 3