The HSE recommend that Airflow Indicators (Pressure Gauges) such as Manometers are fitted where possible to each hood duct, so that the operatives using the system can tell wether it is working properly. This requirement was introduced in HSG258 2008.
The client needs an airflow indicator at every hood. That is because the operator needs some simple indication that the hood is working. The airflow indicator must indicate simply and clearly when the air-flow is adequate.
If there is no manometer fitter at the hood duct, record the need to fir one in the LEV report action plan.
Where an extraction system is designed to extract from some but not all of the attached inlets at one time (i.e. not 100%).
The operator is able by opening dampers, to reduce the airflow to a point where the airflow at some points will be inadequate to properly control the hazardous substance.
In these circumstances the installation of Airflow Indicators is a high priority improvement. It becomes critical when the operator has to adjust a damper to get adequate airflow.
Where an extraction system is designed to connect to one or more inlets and is designed to extraction from 100% of the inlets operating simultaneously.
On certain types and locations of hood ducts it is not possible to fit an airflow indicator that would be visible to the operator, in which case the system manual and operative training should include reference to how the operatives should be aware of the adequacy of the airflow during use and how to respond to any suspected deterioration in performance.
Extraction systems designed to extract from all of the inlets at once. LEV problems that result in reduced airflow are caused by blockages, build of material or component deterioration such as filter blinding or ductwork damage. For all LEV, the installation of Airflow Indicators is recommended as they will immediately show the operator or supervisor that there is a problem.
In addition to the problems detailed above that result in reduced airflow on this type of LEV system to the operators are able to control the airflow by opening and closing dampers. The user can inadvertently reduce the airflow to a level that does not control the hazardous substance properly. For LEV with blast gates, the installation of Airflow Indicators is essential. HSG 258 s106 states: 'It becomes critical when the operator has to adjust a damper to get adequate airflow'.
Filtered LEV systems filters of the type where a filter pressure drop can be measured across the filter should have a Differential Pressure Guage fitted and calibrated to show the range of acceptable pressure drops and indicate when the filter is failing. This will immediately show the operator or supervisor that there is a problem.
For LEV systems serving more than one hood, a filter guage should be fitted, in addition to the hood airflow indicators. If the LEV is connected to one hood only then the system needs either the filter gauge or a hood airflow indicator.