One of the main problems we encounter when carrying out a thorough examination and test of a local exhaust ventilation (LEV) system is the lack of “baseline data” – i.e. information on what the pressures and velocities should be when the system is operating in accordance with the design specification. Providing the system has been designed competently, if the measurements taken during the test match the baseline data, this tells us that it is working satisfactorily.
The main reason the information isn’t available is that no testing was carried out when the system was installed and commissioned. HSEs guidance in HSG258 is quite clear that a proper commissioning test should be undertaken to
- verify that the system is operating in accordance with the design specification
- ensure that it is controlling the contaminants effectively
- establish baseline data against which future test results can be compared
It would, perhaps, be wishful thinking to expect that, in future, all systems will be properly tested on installation. In our experience it is still not being done for many new systems we’re coming across. Hopefully the situation will improve over time.
However, we are still left with the problem of what to do when testing systems where no baseline data is available. One answer is that the testing company should recommission the system to generate baseline data for future surveys. In the ideal world we should do this, but the difficulty is that this would often increase the time needed to complete the work and, where the testing is being carried out by an outside organisation, lead to a higher cost, which the client might not be prepared to pay. Too many testing companies will obtain engineering test data and report it to the client without any evaluation of whether this indicates satisfactory performance and demonstrates adequate control. A company proposing to carry out a thorough test to recommission the system is put at a severe competitive disadvantage.
There isn’t a simple answer to this, but HSE have now provided some additional guidance on how to approach this problem on their website. They recognise that we need to be flexible, depending on circumstances. The nature of the system being a major consideration.
With simple systems with a single hood, the most important considerations are whether:
- – the contaminant is being captured
- – the contaminant does not settle out in the ductwork
- – it is not being returned to the workplace
Establishing whether this is the case is not difficult for a competent occupational hygienist or engineer, using standard tests (smoke, dust lamp, velocity measurements, air sampling etc. as appropriate). Once this has been ensured other engineering measurements can be carried out on the system, such as static pressures. The system has then, effectively been recommissioned and baseline data established for future surveys.
It isn’t so easy with more complicated, multi branch systems. HSE’s advice in such cases is
“you should still undertake a thorough examination and test, which will provide information on the current performance of the system……
However, it is accepted that such a report may not fully meet the advice in the (COSHH) ACOP, which recommends reference to intended operating performance, and you should indicate this in your report. You should also suggest that the user assesses whether the performance you have reported is providing adequate control*. Your examination should also identify any adjustments or repairs that you believe are needed……
Where you cannot make a professional judgement on the design performance standards of the LEV, or your assessment suggests that the exposure control may not be adequate, you should clearly indicate this in your report to the employer (client). The report should also clearly indicate where no information on intended performance was available.”
If all testing companies follow this advice, we will at least have a “level playing field”. However, many occupational hygienists may not feel comfortable leaving the client to decide whether the system is achieving adequate control.
* my emphasis