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Does your cyclone make the cut?

Introduction

Respirable dust is considered anything below 10µm in size. This particulate is high risk because it penetrates deeply into the lungs. Respirable hazards for dust come from many dusts within the workplace and for that matter the environment. Particulate matter of various sizes can be generated from processes such as grinding, blasting, or mixing. Examples of common harmful dust materials are coal, silica and wood. This article looks at cyclones, commonly used when sampling for respirable dusts and discusses points to be aware of in their use.

Air sampling for respirable dusts

The normal method for sampling respirable fractions of dusts within the workplace is using a personal sampling pump which draws a volume of air at an accurate flow rate. The pump is normally worn on the belt and connected to a tube to a sampling head worn within the ‘breathing zone’, that is close to the mouth, such that the air sampled best represents the air we breathe. The sampling heads take various forms for sampling dusts depending on the type of particulate being sampled. The dust itself is then collected on a filter within the head, typically made from glass fibre. Glass fibre is typically used because it has a very high collection efficiency, even at very small particle sizes.

About cyclones

For respirable dust, a sampling head called a cyclone is used to remove specific size fractions of dust. The use of a ‘Cyclone’ – a precisely designed and crafted chamber which uses the effect of centrifugal force to allow larger non-respirable particles to be cast out, leaving only respirable particles behind.

The larger particles fall into a ‘grit pot’, leaving the respirable fraction to be collected on the filter. These cyclones have a collection efficiency often referred to as the D50, that is defined by the median particle size, which in the case of cyclones is 4µm.

Flow affecting measurement

The flow rate is very important to get the right size cut i.e. D50. During development of the cyclones, in depth research is performed to ensure that the cyclones get the correct size cut and this is dependent on flow rate. Each cyclone will have a specific air flow rate and the cyclone will only get the correct size cut when run at that exact flow rate.

Orientation is also a consideration when wearing the cyclone. Cyclones should be worn vertically for the cyclone to operate effectively, therefore it is often best to allow them to be clipped in a single position, allowing them to hang loose when the person wearing them is moving around. Ensure also that after the measurement, the cyclone is not ‘tipped’ over, which would allow some contents of the grit pot to pass through to the filter, drastically affecting sample weight.

Variation in Flow

Air sampling pumps therefore have to provide a very steady flow rate. Flow rates for the Casella manufactured ‘Higgins Dewell’ cyclone above are required at 2.2L/min. This air sampling head has been around for many years and academic papers have compared and proved that this design of cyclone correlates to the standard respirable curve defined by EN 481.

Different cyclones have different designs and flow rates to provide the same respirable size cut, therefore the flow rate may change for each design. You should check with the manufacturer of the cyclone you are using in order to verify that the flow rate used is correct. It has been the case recently that a manufacturer of cyclones updated their recommended flowrate based on upon the latest research. This research showed one manufacturers cyclone oversampled by 30% and the recommended flow rate had to be changed from 2.2L/min to 3.0L/min to provide the correct size cut.

On the Pulse

Flow rate is a key consideration when using cyclones, but another factor is the little known issue of pulsation. The sample air enters the cyclone and begins to rotate and descend downwards. As it spins the larger particle sizes follow a trajectory whereby they are effectively spun outwards to the wall surface and drop out by a process known as inertial impaction. The smaller sized particles (of interest) remain suspended in the central vortex airflow which carries them upwards and out of the cyclone to the filter media.  The rotational velocity and hence flow rate are critical factors in determining the cyclones size cut.  Pulsations in the flow alter the effective size cut and various research(1&2) papers have shown that the pulsations from some sampling pumps affect the cyclones collection penetration by over 10%.  Respirable sampling guidance notes from international bodies (such as NIOSH 0600 and HSE MDHS14-4) identify the importance of using a known low pulsation level. An international standard, ISO 13137 defines the Requirements and Test Specifications for personal Air Sampling pumps in respect to details such as flow stability and maximum pulsation level.

A twin head diaphragm pump has flow pulsations at twice the rotation frequency and at typically half the magnitude of a single headed pump, however, airflow pulsation is an unavoidable consequence of any diaphragm pump.  By good damper design, the amplitude of flow pulsations can be minimised, but never fully eliminated.

Some sampling pumps lack effective pulsation damping. Whilst this may be irrelevant to some sampling strategies, in a size selective sampling application this can lead to undesirable sampling errors. Personal air sampling pumps from different manufactures have pulsation levels that vary drastically, with some models up to 70%(1), well above the required value of less than 10%.  Therefore a sampling pump with a pulsation level of <10% should be selected when sampling for respirable dust using a cyclone. An example of this is Casella’s Apex2.

For the first time, a flow meter can automatically and reliably warn of pulsation levels exceeding 10% (according to ISO13137).  The Casella flow meter, called the Flow Detective, gives a warning when the pulsation is above 10%, where the measurement accuracy will begin to reduce and it is recommended that efforts are made to reduce the pulsation levels.

Being aware of a high pulsation level makes it possible to take precautions such as adding an additional flow dampener to the sample line, operating the pump with longer and more flexible tubing, operating with a higher inlet pressure loading to reduce pulsation level, or use a different sampling pump!

Conclusion

Cyclones are a long trusted and validated method for measuring respirable dust. A number of factors can affect their use including flow rate, pulsation, orientation and handling. Being aware of the issues and ensuring consistent use of the cyclone and sample will provide improved and repeatable air sampling data.

 

Literature References

Casella Solutions – https://www.casellasolutions.com/uk/en.html

The Annals of Occupational Hygiene – Journals –NCBI Publications, “Evaluation of Pump Pulsation in Respirable Size-Selective Sampling”: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4607266/pdf/nih
ms726843.pdf

The effect of flow pulsations on the performance of cyclone personal respirable dust sampler: https://www.sciencedirect.com/science/article/pll/002185029190082S

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