Isokinetic sampling is very tedious and we, as people who try to serve our clients best interests are looking for ways to minimise costs, and in doing this we are having a serious look at the benefits of doing isokinetic sampling.  


Questions we ask

  1. Is it worth the expense?  

  2. What useful information is obtained?

  3. Is there not an easier way to get similar information?


These comments are based on the South African Industry, and experience is predominantly on mines falling under the Department of Minerals and Resources, Mine Health and Safety Act.

The British Standard Test method for Isokinetic sampling is BS 3405.  This standard is unfortunately only available to be purchased and cannot be published on a web site.

Isokinetic sampling for some reason seems to have become over complicated and technical.

The main question to answer is : What are the reasons for taking "Isokinetic samples"?  

As mentioned already it is usually done to calibrate instrumentation or just to determine the dust load in a stack.  

From our experience the latter is less common, and almost always isokinetic sampling is required when the load in the stack or duct isvery low, i.e. when the instrument is reading at the bottom of its range, or the dust plant is working well.

Any mine needs to weigh up the costs versus the benefits of isokinetic sampling.  Very often the benefit side of the scale is a bit on the light side.

The main difficulty with isokinetic sampling is to try and make the sample as representative as possible of the dust in the stack or duct,without excessive costs.

The arguments are that the perimeter of the sampling tube will effect the air flow quite drastically in and around the tube unless the flow is very similar past the outside and into the tube.

The above problem is very significant if the area of the perimeter is similar to that of the area of the sample.  
However if the sample area, i.e. the diameter of the tube is larger, then the disturbance caused by the perimeter is less significant because more dust is collected.

The arguments for the disturbance are as follows:

  1. If the sampling velocity in the tube is less than the velocity in the duct then the tube will become pressurised and particles will want to be expelled from the tube.  
    The particles that are more easily dispelled are the small particles, and as a result a greater percentage of larger particles are collected.  
    So the particle distribution will not be representative of that in the stack or duct.

  2. If the sampling velocity in the tube is greater than the velocity in the duct then the tube will suck extra particles into the tube, thus collecting a higher percentage of smaller particles, also not a representation of the dust in the stack.

The disadvantage of having a larger sampling tube of about 2 cm is that a higher volume of air has to be sampled.  This requires a larger vacuum pump at higher capital costs.

Practicalities of taking samples

If you have a stack and you are required to monitor it for environmental reasons then you will purchase an instrument to measure this for you.  

Why then would you ever need to do isokinetic sampling?  
Unless the accuracy of the instrument is disputed, in which case the suppliers should be contacted, so that they can provide the best method of testing their instrument.

At the prices of these instruments they should be accurate enough, and the suppliers should be able guarantee accuracy for a certain number of years.

Stacks are often very high up with access being difficult.  
Then there is the problem of having access to two sides of the duct so that readings can be taken at 90 deg to each other.  

This requires a platform and two holes in the duct.  More often than not there is only one hole in the duct.

There are many other standards applicable in different countries, and these all have small variations.

The uses of isokinetic sampling are quite varied, and often are just a check to see if the instrument, bag filter, or scrubber is working.  In places where there is no opacity instrumentation, isokinetic sampling is the only way to determine the dust load in a stack.

Isokinetic sampling is almost always performed when the process is working well, and the emissions are very low, often tending towards the invisible range.  
This means that very little dust is often collected, thus making accuracy in the sampling measurement very important.

The equipment is very expensive and the methods are long and very systematic.  One has to weigh up the cost of doing a test with the benefit obtained from the result.

The argument of particle size distribution makes sense, but often the duct is after a bag filter, where particle sizes are quite constant, unless a bag is broken or there is some other process problem in the plant, and then isokinetic sampling will not be required because the plant is visibly out of specification.

Comments on the equipment and method of isokinetic sampling.

The equipment used for isokinetic sampling is not identical for every application with many factors such as temperature, and moisture content, effecting the method and equipment used.

In each case an elaborate method of removing the moisture and compensating for the temperature is used, each with it's own inaccuracies, and increased costs.

The effects of temperature and moisture content on the sample are very significant, and their influence on the accuracy of the sample should definitely be considered. The issue is that there needs to be an easier, cheaper and more practical way.

An average velocity is taken, either at the beginning or by sampling throughout the period.

The irony of the limit set on stacks is that by diluting the dust, you can achieve your limits.  So instead of a major capital cost, a small variable cost that puts in clean air into your stack at any point would lower your readings.

I have heard of an operation where two stacks were next to each other, one was above the limit and the other was below.  To solve this problem, the stacks were simply connected with a horizontal pipe thus balancing the air flow between them and keeping both stacks below the legal requirements.  Even though exactly the same amount of dust was being emitted.  This does not make sense, but it is how it is done.

This also puts the limit that is sometimes mentioned and applied of no "visible dust", under dispute.