Quick, robust and safe

Oliver Schirk

Benzene has a carcinogenic effect on the human organism. Long-term exposure affects bone marrow and blood production and can cause leukaemia. If benzene vapours are inhaled, it can be assumed that around 50 percent will be retained in the body. Benzene is lipophilic and is therefore absorbed by fat and nervous tissue throughout the body. In Germany, the applicable TRK value (Technical Guideline Concentration) is 1 ppm (3.25 mg/m³).

Due to the fact that it occurs in many different forms and has serious physiological effects, benzene needs to be measured at a variety of workplaces. In refineries in particular, benzene measurements represent a recurrent and challenging task. Especially when refineries are shut down for inspection work, benzene is one of the most important substances to be measured. Typically, confined space measurements need to be performed prior to entry. The measurement process needs to satisfy a whole range of requirements:

We will now look at three measurement systems which are frequently used in refineries, and assess them on the basis of the aforementioned criteria.

The principle of measurement in a photo-ionisation detector (PID) is based on the ionisation of the analyte contained in the measurement chamber. Most PIDs generally have a detection limit of around 0.5 ppm and below. Some PIDs are also sensitive enough to detect low benzene concentrations. PIDs equipped with a pump are quick to respond and produce a measurement result within just a few seconds. For this reason, PIDs are an ideal choice when it comes to source or leakage detection. Changes in the analyte concentration can be detected quickly. Many PIDs allow alarm limits to be set so that the measurement instrument will alert the user via an audible and visual alarm if a particular concentration is exceeded. In addition, many PIDs are capable of data management. The Dräger Multi PID 2, for example, offers the user the choice between different storage intervals; with a storage capability of up to 15,000 data points, the minimum, maximum and average concentrations can be determined and stored for any interval during the measurement process. The GasVision software then allows straightforward analysis of the data in graphic or tabular form. Events (e.g. alarms, calibration etc.) are stored and visually highlighted.

One aspect that must be given consideration when using a PID is the fact that daily calibration is normally necessary. The reason for this is the fact that the UV lamp can become contaminated, which in turn may result in the system becoming incorrectly adjusted. In the interests of quick and reliable calibration, it is important to ensure that the instrument chosen offers a straightforward calibration process. The Multi PID 2 features a special softkey for calibration, so users do not have to find their way around complex menus but can perform the entire calibration process, i.e. setting the zero point and concentrations, in just a few simple steps. Another thing to remember when using a PID is that, because it works by ionisation, the instrument offers no selectivity whatsoever. This means that organic substances are detected as a sum. In other words, in the presence of other substances besides benzene that can be ionised by the PID’s UV lamp, this system-based cross-sensitivity distorts the detection of benzene. To circumvent this problem, some manufacturers now offer scrubber tubes specially for benzene which let benzene past selectively while adsorbing other aromatic and aliphatic compounds. However, this puts paid to the quick response advantage and, if the scrubber tube is overloaded, the other substances will also break through and give an incorrect measurement result.

Dräger offers a range of more than 250 different tubes, meaning that this method can be used to measure a whole series of other substances besides benzene which are relevant in refineries and other work areas. The principle of operation is based on a sample of air being sucked through a reagent system situated inside the tube. The length of the discoloration is a direct indication of the concentration of the analyte. The concentration can be read off on the scale printed on the tube. A number of selective Dräger tubes are available for measuring benzene, i.e. concentrations of aromatic and other hydrocarbons, even in the range of several hundred ppm, will not affect the measurement. Depending on which pump is used, there is a choice of manual or automatic pump operation. Many users believe that the key advantage of the manual Dräger accuro pump is that no source of power is needed. As a result, there is no need to recharge or purchase batteries. From an explosion protection point of view, a system which runs without a power supply is also less problematic. Tubes do not require any form of calibration on the part of the user.

The length of the reading on the tubes can be checked with the naked eye. Obviously, subjective errors in reading the result cannot be completely ruled out, and this is particularly relevant in poor light or ambient conditions such as rain, etc. In such cases, it is helpful to compare the tube display against an unused tube from the same pack. A new tube is used for every measurement. Measuring low benzene concentrations takes several minutes.

The chip measurement system (CMS) was the next logical step in Dräger tube technology. Based on experience gained in developing chemical reagent systems for Dräger tubes, a completely new, miniaturised tube system was designed. Ten miniaturised tubes are safely housed in a clear plastic chip, which protects them from breaking. Each chip allows 10 measurements of a particular substance, and is therefore comparable to one pack of the conventional Dräger tubes. The reagent system inside the chip is exposed to the air to be analysed by means of the CMS analyser. The chip is inserted into the CMS analyser, a measurement channel in the chip is activated by a sliding switch and the air sample to be analysed is drawn through the measurement channel. The reagent system inside the measurement channel changes colour; the change occurs either quickly or slowly, depending on the concentration of the analyte. An optoelectronic system based on diode technology analyses the speed of the colour change and calculates the analyte concentration. The concentration is then displayed as a numerical value on the instrument’s display. The time taken for the measurement result to be shown depends on the concentration: high concentrations cause the measurement channel to change colour more quickly and therefore produce a quicker measurement result. With regard to the measurement of benzene, this means that concentrations of 10 ppm will produce a measurement result within 30 s, while concentrations of 0.2 ppm will give a result after 5 min. In other words, users receive an earlier warning of high concentrations. The analyser stores up to 50 measurement results together with their respective dates, times and location codes, dispensing with the need for any further documentation of the measurement result. The CMS does not require any user calibration. What is more, operating the CMS is considerably easier than using detector tubes: there is no need to break off the tips of the glass tubes, nor do pump strokes need to be counted or a stain length analysed.

The CMS, like the Dräger tubes, is a batch measurement system. However, it delivers a measurement result more quickly than the detector tubes. Significantly less consumable material is needed for the CMS than for measurement with tubes. So far, 55 different chips are available, though this does not yet match the number of substances which can be measured using the detector tubes.

Ultimately, it is up to each user to decide which measurement system best suits their needs. One difference between the PID on the one hand and direct-reading tubes or the CMS on the other hand is the significantly lower purchase price of both the latter measurement systems. The fact that no calibration is necessary and the CMS in particular is extremely easy to operate is likely to count as a major advantage in favour of the two batch measurement systems. In terms of selectivity and flexibility too, tubes and CMS are superior to the PID. For checking sum parameters, PID is the system to choose. For leakage detection, the PID is also more suitable than tubes or CMS, thanks to its faster response time.

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Information about benzene

VDI guideline: VDI 2100 Blatt 1