New gravimetric measuring ­system with on-site evaluation

1 Introduction

Measurement of particulates (“dust”) content can be necessary at industrial facilities for a number of reasons. At larger plants (power-generating, waste-incineration and cement plants, for instance) there is the necessity of continuously monitoring waste gases for emissions content. At installations with a lower emissions potential, it is nonetheless necessary to check the concentrations of specific pollutants by means of individual measurements made at regular intervals. Measurements of particulates content can also be performed in a similar manner on filter systems and separators, for the purposes of acceptance inspections and/or performance trials.

Various measuring methods (e.g. optical or triboelectric) can be used for continuous measurement of dust concentration. All the measuring instruments used for this have in common that they need to be calibrated under specific plant conditions after installation and, generally, at intervals of three years subsequently. This calibration operation is performed by means of gravimetric control measurements, which are also used for the above-mentioned individual measurements.

For this purpose, a particle-laden sample of gas is taken under defined conditions from the waste gas flow and fed through a filter sleeve or a plane filter. The unladen weight of the filter is determined after its elaborate pre-conditioning. Conditioning is then repeated, and the weight of the laden filter then measured. This is normally done in an analytical laboratory.

Every individual on-site sampling operation normally takes 30 min. Not less than 15 samples taken on three days are necessary for control measurement for calibration purposes, during which the half-hourly averages of the continuously operating measuring instrument are recorded simultaneously.

The necessary procedure is standardised under DIN EN 13284, Part 1 and VDI 2066, Part 1, inter alia, in order to assure the obtainment of representative results. Immediate statement of the particulates content at the plant is not possible during the measurements, however. The actual particulates content can be stated only once the filters have been weighed under defined conditions in the analytical laboratory. This, under certain circumstances, can take several days. This time lag is unsatisfactory in some cases, particularly in the context of acceptance and/or control measurements abroad. Parallel evaluation in the immediate vicinity has remained impossible up to today, despite the fact that measurements of particulates contents of this type have been performed since as long ago as the 19th century.

2 Current state of the art

The technological state of the art is characterised by the standard procedures and instruments from a range of suppliers and now in use for decades. It is necessary to differentiate between sampling systems, equipment for pre- and after-treatment of the samples, and the actual weighing systems.

This standard reference-measurement procedure is described in detail in DIN EN 13284, Part 1 and VDI 2066, Part 1. The principle of gravimetric measurement is shown in the following illustration (Fig. 1). Before measurement is started, the filter sleeve or the plane filter from the measuring apparatus is submitted to pre-treatment and then weighed. Under DIN EN 13284, Part 1, the temperature in the drying cabinet for the filters and for any other items to be weighed should be 180 °C, and cooling in the desiccator should extend for not less than eight hours.

Since the difference between the weights measured before and after lading of the filters is used to determine particulates content, DIN EN 13284, Part 1 requires that after-treatment be extremely similar. Drying after removal of the dust should take place for more than one hour at 160 °C. The cooling time in the desiccator down to the temperature of the room in which weighing is performed should be eight hours. Other temperatures may be selected, depending on the properties of the particulate and, in particular, its thermal stability.

Extractive sampling for the capture of particles must be conducted isokinetically. This means that the taking of the volumetric side-stream must be accomplished at approximately the same velocity as that of the main flow of waste gas, in order to preclude segregation phenomena during sampling. A nozzle-diameter appropriate to the suction rates to be established must be used for this purpose. DIN EN 13284, Part 1 requires the use of nozzles with a diameter of not less than 8 mm. Knowledge of flow conditions in the measuring cross-section is also required.

Measurement using a pitot or Prandtl tube with a pressure-gauge connected must be performed if the waste gas velocity at the measuring point is not known. Total pressure and static pressure are read by the hook-shaped probe of the Prandtl tube, which faces into the waste-gas flow, and local velocity calculated from the differential pressure and gas density. Measurement is performed using a differential-pressure gauge. DIN EN 13284, Part 1 requires that this measurement be made continuously, or at intervals of not longer than five minutes, during sampling.

The value for the dry volume of gas for measurement suctioned off is available on the spot at the end of the measurement. Weight difference is determined following after-treatment of the filter in the laboratory. Particulates concentration in mg/m³ (dry, standard temperature and pressure) is calculated using this and the volume of measuring gas suctioned off.

The gravimetric particulates measuring systems available on the market achieve, in combination with the laboratory equipment used, measuring accuracies ranging from approx. 0.1–1 mg/m³.

3 The GMD 13 gravimetric

measuring system

Dr. Födisch Umweltmesstechnik AG’s GMD 13 is a compact and highly auto-mated system for the isokinetic gravimetric measurement of particulates content in waste gas ducts and stacks with reference to DIN EN 13284, Part 1 (Fig. 2). The system is a further development of the established GMD series, which has been successfully marketed by this company for the purpose of gravimetric control measurements for a number of years now.

The device’s integrated hot-weighing system makes it the world’s first measuring system to enable the user to state information on the particulates concentration measured gravimetrically only a short time after measurement. Additional laboratory equipment for gravimetric measurement is then no longer necessary; transport and the necessary time are therefore also eliminated. The simple and reliable structure of the system, in combination with the project-orientated software package, also minimises error sources during measurement.

The GMD 13 registers measured-gas velocity, measured-gas pressure and the volume of measured gas taken. Measured-gas moisture content can, in addition, also be measured using a separate sampling probe. All the variables needed for determination of particulates content under standardised conditions are thus registered by the system, and sampling of gas for measurement isokinetically regulated fully automatically. Figure 3 shows the GMD 13 measuring apparatus during particulates sampling on a waste gas duct.

The equipment’s control system features two additional mA measuring inputs (0 – 20 mA). These can be used during sampling to record the measured data supplied by any automatic particu-lates-measuring systems installed, thus eliminating the need for a separate recorder. In the case of plants on which no continuous measuring system is installed, it is also possible to operate a directly indicating particulates-measuring instrument or a dust sensor in parallel (e.g. the Dr. Födisch Umweltmesstechnik AG PFM 13C). This instrument is actively supplied at 24 V by the GMD 13. Parallel operation makes it possible to automatically register and evaluate changes in system conditions as a result of operation (Fig. 4).

3.1 On-the-spot hot weighing

The new gravimetric measuring system permits easy, safe and reliable on-the-spot handling and use of the entire system. The precision hot scale is kept in its carrying case for transportation (Fig. 5).

The essential development innovation of the GMD 13 takes the form of the patented process used for hot weighing of the filter as an integrated element in the measuring system. This eliminates elaborate conditioning prior to and after dust sampling and makes it possible to determine the dust concentration measured in real time at the location. Conditioning is now superfluous, because the filter is heated so intensively during weighing that its entire water content is expelled. The filter can be heated to up to 160° C during the weighing operation using the precision hot scale.

The filters used (Fig. 6) consist of extruded glass or quartz fibres and require extremely careful handling. To assure this, the filter used is protected by means of a robust filter holder, which is weighed together with the filter during hot weighing. Weighing of the complete filter element is permitted under DIN EN 13284, Part 1.

Before weighing, the precision hot scale is tared and connected directly to the GMD 13’s control system on the measuring unit. Weighing using the precision hot scale is performed prior to and after gravimetric measurement in a clean, draft-free and vibration-free room (e.g. an office, a workshop, a hotel room, etc.).

The control system guides the user through the weighing process, and receives the measured data. No settings on the precision hot scale are necessary. Weighing is performed semi-automatically at an accuracy of less than 1.0 mg. Completion of weighing is signalised by means of an audible tone.

Weighing accuracy can be improved within a rational period of time to 0.4 mg by means of multiple weighing of the same filter element. A stable average becomes established after as few as 3–5 weighing operations, making it possible to achieve accurate measuring results.

Figure 7 shows typical measured data obtained from multiple weighing operations on two measuring days, each of eleven weighing operations. The weight differences between the averages determined are less than 0.25 mg even after only the fifth repeat weighing. Even severe fluctuations caused, for example, by a large deviation in an individual weighing (see indication in Figure 7) influence the average only insignificantly.

The time requirement for the first weighing operation varies between 5 and 30 minutes, and the time necessary for further weighing operations between 3 and 15 minutes per operation.

3.2 Project-orientated software

The software package, which has been revised compared to previous models, functions on a project-orientated basis. Within a project, all the measurements and settings for one measuring point are summarised and jointly evaluated by means of various filters. In addition, the results of a weighing operation in accordance with DIN EN 13284, Part 1, can be entered into the GMD 13’s measuring unit and used on a comparative basis for evaluation. During sampling, the pressure drop across the filter and, optionally, the signals on the mA inputs during measurement, can be shown in graphic form on the measuring unit’s display (Fig. 8). The device’s measured data can be outputted on-the-spot, or written to a CompactFlash memory card, to permit their further use on a PC.

3.3 Measuring procedure and results using the GMD 13

The procedure for gravimetric measurement using the GMD 13 is as follows:

1. Creation of a new project and planned filters/measurements using the instrument’s software; preparation of the required filter holders, with the filters.

2. Setting up of the precision hot scale in a dry, draft-free, vibration-free place.

3. Weighing of the filter holders and filters using the precision hot scale.

4. Setting up of the measuring system at the particulates sampling point.

5. Measurement of absolute moisture content in the waste gas at the particulates sampling point using the moisture probe and/or adoption of a known measured value and entry of gas parameters (O2, CO2).

6. Measurement of waste gas velocity using the dust probe and installation of the sampling nozzle.

 The GMD 13 supports selection of a suitable sampling nozzle.

7. Entry of the boundary parameters for measurement (measuring time, cross-section, flange depth, measuring points).

8. Insertion of the filter element into the filter mounting of the dust sampling probe.

9. Start of measurement/particulates sampling.

 Any change of measuring points is acoustically signalised and visually displayed, depending on specification.

10. Repetition of measurements using other filter elements (as described in Steps 8 and 9 above).

11. Weighing of the filter elements using the precision hot scale (same scale, same weighing location).

12. Computation of the results of the particulates sample by the GMD 13.

 The results are outputted on the display, by means of a printer, or by means of read-out of the CompactFlash memory card on a PC.

Figure 9 shows a comparison of the particulates concentrations determined by means of hot weighing using the GMD 13 and by means of weighing in accordance with DIN EN 13284, Part 1 (32 measurements on the particulates duct).

4 Conclusion

The Dr. Födisch Umweltmesstechnik AG GMD 13 is a compact and highly automated system for isokinetic gravimetric measurement of particulates content in waste gas ducts and stacks. All relevant variables are registered under standardised conditions for determination of particulates content and measuring-gas sampling is isokinetically regulated fully automatically. The system’s simple and reliable structure, in conjunction with the project-orientated software used, assure accurate measuring results.

The device is the first measuring system in the world featuring integrated hot weighing and the capability for on-the-spot evaluation with no additional laboratory equipment. It also permits operation in parallel with a directly indicating particulates-measuring instrument and/or a particulates sensor, signifying that system changes caused by operating conditions can be automatically registered and evaluated.

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