Dependable fill-level control in coal mining

Dependable fill-level control in coal mining

Mine workings and opencast pits are well known for their harsh working conditions. This applies equally to the technology deployed. For this reason, durable and reliable measuring instruments are required to monitor groundwater.

Ten percent of worldwide coal deposits are to be found in Australia. As the leading coal exporter, coal mining is one of the most important economic factors on that continent. Mining of the raw material, however, is not without its pitfalls. The operator of an Australian opencast approached STS as they were seeking a pressure transmitter for fill-level monitoring at depths of up to 400 meters.

Mining operations have a heavy influence upon groundwater. The aquifers surrounding the coal mine become drained, which leads to a sinking of the depression cone. This sinking alters the natural hydrological conditions underground by creating paths of lowered resistance. What this then leads to is water penetrating the open pit and underground workings. As a result, the constantly inflowing water needs to be continuously pumped out of the pit to ensure a smooth and safe extraction of the raw material.

To control the groundwater level and the pumps used for drainage, the operators of the opencast needed a pressure transmitter to monitor the fill-level according to their requirements. Stipulated was a pressure range from 0 to 40 bar (400 mH2O) ambient pressure, as well as a cable length of 400 meters. The solution offered by STS at that time, the ATM.ECO/N/EX, read only to 25 bar and had a cable length of 250 meters.

But since STS is specialized in customer-specific pressure measurement solutions, this challenge was to prove no major obstacle. In short time, the failsafe ATM.1ST/N/Ex pressure transmitter for fill-level was developed, which precisely meets the pressure requirements and is equipped with a 400-meter-long Teflon® cable. It is also convincing in its accuracy of just 0.1%. STS decided upon development of the new pressure transmitter for a Teflon® cable, a sealed cable gland and an open aeration tube (PUR is too soft for this). In addition, there is also a screw-on ballast weight to ensure a straight and stable measuring position. The stainless steel strain relief, which can also be screwed on, helps to relieve strain on the electrical cable. As the device designation indicates, it also carries the EX certification for use in explosive areas.

ATM.1ST/N/Ex with strain relief (left) and ballast weight (right), each screw-on.

Being an expert in customer-specific pressure transmitters, STS was able to supply the ATM.1ST/N/Ex in less than three weeks.

The features of the ATM.1ST/N/Ex in brief:

  • Pressure range: 1…250 mH2O
  • Accuracy: ≤ ± 0.1 / 0.05 % FS
  • Total error: ≤ ± 0.30 %FS (-5 … 50°C)
  • Operating temperature: -5…80 °C
  • Media temperature: -5…80 °C
  • Output signal: 4…20 mA
  • Materials: Stainless steel, titanium
  • Electronic compensation
  • Common process connections available
Reliable groundwater and surface water monitoring in Romania

Reliable groundwater and surface water monitoring in Romania

A seamless control system with alarm function is required to perform precise water level measurements and to make reliable forecasts on potable water supply, as well as to anticipate floods. Together with its partner MDS Electric Srl, STS has implemented a comprehensive system for groundwater and surface water management in Romania.

Romania draws a major part of its potable water from surface waters such as the Danube, as well as from groundwater resources. A sound management of these natural resources is therefore of huge importance.

To safeguard potable water supply and to protect from flooding, the nation has invested in a comprehensive hydrological measurement infrastructure.

Figure 1: Groundwater measurement point 

In collaboration with its Romanian partner, MDS Electric Srl, over 700 data loggers and more than 350 data transmission systems have thus been installed throughout the country in recent years – also including remote areas. For this reason, the primary investment was in battery-operated measuring instruments, which monitor the current situation on the rivers of the Danube region and also the groundwater resources across the country.

Requirements-specific measurement solutions 

This was a complex undertaking, since each of the submersible probes and data transmission systems deployed required a different assessment and intervention to comply with their respective conditions. An automatic alarm function was also indispensable in this case, should predefined limit values be exceeded.

The permanent monitoring of water levels at important nodes across the potable water supply, as well as the rivers of the Danube region, hinges upon a multitude of requirements:

  • An automated and dependable data transfer via M2M protocol
  • Automatic alarm function when limit value is exceeded
  • Monitoring of water level and temperature, as well as ambient temperature in some instances
  • A server solution with functions for visualizing, evaluating and processing the measured data, as well as the integrated database
  • Easy installation and maintenance
  • On-site support service

For the implementation of this large-scale project, STS opted in pressure and temperature measurements for the DL/N 70 and WMS/GPRS/R/SDI-12 data loggers, or – depending upon requirement – the DTM.OCS.S/N digital data transmitter with Modbus interface to ensure highly precise water level measurement to a 0.03 percent accuracy at critical points.

In association with our local partner MDS Electric Srl, STS was able to realize the entire water level monitoring system from a single source. Each installation point was evaluated on-site by experts from MDS Electric Srl and STS, in order to install a custom solution at each of those individual measurement points. The long-term stability of the pressure measurement technology deployed is also guaranteed. The Modbus transmitter DTM.OCS.S/N excels in this area with an excellent long-term stability of less than 0.1 percent total error per annum. Because of its low energy consumption and robust design, this sensor performs largely maintenance-free for years on end.

Further advantages of the DTM.OCS.S/N in brief:

  • Pressure range: 200mbar…25bar
  • Accuracy: ≤ ± 0.15 / 0.05 / 0.03 % FS
  • Operating temperature: -40… 85 °C
  • Media temperature: -5…80 °C
  • Interface: RS485 with Modbus RTU (standard protocol)
  • Simple implementation in existing Modbus systems
  • Easy adjustment of span and offset
Polluted sites: Groundwater decontamination requires robust level sensors

Polluted sites: Groundwater decontamination requires robust level sensors

Whether it be old landfills, coal tips, former military sites or refineries, what remains behind is contaminated ground, which is a danger to both humans and the environment. In the rehabilitation of these sites, level sensors are required which are resistant to the often aggressive hazardous substances encountered.

Contaminated sites are not only characterized by adverse health or environmental changes in the soil. In the absence of safety measures (as in old landfills) and depending on soil conditions, hazardous substances are flushed by rain into the groundwater. Depending on the type of usage, a number of different hazardous substances can be found, including, among others:

  • Heavy-metal compounds: Copper, lead, chromium, nickel, zinc and arsenic (a metalloid)
  • Organic materials: Phenols, mineral oil, benzenes, chlorinated hydrocarbons (CHC), aromatic hydrocarbons (PAH)
  • Salts: Chlorides, sulfates, carbonates

Decontamination of the groundwater supply

In the rehabilitation of contaminated sites, not only is cleansing of the soil of great importance, but also the control and purification of the groundwater. Without reliable level sensors that can withstand the adverse conditions, this would not be possible.

The decontamination process usually proceeds as follows: The contaminated groundwater is pumped to the surface and then treated. As filtered flush water, it is next returned to the source of contamination. To prevent this flush water from flowing to a margin away from the contamination source, active hydraulic methods are used for protective infiltration. Water is injected into the ground via several wells situated around the actual decontamination process. The pressure conditions arising here to some extent form a barrier wall and cause the flush water to flow towards the source of contamination. For controlling and monitoring this process, level sensors will be required.

Figure 1: Flow of a decontamination process

Level sensors are of course also used after the remediation work. Long after completion of this work, the affected sites will be monitored to check for any noticeable changes in the water level or the direction of flow.

Level sensors are also used when actively running applications potentially damaging to the environment. Newer landfills are now built like an impermeable basin. The groundwater level below the landfill is lowered, so that no water can flow into adjacent areas in the event of leakage. Here also, the respective water levels are to be monitored by level sensors.

Level sensors in contaminated waters: Highest demands 

Operators in the field of decontamination of polluted sites should be very careful in choosing suitable level sensors. Due to the large number of substances that can be dissolved in the water, there is no single solution that works reliably in every instance. There are several aspects to consider, which we next briefly outline.

Materials

Housing

In most applications, a high-quality stainless steel, as used by STS, is sufficient to protect the measuring cell from aggressive substances. If this were to come in contact with saltwater, then a titanium housing would be chosen, but where galvanic effects are to be expected, a level sensor made of PVDF should be the selection.

Figure 2: ATM/NC chemically resistant level sensor with PVDF housing

Probe cable

Far more critical than choosing a suitable housing, in our experience, is the choice of probe cable. Because of gradual diffusion processes, the progress of destruction is not immediately apparent. Often, this is not visible from the outside even when already damaged. Special caution is therefore required when consulting resistance tables, since these usually say little in particular about probe cables. In the middle of a probe cable is a small air tube, which serves for relative pressure equalization. If the cable material is not one hundred percent resistant, however, raw materials may diffuse through the cable sheath and travel across the air tube into the sensor chip.

Depending upon the substances anticipated, STS users can resort to PE, PUR or FEP cables. The latter can also be used at very high temperatures of up to 110 °C.

Installation

Cable routing

Old landfills and industrial sites are harsh environments, where not only hazardous substances can impair the functionality of the level sensors used. Care must be taken that the cable sheath is not damaged by mechanical burdens (such as debris). Chafing and kinking points should also be avoided. It is therefore recommended to use special protective tubes, such as those offered by STS, when routing cables.

Strain relief

The compression rating of level sensors varies from manufacturer to manufacturer. At STS, all level sensors are pressure-resistant up to 250 meters as standard and their cable is designed for normal tensile strains up to this depth. Nevertheless, operators should consider the use of strain relief in difficult installation conditions.

Mounting

If the sensor is used in flowing waters or tanks with agitators, it can be supplied either with a G 1/2” thread at the cable outlet (pipe mounting) or with a compression fitting (15 mm).

Explosion protection

In applications where a number of hazardous substances are to be expected, it is imperative to also pay attention to explosion protection. Information about this is given by the international standards-compliant ATEX directive.

Correcting Water Level Data for Barometric Pressure Fluctuations

Correcting Water Level Data for Barometric Pressure Fluctuations

Piezometric surveys of Otavi karst aquifer – data analysis through barometric efficiency calculation

The main concepts for identifying and removing barometric pressure effects in confined and unconfined aquifers are described. Although it is commonly known that barometric pressure changes can effect water level readings, few articles and procedures are provided to correctly manage piezometric data.

Knowing the barometric efficiency reduces errors in calculating piezometric surfaces and drawdowns in the piezometers during pumping tests. Stallman (1967) suggested furthermore, that air movement through the unsaturated zone and the attendant pressure lag, could help to better describe the aquifer properties. Rasmussen and Crawford (1997) described how barometric efficiency varies with time in some aquifers and how to calculate the corresponding barometric response function (BRF). They also showed that this last parameter is related to the degree of aquifer confinement.  Finally we present an application of the procedure in an unconfined karst aquifer located in northern Namibia (Otavi mountainland) where a set of four absolute transducers have recorded water level changes and earth tides during a 10 months period at 1 hr. interval.

General framework

The area under investigation is in the SE part of a 6000 sq. km plateau with average elevation of 1300-1500 m a.s.l. and hills reaching 2000 m (see below).

Rock formations are made of thick dolomitic limestone beds with stromatolites (500 b.p.). The strata have been folded into a number of synclines and anticlines generally striking east – west. The southern part of the study area is bordered by a long fault with various mineral occurrences (copper, vanadium, lead, zinc).  Due to the high fracturing, low vegetation cover and lack of soil, surficial runoff is almost nil. Two natural water basins, collapsed dolines, of 100-200 m in large, are located farther north and outside the project area. The mean annual rainfall is 540 mm (1926 – 1992) with peaks during summer, between December and March. Since mid ‘70s and until the year 2000 the area suffered a fall in precipitation that, together with mining activity (Kombat, Tsumeb, Abenab) were responsible for the lowering of the water table of as far as 20-30 m in some places.

From 2005 on, this trend has reversed due to the reduced activity of  the mines and a new meteorological regime.

Hydrogeological framework

This region is well known for its karst features, and hosts some wide underground lakes located between 70 and 120 m below ground surface.

The area is also classified as one of the most important aquifer of the country (Dept. of Water Affairs, MAWRD, area E-F). To glean more valuable insights into this particular environment and locate alternative positions for water boreholes we prepared two piezometric maps (2007-2010) and installed 4 water level transducers in some water points at 2-4 km distance in Harasib farm (fig. 13).

Fig. 13 Piezometric map (February 2007) and position of three water level loggers

The 2007 piezometric surface shows a recharge area, coincident with the topographic highs and feeded by rain infiltration. From this point, underground flow directions are to SW and SE. During this stage we focused our researches to define: 

  • Type of aquifer
  • Aquifer connections between Harasib and Dragon’s lakes
  • Recharge

Chemical analysis of surface and deep waters were conducted in 2007, while continuous barometric pressure and water level readings were made during a ten months period, between September 2010 and June 2011. The aquifer recharge starts when cumulative rain exceeds 400-500 mm. The thickness of the unsaturated part ranges from 40 to 100 m. Considering this value close to the average annual rainfall, and that the aquifer is karstic and highly fractured, one should note that one or two years of scarce precipitation is enough to decrease dramatically the exploitable yield.

Barometric efficiency (BE) and barometric response function (BRF)

Fig. 16 Values of dry period (September – January)

The water level readings have been analysed with the software BETCO (Sandia National Laboratories), to remove the effects of the barometric pressure changes. The measured and corrected values are depicted in fig. 16 and refer to the dry period (September – January) while fig. 17 shows the barometric pressure versus water level changes, used for the calculation of the barometric efficiency.

Fig. 17 Difference in barometric pressure and water levels during the dry period (Sept.-Dec. 2010)

In all examples we notice that:

  • There is a good correlation between measured and corrected values, even if with lower amplitude
  • There still is a variation diminishing in the corrected values; being excluded skin effects phenomena this behaviour could be ascribed to other non-barometric effects (earth tides, double porosity)
  • The initial barometric efficiency values are quite similar (0.55 – 0.61)

In fig. 18 is depicted the barometric response function (BRF) that characterises the water level response over time to a step change in barometric pressure; essentially BRF is a function of time since the imposed load.

Fig. 18 Barometric Response Functions for the three water points. The curves are similar (especially Dragon’s Breath and Harasib lake) suggesting an unconfined aquifer with perhaps a double porosity component.

A good agreement is observed for all three water points. In Dragon’s Breath lake e.g, there is a quick rise to 0.5 and a longer term decay to a lower value (0.2 – 0.3 after 20 hrs), due to the slow passage of air through fractures. The balance between external pressure and the aquifer is reached at 0.1 value.

The shape of the three curves indicate an unconfined aquifer with good hydraulic connections especially between Dragon’s Breath and Harasib lake, this last one at 2 km distance.

The correlation has also been proved by isotopic and chemical analysis made in 2007 (prof. Franco Cucchi, Dept. of  Geology, Trieste University).

Generally speaking the collected data confirm the unconfined behaviour of the aquifer, overlaid by a thick and rigid unsaturated layer, well fractured and hydraulically connected. The initial barometric efficiency is higher than the final.

Earth tides and sensor readings

Fig. 19 Water levels asl in the underground lake. The enlargement above shows small cyclic differences due to earth tides.

Regarding this last topic, data collected are still scarce but we think it is nevertheless interesting to illustrate some thoughts. When inspected in detail the curves show a distinctive zig-zag pattern with peaks every 10-12 hrs (fig. 19). This behaviour supports the effect of earth tides, producing slight changes in the volume of the fractures and pores and hence in the groundwater potential. The Fourier analysis (Shumway, 1988) shows the harmonic structure for the three water points in fig. 20 and the tide components in fig. 21.

Fig. 20  Harmonic structure for the three water points 

Fig. 21  Tidal magnitudes for the main harmonic components (values in ft)

The area close  to Harasib lake has the higher values for the M2 component and this can be considered as an indication of a higher transmissivity zone (Merritt, 2004). This fact is partly confirmed by the presence of a local fracture elongated ENE-WSW very close to Harasib lake.

Concluding remarks

Water levels fluctuations in aquifers are not only due to recharge variations. Barometric pressure and tides are among the main concerns. Knowing barometric pressure variation for a particular site, helps to validate a piezometric map or a pumping test. Modern pressure transducers vented to the atmosphere are recognised to be extremely useful when installed into boreholes. Recordings are different following the type of aquifer and the graphs can be diagnostic of the degree of confinement for the monitored levels.

Useful parameters that characterize this behaviour are the barometric efficiency (BE) and the barometric response function (BRF). The latter characterizes a deep unconfined aquifer when values are initially high and approximate 0 on the long term response, conversely the aquifer is confined/semiconfined when values stay constant or approximate 1 on the long term response. Removing barometric effects is sometimes necessary to correctly interpret a pumping test or dress up a piezometric map. Finally a particular analysis of the water level data allows to calculate the harmonic components due to tides and hence some hydrogeological features.

This theoretical approach has been applied to the data gathered for a project study of an unconfined karst aquifer in northern Namibia. Water levels have been monitored during a 10 months period, with hourly readings and by means of four transducers. The data confirmed the general assumptions obtained during preceding investigations and have underlined the importance of the use of such instruments for aquifer assessment, showing particularly:

  1. The role of the recharge due to rainfall and high transmissivity around Harasib lake area
  2. The good hydraulic connection and conductivity for the aquifer
  3. The lack of confining layers (it’s a deep and rigid unconfined aquifer)
  4. The storage effect of the unsaturated part, above the water table, that starts draining when rain exceeds 400/500 mm
  5. The other pressure effects, such earth tides, can be highlighted using water level transducers

Acknowledgements

Namgrows stands for Namibian Groundwater Systems, a project set up by the author and the colleague Gérald Favre, with the participation of geologists and cavers from 4 different countries (Italy, Switzerland, Namibia, South Africa). The project was supported in Namibia by eng. Sarel La Cante and his wife Leoni Pretorius (Harasib farm).

The company STS – Italia sponsored us by providing the water level sensors and its technical support.

I also wish to thank prof. Todd Rasmussen (The University of Georgia, Athens)  for providing his valuable insights into the data and particularly those regarding the barometric efficiency and earth tides.

 Source: Dr. Alessio Fileccia / Consulting Geologist

Level loggers monitoring water levels in Venice

Level loggers monitoring water levels in Venice

The Piazza San Marco will never flood: Level data loggers from STS are in use to continuously measure groundwater levels at the Piazza San Marco. These are particularly robust and are also suited for application in various scenarios.

In 2003, the company of S.P.G. began to install several groundwater dataloggers at the Piazza San Marco in Venice. These were designed for the specific demands and possess, above all, the attribute of withstanding several days submerged in saline waters, since on rising tides, Piazza San Marco is regularly flooded. The site operates in conjunction with efforts initiated by the water regulatory authority for protecting the lagoons and the city of Venice from flooding.

The appointed consortium of Venezia Nuova earmarked the wharfage opposite the Piazza San Marco with innovative technical features. The challenge consisted of monitoring the flow of groundwater, which was by degrees shifting from the site area to the buildings located behind. At the client’s request, level data loggers from STS were installed to continuously measure the fluctuations in groundwater levels.

The groundwater datalogger permits a simultaneous measurement of level, temperature and conductivity in ranges of 0…50 cmWS to 0…250 mWC, -5 to 50 °C and 0.020…20 mS/cm. When required, the end user can at any time retrofit a data transmission unit. The logger features a simple, user-friendly operation, an extended measurement memory for up to 1.5 million readings and a probe diameter of only 24 mm or 10 mm.

The plug-in units also allow for the possibility of cable extension. New software functions can also be updated without their requiring inconvenient return through the end user. The standard lithium batteries can be changed on site in no time. Data can be transferred in ASCII or XML format and further processed using standard software such as Excel. Variable data-saving intervals dependent on pressure or time allow for versatile measurements.

Through the use of various materials including stainless steel, titanium, PUR, PE or Teflon cable, a high medium tolerance is attained for the most varied of applications, such as landfills, contaminated sites, pump trials, high-water alarms and discharge/overflow logging in rain overflow basins.

Original publication: Konstruktion magazine