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
Mud Logging Requires High-Performing, Rugged Pressure Transmitters

Mud Logging Requires High-Performing, Rugged Pressure Transmitters

The term mud logging refers to the analytical methods that are performed on drilling mud during drilling operations. Powerful and, above all, rugged pressure transmitters are paramount to the process.

The words “mud” and “logging” already provide a good, albeit incomplete, description of the process involved: mud loggers (also surface-logging specialists) are tasked by drilling companies to create detailed records of a borehole. Mud loggers analyze the information brought to the surface during the drilling process, which is why many companies also use the term surface logging services (SLS). The drilling mud is the most important component of mud logging as it carries the information from the depth of the borehole to the surface, where the cuttings (i.e. pieces of formation rock) contained in the circulating drilling medium are examined.

These findings provide a depth-dependent protocol to determine the depth position of hydrocarbons, identify borehole lithology, and monitor natural gas that may enter the drilling mud. Further objectives of mud logging are estimating the pore pressure and porosity as well as permeability of the drilled formation, collecting, monitoring and evaluating hydrocarbons, and assessing the producibility of hydrocarbon-bearing formations as well as keeping a record of drilling parameters. This data is important to ensure safe as well as economically optimized drilling operations.

Mud logging takes place in real time in mobile laboratories that are set up at the drilling site. The real-time data is directly used for drilling control. Mud logging services are usually carried out by specialists contracted by the drilling company. STS is providing pressure transmitters to some of these providers of surface logging services.

Pressure sensors used in drilling processes: durability is key

In order to monitor the drilling process, mud loggers mount various sensors on the drilling apparatus. The detection of even minor losses of drill pipe pressure requires a very high degree of accuracy. Moreover, an immediate response is necessary as well to prevent fishing times, lost-in-holes as well as the risks and costs associated with abnormalities.

Drilling sites are rugged environments and as such can be very demanding on the sensor equipment. The two most important factors in this regard are the mud itself and the vibrations that are to be expected in drilling operations.

Image 1: ATEX certified pressure transmitter for Mud Logging applications

To deal with these harsh conditions, STS provides companies offering surface logging services with the ATM/ECO/EX with customized housing. The ATEX-certified pressure transmitter is optimized for high pressure ranges. The vibrations occurring during drilling processes largely affect the area between the tube and the process connection. STS solved the issue by double welding the connection. Moreover, the stainless steel tube is thicker than is usually the case (26,5 mm). Other than the high pressure ranges and the vibrations that have to be accounted for, the mud presents another challenge by potentially clogging the pressure channel. To prevent clogging, we made the channel a bit wider (10 mm). Normally, a wider pressure channel can put the pressure diaphragm at risk. However, since mud loggers largely work with static pressures, this is not an issue.