Mining industry

Fitness cure for ochred pumps in mining

With an output of approx. 182 million tonnes (2004), lignite contributes to more than 26 per cent to Germany's power generation. Therewith it is the most important national energy source. By the combination of opencast pits and power stations, plants based on lignite offer a very high security of supply, which is from the long-term political perspective a very important aspect. In Germany, there are three large lignite mining regions: The Lower Rhine Basin, the Central German area and the Lausitz area.

MIBRAG has approximate 500 pumps in operation for dewatering their two opencast pits, Profen and Vereinigtes Schleenhain.

Fitness cure for ochred pumps in mining
Cleaning with Inox Cleaner ensures a high level of water pumping performance in surface mining

Lignite is traditionally won from the Central German area in surface mining up to 100 m deep. For each tonne of coal, several cubic metres of water need to be pumped out. In order to ensure the required draining, the submersible pumps installed in the filter wells need hydraulics that is as clean as possible. By means of the special cleaning product Inox Cleaner, the original performance ratings can be recreated reliably without having to dismantle the pump, even with units that have become heavily ochred – an experience MIBRAG has made in practice.

Figures 1-3: The surface mining as a ‘Workplace’ is no simple task for submersible pumps. Ochre, calcium and other substances soil the hydraulics.

„Following the positive reports from other pump operators, we installed a stationary cleaning facility consisting of a slewing pillar crane, two dip tubes and equipment for the measuring of pressure and flow. Even with submersible pumps ochred and/or covered with calcium heavily, a significant improvement in performance can be detected just after a few minutes residence time of the Inox Cleaner. And generally the output level of a new pump is nearly reached again after 30 minutes” says Rüdiger Worms, head of the 125 strong Department of Reconnaissance and Dewatering at MIBRAG. Frithjof Kellner, Head of the Central Pump Office Team, added: “In the past, we used external service providers to maintain and clean our pumps. Today, we can clean pumps, being mechanically and electrically faultless, by a cheaper and highly efficient way ourselves, generally achieving 90 to 100 per cent of the characteristic values of a new pump.“

Figures 4-6: Left, the stationary cleaning facility; top and right, cleaned pumps

The Mitteldeutsche Braunkohlegesellschaft mbH (MIBRAG), whose headquarter is in Theißen, operates the opencast pits Profen and Vereinigtes Schleenhain and employs approximate 2000 persons. The company also runs power plants at Deuben, Mumsdorf and Wählitz and a lignite factory for pulverisation in Deuben (all within the Saxony, Saxony-Anhalt and Thüringen regional triangle).

The basic requirements for operating opencast pits are stable slopes and suistainable working levels for the mining equipment. Therefore it is necessary to drain the water-bearing layers located above the coal, which also entails a sufficient reduction of the pressure level under the deepest coal seam - a process which is called ‘sump dewatering’. By means of various wells, the ground-water table is lowered in accordance with the development of the opencast pit.

And huge volumes have to be transported at MIBRAG. On an average of 4000 litres of water has to be pumped to extract one tonne of lignite – at least 75 million m³ of water every year! Over 200 wells are operated in both opencast pits and approximate 500 pumps are ready for the drainage process. Among them are units from Odesse, KSB, Grundfos, Pleuger, Ritz and EMU.

Iron slurry: The enemy of any submersible pump

The opencast mine as a "Workplace" is a difficult task for submersible pumps, only from the point of view of the fine sand particles that also get extracted together with the water. The sump water that is pumped out is furthermore characterised by the varying natural loads of bivalent iron (II). In combination with atmospheric oxygen, which it already comes into contact with in the well, parts of the iron loads are oxidised into trivalent iron (III). Iron hydroxide compounds are precipitated and entail ochred pumps. The soggy brown iron hydroxide compounds are deposited wherever flow turbulence occurs in the hydraulics of the submersible pump - in particular at the impellers and within the inlet and outlet areas. As a result of this clogging, the performance of the pump decreases continuously and the energy is used worse and worse. During the final stage, not only the pump hydraulics but also the risers may become completely blocked.

Apart from that, mining also exposes underwater pumps to rough conditions. Unlike conventional drinking water wells that are characterised by relatively constant supply conditions, the aim of the dewatering process in mining is the lowering and maintenance of the water level under the coal seam. "When the ground-water table is lowered, the supply conditions for the pumps are also altered – greater heads, smaller flows." says Rüdiger Worms. "We have to last many different types of pump, from the 4 inch pump (5 m³/h) right up to the 8 inch pump (100 m³/h) – that is why we have 500 pumps for 200 wells what initially appears to be a surprisingly high number."

In addition, several pumps feed into a common piping system. The varying pressure ratios can have a knock-on effect on the individual pumps and influence their output performance.

One of the consequences of this mode of operation is that a certain hydraulic reserve should be ensured when selecting the pumps (this ensures that the pumps do not "constrict" each other).

Pump efficiency (almost) as good as new

It is even more important that the pumps do not lose in efficiency and thus in performance due to clogging caused by natural iron slurry in the sump water and/or calcifying. The operating lifetime of the pumps varies significantly, depending on the chemical composition of the water in each situation; some waters will contain high levels of iron, others calcium. "We monitor each individual well and all the pumps installed there. If the flow drops below a critical value or the operating water-level rises, we remove the pump and replace it by another one" says Frithjof Kellner. "At the end of the day, we need to ensure that the opencast pit remains dry."

If the pump experts establish that the pumps are mechanically and electrically faultless but that their performance capacity is restricted due to calcium and / or ochre deposits (poor level of hydraulic efficiency), the unit will then be cleaned. For some time now, MIBRAG has been using a new type of process to clean the pump hydraulics. Without having to dismantle the pumps, the unit practically cleans itself by pumping the special Inox Cleaner in a cycle.

While conventional cleaners need up to 24 hours to take effect and to remove the ochre, calcium and rust from the pump hydraulics, the Inox Cleaner requires just 30 minutes of cleaning time (depending on the degree of dilution). The time that needs to be allowed for cleaning depends of course on the respective parameters – the type of pump, number of stages and the composition of the deposits. The decisive aspect for MIBRAG is: "We no longer have to dismantle a pump in order to clean it – and we still obtain very good cleaning results!" And should it turn out that there is in fact some minor mechanical damage, the pump has already been thoroughly cleaned and is therefore ready for replacement parts to be fitted. What are the cost benefits for MIBRAG: "We give fewer pumps to external service providers and postpone the need to carry out mechanical maintenance, which can after all cost 50 to 60 per cent of the new value." After the positive initial experiene , MIBRAG itself now wants to attend to all pumps that merely require cleaning. That saves on external costs and guarantees internal jobs.

  „Even if submersible pumps are ochred and/or covered with calcium heavily, a significant improvement in performance can be measured just after a few minutes residence time of the Inox Cleaner!" (Rüdiger Worms, Head of the "Department of Reconnaissance and Dewatering" at MIBRAG mbH).
   „Until now, in order to really be able to clean a pump well, the unit had to disassemble into its individual parts so you could gain access to the hydraulics. With the Inox Cleaner, this can be done considerably faster in a bypass cycle without dismantling the pump."

The main features of Inox-Cleaner ®:

  • removes ochre, calcium, manganese, trace of corrosion, tinder and greases all by itself
  • designed for stainless steel (dilution factor 1:3 to 1:50 depending on level of soiling; when highly diluted also suitable for cast iron)
  • contains high quality corrosion inhibitors and does not foam up
  • the cleaner is biologically degradable (in accordance with OECD direction 301-A)
  • no disposal costs (after neutralisation, the liquid is disposed of via the drainage system)
  • Classified in the low water pollution class 1 (i.e. handling is also possible without problems in protected water pollution areas)
  • also suitable for water pipes and piping systems
  • Can be used in immersion, spraying or circulation systems
The Materials Testing Office of North Rhine Westphalia (MPA NRW) tested if and to what extent (a) an impeller (b) rubber parts of a submersible pump were altered in 1:3 diluted Inox Cleaner over the course of a 48 hour test run. The result? The rubber parts were not altered in any way and there were no signs of swelling. After the pump impeller had been dismantled, a visual inspection was carried out on the welded joints. In spite of the fact that the welded joints had not been post-treated, no excessive corrosion deterioration could be detected.
The Institut Fresenius examined the Inox Cleaner in terms of the [German] Foodstuff and Commodities Act (LMBG). The result? The cleaner can be mixed with water and be rinsed away residue-free.

Last, but not least: Approval for drinking water areas in accordance with DVGW (German Association for Gas and Water) data sheet W319.