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Submersible pumps.

Mine dewatering using modern submersible technology

In 1948 Swedish company Flygt released the world's first heavy-duty submersible pump, making a great advance in mine dewatering. It became possible to pump unsettled mine water because now there was a pump capable of pumping highly polluted and abrasive water.

Submersibles opened up a new pump technology and a new pumping philosophy, which include the following major advantages:

* No risk of flooding the motor, as in the case of traditional pumps

* The ability to pump unsettled mine water containing abrasive particles

* Portability

The efficient dewatering of mines is a matter of coordinating a system of pumps with appropriate capacities. The choice of pump type and pumping system is highly dependent on the depth and lateral extent of the mine.

Since the water nearly always contains abrasive particles and there is also an ever present risk of flooding, submersible, highly wear-resistant pumps are used today wherever possible.

Dry installed, clean-water pumps continue to dominate for use in main pumping stations. However, in recent years, more powerful submersibles have come on the market. Thus, mine dewatering can now, in many instances, be based entirely on submersibles.

General design

The basic material used in the construction of this kind of pump is normally aluminium for portability but other materials are rubber or, with some restrictions, polyurethane lining, high-chrome steel and stainless steel. The choice of pump depends upon the specific application.

Starting from the bottom of the pump, the liquid passes a wide-based, spring-loaded strainer before it reaches the pump inlet. Impurities that pass through the openings of this strainer will also pass through the rest of the pump, virtually preventing clogging.

The semi-open impeller is made of high-chrome steel with a hardness of Rockwell C 60. Stainless steel impellers are also available for corrosive conditions. The impeller is trimmed against the rubber-lined pump housing bottom. By this design, it is possible to adjust for wear, thus maintaining maximum performance.

Liquid is guided by the large diameter rubber-lined diffuser past the motor for cooling purposes and leaves the pump through the rotatable discharge outlet on the side of the outer casing.

The tandem face seal arrangement on the shaft, operating in an oil chamber, seals the motor off from the hydraulic end. The oil acts as lubricant and cooling medium and provides an extra buffer between the pump liquid and the motor.

The completely encapsulated and dry-running motor is of a single- or three-phase, squirrel-cage type, with either two or four poles. Most motors have series-connected thermal switches built into the stator windings, to protect against overheating.

The junction chamber on top of the pump, in which the cable leads are connected to the terminal board is sealed off from the motor, to prevent burn-out should moisture enter due to damage to the cable. Water sealing and strain relief functions are separated.

Face pump

Small, submersible units are used to keep water out of working stopes and pump it to some larger, more centrally located stage pumps on the same level. These small submersibles are in a range of sizes up to 6 kW, with a maximum 102 mm discharge. The basic requirements for a face pump are wear resistance, submersible, able to withstand dry running, small dimensions and portability.

Stage pump

The characteristics of a stage pump are mainly the same as for the face pump, but of medium size. The capacity is greater as this pump installation handles water coming both from a number of face pumps and by gravity in drainage ditches to the stage pump.

Stage pumps are preferably located at suitable intervals along drifts or ramps, in order to collect roadway drainage. In such a system the water coming into the roadways can be pumped to the next pump instead of flowing under gravity in ditches, and there accumulating solids. This will also reduce the need for road maintenance, particularly for damage from erosion.

Feeder pump

The feeder pump's main task is to pump between levels. The installation serves as each mine level's main pump station. However, the major difference between the feeder and main pump is that a feeder pump installation is not permanent, unlike the main pump station. Also a feeder pump installation has to operate with static heads of 50-70 m, while a main pump has to operate at significantly higher heads.

The water collected at the feeder pump comes from face/stage pumps and by gravity through ditches and is thus unsettled water.

As this pump installation is mostly found at the development stage of a mine, it has to be regarded as semi-permanent. The two conditions of unsettled water and semi-permanence require mobility but there is no need for the excavation of costly settling sumps or tanks.

Submersible heavy-duty pumps in the 20-40 kW size range are common here. They must:

* Have a duty between 50 and 70 m

* Be portable

* Be able to pump unsettled water

Feeder pumps should be designed for tandem operation. This is an important feature in modern mining, where vertical shafts are replaced by ramps. In a modern mine lay-out, total pump head is becoming more important than the static head. This means that the pumps selected must be flexible in head requirements.

Temporary shaft pump

When sinking shafts, pumps have to be able to follow the work, pumping from different levels up to the surface. It is also important to have a slim-line pump design to save space. Submersibles are becoming more popular in this application (often the only solution), where they offer space saving and flexibility.

The temporary pump system for shaft sinking is mostly a two-phase system consisting of in-shaft pumps as well as pumps in the temporary stations. The location of a temporary pump station is determined by:

* The location of the major aquifer

* The distance that the in-shaft system effectively can lift the water

* The characteristics of the pumps available for the temporary station

* The excavation of slots as part of the shaft sinking task

Instead of the costly excavation of slots, only used for a temporary station, shaft water rings are used to collect both the water which seeps through the shaft lining and water coming from the in-shaft pumps.

The static pump head for the in-shaft pump is not normally more than 40 to 50 m. In order to reduce pump maintenance, in such extremely tough conditions as shaft sinking, frequent rotation of the pumps and a high level of back-up is necessary. For this reason, the pump system chosen should have a high degree of flexibility, allowing the pumps to be installed both as in-shaft units and in the temporary station.

Pumps for shaft sinking must meet the following requirements:

* Submersible

* Flexible, in respect to easy service (portable) and have wide variations in pump head

* Slim-line, thus space saving

* Able to handle abrasive water

Stationary shaft pump

For service and maintenance reasons, complex pumping systems are not normally placed at the bottom of a shaft. Shaft pumps must be able to operate over long periods without any attendance, compared to the other installations described.

The main character of a shaft pump application is the same as for a feeder pump. The required pumping capacity varies, of course, but is normally less in comparison to the feeder pumps. Most of the water has already been collected at various locations above the shaft bottom. Another difference is that a stationary shaft pump should have more of the characteristics of a sludge pump, since much of what is collected at the shaft bottom is sludge with a high solids content.

This kind of pump works with a limited constant flow and high heads, and pumps liquid containing abrasive solids, such as sand and grit up to 15% or more by volume. It should therefore be equipped with a torque flow impeller and a pump housing of cast Ni-hard.

Much of the sludge accumulated at the shaft bottom comes from the skip loading station above. Even in a well designed shaft pump installation, the shaft bottom must be cleaned out at regular intervals.

Main pump

While the face, stage and feeder pump installations are the mine's water veins, the main pump station is the mine's water heart.

There are three usual options for the main pump station:

* The horizontally mounted, multi-stage, centrifugal, clean water pump driven by a non-submersible electric motor

* The horizontally mounted, non-submersible, electrically driven, centrifugal pump of heavy-duty design

* The fully submersible electric pumping unit

The major drawbacks of the non-submersible alternatives are the costly requirements for bulky settlers or holding tanks, and the risk of flooding the motor.

When designing a settling sump, there is a trade off between its size and the estimated peak capacity needed. If the sump is too small, the 'clean water' pump installed has to pump mine water containing solids, which will rapidly destroy its hydraulic parts, with the further risk of flooding the non-waterproof electric motors.

The cost of a pump station, including settling and clean water sumps, pump chambers, etc., makes a good case for the use of larger pumps. If, however, space for pumping facilities is available on the intermediate levels, it could be more economical to have multiple lifts, due to reduced sump excavation costs.

In situations where space is restricted, a system of series-connected pumps is an alternative for the main pump station. However, the question then is which design criteria should be used for the pipework and fittings of the lower sections of the system. This lower section must be protected against the gravity head above the station in case of failure of the upper pump(s). Check valves can be installed, but any valves can fail, and then the savings from a series configuration, over using sumps, could quickly turn into a loss.

Summarising, the multi-stage clean water pump arrangement has a number of weak points:

* Unable to pump contaminated water including abrasive particles, thus requiring expensive settling and clean water basins, and costly handling and disposal of the accumulated sediment

* Since the electric pump motor is not waterproof, there is the risk of a complete breakdown if flooding occurs and that presents a situation when maximum pumping capacity is needed more than ever. Separate pump chambers are required

* None of the common alternatives are easy to service

* Limited mobility. Such a non-submersible system is permanent/stationary

* A drainage system is required in order to drain water downwards before pumping it up again

* The cost of horizontal drainage has a tendency to increase since the location of larger pump stations has to be protected from damage from the working stopes

To overcome these problems, large heavy-duty, high-head submersible main pumps can be used in combination with submersible feeder pumps. The submersible main pump (pump and motor combined in a single completely submersible unit) has appreciable advantages over today's common main pump arrangement. The submersible heavy-duty concept will eliminate each of the disadvantages of a dry-installed, non-submersible centrifugal pump:

* Heavy-duty submersibles do not require settling and clean water basins since they can handle contaminated water containing abrasive particles. There is no handling cost for accumulated sediment

* They are completely impervious to damp and water

* Routine checks can easily be carried out at the installation. As the pump is easy to transport, the entire unit can be taken to a workshop for major overhaul and repairs

* Mobility means less stand-by equipment is needed

* Since submersibles do not require special foundations, settling basins, etc., a simple drainage circuit preventing water from draining downwards is all that is required

* No costly excavation of protection chambers is needed for the main pump installation. Its location is part of the mine's development stage closer to the working areas, therefore there is less expense involved in the horizontal drainage circuit

Submersibles intended for installation underground are currently available in sizes from the smallest face pump of 1 kW up to the largest typical main pump of 90 kW with a maximum output of about 9,000 litres/min, or for the high-head version a total head of 200 m. A 180 kW pump is also available with a maximum output of 3,000 litres/min, or a total head of approximately 350 m. The successful development of increasingly larger submersibles is a result of the demand to simplify and reduce the cost of pumping stations, settling basins, and the like.

Open pit dewatering

The superiority of submersibles in open pit mining is already well established. Two main systems are employed, depending on the layout of the mine and the local groundwater and rainfall conditions.

In fairly flat and level mines covering an extensive area, small and medium size, wear-resistant, submersibles can be installed in simple pumping stations or sumps. As a rule, the station will merely consist of a pit or a prefabricated perforated concrete pipe.

These pumps deliver water to a main pumping station which then pumps it out of the mine. In principle, the main station can be of the same simple design as the smaller stations and the same types of pumps can be used, although of larger sizes.

In deeper mines with relatively moderate lateral development (small pit bottom diameter) it is normally preferable to install all the pumping capacity at one location. At places where the water inflow is highly irregular and at times extremely large, submersibles can be installed on rafts that follow the level of the water as it rises and falls.

Since wear-resistant submersibles with comparatively large capacities at delivery heads of 300-350 m are now available, water can often be pumped out of the mine in a single stage. Where this is not possible, an intermediate pumping station of the same simple type can be arranged, or non-submersible booster pumps can be used. However, these must be adequately protected in tough climates and installed at a sufficiently high level not to become flooded. Such flooding has occurred in areas with periods of heavy rainfall, with consequent costs.

Pumps in gassy areas

For submersible pumps to operate in coal or other mines where explosive gas may be a problem it is essential that the temperature of its external surfaces do not exceed the permitted maximum. This can be achieved by choosing the correct material quality and designing the equipment for good heat transfer.

Electrical arcs and sparks must also be avoided, so the enclosure for the electrical equipment must be designed in such a way that it can withstand the pressure created if an explosion occurs inside. Sparks created inside the same enclosure must also be prevented from propagating to the outside. This is accomplished by using closer tolerances and increased slit depth in the design.

Today almost all sizes in the standard submersible line are also available in a corresponding permissible variant for explosive atmospheres.

Bengt Anden, Marketing Manager, Flygt Overseas
COPYRIGHT 1995 Aspermont Media UK
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Copyright 1995 Gale, Cengage Learning. All rights reserved.

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Title Annotation:mine dewatering
Author:Anden, Bengt
Publication:Mining Magazine
Date:Feb 1, 1995
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