STUDENT EDITION

 

A critical evaluation of the water reticulation system at Vlaklaagte Shaft, Goedehoop Colliery

 

 

R. Lombard

University of Pretoria

 

 

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SYNOPSIS

Water is a very important component in the production process at underground coal mines. Current unfavourable economic conditions have forced the coal mining industry to identify and address every possible bottleneck preventing optimal production. An increase in water-related downtime was identified as one of the bottlenecks at Goedehoop Colliery's Vlaklaagte Shaft. The purpose of this project was to identify the various causes that contributed to the high downtime (501 hours in 2013, which led to a potential profit loss of R12.9 million) and to suggest possible solutions.
After a thorough investigation the main causes of water-related downtime were identified as low water pressure and low water flow caused by pipe leakages and bursts. The main root cause for the low water flow and pressure was identified as being the low pressure resistance (1600 kPa) of the thin-walled galvanized steel pipes used in the underground inbye water reticulation system. The pipes were selected according to the previous 1000 kPa pressure requirement for the continuous miner. However, the pressure requirement changed to 1500 kPa, which resulted in the pipes being exposed to much higher pressures than designed for.
The water reticulation system was reviewed and current and future underground pipe layout and water requirements were determined for the shaft. The time frame in which the water consumption would be the highest was determined to be between 1 January 2014 and 7 September 2014. Machine and sprayer specifications were used to determine the water consumption at the shaft.
Three different solutions were considered to solve the water-related downtime problem and to ensure the efficient supply of water to the newly open sections. Permanent underground concrete dams, semi-mobile dams, or new pipe columns with a higher pressure resistance of 3200 kPa were considered. A trade-off study (taking into consideration cost, time to completion and ease of implementation, maintenance requirements, safety, and flexibility) was completed to determine which of these solutions would be most viable.

Keywords: water reticulation, down time, pipe bursts, leakages, cascade dam system, permanent dams, portable dams

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Mine background

Goedehoop Colliery is situated approximately 40 km east of Witbank in Mpumalanga Province. Currently Goedehoop has two underground shafts - Vlaklaagte Shaft, which is situated in the southern part, and Simunye Shaft, which is situated in the northern part - which consist of 11 sections. The bord and pillar mining method is employed for coal extraction and each section is equipped with one double-boom Fletcher roofbolter, one feeder breaker, three 20 t shuttle cars, and one continuous miner (CM).

Goedehoop produces 8.7 Mt of run-of-mine (ROM) yearly, of which 5 Mt are saleable. Ninety-nine per cent of the coal from Goedehoop is exported through Richards Bay (Becht, 2010).

Vlaklaagte Shaft is currently mining only the No. 4 Seam, as the No. 2 Seam has been mined out. The shaft produced approximately 320 634 t of coal per month in 2013 and made a profit of R120 per ton due to the high-quality coal (on average 27.5 MJ/kg) that is extracted at this shaft (Du Buisson, 2013). The shaft consists of six sections: Section 1 (Simunye), Section 2 (Magwape), Section 3 (Siyaya), Section 4 (Ngwenya), and Block 7 (Section 5/6 and Section 9/10).

The main water source for Vlaklaagte is the Komati Dam. Recycled water from surface is supplied from the return water dam (RWD) to underground sections 1 to 4 via a pipeline running alongside the conveyor belt. Sections 2 and 4 have been developed more than 8 km away from the RWD.

 

Water requirements

Water is utilized for many purposes, including dust suppression, cooling, and cleaning (Table I).

 

Current water reticulation system at Vlaklaagte Shaft

As indicated in Figure 1, clean water was supplied to the No. 4 Seam underground sections (via 200 mm galvanized steel pipes) from the surface water cleaning plant until 27 July 2013. The raw water dam received water from the Blesbok reservoir, and the water was then pumped to the water cleaning plant to process the water to drinking quality. However, the pipes that supplied clean water to underground workings from the raw water dam corroded. As a result, recycled water from the RWD (via 200 mm galvanized steel pipes) was used as a substitute. A filtration system consisting of 2 μm sieves was installed to remove solids (which cause blockages in the CM and belt sprayers) from the recycled water.

Since 28 July 2013, water has been supplied to the No. 4 Seam from the RWD. The water cleaning plant therefore only supplies water to the change houses on surface, as recycled water is now being used to supply the underground workings.

Surface pump and pipe layout

The surface pump and pipe layout consists of a centrifugal pump (pump 2) which pumps water into a 23 000 litre tank. The water from the tank is pumped by a five-stage, 65 kW multi-stage pump (pump 1) to the underground sections. Figure 2 shows the surface pump and pipe layout. Standard 200 mm pipes are used on surface.

 

 

Figure 3 is a schematic illustration of the surface to underground pipe layout, including dimensions that are required to calculate the available head.

 

 

Underground pipe and pump layout

Figure 4 indicates the underground pipe layout and positions of different water users in the different underground sections at Vlaklaagte. Recently a 150 mm standard galvanized steel pipe size was selected and these pipes were tested to withstand a maximum pressure of 1600 kPa (Louw, 2013).

 

 

Summary of water requirements at Vlaklaagte Shaft

Table II is a summary of the water consumption at sections 1, 2, 3, and 4 of Vlaklaagte Shaft (31 December 2013).

 

Water problems experienced at Vlaklaagte

The water-related problems that led to downtime, may be attributed to the following facts.

➤ Water is pumped over very large distances, which means that major pipe friction losses need to be overcome. The pressure that is required at the CM has changed over the past years. Previously the CM required only 1000 kPa of pressure to operate. Pipes were selected according to this pressure requirement, and thin-wall galvanized pipes, which can withstand only 1600 kPa, were chosen. However, the pressure requirement at the CM changed to 1500 kPa, which exposed the pipes to much higher pressures than they were designed for. No action has been taken so far to change the water reticulation system to adapt to this higher pressure requirement

➤ Vlaklaagte is an old shaft and therefore has an ageing infrastructure, including pipelines. The old infrastructure and increased pump pressures are the main causes of frequent pipe damage and leakages leading to low water flow and low pressure (or no water flow and no pressure) at the face

➤ Changes made to the water reticulation system over the past years (such as changes in the pipe sizes in underground sections, the change from clean water to recycled water, and changes to pump settings and the installation of new pumps) were not well documented

➤ New underground mining blocks, such as the extension in Block 10, for which the current water reticulation was not designed, are being accessed further away from the shaft and the RWD.

Data from the water-related downtime logbook was sorted and analysed to determine the extent of the problem and to identify possible root causes leading to the high downtime. Block 7 (Section 5/6 and 9/10) was excluded from this investigation as Block 7 has a separate water reticulation system in place.

Table III indicates the total hours of production lost by each section from 1 January to 31 December 2013 due to water-related downtime. Sections 3 and 4 contributed the most to the total downtime of 501 hours. Solving the problems causing the high downtime in these two sections can eliminate 74% of the water-related downtime. Sections 3 and 4 were therefore selected for further investigations.

 

 

A summary of the combined impact of the different causes on both Section 3 and Section 4 is shown in the pie diagram (Figure 5). The chart clearly indicates that low water flow and low water pressure are the two main causes for downtime in these two sections.

 

 

Production losses due to downtime

Every time production stops the mine loses potential profit. The total potential profit lost in 2013 due to water-related downtime was calculated as indicated in Table IV and totalled R12.9 million (Du Buisson, 2013). An intervention was required to stop losses due to water-related problems and to ensure that the water requirements over the life of the shaft are met so that water problems do not occur in the future.

 

Objectives and methodology

The objectives and methodology are presented in Table V.

 

Results

The current water reticulation was reviewed to quantify the reasons for the pipe bursts. The future water reticulation system was also reviewed in order to determine the final pipe layout and underground dam placement.

Analysis of current water reticulation system

The pipe layout in Figure 5 can be analysed thoroughly by using the Bernoulli steady-state energy equation (White, 2011):

Each term in the equation is a length or a head.

α = Kinetic energy correction factor (in problems common to assume that α= 1)

P₂ = Pressure required at the end of the pipe system (at the CM)

P₁ = Pressure at the inlet

V₁ = Velocity of the fluid entering the pipe (zero because static water is pumped out of the dam)

V2 = Velocity of the fluid required at the end of the pipe system (at the CM)

∆z = Height difference/ elevation difference (m).

Equation [2] can be used to correlate the head loss to pipe flow problems (White, 2011).

where

f = Friction factor

D = Inner diameter (m)

K = Minor losses (read off from the table in Appendix H)

g = Gravitational acceleration (m/s²)

V = Velocity of medium flowing through the pipe (m/s).

Every pipe section has a different flow rate because of the location of the different water users, which results in different frictional losses within each pipe section. A number was allocated to each pipe section in order to differentiate between them (as indicated in Figure 6).

 

 

Table VI details how the friction losses within each pipe section were calculated using Bernoulli's steady state energy equation. For all the calculations in Table VI it was assumed that e = 0.15 mm and μ= 0.001.

As seen in Table VI the friction losses within the system amount to approximately 31 m. The required head of the pump can now be determined by using Bernoulli's equation (Equation [1]). Taking into consideration that:

➤ The static head available (as indicated in Figure 4) is 40 m

➤ Pressure in the pipes should not exceed 1600 kPa (or 163.2 m)

➤ The allowable head for the pump can be calculated as 123.2 m (163.2 m - 40 m)

➤ The frictional head loss in the total length (21 460 m) of pipe is 31 m

➤ P₁ = pgh (h = 2 m, as indicated in Figure 4 the water level in the tank is approximately 2 m above the pipeline exiting the tank)

➤ P₂ = 1500 kPa (the pressure required at the CM is 1500 kPa)

➤ V₁ = 0 m/s

➤ V₂ = 0.13 m/s (derived from the required flow rate of 135 l/min for the Bucyrus CM)

It can therefore be concluded that the pump pressure required for supplying water at the required pressure and flow rate to the four underground sections will cause pipe breaks and bursts. The required pump head (142.21 m) exceeds the allowable head of 123.2 m. No pump will therefore be suitable in this application. Three solutions to this problem were considered:

➤ To replace all the thin-walled pipes with thick-walled pipes with a higher pressure-holding capacity

➤ An underground cascade dam system using permanent underground dams

➤ An underground cascade dam system using semi-mobile underground dams.

The solutions needed to be implemented to satisfy the life-of-mine (LOM) water requirements. Therefore the LOM pipe layout and maximum future water requirements needed to be determined.

 

Summary of maximum future water consumption at Vlaklaagte Shaft

The maximum future water requirement for the shaft was determined to be during the period when sections 2 and 4 moved to block 10 and Section 1 had not been closed yet. A summary of the future water consumption for these four sections is given in Table VII.

Future underground pipe layout

The final pipe layout, including final pipe distances for the LOM of Vlaklaagte Shaft, is illustrated in Figure 7. In Figure 8, each pipe section was numbered to facilitate the analysis of the layout.

 

 

 

 

Analysis of future pipe layout

Table VIII shows details of how the friction losses within each pipe section were calculated with the use of Bernoulli's steady-state energy equation. The total frictional losses were calculated to be approximately 68 m. Table VIII can be used to determine where the underground dams should be placed and how many dams would be required. The placement was determined by calculating the distances over which the pipe's maximum pressure rating will be exceeded.

The pipe layout (Figure 7) is too complex to analyse as a single network. The network was therefore divided into five different legs in order to determine how many dams will be required and where the dams need to be placed. The logic behind determining when a dam will be required is simple: the pump needs to supply 153.22 m head at each outlet (spray), but the pipes can only withstand a maximum of 163.43 m, therefore whenever the pump needs to overcome frictional losses exceeding the difference (163.43 m - 153.22 m = 10.21 m), the maximum head that the pipes can handle is reached and a dam is required. The calculation for legs 1-5 are presented in Tables IX - XIII. As seen in the tables, seven dams will be required in order to ensure that the maximum pressure of 1600 kPa is not exceeded. The locations of the dams on the underground pipe layout, for all five legs, are shown in Figure 9.

 

 

Trade-off study

The three possible solutions were traded off, using five criteria: cost, time to completion and ease of implementation, maintenance, safety, and flexibility.

Based on their importance and the preferences of Vlaklaagte Shaft, the criteria were weighted as set out in Table XIV. The solution that scores the highest in the criteria will be recommended for Vlaklaagte.

 

 

Summary of how solutions performed against the criteria

A summary of how the three solutions performed against the criteria is given in Table XV. This table forms the basis for rating the solutions.

After taking Table XV into consideration, the solutions were rated according to the evaluation rubric that was drawn up as indicated in Tables XVI-XVIII. According to the evaluation rubric, building permanent underground dams scored the highest with a value of 73.8.

 

Conclusions

The water-related downtime problem at Vlaklaagte Shaft was quantified through a thorough investigation of the downtime logbook. The main causes of water-related downtime were identified as low water pressure, and low water flow caused by pipe leakages and bursts, the main root cause being the low pressure resistance of the thin-walled galvanized steel pipes used in the underground inbye water reticulation system, which cannot withstand the increased pressure now required by the CM. The ageing infrastructure and increased pump pressures are also contributory factors.

The current water reticulation system was reviewed and an underground pipe layout was drawn up for the shaft after on-site investigations. The water consumption of the current water reticulation system was determined from machine and sprayer specifications. The LOM plan was used to determine the maximum LOM water requirements, and the time frame in which the water consumption would be the highest was determined.

Three different solutions were considered to solve the water-related downtime problem and to ensure the efficient supply of water to the newly opened sections. Permanent underground concrete dams, semi-mobile dams, and new pipe columns with a higher pressure resistance of 3200 kPa were considered. The dam placement was determined by calculating the friction loss within each pipe section using Bernoulli's energy equation. The conclusion was that seven underground dams should be placed to ensure that the maximum pressure of the pipes (1600 kPa) is not excee ded.

The solutions were compared using an evaluation rubric. Building permanent underground dams was determined to be the cheapest solution (R438 397) and can be implemented in the shortest time (49 days). Cost and time to completion were critical for the solution to be a viable option. The payback period for the cost associated with building underground permanent dams was determined to be 0.035 years, and the solution will save the mine R12.9 million. Building permanent underground dams was therefore identified as the best solution for implementation.

 

Recommendations

It is recommended that seven permanent underground dams should be built at Vlaklaagte Shaft to solve the water-related downtime problem and ensure the efficient supply of water to the newly opened sections.

 

Suggestions for further work

➤ A sensitivity analysis should be done on the weighting factors of the different criteria used to trade off the three possible solutions. This will give an indication of how changes in the weighting of each criterion would affect the outcome of the trade-off study

➤ Studies can be done on a more effective recording system for water-related downtime and for recording changes made to the water reticulation system.

 

Acknowledgement

I would like to thank Prof. R.C.W. Webber-Youngman, my supervisor at the University of Pretoria, and Charl Du Buisson, my mentor at Goedehoop Colliery, for their guidance and support.

 

References

Becht, E. 2010. General Manager, Goedehoop Colliery. Presentation.         [ Links ]

Du Buisson, C. 2013. Shaft Manager, Goedehoop Colliery. Personal communication.         [ Links ]

Horac, T. 2013. Foreman, Goedehoop Colliery. Personal communication.         [ Links ]

Lottering, R. 2013. Consultant, Barloworld. Personal communication.         [ Links ]

Louw, N. 2013. Mine Overseer, Goedehoop Colliery. Personal communication.         [ Links ]

Pieterse, C. 2013. Section Engineer, Goedehoop Colliery. Personal communication.         [ Links ]

White, F.M. 2011. Fluid Mechanics. McGraw-Hill, New York.         [ Links ]

 

 

Paper written on project work carried out in partial fulfilment of B. Eng. (Mining Engineering)