HYDRAULIC STOWING

This process is widely used in India in those collieries which are situated within 16 km of rivers giving plentiful supplies of sand, the commonest stowing material in our mines.

The following factors have made stowing possible in many Indian mines:—

1) Availability of sand from rivers flowing near the collieries within 16 km.

2) Roof and floor of seams are not affected by water.

3) Seams not being very deep, humidity is not a major problem.

4) Mines are usually at depths exceeding 100 m and the seams are inclined. Hydraulic sand stowing is not successful where the seam is at a low depth from the surface and is flatter than 5⁰.

From the stage of collecting sand at river end till the sand is packed in the goaf, the following operations are necessary:—

1) Gathering of sand at the river bed.

2) Transport of sand from river end to the bunkers on surface at the colliery.

3) Transport of sand hydraulically from the bunkers to the underground stowing site through pipes.

4) Stowing of the sand in the area from where coal has been extracted.

Before introducing sand stowing arrangements at any colliery it is necessary to make bore holes in the river bed to ascertain the depth of sand, to estimate its reserves and to explore the possibility of continuous supplies to meet the demands. One te of coal extracted needs theoretically nearly 1.3 te of sand in a virgin area. Where old workings have to be stabilised and the pillars to be extracted, the ratio may extend to 2.5 te of sand for every te of coal extracted from old workings. In workings which arc developed by bord and pillar method, if the depillaring is-to be in conjunction with stowing, nearly 1.8 te of sand per te of coal extracted during depillaring will be essential. In longwall advancing workings, nearly 1.4 te of sand per te of coal raised is required. Wastage or losses account for additional 10- 15%.

Sand is brought to the sand storage bunkers on the surface in any one of the following ways:

1) By trucks or wagons.

2) By aerial ropeway (Fig. 10.1).

3) By tipping tubs pulled by haulages if the river bank is near.

4) By pontoon mounted sand pumps discharging into separators. (Fig. 10.2).

5) By sand slushers and scrapers which feed the sand to a river bank bunker and from the bunker the sand is supplied by aerial ropeway to the sand storage bunker at the colliery.

6) By dredgers discharging sand into bunkers.

The sand storage bunker is always situated to the rise side of the underground area to be stowed.

Fig. 10.4 shows the general layout of the stowing arrangement from bunker to the pit-bottom. The bunker is situated on one side of a shaft and a drift inclined at 1 in 3 or 1 in 4 is driven from the mixing chamber (situated directly below bunker) to the shaft carrying the stowing pipe range. Sand from the bunker drops through a chute into a "mixing cone" (in fact, an inverted cone) fitted below the floor of the bunker. Its opening is controlled manually by a rack and pinion arrangement. Sand and water are mixed in the mixing cone in. predetermined proportions. The water is supplied by 125 to 175 mm dia. water pipes from the surface reservoirs, situated close to the mixing chamber and provides sufficient head of water for flow through the pipes leading from reservoir to mixing cone. In practice, sand bunkers have a capacity of 2 days' requirement of sand, and water tanks, a day's requirement.

Mixing Chamber

The place where the mixing cone is located is called mixing chamber. The size of the chamber should be sufficient to accommodate the desired number of mixing cones and pipe ranges. The chamber has an access either through an incline from the surface or by a cage operated by a small hoist in the case of a stowing shaft. On the mixing cone there is a screen to prevent pebbles or stones larger than 25 mm size from going into the shaft range with the sand-water mixture. These rejects have to be picked up and collected in the chamber from where they are removed to surface. The chamber should have sufficient lighting and as the work goes on in humid conditions all fittings and cables should be moisture proof.

Signalling or telephone arrangement is provided for communication between the mixing cone operator and the underground stowing supervisor so that supply of sand, water, or both 'can be stopped or adjusted according to the underground requirements. In the mixing cone, a vertical free fall of sand to a minimum of 600 mm from the bunker chute is believed to avoid heaping up of sand on the screen. The water is admitted to the mixing cone by surrounding it with a circular perforated pipe (garland pipe), it is preferable to line the mixing cone with rubber, e.g. old conveyor belts, to save iron from abrasive sand.

To measure the input of sand and water in the mixing cone with a view to have control on the stowing operations, the mixing chamber is equipped with:-

1) Water meter: - Kent 'velocity meter is commonly used on some installations; alternatively a V notch or a venturimeter may be used: and

2) Lea Recorder: - The sand input may be calculated in terms of the area of chute opening which can be recorded by a Lea chute opening recorder.

Stowing pipes and their layout

The sand water slurry pipe, from the mixing chamber downwards, may be installed in a borehole, in a steeply dipping stone drift or in a shaft.

The pipes used for sand stowing range are of C. I., mild steel, hot roiled seamless tubes or alkathene. They have flanges for joints (except in boreholes). C.I. pipes are heavy, have a low tensile

strength of only 15 kg/mm² and are used for more or less permanent installations in drifts, shafts, main cross-cuts, etc. Their use is not favoured at the face. Sizes in use are .125 to 150 mm bore, 3 m long, and with 20-25 mm thick walls. For facility of turning, the marks I, II, III & IV and an arrow for direction of turn, are cast on the pipes during manufacture. The life of C.I. pipe is observed to be 4 lac. te of sand for 20 mm thick walls and nearly 8 lac te for 25 mm thick walls.

M. S. Pipes are lighter than C. I. pipes and have a tensile strength of 45 kg/ mm- . M. S. pipes are preferred to C. I. pipes for face pipe range due to their comparative lightness. M. S. pipes are usually with 12 mm thick walls, and of 5.5 to 6m lengths. Their life is 6 lac te approximately.

Alkathene pipes are much lighter than M. S. pipes and greatly resist wear by sand or siliceous mill tailings. They can be threaded for fitting flanges and pipes lengths can be joined together by heating with a special heating-cum-joining device. Jaduguda mines have replaced M.S. pipe range from the surface to underground slopes by Hasthi pipes (trade name of one make of alkathene pipe) some years ago with encouraging results.

Stowing pipe range consists of three main portions:

1) Shaft range, drift range, or borehole range.

2) Underground roadways range.

3) Face pipe range.

The shaft range is carried in a shaft which is not the main coal winding shaft with a view lo avoid interruptions to raisings when pipe ranges have to be changed, repairs to be done, or pipe jams to be cleared. The ropes are badly affected with leakage of sand, and specially when pipes burst, as sand adheres to the greased surface of ropes and abrades it. Main intake shafts and main intake roadways are also avoided as water leakage increases humidity.

Roadway pipes are, as far as possible, kept off the main haulage roads as —

1) Leakage of water may wash away tram line packing,

2) Clearing up of pipe jams introduces quantities of sand into roadways, and

3) Dangers to pipe range arise from derailment of tubs.

Pipes are laid out such that the line is dipping all the way and stepped or staircase layout is preferred. Sharp bends are avoided and standard bends of 90⁰, 45⁰, 30⁰, and 15⁰ are kept at hand and used instead of bent pipes. Smaller deviations of 5" to 10" may be covered by suitably adjusting thickness of packing at the joints. Instead of one large bend, 2 or 3 small bends are preferred, as replacement of a worn-out bend is then easy.

Valves are not used for controlling branches in stowing ranges as they will be easily worn away. Y Pipes are used for branching off arid the range that is not needed is blanked off. Bifurcators of either “plate type” or "hinge type" may sometime be used, but they are somewhat uncommon.

Pipe Joints

Fibre or asbestos washers are used between flanges. A ring of lead sheet or of signalling wire, having hessian cloth wrapped around it and soaked in coal tar is a good packing between flange joints. Joints without coal tar rot after some time and start leaking. The flanges should be screwed upto the last thread on the pipe end. As the pipe wall thickness is less at the threads, the threaded portion is the first to leak due to wear. Pipe leak is the common cause of interruption in the flowing of sand and of pipe jams. The joints should be occasionally tested to a pressure of 15 to 20 kg/cm² of hydrostatic head. This can be easily done by putting in a blind flange at the end of the pipe and filling up the pipe with a known quantity of water as measured by water meter. In very long ranges, the pipes should be tested in different stretches for their ability to withstand pressure.

Where workings are on the rise side of a shaft, entry of stowing range by the shaft will not be helpful for stowing sand in the goaf on the rise side and stowing ranges can be installed in bore holes drilled on the rise side. Pipes with screwed sockets are lowered down the borehole and are suspended by clamping them to wire ropes anchored at the surface.

Support of pipes

In the shaft the pipe is supported on stool pipes placed every 30 to 40 m apart (Fig. 10.5). The stool pipes are supported by double buntons set into the side of the shaft. At intermediate points, the shaft ranges are steadied by single buntons set 8 to 10 m apart in-between the double buntons.

At the bottom of the range in the shaft the change of direction of the pipe range is effected by a stool bend. It is usual to fix a stool pipe on a double bunton at the bottom of a shaft range and to attach this to a stool bend. Along the roadways the range rests on the floor or on sleepers or on brick-work wherever necessary for smooth gradient. It is essential that the range be fixed to the ground securely.

Wear in Pipes

In a shaft range there is more wear on the upper portion of the range than at the lower. The upper portion of shaft range is generally changed after transport of 2, 00,000 to 4, 00,000 te of sand (C.I. pipe, 25 mm thick walls). Lower portions are known to pass even 8 lac te of sand. To economise and to utilise the shaft range to the full extent, it is considered advisable not to change the whole length of shaft range at a time but in parts, say, length between two stool pipes at a time. The safe wall thickness in C.I. pipes is 6 mm with a factor of safety of 3.5 for a depth of 400 m. It is not necessary to rotate the pipes of shaft range as the wear is uniform throughout the bore of individual pipe length. Where the pipes are in inclined position, e.g. in the drift and in the underground roadways, maximum pipe wear is in the bottom portion covering an arc of 90°. Hence rotation of pipes should be completed in four stages for uniform wear of the pipe walls. At one time 10 to 15 pipes of a range could be turned. After the turning the joints should be checked, because, those with taper, become slack.

Hydraulic profile and H: L ratio

It is essential that the pipe layout in a hydraulic stowing installation should conform to a correct hydraulic profile. Incorrect profile will cause cavitation and then the full available head cannot be pressed to use; high local velocities will cause considerable wear on the pipes and the entrapped air will set up pulsations in the system.

The length (L) of the pipe range through which sand: water slurry can be transported depends mainly upon the vertical head or height between the mixing chamber and the point at which slurry is discharged (H). The ratio H/L (height/length) available in practice for a stowing range indicates its efficiency. Generally this ratio is 1/7 for reasonably good stowing rate.

Underground Stowing Arrangements and Operations

Before a goaf is packed a boxing (a barricade of bamboo matting) should be constructed as near the face as possible, leaving space for conveyor path, coal cutting machine and roof supports. On a longwall face the width between previous boxing of sand pack and new boxing under construction is generally 4 to 5 m. With smaller widths of packs, cost of boxing becomes high and face work gets disrupted frequently. Materials used for boxings of narrow packs 1.5 m to 1.8 m in width should be stronger than those for packs of larger widths, say 3.5 m to 4.5 m, as the pressure developed on boxing in narrow packs due to sand is more. Bamboo matting or hessian cloth is generally used, and in some installations coir matting has been tried. The latter, though costly, can be used 3 to 4 times and has thus an economic advantage over the bamboo matting or hessian cloth which can be used only once. Wooden props or telescopic rails are erected along the new boxing line and the hessian cloth or bamboo mating is fixed to it by wire nails or strings from the floor to the roof. In some cases props are placed slightly inclined. For this, two chalk lines are marked on the roof by stretching strings to indicate position of the top and bottom ends of props. Ideal inclination would be the angle of repose of sand, but due to limited space it is not practicable and the prop has an inclination of 10° with the vertical, the top end leaning towards goaf. In such cases recovery of props and coir mattings is easy. If the boxing is made vertical, the sand buries the coir matting which is, therefore, difficult to recover when props or rails are withdrawn. The dip side boxing is made stronger with double bamboo matting, and sometimes, with wooden planks or coir matting for a simple reason; the dip level may be drainage level or it may carry gate belt or tracks for tubs and bursting of the packing may foul the level road. Chocks are preferred to props on such dip side packing. When the new boxing is complete, the old boxing is dismantled and props, coir matting, planks, etc., are recovered from it, unless the old boxing is required to protect equipment of gate roads.

Telephone is extended to a convenient place near the new boxing when stowing of goaf is to be undertaken.

At the commencement of stowing the stowing range is extended so as to keep the end of stowing pipe nearly 7.5 m away from the dip side boxing. The velocity of sand: water mixture would depend upon the pressure head available. A nozzle is attached to the pipe end to increase the throw in special cases. Without nozzle the throw in one case was 3 m but with the help of a nozzle it was increased to nearly 7 m. The nozzle is of mild steel and tapers from 125 mm dia. to 85 mm dia. within 1.2 m length.

To stop the stowing, the flow of sand into the mixing chamber at the surface should be stopped first and then, after 3-4 minutes, the flow of water should be stopped when the discharge through the nozzle is only of water. This avoids pipe jam.

Concentration of sand and water

In most of our installations for depths of 200 to 300 m the average ratio of sand: water may be 1: 1.5 by volume though it varies with individual installations.

Sand	Water(by vol.)	Sand	Water
1 :	:1.5	1 te	935 litres
1 :	:2	1 te	1100 litres

For deeper mines the sand: water ratio has been more favourable, nearly 1: 1.3 by volume.

Rate of stowing

For a given stowing pipe range there is a maximum quantity of the mixture which can flow; if more quantity is fed, it will overflow the mixing funnel. If less quantity than this maximum is fed, the vertical range will not run full bore; as the mixture flows down the pipe, a partial vacuum will be created and the air will be drawn in, leading to a pulsatory flow and consequent jamming of the pipes. High velocities will be created in the upper section of the vertical range causing excessive wear on pipes. Thus it is desirable that just the maximum quantity of the mixture should flow in the range, ensuring a full bore flow in the vertical range. Rate of stowing varies from installation to installation. A rate of 80 to 100 te of stowing (sand only) per hour is common in many mines at 200 to 300 m depth with nearly 1100 litres of water per te of sand.

Discharage of sand is given by the formula;

Qs = Qm X S 

S + W

Where

Qs = sand (m³) discharged/hr

Qm = sand & water mixture (m³)/hr.

S and W are proportion of sand water respectively (by vol.)

CMRS, during its experimental work at Ballarpur colliery (depth of coal seam 45 m from surface) has arrived at the conclusion that a high rate of stowing at a water; sand ratio of less than unity (nearly 0.9 water: 1 sand) by volume can be obtained even with 125 mm dia. pipe range, during the experiments carried out at Pure Jambad colliery CMRS claims to have achieved a water: sand ration of 0.9: 1 in pipe range of 125 mm and 150 mm bore and the H : L ratio was 1 : 11.4.

Pipe jams

Pipe bursts take place when pipes are worn out and they are not able to withstand high pressures. If extra pressure is created due to water ram, that may burst a worn-out pipe. When the valves of water pipe are not closed well after stowing it is possible for an extra pressure to be created due to water head.

In the mixing chamber heaping of sand on (he screen, overflow of water at the cone and froth on top of the mixing cone are some of the indications of pipe jam. If it is of minor nature, sand is stoppped and clear water flushed; the jam may clear. Hammering the pipe at suspected places of jam may help in some cases. If the methods do not help, pipes are opened out at suspected places and the sand deposition scraped.

Pipe jams are completely avoided if

(i) air removal devices are incorporated in the installation and the full-bore flow is obtained,

(ii) the pipe layout is in correct hydraulic profile,

(iii) the size of the largest particle of the solid does not exceed l/3rd D (preferably 1/4 to l/5th D), where D is dia. of the pipe, and

(iv) the average velocity of the mixture is above the critical velocity of deposition.

Installation of booster pumps for increasing the pressure of sand-water slurry to stow on the rise side is not very common. As a thumb rule; for every 10m length of a horizontal pipe range there is a loss of 1 m head and the velocity of slurry has to be kept at a minimum of 2 m/sec to avoid sand deposition during flow.