6. Construction of the biogas plant
The overflow of the biogas plant must be higher than the slurry bed or the slurry distribution channel. The inlet must be lower than the stable floor. The biogas plant should be so far from trees that roots will not grow into its brickwork. It should not be in areas where heavy machinery move frequently. Biogas plants are not meant to be a playground, still they should be safe for children and animals.
A gasplant of a rural biogas unit is standardized and preferably a fixed dome plant. Once the decision for standardization is made, modifications are only allowed in order to join existing local structures. The plant itself is not to be changed.
The size of the plant depends on the substrate available. In practice its volume is chosen according to the number of cattle or pigs and their stabling. In case of doubt, the energy demand may also be considered. The biomethanation process is rather hardy and robust and does not require defined loading rates. Therefore, it is possible to consider only a few standard digester volumes. The standard volumes of digester and gasholder have to be estimated in each project area according to gas production rates and general gas consumption patterns.
Larger gasplants have longer retention times and, therefore, higher gas production rates. Nevertheless, the amount of daily fed substrate has more influence on gas production than the volume of the digester. In case of doubt, criteria used are the investment costs and security of gas supply. Larger gas plants have higher gas storage capacity.
The most common size in the Arusha Region is the 16 m³-plant which can provide gas for cooking and lighting for a normal family. The 12 m³-plant is reserved for places of little gas demand, e.g. small families, or where ground temperature is above 24ºC and therefore, retention time could be less. 30 and 50 m³-plants provide gas not only for household use but fuel for big institutional kitchens and special appliances like refrigerators, incubators, hatching heaters or power engines, etc. Structural drawings for the standard plants are to be found in the Appendix.
The Principle Design
The standard fixed dome plant has a half-bowl spherical shape with flat bottom and a top opening. The outer walls rest on a foundation ring beam. The floor has no static function. The upper part of the sphere is separated from the lower part by a joint, called the "weak ring". Gas tightness of the upper part is achieved by a crack-free structure and a gas-tight inner surface plaster.
The Inlet pipe is connected to the spot of dung disposal in the stable. The outlet pipe connects the digester with an expansion chamber of reduced spherical shape. The overflow of the expansion chamber - really the final outlet of the gasplant - leads to the slurry disposal system, i.e. the distribution channel, storage tank or compost pit.
Fig.14: Principle of statics of fixed dome plant
The plant consists of a non-load bearing bottom (A), the lower slurry~ tight digester (B), the upper gas-tight gas storage part (C), the neck (D) and the gas-tight lid (E). Gas storage part and digester are separated by the weak-ring ( 10) in order to allow free reaction of the strong-ring (3) and to prevent cracks which have developed in the lower part of the digester to "grow" into the gas storage space.
The plant rests on a foundation ring ( 1 ) bearing mainly the vertical loads of' the construction and the soil cover (7). The surrounding soil supports the construction to resist gas pressure (5) and slurry pressure (6). Concrete at the outside of the lower layer of bricks (2) helps to reduce tangential forces at the foot point (9). The ring forces of the upper part are absorbed by the strong-ring (3).
The Reference Line
Because the slurry is a liquid, the biogas plant follows the physical law of communicating tubes. A reference line is used in construction to keep the exact levels, which are of outmost importance for the functioning of the system. Main vertical measurements of the working drawings are given in relation to the reference line. The reference line is 35 cm above the overflow of the expansion chamber and marks the lowest possible point of the stable floor from where the dung is pushed into the mixing chamber. It is also the minimum level for soil covering of the dome.
At the site, the reference line is marked by a string passing over the centre of the digester, preferably in direction from inlet to outlet. The string is fixed in absolute horizontal position with a spirit or hose-pipe level. The pegs for the reference line should be sturdy and well protected during construction time. In order not to lose the level of the reference line it is advisable to also mark it on a tree or a building near to the plant.
In case there is an existing stable, a horizontal string is fixed from the lowest point of the floor to the place of the proposed overflow of the expansion chamber. The overflow might be 35 cm or more below this string. The convenient overflow level might be decided and the string of the reference line is tied 35 cm above that point. In all cases, it will be at, or below, the lowest floor level of the stable. In case of a 16 m³ standard plant, the centre of the digester is 3,30 m away from the inlet point. The point of overflow is 5,00 m away from the centre.
In case a new stable will be constructed, the point of overflow of the expansion chamber might be decided according to the convenience of slurry disposal. A horizontal string 35 cm above this point forms the reference line. The lowest point of the stable floor might be on the same level or preferably above the reference line; but never below this level. In case of a 16 m³ standard plant, the centre of the digester is 5,00 m away from the point of overflow. The inlet chamber attached to the stable is 3,30 m away from the centre.
Digging the Pit and Casting the Foundation
For safety of the labourers, the sides of the pit must be sloped according to the soil properties. Excavated soil should be placed 1 m away from the rim of the pit. Place of inlet and expansion chamber should be kept free from excavated soil.
The pits of the digester and the expansion chamber are excavated in their proper sizes and positions down to their respective final depths. If soil is soft or of unequal strength, stone or sand packing below the foundation is required. Provide drainage facilities in case of ground or hill water.
Fig.15: The reference line
The reference line (RL) is marked by a string during construction to maintain proper levels of essential parts of the gas plant. The lowest point of the stable floor (SF), i.e. the lowest point of the urine drain, must be 35 cm above the overflow point (OP) in order to allow sufficient depth (min. 15 cm) of the inlet chamber. On uneven ground it may be required to fix the string l m above the real reference line. Then, 1 m must be added to all measurements. The reference line may be lower than the stable floor (1). It should never be higher as to avoid lifting up the feed material for filling the plant. The reference line also marks the necessary soil cover above the dome (4).
The foundation ring is excavated immediately before filling the concrete of the foundation. A mixture of 1: 2: 4 (cement: sand: aggregate) is used and the concrete is firmly rammed. Casting of the foundation should be done early in the day as to allow sufficient time to place the first two layers of brickwork into the fresh concrete at the same day. These two layers are back-filled with a lean concrete mixture of 1: 3: 9.
Brickwork of Spherical Wall
The centre point at the bottom of the digester is the heart of the construction. The centre peg should be firmly driven in at proper position and level according to the reference line. A nail on the head of the peg marks the exact centre.
To construct the spherical masonry wall, a guide stick is used which keeps the radius constant and helps to create an absolute half bowl shape. Each brick of the wall is laid against the nail of the radius stick. It is easier to do than to describe. Just start putting brick by brick, keeping the top of the brick in the same slope as the direction of the radius stick, which is radial, pointing to the centre. Automatically, brickwork will turn out in spherical shape.
Bricks must be of good quality, preferably of 7·12·23 cm in size. If bricks are less than 5 cm in thickness they should be used in flat layers. The wall becomes then 10-12 cm thick and more bricks will be required. The bricks are soaked in water before laid into 1 cm mortar bed of mixture 1: 1/4 : 4 (cement: lime: sand). Gauge boxes are used to measure the volumes for mixing the mortar. Only sieved and washed river sand is permitted; otherwise the amount of cement must be increased if only quarry sand is available. Vertical Joints should be "squeezed" and must, of course, be offset. The inner edge of the brick forms always a right angle with the radius stick.
Inlet and Outlet Pipe
Inlet and outlet pipe must be placed in connection with brick-laying. It is not possible to break holes later into the spherical shell; this would spoil the whole structure. The pipe rests below on a brick projecting 2 cm to the inside. Above, it is kept in position by being tied to pegs at the rim of the excavation.
The inlet pipe is of 10 cm (4") diameter. Its upper side is in line with the top of the weak ring. The outlet pipe which connects the digester with the expansion chamber is of 15 cm (6") diameter in order to avoid clogging. It starts at the bottom at the 4th layer of bricks and continues above the dome of the expansion chamber to allow poking in case of blocking. A collar of cement mortar 1: 1/4: 4 at the outside of the wall seals the Joint between the outlet pipe and the brickwork. At the level of the expansion chamber it is cut out to allow for slurry flowing in and out.
Outside Plaster of the Lower Part
Only sieved and washed river sand is to be used for plaster. After brickwork has reached the level of the weak ring, smooth plaster of 2 cm thickness and of 1: 1/4 : 4 mixture is applied all over the outside. The plaster should harden over night before back-filling of soil is done.
The outer plaster protects the brickwork against roots growing into the joints. It forms also a smooth surface which reduces friction between soil and structure and thus, reduces static stress of the brickwork.
Fig.16: Construction of the lower part of the sphere
(1) Foundation ring of concrete 1: 2: 4; (2) First two layers of' bricks laid in cement-lime mortar 1: 1/4 : 4; (3) Supporting concrete ring 1: 3: 9; (4) Brickwork up to the bottom of the weak ring laid in mortar 1: 1/4 : 4; (5) 2 cm thick outside cement-lime plaster 1: 1/4 : 4; (6) Backfilling soil rammed in layers of max. 30 cm height.
For measuring the correct mixtures a gauge box is used (7). The brickwork is erected with the help of a radius stick (8). The radius stick is set at the centre of each brick. The surface of the brick follows the direction of' the radius stick (9). It rests with a groove at the nail of the centre point (10). Because the floor has not yet been laid, the peg of the centre point is 3 cm above the excavated ground. The upper nail (11) of the radius stick (11) marks the inner edge of the brick. The measure of the stick is reduced by 4 cm for placing the headders of the strong ring (12). When laying the bricks, they are first knocked horizontally, then vertically.
Fig.17: Inlet and outlet pipe
The outlet pipe (Ã˜ 6" ) rests on a flat brick (1) above the 4th layer of the spherical wall. At the outside of the wall it is surrounded by a mortar collar (2). The inlet pipe (0 4") penetrates the weak-ring (3). The pipe is not allowed to be higher than the top of the weak-ring, because it would then disturb the strong-ring. From the outside it is sealed only by the plaster of the lower brick work. At the top, the pipes are kept in position by pegs (4).
Once the digester is in use, the brickwork is under high pressure from the inside and therefore, must be supported from the outside by firmly back-filling of sand. The first two lines of brickwork are back-filled by lean concrete, mixed In ratio 1: 3: 9 as described before. Back-filling is done one day after the outside plaster is completed; layers not exceeding 30 cm are firmly rammed. Only sand or non-bounding soil is suitable for back-filling. Pure sand may be washed in instead of ramming.
The weak-ring separates the bottom part of the digester from the gas storage part. The weak-ring shall prevent vertical cracks of the bottom part entering the upper part which must be gas-tight as it is the gas storage space. Vertical cracks are diverted into a horizontal crack remaining in the slurry area where it is of no harm to the gas-tightness. The weak ring acts as a swivel-bearing allowing free movement of the above strong ring.
The weak ring is formed by a 5-7 cm thick layer of lean mortar having a mixture of 1: 3: 15 (cement: lime: sand). The top of the weak ring restores the horizontal level. It is interrupted only by the inlet pipe passing through.
Brickwork above the Weak-Ring
The upper part starts above the weak ring with a strong ring to receive tension forces from the dome. It can be seen as a foundation of the upper part of the spherical shelf. It consists of a row of headder bricks with a concrete package at the outside. In case of soft or uncertain ground soil one may place a ring reinforcement bar (Ã˜ 10 mm or 2·Ã˜ 6 mm) in the concrete of the strong ring. The brick of the strong ring should be about three times wider than the brickwork of the upper wall. In practice this would mean a width of the full brick laid in headders if the spherical wail is of quarter brick. The radius stick must be changed to reduce the radius by 4 cm for Placing the bricks of the strong ring properly.
Fig.18: Wall construction of the upper part of the dome
A row of temporary bricks is laid on the backfilling soil to mark the outer edge of the weak-ring (9). A 20 cm-wide strip of lean cement-lime mortar 1: 3: 15 is levelling the brick wall below and forms the weak ring (1). It should not exceed 7 cm in thickness. A row of full brick headder projects 4 cm to the inside, forming the base of the strong-ring (2). For that layer, the nail of the radius stick is put back, shortening the radius by 4 cm. After 4 layers of bricks (3), a wedge of concrete 1: 2: 4 completes the strong-ring (4). Then, brickwork continues up to the neck (5). Outside plaster 1: 1/4 : 4 (6) must be cured for 4 days before backfilling of soil is done (7). The soil is rammed in layers not exceeding 30 cm.
Brickwork continues after the strong ring until an opening of precisely 64 cm in diameter remains. Bricks must be chopped to get the exact round form of this size. The bricks rest now in a slanting position and might fall down before the layer forms a closed ring. Therefore, at least the first and last brick of a layer must be temporarily supported. This can be done by clamps, hooks or leaning poles.
Above the headder bricks of the strong ring, 2-3 rows (30 cm) of the following brickwork are covered with concrete of 1: 2: 4 mixture, forming a wedge to the strong-ring which Joins the outside plaster of the upper part. The concrete ring and the plaster is cured by sprinkling water for 4 days before back-falling is done and masonry work of -the neck continues. In this time construction of the expansion chamber is done.
Fig.19: Temporary support of brickwork
There are several methods to prevent slanting bricks from falling down during construction of the upper part of the sphere. The radius stick ( 1 ) supports only the actual laid brick. The last but one and the first brick of a new round must be supported. Near the top, every brick must be held in position until the round is completed. A suitable solution for the lower part above the weak-ring are poles leaning against the brick. There are some bricks tied to it for ballast (2). Sticks placed under the brick (3) are not advisable as they may be too tight and thus loosen the bond of the fresh mortar. A rope with a brick at one end and several at the other end (4) is a rather clumsy solution. Steel hooks with brick ballast (5) are the best solution, especially for the top-most rows as they leave room for the mason to work freely.
Fig.20: Concrete Blocks for dome construction If bricks are not available, concrete blocks of brick-size may be used instead. As they are difficult to chop, they should be shaped to suit the curve of the sphere. It saves mortar, if the heads (1) and the outer side (2) are angled. The shown dimensions are only a proposal and have not yet been tried out by CAMARTEC.
The Expansion Chamber
The foundation of the expansion chamber may be done like that of the digester or more simply by using a flat concrete slab of 7-10 cm thickness.
The lower part up to the overflow level is of spherical shape constructed with the help of a radius stick like building the digester. Above the overflow level, the structure continues, covering the pit in a flattened shape.
The overflow opening is the size of a manhole. During construction it is closed by brickwork laid in mud which can be easily removed after completion of the shell. For the safety of children and small animals, the manhole is provided with a cover. Only the overflow hole is left open.
The digester outlet pipe extends over the brick dome to allow poking from the outside. There is a side opening of a 20 cm height to allow slurry flowing into and out of the digester. The lower part may be plastered from the inside for better water tightness. The outside plaster is merely for beautification but also against mechanical wear and tear.
The expansion chamber can also have the shape of a covered channel. This is of advantage when slope is not enough to distribute the slurry.
Fig.21: The expansion chamber
The standard expansion chamber is of spherical shape (1), made from brickwork, plastered from the outside only. For the part above the overflow level the radius stick is reduced by 5 cm for each row as to flatten the dome (2). The outlet pipe passes through the dome in order to allow poking through from the outside. The - slurry opening (3), 2 cm above the bottom of the chamber (4), is 20 cm high and cuts out half the pipe. An expansion channel (5) is used to gain height at the point of overflow (6). It is also very handy in case of compost preparation. The volume of the expansion chamber represents the gas storage capacity. In case of a prolonged expansion channel, only the part above a 3% slope may be calculated (7). The bottom of the canal remains horizontal to allow sedimentation (8). A higher slurry level inside the expansion chamber (9) allows a smaller radius but increases the gas pressure. A higher neck is also required to keep the gas outlet pipe above the point of overflow (10). Therefore, this is not recommended.
Water-proofer is added to the cement for gas tightness. Water-proofer based on plastic is preferred over crystalline components because of greater elasticity. To obtain gas-tightness, twice the manufacturer's recommendations for water-tightness is added to the cement.
The use of water-proofer has solved the problem of gas tightness of the plaster. Any kind of surface paint is difficult to apply because the structure is still "sweating" for a long time and the surface will not be as dry as recommended for painting. Other methods, like paraffin coating require higher skill and close supervision. Bituminous paints are washed away in little time by the movement of slurry. Further, materials composed of carbohydrate are principally affected by methane bacteria.
The Neck and the Lid
The gasplant is closed on top with a removable concrete cover of conical shape and 20 cm thickness. The mixture is 1 : 3 (cement : sand ) with water-proofer added to it. Casting of the lid is done in a mould which is also used to shape the supporting surface at the neck.
The gas outlet (¢ 3/4") passes through the centre of the lid. It has a steel ring collar welded on to prevent gas leakage between the pipe and the concrete. Two handles made from iron bars are normally provided for lifting the cover. As they may hinder later tinning the wedges and pressing the lid into its clay bedding, a steel ring which folds down is preferable.
At the neck, the frame of the lid is formed by pressing the mould into the mortar bedding. For preparing the cone in proper shape, the same mould is used in which the lid has been cast.
The gas outlet should be well above the highest slurry level in order to avoid slurry particles entering the gas Pipe. Therefore, the conical support of the cover is raised 2 layers of bricks above the dome structure. For chocking the lid, three pre-manufactured devices are fixed into the brickwork.
A Ã¸ 3/4" gas pipe of 30 cm length with threads on both ends is fixed horizontally in the brickwork of the neck and later connected to the pipe coming out of the lid by a piece of flexible hose pipe. The pipe projects only 6 cm into the inner space of the neck in order not to hinder the placing of the lid. The top of the lid should be above the terrain to prevent grass roots from growing into the clay for sealing the lid.- There is a top-most lid above the water bath. The lid and the neck could also be made from pre-fabricated concrete rings but it is difficult to make them in pieces not exceeding 70 kg of weight.
Only sieved and washed river sand is to be used for any plaster. The inner plaster of the lower part of the digester is for water-tightness. It consists of 2 cm cement-lime-plaster of mixture 1: 1/4 : 4, applied in two successive layers. Its surface is wooden trowelled because a rough surface is a better growing place for the bacteria Inlet and outlet pipe should be closed with paper or rags during plastering.
Fig.22: Construction of neck and lid
(A) The neck is built with the help of sticks having the length of the inner diameter of the cylinder. Separate measuring sticks are used for the different diameters (d1, d2). When keeping the wall vertical (1), a proper round shape is controlled by rotating the stick around the centre (2).
(B) For shaping the cone to receive the lid, the same mould is used as for making the lid (3). The mould is a conical ring made from mild steel (4). Two handles are fixed for easy turning while shaping the cone of the neck. The mould might be not exactly round. Therefore, in case the gas outlet passes through the lid, the mould should be marked in order to form the cone according to the direction of the gas pipe (5). The lid has two handles of steel (min 8mm thick) which can fold down as not to hinder the fixing of the wedges. The gas pipe has a steel plate collar welded to it to prevent gas leakage alongside the pipe (7).
Fig.23: Details of the neck and the lid
The gas pipe may pass through the lid (1) or the neck (2). When it passes through the lid, it will be connected to the gas pipe at the neck by a rubber hose (11). I! the gas pipe passes through the neck below the lid, an additional layer of bricks (4) is needed to preserve sufficient height above the highest slurry level (3). The gas pipe lies lower than when passing through the lid which might result in saving a water trap
In case settling of the structure can be expected, a rubber hose connection is advisable (12).
Above the cone there are 3 pieces of 3/4" pipe (5) to receive the 14 mm steel bars (6) for wedging in the lid (7). There are two wire anchors fixed to the case-pipe (8). In case no anchors are used, an additional layer of bricks is required above (9). The inside of the lower part of the neck widens downwards to allow using a ladder without narrowing the manhole (10). The part below the lid must be gas-tight, above, it must be water-tight. The neck is covered by a removable top-most lid 01° 5 cm concrete (13). A hole of 4" diameter is kept in the centre to control the water level above the lid. It might be made by a piece of pipe which can be covered with a tin.
The plaster of the upper part has a smooth surface for better gas-tightness. It is applied in seven courses which must be completed within 24 hours. Because the mortar is to be waterproof there is no bond once the mortar has dried. The plaster consists of the following layers:
1. Cement-water brushing
2. 1 cm cement plaster 1: 2 1/2
3. Cement water brush in 9
4. 1 cm cement-lime-plaster with water-proofer mixed 1: 1/4 : 2 1/2 + WP
5. Cement water brushing with water-proofer applied consecutively
6. Cement-lime-plaster with water-proofer made of fine sieved sand, mixed 1: 1/4 : 2 1/2 + WP and applied consecutively
7. Cement screed (New) made of cement-water paste with
water-proofer, applied consecutively.
The first base for the flooring is formed by dropped mortar from bricklaying and plastering. A 3 cm cement screed (mixture 1: 1/4 : 4) applied to the ground would be sufficient In case of laterite or volcanic mourrum soil. If the structure itself is sound and solid, water losses are a temporary problem until sludge particles have sealed the surface sufficiently. In case of unstable soil, e.g. black cotton soil, high ground water table or hill water flows, a water tight floor should be achieved right from the beginning. A 30 cm thick layer of rocks covered by 5 to 10 cm of concrete might be necessary to create a proper floor before cement screed can be applied.
In areas where generally unstable soil is found, foundation and flooring is integrated by forming a conical or spherical shell of 12 cm thickness under the digester and if necessary under the expansion chamber as well. To avoid floating up of the whole biogas plant and when lowering the ground water table by pumping is not possible, the construction must be flooded until the top structure is ready and soil covering of 35 cm in height has been packed above the top of the sphere. The mason has then to work while standing in the water.
Supplement structures are inlet chambers, slurry channels and gas control chambers. It takes a surprising lot of time to construct these little items. They can be built in brickwork or made from pre-fabricated concrete. When the biogas plant is directly connected to the stable - and this should be the usual case - the inlet chamber consists of the dung and the urine/water collection box. The control chamber houses the main gas valve and the gas control or testing unit. It is a rectangular concrete box with cover to protect the accessories and is placed directly beside the neck of the digester. Pavements for wheelbarrow transport, erosion control or just for beautification are also part of the unit. They may be done from tiles of 1: 2: 4 concrete mixture and 30 30 5 cm in size.
Fig.24: Construction below the ground water table
During construction, ground water must be kept away. The foundation rests on a stone packing (1) to drain the water into the pump-sink (2). A layer of gravel prevents blockage of the drain. A vertical pipe (3) allows pumping even when backfilling has bee done. The lower part of the pipe is surrounded by a stone packing (4).
If the ground water is only slightly higher than the digester bottom or if no pump is available, the construction must be flooded. The floor must be of solid concrete (5). Water must be kept away until the outer wall has reached above the ground water table and has been plastered from the outside. One or several bottles without bottoms are placed into the concrete (6). When scooping of? water has stopped, ground water passes through the bottle and floods the floor (7). After completing the masonry work and covering the dome with soil, the bottle is closed and water might be taken off the digester bottom (8). The cap of the bottle is covered with cement mortar (9). In case of high ground water table a conical or bowl-shaped solid concrete slab is required.
Fig.25: Shape of the digester bottom slab
A flat bottom (A) is the weakest in view of statics, but it is a great advantage for the mason to work on an even floor. The centre point can be pegged-in easily (1). A conical shape (B) is much stronger and as well easy to construct. The steeper the slope, the stronger the structure. The centre point must be high above ground in line with the foot point of the brick wall (2). The strongest solution is a bowl-shaped bottom (C). The radius of the digester can be reduced because of the additional volume gained below the centre point. It is uncomfortable to work on a curved base and scaffolding becomes necessary because of the increased height to the top of the dome. Therefore, solutions B and C are used only when ground water pressure is high or the sub-soil is too soft for only a ring beam foundation.
The Piping System
Pipes should be short and straight, preferable 30 cm under ground. Biogas contains water vapour. If the gas cools below the dew point of the water vapour then condensation forms. The water always collects at the lowest point in the pipe. Therefore, pipes are laid in slope of min. 1%. At the lowest level is either the biogas plant itself or an automatic water drainage device, called the water trap. If pipes are not laid in slope and do not have a water trap at the lowest point, the gas supply system will collapse after only a few weeks. Therefore, the bottom of the pipe trenches must as well be even, otherwise water might collect at hollows blocking the gas-flow. The best way of placing a water trap is to avoid it by careful planning of the pipe line.
Fig.26: Installation of gas pipe
All joints have to be sealed by grease and hemp of 5 clock-wise turns of teflon tape (1). The water trap collects condensed- water and is needed at each of the lowest points in the pipe line (2). The length of the open water pipe should be 40 cm more than the water column of the highest gas pressure. The most common automatic water trap is a U-pipe below the line (3). Cheaper and easier to control is the asymmetric U-trap (4) where the open water pipe ends above ground. The maximum length above the pipe line is half the highest regularly appearing gas pressure. The pipe-in-pipe trap (5) functions like the U-trap (3).
Fig.27: The test-unit
The test-unit is placed near the plant in a pre-fabricated concrete frame (1). It houses the main valve (2) and two T-joints on each side (3). Normally, the T-joints are closed by plugs (4). For testing the plant (5) a pressure gauge is connected to the T-joint between the valve and the plant. The manometer is made from transparent plastic tubes fixed to a scaling board (6). The main valve is opened (7) and pressure can be read. To test the piping system (8), the pressure gauge is connected to the T-joint after the valve. The main valve is closed (9). Air is blown into the pipe by mouth or compressor (10). When the valve at the inlet point (11) is closed, pressure remains in the pipe. A pressure drop indicates leakage. For testing gas consumption (12), a gas flow-meter is installed between the T-joints (13). When the main valve is closed (14), all gas has to pass the meter and can be measured.
Gas pipes should not pass roads or trenches with potential danger of soil erosion. If this is unavoidable, pipes must be protected by concrete casing. All gas pipes are of 3/4" diameter and preferably of galvanised iron. Whenever possible they lay 30 cm under ground. Joints must be sealed by grease and hemp (not sisal!) or by 5 layers of teflon tape. Bends and junctions should be kept to the minimum because they reduce gas pressure and are potential points of leakage. Unions are especially harmful and should be avoided unless absolutely necessary. Their outer threads must be sealed with hemp or teflon, their inner threats with silicon latex.
For long distances without junctions or joints, PVC or PU pipes which are suitable for underground installation may be used. They are cheaper, but pipes of smaller diameters might be attacked by rodents. Care must be taken by Joining plastic pipes with G.I. pipes. Avoid flexible hose pipes, if not avoidable, use fibre reinforced material.
Before fitting the pipe, dust must be blown out of each piece of pipe or fitting. Pressure tests are to be undertaken for every 30 m of piping installed. If the gas pressure of 1.40 m W.C. does not hold for 10 minutes, all joints must be checked for leakages by help of soap water. Pressure tests can only be carried out under steady temperature conditions. Direct sunlight and alternating cloudy periods have great Influence on the temperature and hence, gas pressure inside exposed pipes. The final pressure test is done with all the accessories connected.
For gas production-, pressure- or leakage control a test unit is permanently Installed directly behind the gasplant inside the control chamber.
Filling and closing the plant
The initial filling of the gasplant has to be done via the inlet pipe in order to avoid bulky substrate entering the plant which later might block the outlet pipe.
The lid should not be closed until the plant is filled above the Inlet opening, preferably after it is filled up to the level of the bottom of the expansion chamber. Sealing of the lid is done by fine clay applied on the supporting surface at the neck. In order to keep the clay moist the lid will remain constantly under water when the gasplant is in operation. The lid is chocked with wooden wedges in order to resist the gas pressure from below and to press the sealing clay firmly between rest and cover.
Fig. 28: Closing the lid
The lid is sealed by clay which is kept moist by a water bath above. The clay is dried and groined before being mixed with water into a putty like paste. It is then applied by hand approximately 2 cm thick ( 1). The lid is placed into the clay bedding and rammed in (2?. Iron bars are put into the case-pipes of the neck and the lid is fixed by wooden wedges (3). In case the gas pipe passes through the lid, the pipe is connected by a rubber hose, The neck is then filled with water (4) and the topmost lid is laid above (5).