The nozzle is the part that guides the water into the runner. The runner will work most efficiently if all the water hits its circumference at the proper entrance angle (see box 4.7). The bent side (see fig. 4.15 and fig. 4.17) has a special curved form to achieve this.
The 4 sides of the nozzle can be made from 2 mm mild steel. This is easy to cut and bend, while it is just thick enough for electric arc welding using 2.5 mm rods. If 2 mm steel is not available, thicker steel can be used. Then make the nozzle in such a way that the inner dimensions remain the same so that the flow and power output will still fit with the characteristics given in par. 4.1.
The runner side and bent side are just rectangular pieces, see fig. 4.15 for the dimensions. The free side and alternator side have a more complicated form which can be copied onto the material in the same way as with the side disks: Make copies of fig. 4.15, glue them onto the steel sheet and mark the relevant points with a centre punch. Arrange the 2 pieces of paper in such a way on the steel that the centre of the runner can also be marked since this makes it easier to mark the inner radius with compasses or the modified vernier callipers.
|Fig. 4.15: Sides of the nozzle.|
As cut out, the alternator side and free side have a radius of 37.5 mm, leaving no clearance with the runner. When fitting the assembled nozzle onto the runner, these edges will be filed off or grinded off a little until it fits well with only a small clearance. In this way, leakage of water through this clearance will be minimal and thus efficiency will be best.
When alternator side and free side are cut to shape as described above, welding them onto the bent side may become tricky since near the tip, there is so little material left. Then the whole tip may melt and end up in a large droplet of steel, meaning that the carefully designed shape is lost. So when cutting out the free side and alternator side, better leave a lump of material near the tip (not drawn in fig. 4.15. This edge must be filed off before assembling the runner sides. Having this rounded edge is not essential but without it, the charger will consume roughly 10 % less flow than calculated in par. 4.1 and consequently power output will be 10 % lower (but it might still be higher than expected if overall efficiency turns out to be better than expected).
When making the sides, take care not to damage the inside surfaces. This is especially important for the areas near the runner, where water will have very high velocities and any disturbances will cause high friction losses. So for bending the bent side, do not hammer directly at the inside surface. Better find a piece of pipe that has about the right radius and hammer the tip of the bent side around it while holding a heavy object against the other end. Use the free side or alternator side as a reference. For easy welding, the bent side should fit closely against the free side and alternator side. This might take quite a while and if it looks too bad, hammer it flat and try again. Bending it to a perfect fit is nearly impossible so for welding, have it clamped against the alternator side and bent side.
The free side has a hole in it through which the blocking timber can be fixed with a wood screw. When there is no blocking timber, this hole should be closed and therefor an M6 thread is tapped into it so that a short bolt fits into it. When assembling the four sides of the runner, check with the runner which side will become the free side. This depends on the direction of rotation of the alternator.
The shape of a blocking timber resembles closely the shape of the alternator- or free side of the nozzle. In fig. 4.16, two shapes of blocking timber are drawn. The difference between the two is that the more complicated type guides the water right up to the tip of the nozzle. This means that the water jet coming from the nozzle will be equally wide everywhere and therefor the force exerted on a runner blade will be constant.
|Fig. 4.16: Two possible shapes for a blocking timber.|
With the simple wedge-shaped type, the water jet is not guided all the way. Once the water jet has passed the edge of the blocking timber, it can expand in width as the bent side tries to deflect it towards the runner. This means that near the tip of the nozzle, runner blades are exposed to a wider water jet (in axial direction) and thus to larger forces than expected based on the thickness of the blocking timber. Therefor the more complicated type (top drawing in fig. 4.16) is needed when forces on blades are becoming critical, so at a head of 15 m or higher. Because it guides the water jet better, this type will also give a slightly higher efficiency.
Addition to internet version:
Instead of fitting a suitably sized blocking timber inside the nozzle, one could also adapt nozzle design such that the flow through it, can be adjusted. This can be done by making part of the inward side hinge towards the bent side. See:
for a design. Such an adjustable nozzle comes in handy for:
- A demonstration charger that will be installed temporarily at sites with different heads.
- When available flow is less than the charger needs. Then one could adjust the nozzle such that it takes just a bit less flow than the canal provides so that the penstock remains full of water and the charger produces some power. Without reducing the flow, air would enter the penstock pipe, the charger would operate at a reduced head and a reduced flow and power output would be minimal.
The space between the alternator side and free side must be 51 mm (nozzle width is 51 mm). The bent side and runner side are 53 mm wide so at each end, they overlap 1 mm with the alternator side and free side. This way the bent side and runner side overlap enough to make assembly easy, while still there is enough space left open to make a strong weld that penetrates nearly the whole thickness of the material.
Assembling the 4 sides is a job that is best done by 2 people. One can hold two parts in proper position with respect to one another while the other makes two small welds just to fix them. For easy, accurate working, sides can be clamped onto a block of steel or wood with a right angle. Especially for welding the bent side, it is a great help to have someone press it down so that the seam closes completely. Only when all sides are fixed and all angles are checked, the seams can be welded completely.
|Fig. 4.17: Nozzle sides assembled.|
Onto the assembled runner sides, an extension pipe can be welded that serves the following purposes:
The easiest way is to use a piece of 2.5" pipe with a 2.5" conical pipe thread cut into it on one end, see fig. 4.18. Such pieces of pipe with screw thread are made to order by specialised workshops. In the Philippines, hardware shops sold `standpipes' that consisted of a short length of pipe with thread at both ends, which were long enough to cut into two and use both ends. Or maybe somewhere there is a long length of pipe with screw thread at the ends that serves no purpose and the owner will allow you to cut it off. For an alternative if such a pipe cannot be found, see box 4.12.
For making the rectangular cross section, the pipe is cut in some 10 mm lengthwise at the places where the corners will come. These cuts will make that the pipe will bend easier there. To make sure that the screwthread end will retain its shape during hammering, screw the socket (see below) onto it. Find a heavy hammer, hold the pipe over the wedge-shaped point of an anvil (or another heavy object) and hammer on the triangular areas that should become flat, see fig. 4.18. Once it fits well, the nozzle itself can be welded onto it and the lengthwise cuts have to be welded again. If the pipe is galvanised, first file off the zinc layer up to 10 mm from the weld itself to prevent poisonous fumes to come off. The same goes for welding on the socket, if that was galvanized.
|Fig. 4.18: The extension pipe.|
Now a 2.5" socket can be screwed onto the nozzle and on this socket, an especially made piece of pipe can be welded that fits tightly into the PE (Poly-Ethylene, a kind of plastic) penstock pipe. If the size of the penstock pipe is known already, this piece can be made right away. Often PE pipe is not completely round so measure the largest and smallest diameter and take the mean to find an accurate figure. Multiply this by 3.14 to find the circumference. Cut out a piece of 2 mm steel sheet of about 100 mm wide and as long as the circumference that was just calculated. Bend it round, weld the lengthwise seam and carefully file it flat to prevent leakage. If the diameter of this pipe differs too much from that of the socket, it can not be welded onto the socket directly. Then make a ring of ca. 8 mm steel rod that bridges the gap between them and weld the 3 parts together. To make it easier to fix or take loose the socket from the charger, one could weld handles onto it. Take care to make only light welds since welding at one point will make it lose its shape. In fact one small piece of steel welded on somewhere around the circumference is already enough. One could hook on a rope there, wind it round the socket one turn and fix it to a long piece of wood. Now the piece of wood can be used as a lever for turning the socket.
Because of the thickness of the steel sheet itself, the piece of pipe onto which the PE pipe will be fixed, will have an outer diameter that is 2 mm larger than the inner diameter of the PE pipe. This assures a good tight fit but it can be difficult to get it into the penstock pipe. So bevel the end and also the inside edge of the penstock pipe. If it still doesn't go in far enough, heat up the PE pipe in hot water or hold a piece of board over the socket and hammer it into the PE pipe.
Disadvantages of a screwthread connection are:
|Box 4.12: A flanged joint.
Instead of using 2.5" pipe with a ready made pipe thread, one could make flanges, see fig. 4.19. Flanges of 3 mm thick steel sheet, an outer radius of 60 mm and 6 M6 bolts at a radius of 50 mm should do the job. To make it waterproof, fit some layers of thick paper in between. The flange on the nozzle should have an inner radius of 38.2 mm (or 36.7 mm if you make the extension pipe out of 2 mm steel sheet, see below). The inner radius of the other flange follows the radius of the piece of pipe onto which the penstock pipe is fitted. Have the pipes stick into the flanges only about 1 mm. In this way, they can be welded both at the outside and at the inside.
To connect the nozzle to its flange, still a piece of 2.5" pipe could be used and then this should be cut in lengthwise and hammered to a rectangular cross section like described above. But now a piece of only 65 mm long will do since there is no screw thread that can get deformed when bending it to a rectangular cross section
Instead of 2.5" pipe, one could also make a piece of pipe from a piece of 2 mm (or thicker) steel of 65 x 224 mm. Plan on having the seam right through the middle of one of the 4 triangular flat areas. Then measure off where the corners of the nozzle itself will end up and draw the triangular areas that should remain flat. Bend it to shape by hand over a protruding piece of steel. The lines in fig. 4.18 show where the strip should bend in order to get a round cross section at the flange side. With 2 mm steel, there is little need to cut it in at the corners in order to have it bend easier but there is no harm in it either. Making this piece of pipe from steel sheet has the added advantage that the charger will end up a little lighter and more compact.
Making a tricky connection just to keep chargers interchangeable might seem a bit ridiculous if this is the first charger to be built and there is no other one within thousands of km. Still it is worthwhile to think ahead, maybe in 2 years time there are quite a few running. Also chargers will be are installed in remote villages where no welding can be done. Then it is handy if a spare charger could be fitted to the penstock pipe if the first one needs repair.
Another reason for such a removable connection is that with small penstock pipes, it might be difficult to fit and remove a blocking timber through the narrow piece of pipe where the penstock pipe fits on. Small size penstock pipes can only be used at quite high head and under these conditions, the widest blocking timber is needed.
To keep chargers interchangeable, of course all chargers in one area should have either a screwthread and socket connection or a flanged connection.