PUMP CONSTRUCTION
The single-acting pump has normally a 3 inch (80mm) PVC barrel and 1.5 inch (40mm) inlet and outlet connections. The 3 inch barrel can be altered to 2 inch for high heads and 4 inch for very low heads. The piston is a single 3 inch leather cup washer with a mild steel back plate and a tapered wooden disc (tufnol or other non-water-exposure-warping plastic might be more suitable in prolonged useage) in front, which is tightened into the cup washer by a nut on the end of the pushrod.
The function of this tapered wooden disc is to produce a grip between the leather cup washer and the PVC barrel which is tight enough to provide good suction and a low seepage, without being so tight as to reduce efficiency below an acceptable level. An appropriate solution is obtained by fitting a suitable number of steel washers between the cup washer and the disc.
A priming system is incorporated to cope with suction heads of greater than 5 metres, or in caes where the leather cup washer has worn and has not been adjusted or replaced.
For short suction lines, only a delivery valve and a footvalve should be required; for longer suction lines then a third non-return valve at the pump inlet is probably necessary. A commercially available footvalve and outlet valve could be used, but for the prototypes so far built at Morogoro, locally-made ball valves were constructed, using solid rubber balls sealing in conical seatings machined in standard pipe reducing bushes. Each valve ball is restrained when open by a mild steel screening which is brazed to the upper reducing bush using oxyacetylene and copper brazing rods.. These features can be seen by reference to the accompanying drawings.
For the inlet pipe, 1.5 inch low-pressure grade galvanised steel pipe (GSP) or ployethyelene (PE) pipe is recommended. PE fittings in Tanzania in 1976 were more expensive than GSP fittings but the pipe itself was cheaper per metre, so for anything over 10 metres length PE was usually cheaper; PE is also easier to lay and has lower pipe friction and therefore pumping losses. However if a PE inlet pipe is used then the footvalve should be fastened to an anchored timber float to prevent it from resting on river or pond bed and sucking in mud and silt.
For the delivery pipe, 1 inch or 1.5 inch low pressure PE or GSP is recommended. 1.5 inch is more expensive but has lower pumping losses and less operator effort.
The PVC barrel is held tightly between the 2 steel flanges by 4 x 10mm diameter mild steel tension studs or bolts (home-made studs from 10mm mild steel bar threaded with 10mm dies and using commercial nuts is a good quality and cost solution). The barrel is sealed against the front flange by means of a rubber sealing ring sitting in the machined (lathe) flange groove. Such sealing rings can be bought commercially or better still can be cut from old vehicle tyre inner tubes.
The unit which comprises the front flange, the inlet and outlet connections, and the priming system (if required) is fastened to the main frame of the machine, and can be made in either of 2 ways:
The original design was made entirely from standard pipe fittings, and was clamped to the machine frame by 2 locally-made mild steel U-bolts. In order to save costs, the modified design (as shown here) was made entirely of standard steel sections and galvanised steel pipe (GSP), welded and brazed into an assembly, and welded to the machine frame. This design resulted in greatly reduced material costs, but requires more skilled man-hours. Which design is chosen will depend on local skill availability and relative pricing of skilled labour and pipe fittings.
PUMP DRIVE MECHANISM..............................................................Pump100.htm
PUMP PERFORMANCE......................................................................Pump110.htm
DRIVE MECHANISM COMPARISON..............................................Pump120.htm
PRODUCTION COSTS........................................................................Pump130.htm
OPERATIONAL COSTINGS...............................................................Pump140.htm
SUMMARY AND CONCLUSIONS....................................................Pump150.htm
REFERENCES......................................................................................Pump160.htm
APPENDIX........................................................................................... Pump170.htm
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