At the site an explanatory sheet is provided which purports to show how the pump operates (Fig 2, right). It seems that the pump is described as a compound system which raises the water pressure to the final high level output in two stages, using first a low pressure (LP) cylinder, and then a high pressure (HP) cylinder. But this is entirely misconceived. Each of the six pumps is in fact a double acting ram pump which increases the pressure of the water in one step. The following notes describe the pumps and provide an accurate explanation of their operation.
Each of the six individual pumps has two piston-cylinder combinations, one at each end of a single iron casting which forms a common cylinder block. The two pistons (or rams) are connected together by side-bars which ensure that the strokes of the pistons are strictly equal: as the inner piston enters its cylinder, so the outer piston is withdrawn from its cylinder by an equal amount.
The piston and cylinder which is nearest to the fly-wheel and crank (the inboard cylinder) has a greater diameter than the outboard piston and cylinder, and it is this difference in size which has lead to the assumption that the cylinders operate at different pressures, and to the idea of referring to the outboard cylinder as the HP cylinder and the inboard cylinder as the LP cylinder. Compound compressors used for compressing gasses can work on this principle, but water pumps cannot, the reason being that water is very nearly incompressible.
The significance of this point becomes clear when stage 'D' of the cycle is considered according to the original explanation (Fig 2). The diameter of the 'LP' piston is greater than that of the 'HP' pump and the strokes of both are the same. Therefore, as the 'LP' piston moves to the right, the volume delivered by the 'LP' cylinder exceeds that received by the 'HP' cylinder and this excess volume must be accounted for - it must go somewhere. The explanatory chart (Fig 2) does not acknowledge this difficulty and does not tell us where the excess volume goes. In fact the explanation is very simple; the delivery valve (outlet valve 2) opens, and the excess volume of water is delivered directly into the high pressure receiver. This is illustrated in stage D of Fig 3. If the volume of the 'LP' cylinder is twice that of the 'HP' cylinder, exactly half of the volume pumped out of the former will be used the fill the latter, and the remaining half will form the output of the pump during this stroke. Then, returning to stage 'A', the contents of the 'HP' cylinder will be pumped out during the return stroke, and the result will be equal volumes of high-pressure output on each stroke of the pump. The machine is thus a double-acting single stage pump.
An interesting aspect of this type of double-acting pump is that it can be made with just two valves; the non-return valve marked 'Outlet Valve 2' on Fig 3 is actually redundant, and all that is needed is the inlet valve to the inboard cylinder, and the non-return valve in the transfer port between the two cylinders (outlet valve 1). The latter is sufficient to prevent water from entering the inboard cylinder from the high-pressure receiver when the pistons move to the left. This economy of parts gives this type of double-acting pump an economic advantage over the more conventional type of double-acting pump which has two identical ram pumps, each with a single cylinder and two valves, operated by cranks set at 180 degrees. The fact that our pump has only one crank represents a further economy of design. However, in the present installation a non-return valve in the position of 'Outlet Valve 2' might be useful in order to isolate the pump from the high pressure receiver when the pump is switched off.
The pumps and their associated pipework are not exactly rocket science, but are nevertheless enigmatic. It is not at all easy to trace the flow-path and to identify with certainty the functions of the various parts. We know that there must be an inlet valve, and also a second valve between the two cylinders, but without an extensive dismantling of the mechanism, or access to historic technical drawings, one must identify the parts and interpret their function using a certain amount of guesswork, assisted by whatever engineering insight one can bring to bear.
Fig 2. How the pumps work according to the existing explanation.
Fig. 3. An alternative explanation of how the pumps work. They are double-acting single stage pumps
| The close-up view (Fig 4, right) gives a good view of the various parts of one of
the pumps - click on it to see the picture full-size. |
An eye-catching feature of each pump is a large and prominent air chamber in the form of a domed cylinder (marked '1' in the lower picture, right). Air chambers of this kind are usually attached to the discharge pipe of reciprocating pumps in order to produce a more steady flow in the discharge system, and to minimise pressure variations at the outlet of the pump. Without such a reservoir, the speed of flow of water in the delivery pipe varies according to the movement of the piston, and the corresponding acceleration and deceleration of the mass of water in the delivery pipe causes peaks and troughs of pressure at the outlet of the pump, which could be potentially damaging. So it is natural to assume that the air chambers serve this function, and are attached to the final output of the pumps. But while discussing this with the engineering staff, I was assured that the air-chambers are actually attached to the inlet system, and this seems to be the case when one examines the two pictures (right) closely. The outlet pipe (with a pressure guage attached) and the delivery pipe (horizontally connecting the two sides of each installation) are clearly identifiable, and they do not connect with the single, centrally located air chamber.
Similarly, the two inlet valves, one on each side of the air-chamber (visible in Fig 4) seem to be more complex than a simple non-return valve, and this suggests that they incorporate something more complicated. Likewise the part where the pressure guage is attached seems to be more than a simple piping elbow, so perhaps it contains a delivery valve after all. It seems that there are still some aspects of these pumps which need further explanation, but I feel that this discussion might make a useful contribution to identifying the type of pump we are looking at, and understanding the various parts of the mechanism.
Fig 4. Detail of one pair of Pumps.
Fig 5. Showing Air Chamber (marked '1') and possible redundant 'Outlet Valve 2' beneath the pressure guage.