Pumping of liquids

Abstract

 There is disclosed a method of pumping a liquid using a pump comprising a piston (1) in a cylinder (12) with an inlet (16) and an outlet which is connected to a discharge pipe (5). The method comprises the steps of moving the piston (1) down the cylinder (2) to draw the liquid into the cylinder (2) and moving the piston (1) up the cylinder (2) to force the liquid into the discharge pipe (5) and adjusting the velocity of the upward stroke of the piston (1) to impart a momentum to the liquid in the discharge pipe (5) sufficient to sustain the movement of the liquid during the downward stroke of the piston (1).

Description

[0001] This invention relates to a method of pumping liquids and in particular to a piston-driven inertia pump which finds application in the pipelining of liquids which contain solid particles such as sand or slurry in suspension.

[0002] Single or multi-cylinder piston pumps are known in which, in the part of the cycle when the piston is being withdrawn, the pumping action is provided by a large surge vessel or more usually by additional piston/cylinder assemblies. The usual requirement is for three single-acting or two double-acting cylinders, probably together with a surge vessel. British Patent 114 170 to Constantinesco describes a pump using a piston working for a part of its stroke on the inside and for the remainder of its stroke, on the outside of a cylinder. The inertia of the liquid expelled from the pump draws in more liquid when the piston is out of the cylinder. The liquid on the high pressure side of the piston is, however, subject to the normal cyclical surge of the pump and the inertia of this liquid is not utilised to facilitate pumping.

[0003] It is an object of this invention to provide an arrangement including a simple pump which requires little maintainance and has a high hydraulic efficiency.

[0004] The invention provides a method of pumping a liquid comprising the steps, in a pump comprising a piston in a cylinder with an inlet and an outlet which is connected to a discharge pipe, of moving the piston down the cylinder to draw the liquid into the cylinder and moving the piston up the cylinder to force the liquid into the discharge pipe characterised in that the velocity of the upward stroke of the piston is adjusted to impart a momentum to the liquid in the discharge pipe sufficient to sustain the movement of the liquid during the downward stroke of the piston.

[0005] In order to achieve the desired cycles of the invention, a ratio of the velocity of the power stroke to the velocity of the return stroke of,the order of 1:4 is preferred, but it will be realised that other ratios may be necessary having regard to the particular circumstances. The power stroke should be such that a shock wave in the pipeline is avoided, and the return stroke should be rapid enough not only to enable upstream fluid to join the downstream fluid and to prevent significant decay in the velocity of the downstream fluid, but also to prevent uncontrolled settlement of solids in cases of movement of slurries in the pipeline.

[0006] The adjustment of the velocity of the piston is preferably obtained by the use of a double-acting hydraulic cylinder and piston arrangement which is connected to the piston of the pump, the hydraulic fluid being supplied to the hydraulic cylinder, for the power stroke, with a low flow and under high pressure, while the fluid is supplied to the hydraulic cylinder, for the return stroke, with a high flow and under low pressure. This would give a rapid "fly-back" on the return stroke which would reduce time during which momentum is lost and minimise the velocity decay of the downstream fluid.

[0007] This effect might alternatively ., or additionally, be achieved by so adjusting the relative cross-sectional areas of the piston rod and piston of the hydraulic cylinder so that the return stroke would be more rapid.

[0008] Further according to the invention a non-return valve is provided in association with the pump, the valve controlling the flow of the downstream fluid, closing on the power stroke and opening on the return stroke.

[0009] In a preferred form of the invention the valve may include a buoyant closure member mounted for movement along a substantially vertical axis, inflow of the downstream liquid causing the un-seating of the closure member and the buoyancy of closure member re-seating the latter upon cessation of flow. Simple guides would be all that is required for the closure member thus avoiding bearings, pins, operating levers and the like. The buoyant closure member, or float, may be covered with abrasion-resistant rubber, while the valve chamber and an inlet chamber may be abrasion-resistant castings. The cylinder itself may be lined with ceramic material.

[0010] The pump may conveniently be provided with means to allow clean water from a clean water supply into the annular gap between cylinder wall and the piston. In this way the piston / cylinder gap would be purged with a clean water supply.

[0011] The clean water is preferably supplied to the piston which is provided with one or more outlets into the piston/ cylinder gap. The clean water supply should preferably be a high pressure supply and in the preferred form of the invention the clean water is arranged to be discharged around the periphery of the piston on the high pressure side of a high pressure sealing elenent which is arranged for sealing, sliding contact with the cylinder walls. In this manner a continuous flow of water away from the seal is ensured so that no solids can reach the seal.

[0012] The piston may be provided with a low pressure seal and an outlet for clean water from a relatively low pressure supply between the high and low pressure seals. This will ensure lubrication of the low pressure seal and further prevent ingress of solids from the slurry or dust from outside. In this manner the wear rate of the piston/cylinder/seal assembly would be kept very low.

[0013] It will be appreciated that the whole assembly can be arranged in alternative positions, but if the axis of the valve is not vertical, the advantage of the simple valve described is lost. An elbow may however be inserted between the inlet chamber and the cylinder so that the piston travels vertically. The advantage would be that there would be no lateral load on the piston seals.

[0014] The pump described above is suitable for a predominantly horizontal system, that is where the static head is small compared with the frictional head. A further embodiment of the invention provides an adaptation of the pump making it suitable for vertical lifts.

[0015] In the further embodiment of the invention the pump is provided with an additional outlet valve and a vessel filled at least partially with a gas, while the discharge line is arranged to be substantially vertical, the vessel being downstream of the outlet valve in the discharge line and the gas therein being compressible to allow ingress of the liquid into the vessel on the delivery stroke of the piston, and the outlet valve being arranged, upon the expansion of the gas in the vessel during the return stroke of the piston, to prevent the ingress of the liquid into the pump.

[0016] The closure member of the outlet valve may conveniently have a negative buoyancy, in which case the direction of travel thereof should be substantially vertical.

[0017] The gas filled vessel, or surge vessel, may be heat insulated and the gas therein may be chosen to minimise thermal losses due to the compression and expansion of the gas. The gas would preferably be nitrogen and a layer of a suitable fluid, arranged to float on the liquid being pumped, could reduce nitrogen solution .

[0018] The invention is further described with reference to the accompanying drawings in which:-

Figure 1 is a section, in elevation, of a piston pump according to the invention; and

Figure 2 is a diagrammatic section, in side elevation, of an alternative piston pump according to the invention.

[0019] In Figure 1 a piston 1 is shown travelling in its power stroke in a cylinder 2. A valve 3 is shown closed by the pressure produced by the piston 1. Fluid contained in the inlet chamber 4 is expelled via the pipeline 5. When the piston 1 starts travelling on the return stroke in the cylinder, the valve 3 is forced open by incoming fluid from the downstream side 6 and.-this fluid divides in the inlet chamber, some flowing into the pipeline 5 and some into the cylinder.

[0020] The next power stroke of the piston imparts momentum to the column of fluid in the discharge pipeline 5. At the end of the power stroke the piston reverses, but the inertia of the column of fluid maintains motion in the pipeline in the original direction.

[0021] The valve incorporates a floating element 7, the direction of travel of which is substantially vertical. In this manner the operation of the valve is extremely simple, simple guides 3a being all that is required.

[0022] The piston/cylinder gap is purged with a clean water supply from a line 12 which conveys high pressure water to the piston crown. This water is discharged around the periphery of the piston on the high-pressure side of the high pressure seal 9 and ensures a continuous flow of clean water away from the seal along the annular gap between the piston and the cylinder so that no solids can reach the seal. The gap between the high pressure seal 9 and the low pressure seal 10 is fed with clean water from a relatively low pressure line 13, ensuring lubrication of the low pressure seal 10 and further preventing the ingress of solids from the slurry or dust from outside.

[0023] The stroke and velocity of the piston 1 is regulated by means of a double acting hydraulic piston and cylinder assembly 8 connected to the piston 1 by means of a piston rod 16. The supply of hydraulic fluid to the cylinder 8 for the power stroke, and the supply for the return stroke could be regulated as follows:

On the power stroke hydraulic fluid is supplied to the cylinder 8 via the line 14 with a high pressure and a low flow to ensure a smooth acceleration of the piston, while, on the return stroke, low pressure and high flow through the line 15 would give a rapid "fly-back" reducing the time during which momentim is lost in the discharge line 5 thus minimising the velocity decay.

[0024] Alternatively, or in addition, this effect may be achieved by adjusting the relative cross-sectional areas of the piston rod and piston of the hydraulic cylinder so that the return stroke will be more rapid, even to the point that fly-back requires no other provision.

[0025] The various functions of the pump may be automatically controlled by interposing a micro processor in the pump circuits, providing a feedback control from the pipeline flow velocity and pressure to the control valves of the hydraulic power unit.

[0026] Charging of the liquid to be pumped, which, in the examples described is a slurry, would be expected to be by gravity from a hopper or sump or could advantageously be by means of a conventional low-head pump. The use of a jet pump would be preferable as the concentration of the slurry could thereby be precisely regulated and the reliability of jet pump would match that of the pump described. In addition, closure of the valve 3 would not cause damage to the charging pump and would not involve complicated circuits with by-passes and additional valves.

[0027] In Figure 2 the arrangement has been adapted to make it suitable for vertical lifts. The pump 100 in Figure 2 finds particular application in the pumping of mine water which, naturally, is contaminated with grit.

[0028] On the power stroke of the piston 102, the buoyant closure member 108 of the inlet valve 106 is closed, a closure member 114, which has negative buoyancy, of an outlet valve.112 is opened, slurry is discharged via the pipeline 105 and a surge vessel 116 is charged, by the pressure produced by the piston. In Figure 2 the piston 102 is shown during its return stroke in the cylinder 104, the inlet valve 108 is opened by the action of the incoming slurry and the inlet chamber 110 is recharged. The outlet valve 112 is closed by the pipeline pressure and the gas 126 in the surge vessel 116 expands and expels liquid into the pipeline 105 to maintain the flow.

[0029] It will be seen that the thrust of the piston imparts momentum to the column of liquid in the pipeline and compresses the gas in the surge vessel 116. At the end of the power stroke the piston reverses, but momentum and gas expansion maintain motion in the pipeline in the original direction. The original velocity of the column decreases under the influence of gravity and frictional forces. The system will work satisfactorily provided the following stroke starts before too much momentum is lost, to the point where excessive solids settlement coccurs or efficiency becomes unacceptably low. Shock waves (hammerblow) which might be caused by the acceleration of the column or the closure of the valve 112 are prevented by the action of the surge vessel.

[0030] The gas 126 in the surge vessel 116 is nitrogen, which would minimise thermal losses due to the compression and expansion of the gas. A layer of a fluid in which nitrogen is insoluble may be arranged to float on the fluid in the surge vessel to reduce nitrogen solution and consumption. The nitrogen is replenished at an injection point 124, the injection of the nitrogen being controlled by a sensor 118 equipped with a delay to prevent operation due to the fluctuations associated with the normal pumping cycle. A low level sensor 120 and a high level sensor 122 would operate an alarm in case of failure of the gas system or supply.

Claims

1. A method of pumping a liquid comprising the steps, in a pump comprising a piston in a cylinder with an inlet and an outlet which is connected to a discharge pipe, of moving the piston down the cylinder to draw the liquid into the cylinder and moving the piston up the cylinder to force the liquid into the discharge pipe characterised in that the velocity of the upward stroke of the piston is adjusted to impart a momentum to the liquid in the discharge pipe sufficient to sustain the movement of the liquid during the downward stroke of the piston.

2. A method according to claim 1 characterised by the specific steps of adjusting the initial velocity of the upward stroke of the piston to a speed below a predetermined velocity above which the formation of a shock wave in the discharge column would occur, and adjusting the final velocity of the upward stroke of the piston above a predetermined velocity, at which velocity the settlement of solids in the discharge column would occur.

3. A method according to either of the preceding claims which utilises a double-acting hydraulic cylinder and piston arrangement which is connected to the piston of the pump characterised in that the hydraulic fluid is supplied to the hydraulic cylinder, for the upward stroke, with a low flow and under high pressure, while the fluid is supplied to the hydraulic cylinder, for the downward stroke, with a high flow and under low pressure.

4. A pump comprising a cylinder formed with an inlet, which includes a non-return valve and an outlet
which is connected to a discharge pipe, a piston slidably located within the cylinder and means to control the velocity of the piston, characterised in that the inlet valve includes a buoyant closure member mounted for movement along a substantially vertical axis for inflow of liquid to cause un-seating of the closure member and the buoyancy of closure member re-seating the latter upon cessation of flow.

5. A pump according to claim 4 characterised in that an elbow is provided between the inlet chamber, and the cylinder, the piston travelling vertically during operation.

6. A pump according to either of claims 4 or 5 which is provided with an additional outlet valve and a vessel filled at least partially with a gas, the discharge line being arranged to be substantially vertical, the vessel being downstream of the outlet valve in the discharge line and the gas therein being compressible to allow ingress of the liquid into the vessel on the delivery stroke of the piston, and the outlet valve being arranged, upon the expansion of the gas in the vessel during the return stroke of the piston, to prevent the ingress of the liquid into the pump characterised in that the closure member of the outlet valve is of negative buoyancy, the direction of travel thereof being substantially vertical.

Drawing