WO2010078627A1

WO2010078627A1 - An improved pump system

Info

Publication number: WO2010078627A1
Application number: PCT/AU2010/000016
Authority: WO
Inventors: John Joseph Garland
Original assignee: John Joseph Garland
Priority date: 2009-01-07
Filing date: 2010-01-07
Publication date: 2010-07-15
Also published as: AU2009238321A1

Abstract

A system and method for pumping a fluid over an at least partially vertical distance is provided. The fluid, such as water, is pumped through a conduit having a substantially vertical portion and gas, such as air, is supplied at or near a bottom region of the vertical portion of the conduit. The addition of the gas to the fluid can advantageously decrease the total dynamic head of the pump in the system, allowing it to operate more efficiently and/or at higher flow rates.

Description

AN IMPROVED PUMP SYSTEM

FIELD OF THE INVENTION

The invention relates to systems for pumping fluids, and more particularly to systems for pumping liquids vertically by supplying or injecting a gas.

BACKGROUND ART

Mere reference to background art herein should not be construed as an admission that such art constitutes common general knowledge in relation to the invention.

Pumps are used to move fluids, such as water, over a distance. A limitation of pumps is that the pump output can only overcome a limited amount of resistance. This is particularly noticeable when pumping a fluid vertically against gravity, such as in a mine.

Mining operations are often subject to large volume inflows of water, and the removal of ore can not usually start if a shaft or area contains water. Typically the mining is delayed until the removal of water is complete as access to the ore bearing surfaces is impossible when pits and/or shafts fill with water which occurs during wet weather.

It is therefore desirable to remove fluids from the mines as quickly and efficiently as possible. This is usually removed by pumping the fluid from the mine to another location. However, a typical pumping operation may require water to be pumped a vertical distance of 10Om₅ or more, and there is a limitation on how far a pump can raise a fluid. If pumping over a greater distance is required, either multiple pumps and/or greater power/capacity pumps need to be employed. This increases costs and the complexity of pumping operations, and can further delay the time it takes for a fluid to be moved.

The higher the capacity of the pump, the more likely cavitation will occur. As an impeller's blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapour pressure, the fluid vaporizes and forms small bubbles of gas. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades. Cavitation in pumps has a number of deleterious effects including:

• reduction in pump capacity;

• a reduction in the head of the pump; and

• damage that can be seen on the pump impeller and volute.

Furthermore, higher capacity pumps are physically much larger and require more electricity to operate, increasing power usage and ultimately costs. In some cases if the load increases after the pump has been installed, the pump may be run outside its recommended operational range, decreasing efficiency and, possibly, causing damage to the pump.

In the case where a booster pump is used, a more complex control system is typically needed to balance the pressures and flow rates at each pump so that both operate around their best efficiency point (BEP). Additionally, having a boost pump results in a system that is usually less reliable and more costly to manufacture and maintain.

It is an aim of this invention to provide an improved pumping system which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides a useful alternative.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a system for pumping a fluid over an at least partially vertical distance, the system comprising: a pump for pumping a fluid through a conduit having a substantially vertical component; and a gas supply for supplying a gas to a diffuser that diffuses the gas into the fluid, wherein the diffuser is located at or near the bottom of the substantially vertical component of the conduit.

According to a second aspect of the invention, there is provided a method of pumping a fluid over an at least partially vertical distance through a conduit having a substantially vertical component, the method comprising the steps of: pumping a fluid through the substantially vertical component of the conduit; and supplying a gas to the conduit through a diffuser that diffuses the gas into the fluid at or near the bottom of the substantially vertical component of the conduit.

As an additional advantage, the injection of air into an inlet of the pump can also be used to decrease cavitation, particularly in high capacity pumps.

The pump may be used with the conduit on either the inlet or the discharge side of the pump. This will typically be determined by the pump's location. For example, when the pump is located at or near the top of the vertical portion (e.g. on the surface in the case of a mining operation), normally the conduit will be on the inlet or suction side of the pump. When used in this configuration, it is preferred that suction cavitation is minimised or avoided entirely.

Suction cavitation occurs when the pump suction is under a low-pressure/high- vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer experiences vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.

When operated with the conduit on the discharge side of the pump (e.g. when the pump is located at the lower end of the vertical portion (e.g. in the mine in the case of a mining operation), it is preferred that discharge cavitation is minimised or avoided entirely. Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high velocity. This velocity causes a vacuum to develop at the housing wall (similar to what occurs in a venturi), which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.

The conduit may have both vertical and horizontal components or portions, but preferably, the majority of the conduit is substantially vertical. The conduit may be a pipe, bore, delivery line, or any other suitable conduit for pumping fluid. The conduit may be partially below ground and may extend to the surface where the fluid can be utilised or expelled. The conduit may have a check valve or foot valve that opens when the pump operates to allow fluid to enter the conduit but closes when the pump shuts off to prevent fluid from flowing out.

Preferably, the gas used according to the present invention will be less dense than the fluid to be pumped.

Typically, the gas is supplied in a compressed form, and a compressor may be provided to compress the gas prior to supplying the gas to the diffuser. The fluid to be pumped is preferably denser than the gas, and even more preferably the fluid is a liquid.

Typically, the fluid will be water and the gas will be air. The air may therefore be compressed by an air compressor which may be located either at or near the diffuser, or fed to the diffuser from another location. Alternatively, the compressed air may be provided in the form of tanks or high capacity canisters of compressed air. If the compressed air is fed to the diffuser from another location, the air may be fed along a second conduit which may be internal, integral with, adjacent to, or external to the first mentioned conduit.

The diffuser is preferably located at least partially inside the conduit, such that it spreads the gas across the flow area of the fluid. Even more preferably, the gas is introduced into the fluid flow as small bubbles substantially evenly across a cross section of the conduit. However, it will be appreciated that the diffuser may also be a simple valve, opening/aperture, or nozzle through which the gas is added or injected into the fluid, and no limitation is meant thereby. According to a particularly preferred form, the diffuser may be a diffusion plate with a plurality of outlets for the gas. Normally the diffusion plate will be oriented substantially perpendicularly to the longitudinal axis of the conduit. Normally the diffusion plate will have a number of flow openings therethrough to allow fluid to flow past or through the plate with minimal friction and/or disturbance.

The diffuser is preferably located at or near the bottom of the conduit or, at least, at or near the bottom of the substantially vertical portion of the conduit. The phrase "near the bottom of the conduit" in this context includes any location or position in the lower half of the conduit (or lower half of the vertical portion of the conduit). Preferably, however, the diffuser will be located in the lower quarter of the conduit (or lower quarter of the vertical portion of the conduit).

The rate of gas supplied to the conduit by the diffuser may be altered or controlled to achieve the desired level of gas introduced into the liquid. For example, when the load on the pump is light, little or no air may be supplied, but when the load on the pump is high (e.g. during wet conditions), a suitable amount of air may be supplied. The rate of gas supplied may be such to avoid annular flow patterns within the conduit, as with annular flow the velocity of the air is faster than the fluid. In particular, the gas volume to fluid ratio is preferably less than 0.75, and even more preferably below 0.52.

The rate of gas supplied may be controlled by the gas supply, or the diffuser, or any other suitable means. In the case of a compressor being used to supply the gas, the rate of gas flow is preferably controlled by controlling the speed of the compressor. An open loop control system, closed loop control system, or manual control may be implemented, and no limitation is meant thereby.

The pump may be located at any suitable point in the flow cycle, but in a first embodiment it is preferably at or near the upper end of the conduit. In the event that the conduit is underground, the pump is preferably at least partially aboveground. Where an air compressor is utilised, the pump and air compressor are preferably both situated above ground within close proximity to each other.

In another embodiment, the pump may be located underground near the source or body of fluid/water to be pumped. The pump may even be submerged within the source or body of fluid/water. Preferably, the suction pipe head cannot exceed a particular predetermined value such as, for example, 10 metres.

Preferably, the pump is a high head single stage centrifugal pump. The pump may be a Sykes XHl 50 which is designed for high head requirements often seen in the mining industry, particularly for dewatering operations.

In order that the invention may be more readily understood and put into practice, one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 illustrates a pump system according to an embodiment of the invention with a magnified portion labelled 'A'.

Figure 2 illustrates a schematic view of a pump system according to an embodiment of the invention.

Figure 3 illustrates different flow patterns in a vertical conduit.

Figure 4 illustrates different flow patterns in a horizontal conduit.

Figure 5 is a plot of the relationship between vertical lift and horizontal transfer distance to achieve a constant total dynamic head of 165m.

Figure 6 is a plot comparing total dynamic head with air injection and without air injection over vertical pipe height.

Figure 7 is a plot comparing water flow rate with air injection and without air injection over vertical pipe height.

Figure 8 is a plot comparing total power usage with air injection and without air injection over vertical pipe height.

Figure 9 is a plot showing percentage increases in flow rate and total power usage (a measure of efficiency) with air injection over vertical pipe height. Figure 10 is a plot comparing total dynamic head with air injection and without air injection over horizontal transfer distance.

Figure 11 is a plot comparing water flow rate with air injection and without air injection over horizontal transfer distance.

Figure 12 is a plot comparing total power usage with air injection and without air injection over horizontal transfer distance.

Figure 13 is a plot comparing showing percentage increases in flow rate and total power usage (a measure of efficiency) with air injection over horizontal transfer distance.

Figure 14 is a plot showing air volume fraction (as a %) at the top of a vertical section of pipe over horizontal transfer distance.

DESCRIPTION OF PREFERRED EMBODIMENT

Figure 1 illustrates a pump system according to an embodiment of the invention. The pump system has a pump 10 above a ground level 11 in connection with a vertical conduit or delivery line 12 located below ground level 11. Adjacent the pump 10 is an air compressor 13 that supplies air along second conduit 14 to a diffuser 15.

The diffuser 15 is located near the bottom of the conduit 12 above a foot valve 16. The foot valve allows fluid to enter the conduit when the pump is operating, but prevents fluid from flowing out when the pump is stopped.

As can be seen in the magnified portion labelled 'A', the diffuser 15 is in communication with the second conduit 14 and diffuses air 17 (or other gas) into water 18

(or other fluid) in the delivery line 12. Although the second conduit 14 is illustrated as being a distance from the delivery line 12, it is envisaged that it could also be inside or adjacent the delivery line 12.

The diffuser 15 is located relatively central to the axis of the conduit 12, allowing for a substantially even distribution of air bubbles 17 in the water 18. However, it is also possible that in this arrangement the diffuser 15 could limit or restrict the flow of water

18 in the conduit 12 too much, such that it adversely increases the load on the pump. Therefore, the diffuser 15 may also be arranged in the side of a conduit 12, or in any other suitable arrangement that allows the gas to be diffused into the fluid 18 in the conduit 12.

The diffused air 17 in the lower part of the delivery line 12 enables the pump 10 to operate more efficiently, and allow it to lift water 18 greater distances. Advantageously, this provides a quicker, and usually easier, process to pump fluids a certain distance. For example, when emptying mine shafts and/or mine pits of water, pumps can be smaller than otherwise would have been needed. The increased efficiency of the pumping systems means operations, particularly mining operations, can begin earlier and have reduced downtime, providing further cost savings as well as increased productivity.

Figure 2 illustrates a pump system having a pump 20 located below a vertical portion of a conduit of delivery line 22. A compressor 21 is provided to supply or inject air via a second conduit 24 to a diffuser 25 near the bottom of the vertical portion of the delivery line 22. In this embodiment, air is supplied downstream of the pump 20 to reduce, or minimise, cavitation.

The system is utilised to pump fluid, such as water 26, over a highwall 29. In many mining operations, the highwall may be any height up to several hundred metres, and is typically at least 50 metres in height. For shorter static head situations, a single stage centrifugal pump is typically used as it is reliable, portable, and relatively easy to operate. As head requirements increase (e.g. mine depths increasing) other, more complex and costly, pump systems are typically utilised including the use of multistage pumps and/or pumping stations with booster pumps. In many cases, a single stage pump is run above a best efficiency point (BEP) and at a severe duty point.

The present invention draws the water 26 to be removed in to the pump through a suction line 27, then pumped up the conduit 22, with air being supplied therein by the diffuser 25, and then along a horizontal portion of the conduit 12 to an outlet location 28. Advantageously, the supply of air to the water being pumped decreases the total dynamic head of the pump system by reducing the static head, providing significant efficiency gains in certain circumstances. Two factors are considered to be particularly important when considering the gas supplied to the fluid within the conduit, namely, pipe wall friction experienced in two phase flow, and the relative velocity of the gas compared to the fluid.

These factors at least partially determine the flow patterns of the gas within the fluid, some of which are illustrated in figures 3 and 4. Considering figure 3, vertical conduits illustrating bubbly flow 31, slug flow 32, churn flow 33, and annular flow 33 are illustrated. Turning now to figure 4, dispersed bubble flow 41, slug (intermittent) flow 42, stratified wavy flow 43, and annular dispersed flow 44 are illustrated in section.

For vertical flow, such as that which would be experienced in a substantially vertical portion of a conduit, the transition of bubble flow 31 to annular flow 34 occurs when the gas volume ratio/fraction is between approximately 0.2 and 0.6, more particularly between 0.25 and 0.52. Preferably annular flow 34 is avoided, and therefore it is preferable that gas volume ratio/fraction is below 0.5, or at least below a predetermined value determined by analysis for a particular gas/fluid system.

Wall friction between the fluid/gas and the conduit can be calculated using models based on theories and empirical results. A homogenous model may be used where it is assumed that the gas/fluid mixture is a homogenous mixture with no 'slip' occurring between the gas and fluid, i.e. it is assumed that the bubbles do not rise faster than the fluid. The density of this homogenous mixture is calculated by:

P_mixlure = oc_GP_G + a_Lp_L

and the dynamic viscosity is calculated by:

Mi, = oc_Gμ_a + a_Lμ_L

where

OCG ⁼ volume fraction of gas phase; PG ⁼ density of gas phase; μo = dynamic viscosity of gas phase; (XL = volume fraction of liquid phase; PL = density of liquid phase; and μ_L = dynamic viscosity of liquid phase.

These parameters can then be used to calculate the Reynolds number and friction factor using the Colebrook equation or another suitable approximation.

The performance of a pump under normal operation can be determined by its pump curve. Typically, for a given pump speed, the flow rate is inversely related to total dynamic head (TDH) or pressure. Therefore, reducing the TDH will increase the flow rate and vice versa. The TDH may be calculated by the following formula:

TDH = SuctionLift + StaticHead + FrictionLoss

The supply/injection of a gas into the flow reduces the density of the mixture and, therefore, reduces the static head term in the above equation. Furthermore, the volume fraction of gas in the fluid flow at any point will be dependent on the pressure at that point. As the pressure in the conduit changes, gas injected at or near the bottom of the conduit will gradually expand in volume as it traverses the substantially vertical portion of the conduit.

In a substantially vertical section of pipe of height Ay, the incremental change in pressure due to static head only is given by:

^ static ~~ P mixture 6 A/

The volume fraction/ratio of gas in the mixture, <X_G, is given by:

a, G , a_G - ^■

P( ^KI - a_a ^U OMhI ) ^J + a_G ° OuIhI

where

P = pressure in the section Δy given in atmospheres absolute; and 0-_GO.M = volume fraction of gas at outlet (typically atmospheric pressure)

Friction losses for a given pipe system are depending on various factors including fluid velocity and the density of the fluid. An equation for the incremental change in pressure due to friction losses in a section, Al of a pipe is given by: Λ P = - P mature F^nc"°ⁿ 2D

where

f= friction factor;

V = average velocity of mixture; and D = diameter of the conduit

Using the homogenous flow assumptions, adding bubbles to the flow results in the fluid/gas mixture having a higher velocity in the conduit to pump the same volume of fluid, substantially due to the fluid sharing the conduit with the gas. The increased velocity is at least partially offset by the lower density of the mixture compared to the fluid only. However, it is appreciated that an increased velocity also increases friction losses (due to V² term in the above equation).

For example, the pump may be a Sykes XHl 50 which is a high head, single stage pump having a maximum head of 185 metres, and a maximum capacity of 150L/sec. Despite the maximum head of 185 metres, to operate around the pump's best efficiency point (BEP), the pump should be operated with a head of less than 155 metres. Furthermore, when heads are greater than 173 metres, the pump is operating out of its recommended operating range. In such circumstances it is envisaged that the system of the present invention would be particular beneficial. For example, it is envisaged that a pump such as the Sykes XHl operating with 175 metres head could be used outside of the recommended operating range to deliver approximately 60 litres per second. With air injection according to the present invention, the head may be reduced by 5 metres bringing the flow rate to 76 litres per second which is not only a 25% increase in flow rate, but would also bring the pump within its recommended operating range.

As illustrated in figure 5, the horizontal transfer distance of a conduit typically has a detrimental effect as the vertical lift has to be reduced to maintain a TDH of 165 metres (as illustrated).

The system and method of the present invention are considered particularly advantageous with: • static head requirements between 100 and 200 metres, and even more preferably between 150 and 185 metres;

• with a horizontal transfer distance of less than lkm, and even more preferably less than 200 metres;

• large diameter conduits (primarily to reduce friction); and

• single stage centrifugal pumps, particularly when being operated outside their BEP or recommended operating range.

As shown in figure 6, the TDH that can be expected for different vertical lift heights without air injection is higher than with air injection. In particular, the TDH reduction is up to 7 metres which, although may not seem large, as illustrated in figure 7 the increase in flow rates can consequently be quite significant, up to 20 litres per second.

Figure 8 shows that there is also an increase in power usage, primarily associated with the increase in flow rate (i.e. from the pump) and with an air compressor to supply the gas/air. In particular, the increase in power usage is around 4OkW, 3OkW of which is due to the power usage attributed to the air compressor.

Figure 9 shows the percentage increase in flow and the percentage increase in total power usage. The intersection between the two curves represents the point at which efficiency is at parity (i.e. there is a 17% increase in flow, and a 17% increase in power usage). The values to the right of the intersection point have an increased efficiency, and those on the left of the intersection point have a decreased efficiency. However, even the decreased efficiency case may benefit from the system and method of the present invention as the flow rate is increased which is often an important factor for mine dewatering applications.

Due to frictional losses, the greater the horizontal transfer distance, the less improvement the air injection system provides. Figure 10 shows how TDH reduction due to the injection of air is eliminated as the horizontal transfer distance increases. As illustrated in figure 11, the flow rate benefit also drops off as the horizontal transfer distance is increased. Figure 12 shows the implications horizontal transfer distance has on power usage, and in figure 13, on efficiency. In figure 13, the intersection between the two curves represents the point at which efficiency is at parity. The values to the right of the intersection point have an increased efficiency, and those on the left of the intersection point have a decreased efficiency. In general as the horizontal transfer distance increases the benefits of air injection decline. For this reason, it is preferred that horizontal transfer distances (if any) are kept to a minimum.

Figure 14 shows the air volume fraction/ratio at the top of the vertical portion of the conduit/pipe. The head reduction due to air injection occurs primarily because the gas/fluid mixture in the vertical section is less dense than the fluid on its own, and although having a less dense mixture in a horizontal section gives no benefit, because of the frictional losses in the horizontal portion of the conduit the pressure at the top of the vertical portion of the conduit can be quite high which compresses the gas bubbles (resulting in a reduced air volume fraction). For example, for a horizontal transfer distance of 350 metres, the air volume fraction at the top of the vertical portion is just 20%, or half of the 40% air volume fraction with no horizontal transfer.

Advantageously, the system and method of the present invention reduces the total dynamic head of pump. In certain preferred situations, it is possible to reduce the total dynamic head substantially. The pump is preferably a single stage centrifugal pump, at least partially because they are considered to be reliable and simple to operate, and are often favoured in mining operations. In cases where single stage centrifugal pumps have to be operated with a high head a lower efficiency, or outside of the recommended operating range, the present invention is considered to be particularly useful at reducing the head and, therefore efficiency and/or flow rate. The improvements are most notable where the pipe/conduit has a relatively short horizontal transfer distance after the vertical portion of the conduit, and also where larger diameter pipes are utilised which result in lower friction due to lower velocities of the fluid and/or gas for the same flow rate.

It is to be understood that the terminology employed above is for the purpose of description and should not be regarded as limiting. The foregoing embodiments are intended to be illustrative of the invention, without limiting the scope thereof. The invention is capable of being practised with various modifications and additions as will readily occur to those skilled in the art.

Accordingly, it is to be understood that the scope of the invention is not to be limited to the exact construction and operation described and illustrated, but only by the following claims which are intended, where the applicable law permits, to include all suitable modifications and equivalents within the spirit and concept of the invention.

Throughout this specification, including the claims, where the context permits, the term "comprise" and variants thereof such as "comprises" or "comprising" are to be interpreted as including the stated integer or integers without necessarily excluding any other integers.

Claims

CLAIMS:

1. A system for pumping a fluid over an at least partially vertical distance, the system comprising: a pump for pumping a fluid through a conduit having a substantially vertical component; and a gas supply for supplying a gas to a diffuser that diffuses the gas into the fluid, wherein the diffuser is located at or near the bottom of the substantially vertical component of the conduit.

2. A system for pumping a fluid over an at least partially vertical distance as claimed in claim 1, wherein the diffuser is located on the discharge side of the pump.

3. A system for pumping a fluid over an at least partially vertical distance as claimed in claim 1, wherein the diffuser is located on the inlet side of the pump.

4. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, further comprising a foot valve.

5. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, wherein the diffuser comprises a diffusion plate.

6. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of claims 1 to 4, wherein the diffuser comprises an air outlet in a side of the conduit that injects the gas into the fluid.

7. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, wherein the gas supply is an air compressor.

8. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, wherein the rate of supply of gas to the diffuser is controlled.

9. A system for pumping a fluid over an at least partially vertical distance as claimed in claim 8, wherein the rate of supply of gas is controlled to achieve a predetermined gas to fluid ratio.

10. A system for pumping a fluid over an at least partially vertical distance as claimed in claim 9, wherein the supply of gas is controlled to keep the gas to fluid ratio below 0.52.

11. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, wherein the pump is a single stage centrifugal pump.

12. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, wherein the majority of the conduit is substantially vertical.

13. A system for pumping a fluid over an at least partially vertical distance as claimed in any one of the preceding claims, wherein the fluid is water and the gas is air.

14. A method of pumping a fluid over an at least partially vertical distance through a conduit having a substantially vertical component, the method comprising the steps of: pumping a fluid through the substantially vertical component of the conduit; and supplying a gas to the conduit through a diffuser that diffuses the gas into the fluid at or near the bottom of the substantially vertical component of the conduit.

15. A method of pumping a fluid over an at least partially vertical distance as claimed in claim 14, wherein the gas is supplied to the discharge side of a pump that pumps the fluid.

16. A method of pumping a fluid over an at least partially vertical distance as claimed in claim 14, wherein the gas is supplied to the inlet side of a pump that pumps the fluid.

17. A method of pumping a fluid over an at least partially vertical distance as claimed in any one of claims 14 to 16, further comprising the step of controlling the rate of supply of gas to the diffuser.

18. A method of pumping a fluid over an at least partially vertical distance as claimed in claim 17, wherein the rate of supply of gas is controlled to achieve a predetermined gas to fluid ratio.