GB2170324A - Pumping systems and control means therefor - Google Patents

Abstract

A control device for a pumping system incorporating fluidic devices has an air inlet 19 leading to a convergent/divergent nozzle 40 producing supersonic flow. A compressive shock wave 43 is produced just upstream of an intake 41 of a diffuser 42. An outlet from chamber 46 may go to vent or a jet pump. Spaced oblique compressive shock waves can be produced in a device with an Oswatitsch intake Fig. 5 (not shown). The device can be used in a pumping system for example one incorporating a reverse flow diverter. <IMAGE>

Description

SPECIFICATION Pumping systems and control means therefor This invention relates to pumping systems incorporating fluidic devices and to control means for such systems.

Pumping systems incorporating fluidic devices are attractive for pumping hazardous liquids because the fluidic devices do not require moving parts which could require repair or replacement with consequent risk to maintenance personnel. One known pumping system incorporates a fluidic device known as a reverse flow diverter RFD. An RFD comprises two opposed nozzles separated by a gap which opens into or communicates with the liquid which is to be pumped and examples of RPD's and their manner of operation are given in British Patent Specification No 1480484.

According to the present invention a control device for a fluidic pump comprises a nozzle having an input for air under pressure and an outlet leading to a chamber, the nozzle being adapted to provide a supersonic flow of air from the outlet, structure downstream of the nozzle outlet in the chamber adapted to produce a compressive shock wave in the air flow, a passage for the air which passed the shock wave, and an outlet from the chamber.

The passage may comprise a diffuser.

The structure may be adapted to produce a plurality of compressive shock waves spaced in the direction of air flow. The device may have a flow axis and the or each shock wave may be oblique to the axis. The outlet from the chamber may go to atmosphere or to a suction jet pump adapted to receive air under pressure and air from said chamber and having a vent outlet.

The invention also includes a pumping system comprising a vessel for liquid to be pumped, a fluidic pump positioned at a level below the level of liquid to be pumped and communicating with a liquid delivery conduit, and control means for effecting alternate pressurising and venting of the pump to effect pumping of the liquid, the control means comprising a control device as defined above.

The pump may comprise a reverse flow diverter inserted between a charge vessel and the conduit.

The invention may be performed in various ways and three specific embodiments with possible modifications will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a pumping system; Figure 2 shows a known controller for a pumping system of Figure Figure 3 shows a controller according to the invention; Figure 4 shows another controller according to the invention; and Figure 5 shows a further controller according to the invention.

In Figure 1 a reverse flow diverter RFD 10 is immersed in a liquid 11 in a vessel 12.

The RFD 10 comprises two opposed, coaxial, conical nozzles separated by a gap which opens into the liquid 11 One nozzle is connected to a charge vessel 13 having an air link pipe 14. The other nozzle of the RFD 10 is connected to a delivery pipe 15 for the liquid 11. The pipe 14 communicates with a compressed air supply line 16 by way of a primary controller 17 and a secondary controller 18 including valve 9 in line 19 and valve 8 in line 20. The charge vessel 13 could be outside vessel 12. Lines 19, 20 include filters and pressure regulators (not shown).

Such a pumping system operates as follows. Initially charge vessel 13 is partly filled with liquid. On operating secondary controller 18 to open valve 8 in line 20, compressed air from line 16 flows through an inlet nozzle in the primary controller, across a transverse bore into a mixing tube in the primary controller 1 7 and thence is vented to atmosphere at 21. Air in passing through the inlet nozzle creates a suction in the pipe 14. As a result liquid 11 in the vessel 12 is drawn through the gap between the nozzles of the RFD 10 and into the charge vessel 13 to enter the end of pipe 14. Excess air passes to vent.

The secondary controller 18 now closes valve 8 in pipe 20 and opens valve 9 in pipe 19 for a predetermined time, for example, 5 seconds, under the control of an electrical fluidic timer unit 7. Compressed air flows from pipe 19, through primary controller 1 7 and pipe 14 to pressurise the charge vessel 13. During this phase the liquid in the charge vessel 13 is urged across the gap in the RFD 10 and along the delivery pipe 1 5.

A fraction (excess) of the compressed air supplied in pipe 14 will escape to vent 21 along pipe 22. At the end of the predetermined time interval the secondary controller 18 again closes line 19, line 20 remaining closed. The charge vessel 13 is thus vented to atmosphere 21 through line 14, and primary controller 17.

After a second predetermined time interval sufficient to allow the air pressure in the charge vessel 13 to fall to a pressure just above the pressure in the vent 21 (usually atmospheric) (the ''blowdown'' period) the secondary controller 18 again opens line 20 to initiate the next pumping cycle.

A known form of primary controller is shown in Figure 2 in the form of a jet pump pair and comprises a drive nozzle 30 connected to line 19 and having a cylindrical nozzle profile 3 1 delivering to a mixing chamber 32 which communicates with a passage 32a leading to a drive diffuser 33 leading to line 14. The mixing chamber 32 communicates with passage 34 communicating with another mixing chamber 35 receiving air from suction nozzle 36 connected to line 20 and communicating with suction diffuser 37 leading to vent 21, the reference to suction indicating that air is supplied in line 20 on that part of the cycle in which liquid is drawn into the charge vessel. With such an arrangement the pressure in line 16 is much higher than the pressure in line 15. Typically a pump with a 5m lift might be fed from a 6 bar g air main.Leakage of air into the vent 21 during the drive phase is avoided by accelerating the air to a high velocity so that its pressure P in passage 34 as it enters the mixing tube 32 is essentially atmospheric, thus avoiding leakage to vent. Flow through the drive nozzle is assumed to be choked so that the mass flow (flow rate) is independent of the pressure downstream of the nozzle. The drive air then passes into diffuser 33 in which sufficient pressure to drive the pump is recovered.

The jet pump driving a fluidic pump uses a long constant area mixing chamber and a conical diffuser. The underlying assumption on which this design (Fig 2) is based is that flow in the diffuser 33 (recuperator) is essentially subsonic.

A jet pump pair (Fig 2) cannot produce an outlet pressure R (at the outlet of diffuser 33) such that R-P is greater than a predetermined value.

Jet pump pairs are capable of high drive pressures but this can only be achieved at the expense of considerable air flow to vent during the drive stroke. The leakage airflow increases until the pressure drop across the suction ejector mixing tube and diffuser 35, 37 reaches a certain value, at which point it is possible for air to flow through the drive jet pump.

By contrast, with the novel arrangements described below a higher normalised pressure output (R-P) can be obtained.

With these arrangements the velocity of air leaving the drive nozzle is supersonic (preferably wave free flow), and the larger part of the velocity head in the airflow leaving the drive nozzle is recovered across one or more compressive shock waves.

A design of a supersonic drive unit is shown in Figure 3. Features of this design are: i) A convergent-divergent drive nozzle 40 (a De Laval nozzle) which expands the air flow from line 19 such that the downstream pressure is essentially atmospheric and the downstream velocity supersonic.

ii) Structure 45 providing a sharp edged 'pitot' intake 41 to a subsonic conical diffuser 42. This intake 41 is arranged such that a plane normal shock wave 43 forms just upstream of the diffuser inlet 41. The supersonic flow from the drive nozzle is decelerated across the shock wave 41 to subsonic velocity and, at the same time, its pressure is increased. The area of intake 41 is less than the area of the outlet of the nozzle 40. The inlet 41 is in chamber 46 communicating with vent 21 or a suction ejector.

The passage 42 could be of uniform crosssection.

iii) The unit is axisymetric and therefore easy to make. Distortion during welding should not be a problem.

iv) The unit is efficient during drive and yet has a very fast 'blowdown'. It is therefore a good controller for high-lift fast-cycling pumps.

v) The unit can be combined with suction jet pump 50 to provide a controller for pumps requiring suction lift. It is suggested that the suction side of such a unit should use a convergent-divergent drive nozzle 40 and convergent/parallel mixing tube 51, further increasing efficiency. The tube 51 leads to conical diffuser 52. Such a unit is shown in Figure 4.

vi) No entrainment occurs during the drive phase. The unit is therefore not a jet pump. It cannot be used as a flow amplifier.

The use of a single plane normal shock in place of a number of oblique shock waves involves some loss of efficiency but makes the device more stable.

In theory the supersonic pitot controller of Figure 3 could be designed to operate with no flow at all to vent during the drive phase. It seems likely that in practice boundary layers within the drive nozzle will be disturbed and may separate from the nozzle wall. In addition some very turbulent mixing will occur around the periphery of the jet between drive nozzle and intake. If this flow were to enter the intake it would adversely affect the flow regime in the diffuser. This problem can be avoided by arranging for the flow from the drive nozzle to exceed the flow entering the intake by a small amount (perhaps 20%). By this means the disturbed air around the jet will be stripped off and the velocity profile across the intake will be uniform.

It can be shown that the most efficient method of recovering the velocity head from a supersonic flow is to arrange for the flow to pass through a convergent-divergent duct designed such that the flow passes through a number of weak oblique shock waves.

The Oswatitsch intake shown in Figure 5 could be used to achieve this result. In theory it would be very efficient but in practice it might be difficult to ensure that the shock pattern was insensitive to small flow changes.

If the shock pattern is not stable the drive flow from the controller would oscillate.

The arrangement of Figure 5 has a solid body 60 suspended in a support vane 61 and provides an annular subsonic diffuser 62. This gives main oblique annular shock waves 63, 64, 65. Waves 63, 64 are spaced in the direction of air flow.

The supersonic pitot controller is capable of providing a high output pressure but requires no restriction in the vent gas path. The drive air is accelerated to supersonic velocity by means of a de Laval or other suitable nozzle.

The greater part of the pressure recovery is achieved by arranging for the flow to cross a stationary shock wave upstream of a subsonic diffuser. In contrast to a jet pump the device described is incapable of entrainment.

The described controller can be used with other kinds of fluidic pump than a reverse flow converter, for example diode pumps and jet ballast units.

Claims (8)

1. A control device for a fluidic pump comprising a nozzle having an input for air under pressure and an outlet leading to a chamber, the nozzle being adapted to provide a supersonic flow of air from the outlet, structure downstream of the nozzle outlet in the chamber adapted to produce a compressive shock wave in the air flow, a passage for the air which passed the shock wave, and an outlet from the chamber.

2. A control device as claimed in claim 1, in which the passage comprises a diffuser.

3. A control device as claimed in any preceding claim, in which the structure is adapted to produce a plurality of compressive shock waves spaced in the direction of air flow.

4. A control device as claimed in any preceding claim, the device having a flow axis and the or each shock wave is oblique to said axis.

5. A control device as claimed in any preceding claim, in which the outlet from the chamber communicates with a suction jet pump adapted to receive air under pressure and air from said chamber and having a vent outlet.

6. A control device for a fluidic pump substantially as hereinbefore described with reference to and as shown in Figure 3, or Figure 4, or Figure 5 of the accompanying drawings.

7. A pumping system comprising a vessel for liquid to be pumped, a fluidic pump positioned at a level below the level of liquid to be pumped and communicating with a liquid delivery conduit, and control means for effecting alternate pressurising and venting of the pump to effect pumping of the liquid, the control means comprising a control device as claimed in any preceding claim.

8. A pumping system as claimed in claim 7, in which the pump comprises a reverse flow diverter inserted between a change vessel and the conduit.