GB2220709A - Fluidic pumps - Google Patents

Abstract

A fluidic pump has primary and secondary vessels 11, 12 connected by pipe 13, a displacement vessel 16 having liquid to be delivered through pipe 21 via rectifier 22 provided with feed tank 23. A drive unit 17 delivers pressure fluid to a line 9 to raise liquid 10 and compress trapped gas or liquid in the space, including pipe 19, between the liquids in vessels 12, 16, and thus drive liquid out of the vessel 16. The driving gas from 17 is therefore separated by the barrier liquid at 11, 12 and the trapped gas or liquid at 18, 19 from the liquid to be pumped at 16, which liquid could by e.g. radioactive. <IMAGE>

Description

Fluidic Pumps This invention relates to fluidic pumps.

In normal fluidic pumps the driving gas comes into direct contact with the liquid being pumped at a quiescent gas-liquid interface. Thus, when the liquor is radioactive the gas may become slightly contaminated and therefore must be exhausted into an active vent system and subseqently scrubbed. This problem may be overcome by the introduction of a barrier liquid into the pumping system. This separates the driving gas from a volume of gas trapped between the barrier liquid and the liquor being pumped. Thus contamination is confined to the trapped gas with the exhaust gas remaining relatively clean.

According to this invention a fluidic pump comprises a primary vessel for containing liquid, a secondary vessel, a pipe communicating the primary and secondary vessels, a drive unit for forcing liquid from the primary vessel through the pipe and into the secondary vessel, a displacement vessel for liquid having a delivery outlet, and a further pipe connecting the secondary and displacement vessels and providing a closed space with the interiors of the secondary and displacement vessels above the liquid therein.

There may be a priming pipe leading into the primary vessel and having one end adjacent a base of the primary vessel.

The drive unit may include a conduit leading into a closed space above the liquid in the primary vessel.

The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of example with reference to the accompanying schematic drawings, in which: Fig 1 is a pumping system; and Figs 2 to 7 show various drive units.

Referring to Fig 1 which shows a biased dual vessel barrier liquid system with a single acting fluidic pump, a barrier liquid 10 is contained within primary and secondary vessels 11, 12. As shown, the secondary vessel 12 is directly above the primary vessel 11 and a rise pipe 13 connects the base 14 of the vessel 12 to near the base 15 of the vessel 11. (The arrangement of the vessels 11, 12 shown is only one of several possible configurations which could be used if necessary, eg nonaxially aligned vessels or sloping pipe vessels). With the primary vessel 11 and a displacement vessel 16 initially full of liquid, a drive unit 17 supplies a positive pressure of air or other gas through pipe 9 to the primary vessel. This forces liquid from the primary vessel 11, up the rise pipe 13 and into the secondary vessel 12.This compresses gas 18 trapped between the liquid in the vessels 12, 16 which are connected by pipe 19, raising its pressure and therefore driving liquid out of the pump displacement vessel 16 along pipe 20 to discharge or delivery pipe 21 via a rectifier 22 for example an RFD (reverse flow diverter) or pair of fluid diodes. The rectifier can receive liquid from a feed tank 23 through pipe 24 and is disposed between pipes 20 and 21. During a delivery period the rectifier tends to prevent flow through pipe 24 to tank 23 but favours flow from pipe 20 to pipe 21.At the end of the drive phase, the high pressure gas 25 in the primary vessel 11 is vented, the barrier liquid 10 falls back into the primary vessel and the displacement vessel refills from tank 23 via rectifier 20 which during this phase tends to favour flow from tank 23 to pipe 20 and tends to prevent flow from pipe 21 to tank 23. Clearly the drive phase should not be so long as to permit the barrier liquid level in the primary vessel to fall below the bottom of the rise pipe 13, which should therefore be near the vessel base 15. The barrier liquid swept volume must be correctly matched to the required displacement vessel swept volume.

The matching will depend upon the pump duty, the initial pressure and volume of trapped gas 18 (which should be kept to a minimum) and the expected degree of heat transfer. Although fundamentally simple the system operating cycle can be optimised and stabilized by introducing various special design features as detailed below.

Pump Priming The barrier liquid is introduced to the primary vessel through a priming tube 26. this should discharge at a level below the bottom of the rise pipe 13 and it should incorporate a valve 27 to close the tube 26 after priming. The priming tube can also be used to drain the barrier liquid if required. Priming should be done with the pump feed tank 23 full and the liquid level 28 in the displacement vessel 16 therefore initially at the level 30 of the liquid in tank 23. The optimum priming method will normally be that of suction priming. Before barrier liquid is introduced, the drive unit 17 applies a suitable suction (eg - 2 psig) to the system. This raises the displacement vessel liquid level 28 above the tank liquid level 30 to balance the suction head. This is particularly useful for low tank levels 30.With the suction maintained, barrier liquid is introduced, either under pressure or due to the suction. Once the barrier liquid level 29 in vessel 11 rises above the base of the rise pipe the trapped gas 18 in vessel 12 is established (at a negative pressure) and the suction pressure in pipe 9 can be removed. Priming is complete when the primary vessel is full. At this point liquid will have risen up the rise pipe to balance the trapped gas negative pressure which will in turn have changed slightly due to the resulting slight compression of the trapped gas. The separation of the primary and secondary vessels should be sufficient that, after priming, the trapped gas negative pressure does not cause the secondary vessel 12 to be partially filled.A further major advantage of suction priming is that during the pump cycle, displacement vessel refill can be maintained at a high rate due to the negative trapped gas pressure, ie the system does not rely solely upon vessel bias for the provision of a negative refill pressure, vessel bias being the pressure head produced by different liquid levels in the vessels 11, 12.

Priming can be done without applied suction, ie atmospheric priming, but this method has several disadvantages. After priming, the trapped gas is at a slight positive pressure. This makes it difficult to fill the displacement vessel at low levels in tank 23.

In addition, negative vessel refill pressures can only be provided by the vessel bias, which must therefore be greater (ie more separation between the primary and secondary vessels). Toward the end of the refill phase, the vessel level bias tends to decrease thus reducing the refill rate and therefore decreasing the overall pumping rate as compared with that for suction priming.

To remove the barrier liquid, the priming tube valve 27 is opened and the liquid either drawn out under suction or forced out by applying a small positive pressure to the primary vessel 11.

The primary vessel will normally be full to its lid at the end of priming, but there will be a different liquid level in the rise pipe depending upon the pressure of the trapped gas. If this is at a negative pressure as in suction priming, then the liquid will be up in the rise pipe, perhaps as high as the secondary vessel. If the trapped gas pressure is positive then the rise pipe level will be below the primary vessel lid.

The barrier system will still operate if the primary vessel is -overfilled so that liquid rises up the air pipe, or underfilled so that there is always an air space at the top of the vessel.

Prevention of gas entrainment Under certain operating conditions it is possible for drive gas to be entrained up the rise pipe at the beginning of the drive phase. This is intolerable as it increases the steady state trapped gas pressure and the system is unstable. The entrainment occurs when there is a significant length of gas pipe 9 from the drive unit which is filled with liquid before the start of the drive phase. When drive pressure is applied, this liquid in the pipe 9 forms a high velocity liquid jet which penetrates to the base of the primary vessel, entraining gas in its wake, some of which gas goes up the rise pipe.

This can be prevented by introducing a plate baffle 31 into the primary vessel at the gas pipe entry. The plate 31 diverts the liquid jet preventing penetration into the vessel. The plate 31 does not interfere with primary vessel priming or refill.

The problem of gas entrainment does not occur when there is little priming liquid in the gas pipe, as is the case with correctly suction primed systems and most atmospheric primed systems. Instead of plate 31, a diffuser can be used.

Preventing liquid overshoot across the secondary vessel If the primary vessel pressure rise at the beginning of the drive phase is too rapid, the velocity of the liquid flowing up the rise pipe can be so great as to form a liquid fountain which can extend across and hit the top of the vessel 12. This can result in barrier liquid being carried over into the displacement vessel 16. To avoid this fountain affect it is necessary to moderate the rate of pressure rise. This requires correct matching of the drive unit output characteristics to the dead gas volume between the drive unit 17 and the primary vessel 11.

An alternative solution is to use a baffle plate at the top of the rise pipe to divert the liquid flow horizontally; or to use a diffuser 13a at the top of pipe 13 to slow the velocity of liquid rise.

Drive unit design If it is permissible to vent gas through a moving part, the drive unit 17 can be formed simply from timers and two solenoid valves. In this case efficiency can be improved by introducing an expansive working phase to the cycle. If moving part venting is not permissible, then a primary controller element, such as a jet pump or vortex amplifier, must be introduced.

If suction priming is to be used, the drive unit must have provision for supplying the necessary suction during priming.

The drive unit could be formed from a sealed displacement system, such as a bellows unit or a piston-cylinder arrangement, operating either with gas as a substitute for the above units, or directly with the barrier liquid. In either case the resulting pressure cycle should be correctly matched to the pump requirements, including the effects of cavitation.

Various forms of controller are shown in Figs 2 to 7.

In Fig 2 there are solenoid valves 40, 41 and timer 39. Drive phase occurs with valve 40 closed and valve 41 open. Expansive working occurs with both valves closed.

Venting occurs if valve 41 is closed and valve 40 is open.

In Fig 3 a jet pump 42 is included and opening valve 41 gives drive and closing valve 41 gives venting.

In Fig 4 a vortex amplifier 43 is used and operation is as for Fig 3.

In Fig 5 a suction jet pump 45 can be used but only in the priming phase and the pump 45 can be generously sized so as not to affect normal operation. Solenoid valve 46 is open during priming and closed during other times. This jet pump could also be used in the arrangement of Fig 2.

Figs 6, 7 show bellows 47, and a piston/ cylinder 48, as means for driving.

Alternative trapped fluids It is probably easiest to use air as the trapped gas 18, but if it is required to maintain a certain chemical environment in contact with the pumped liquid, a suitable gas can be trapped instead.

If system lagging is to be used in an attempt to make the trapped gas cycle more adiabatic, and so more efficient, it may be advantageous to use a monatomic trapped gas.

An alternative to gas systems is to trap a suitably immiscible liquid in place of the trapped gas (or to eliminate the trapped fluid) . The priming procedure would need to be suitably modified but the system would be free from trapped fluid compression losses.

Claims (4)

Claims

1. A fluidic pump comprising a primary vessel for containing liquid, a secondary vessel, a pipe communicating the primary and secondary vessels, a drive unit for forcing liquid from the primary vessel through the pipe and into the secondary vessel, a displacement vessel for liquid having a delivery outlet, and a further pipe connecting the secondary and displacement vessels and providing a closed space with the interiors of the secondary and displacement vessels above the liquid therein.

2. A pump as claimed in Claim 1, comprising a priming pipe leading into the primary vessel and having one end adjacent a base of the primary vessel.

3. A pump as claimed in Claim 1 or Claim 2, in which the drive unit comprises a conduit leading into a closed space above the liquid in the primary vessel.

4. A fluidic pump substantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.