GB2186919A

GB2186919A - Positive displacement pump

Info

Publication number: GB2186919A
Application number: GB08703593A
Authority: GB
Inventors: Roger Charles Hey Sidey
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1986-02-19
Filing date: 1987-02-17
Publication date: 1987-08-26
Anticipated expiration: 2007-02-17
Also published as: GB8703593D0; GB2186919B; GB8604114D0

Abstract

A positive displacement pump comprises two chambers adapted for alternate working to supply the pump outlet, a reservoir 11 and a control system. Each chamber incorporates a piston 3,4 in a cylinder 1,2 and is associated with upper 7,8 and lower 9,10 liquid level or piston position detectors to provide signals when the liquid levels or pistons reach the level of the detectors. The signal from the lower detector actuates a system of valves 16,17 which transfers output from the cylinder having the low level of liquid to the other and primes the former whilst the latter discharges. The signal from the upper detector terminates priming. The pump is capable of delivering accurate volumes of liquid under conditions of high pressure and temperature and is particularly suitable for use in apparatus for determining the permeability of core samples of rock. <IMAGE>

Description

SPECIFICATION Positive displacement pump This invention relates to a positive displacement pump capable of delivering accurate volumes of liquid under conditions of high pressure and temperature.

According to the present invention there is provided a positive displacement pump comprising two chambers adapted for alternate working to supply the pump outlet, a reservoir and a control system, each chamber incorporating a piston in a cylinder and being associated with upper and lower liquid level or piston position detectors to provide signals when the liquid levels or pistons reach the level of the detectors, the signal from the lower detector actuating a system of valves which transfers output from the cylinder having the low level of liquid to the other and primes the former whilst the latter discharges, the signal from the upper detector actuating the system of valves to terminate priming.

The pistons are preferably servo-controlled and driven by stepping motors via a ball screw and gear reduction system which is positively pre-loaded by the pump operating pressure.

With this arrangement a low friction and backlash free drive or transfer characteristic can be achieved-these being the prime requirements for high resolution servo control of the mechanism. In the energised holding condition, the drive will not run back under pressure and thus the pumps can be made to appear as "stiff" hydraulic reservoirs in the system. High resolution flow metering is achieved by averaging the pulse rate of the drive signals applied to the stepping motors which, under constant pressure conditions and over an average corresponding to the pump swept volume, corresponds accurately to the incremental output of the pump.Each cylinder may be equipped with a basic displacement measurement and bar graph type display system which allows detection of end of stroke for automatic control and provides front panel indication of the status of the pumping units.

Flow rates can be metered accurately by measuring the displacements of the cylinders.

The dual piston arrangement, the valves and the control system provide a smooth and uninterrupted flow during the transition between cylinders.

This may be achieved by means of an automatic control system for progressively opening and shutting the flow control valves. These valves, sited between the outlets of the cylinders and the line feeding inlet flow to the sample, are operated by the control system at the time of changeover. When an end of stroke condition (or a change liquid command) is sensed, a servo system selects the next desired liquid source, puts the associated pump motor in the "energised holding" mode and opens the appropriate valve. Overall servo control of line or differential pressure is still maintained at this point by the exhausting cylinder and the valve opening sequence is arranged to be slow enough that the control system can compensate for the small inlet volumetric and compressibility changes that occur.On completion of the opening sequence drive signals are passed to the new cylinder motor and a steady closure sequence begins for the valve assocated with the exhausted cylinder. Completion of the cylinder changeover sequence occurs at the end of this closure when a run back or priming command is issued to the exhaust cylinder. It is likely that this changeover sequence will take between 4 and 10 seconds to complete.

A significant advantage to this pumping arrangement is that it facilitates changeover between any selected delivery cylinders in the system without interruption of flow and without introducing pressure transients in the system. It thus allows a continuous and metered flow of liquid and automatically provides for changeover of liquid.

In normal operation the control valves are actuated by the control system to bring on line the appropriate cylinder or to close it off for re-priming.

During rn-priming the cylinder draws fluid from its associated reservoir via a non-return valve. To facilitate flushing or de-airing at the start of testing a system of manually operated reservoir return valves are provided. These units effectively bypass the non-return priming valves allowing purging of the cylinders into the reservoir. Line pressure is measured by a transducer at this point, the resultant signal being available as both a monitoring and/or pump control signal in the system.

Effective servo control can only be achieved if the drive is free of backlash, i.e. if the pump operating pressure maintains a positive pre-load throughout the entire train of drive components, and if sealing or other Coulomb friction sources are aways easily overcome by the pre-load.

The hydraulic cylinders are designed to be both simple and modular in construction allowing for ease and convenience of replacement in the event of seal deterioration or attack by the treatment liquid.

If the liquid is corrosive, the components exposed to attack may be plated or manufactured from a corrosion resistant stainless steel.

The pump is suitable for operating at pressures up to 1,300 atmospheres or temperatures up to 15p0C. It is particularly suitable for delivering relatively small quantities of liquid at low flow rates, for example, in the range 0.001 to 50 ml/minute with a high degree of accuracy, e.g. to within 0.1%.

These properties make the pump particularly suitable for use in apparatus for determining the permeability of core samples of geological material under conditons of high pressure and temperature.

The invention is illustrated with reference to the accompanying drawing.

The pump comprises two cylinders 1 and 2 containing pistons 3 and 4 servo-controlled by stepping motors 5 and 6 and associated with upper and lower liquid piston position detectors 7 and 8 and 9 and 10.

The cylinders 1 and 2 are charged from a reservoir 11 via non-return valves 12 and 13 and are discharged through outlets 14 and 15 controlled by valves 16 and 17 leading to a common outlet 18.

The cylinders 1 and 2 are equipped with basic displacement measurement and bar graph type display systems 19 and 20 which allow detection for the end of each stroke for automatic control and provide front panel indication of the status of the pumping units.

As shown, cylinder 2 is in the process of discharging. Valves 12, 13, and 16 are shut and valve 17 is open. When the piston in cylinder 2 reaches the level of detector 10 a signal is given which actuates the control system firstly to open valve 16 progressively and then to close valve 17 and open valve 13.

Cylinder 1 then begins to discharge and cylinder 2 is primed from the reservoir 11. When the piston in cylinder 2 rises to the level of detector 8 priming is stopped and valve 13 closes automatically.

The above procedure is repeated, mutatis mutandis, with cylinders 1 and 2 alternatively charging and discharging.

Claims

1. A positive displacement pump comprising two chambers adapted for alternate working to supply the pump outlet, a reservoir and a control system, each chamber incorporating a piston in a cylinder and being associated with upper and lower liquid level or piston position detectors to provide signals when the liquid levels or pistons reach the level of the detectors, the signal from the lower detector actuating a system of valves which transfers output from the cylinder having the lower level of liquid to the other and primes the former whilst the latter discharges, the signal from the upper detector actuating the system of valves to terminate priming.

2. A pump according to claim 1 wherein the pistons are servocontrolled.

3. A pump according to either of the preceding claims wherein the pistons are driven by stepping motors.

4. A pump according to either of claims 2 or 3 comprising an automatic control system for progressively opening and shutting the flow control valves, overali servo control of the pump outlet being maintained by an exhausting cylinder until completion of the opening of a charged cylinder and the closure of the exhaust cylinder.

5. A positive displacement pump as hereinbefore described with reference to the accompanying drawings.