GB2252365A

GB2252365A - Flow control of positive displacement pumps

Info

Publication number: GB2252365A
Application number: GB9102312A
Authority: GB
Inventors: Peter Lester Crowcroft
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-02-02
Filing date: 1991-02-02
Publication date: 1992-08-05
Also published as: GB9102312D0

Abstract

A system for the precision pumping and metering of liquids with a non-pulsating accuracy better than 0.1%, using pumps of inherently low accuracy without flowmeter control. The flow rate is controlled by a positive displacement pump to the inlet of which is applied a pressure (e.g. from another pump or by liquid from a storage vessel subject to air or gas pressure) so that the pressure drop across the p.d. pump approaches zero. The system is particularly valuable for low and variable viscosity liquids.

Description

CONVERSION OF STANDARD P.D.PUMPS TO PRECISION(0.1%) FLOW CONTROL.

This invention relates to the precision flow rate pumping of liquids by the adaptation of standard pumps of low precision.

Automatic precision flow control of low and medium viscosity liquids is at present carried out by flowmeter closed loop control of low accuracy pumps or by reciprocating metering pumps. Mechanical flowmeters are generally accurate to + 0.5% at low viscosities, Mass coriolis meters achieve - 0.2%. An additional error must be allowed for bias and fluctuations of the pump control system. Mitering pumps can achieve + 0.5, ' but the output is pulsating and requires 120 phased pumps and large dampers to bring the 100% fluctuations down to O.S%. Where smooth flow is required or an accurate ratio between flows these pulses are undesirable.

We propose a method for improving the accuracy of continuous Positive Displacement Pumps from the 10% to 20% found in single stage pumps ( on water at 5 bar back pressure), or 12% in the best multi stage pumps 1 on pentane ( viscosity 0.25 cp), to better than 0.1% without the use of flowmeters The error in P.D. pumps is due to slip in the reverse direction, of liquid past pump clearances. When medium to high viscosity liquid is being pumped the slip is negligble and the flow rate accuracy is determined by mechanical accuracy of chamber dimensions and the accuracy of pump speed.Multi stage Progressive cavity pumps smooth out the mechanical variations and precision frequency controlled invertors with synchronous motors remove errors due to mains variations.

slip arises from back pressure. In a normal installation back pressure would arise from pipe, valve and orifice losses and operations creating pressure. We propose a pumping system in which the force required to overcome back pressure is provided by a seperate pump or system, and the control of flow is achieved by a P.D. Pump driven at an accurate ( 0.05) speed, with the pressure drop across the P.D.Pump being used to control the output of the first pump ( back pressure pump). The control system is designed to keep the pressure across the P.D.Pump at or near zero.

By this means the P.D. Pump acts as a driven flowmeter, but also, should the control pressure deviate from zero, the P.D.Pump then acts as a pump to achieve the correct flow rates albeit at a slight loss of accuracy due to slip. This slip error will be proportional to the deviation from zero of the control pressure. However a control error of 5 produces a total error of only 0.12 ( where back pressure of 5 bar gives 6 slip). By comparison a flowmeter of 0.5 accuracy operating a pump control of speed to 0.5 ( ten times better than we require of the pressure balancing system) would have 1% accuracy.

The following are examples of the results from applying balancing pressure to Progressive cavity pumps.

1. Assume a constant back pressure of 5 bar on a progressive cavity single stage pump on water. Slip with no forward pressure is 13%. Apply a constant inlet pressure of 4.8 bar reducing the pressure across the pump to 0.2 bar.

Slip re ces linearly to 0.5%. If liquid of viscosity 5 cp is used instead of water then slip reduoes to 0.1%.

2. Assume a constant back pressure of 5 bar on a 4 stage P.C, pump, Slip with zero forward pressure is 3 on water. The following results from forward pressure. 4.8 bar on water, slip is 0.12%. 4.8 bar on pentane ( viscosity 2 0.25) slip is 0.48%. 4.8 bar on catalyst ( viscosity 20 cp) slip is 0,006%, 3. Assume a constant back pressure of 5 bar on an 8 stage P.C. pump.Slip with zero forward pressure is 0.5% on water and on pentane 2%, Apply a forward pressure of 408 bar and the slip on pentane reduces to 0.08%.

4. Assume the severe conditions of a 4 stage pump on pentane with back pressure fluctuating between 1 and 10 bar. Assume a 1% control system which balances forward to back pressure to 0.1 bar, Slip with pentane at 5 bar is 12%. Slip with back pressure balanced out to 0.1 bar is 0.24.

Systems for producing balancing pressure are typically:a. Air pressure can be used when the liquid can be pressurized in storage vessels. The differential pressure across the P.D.Pump operates a pilot valve to control the vessel pressure until the differential pressure is reduced to zero. Continuous fast acting systems can be achieved by using duplicate small storage tanks which automatically alternate between delivery of the liquid and receipt from bulk storage, thus reducing the time required to change forward pressure.

b. A pump producing a pressure output, such as a centrifugal pump may be used to provide the balancing pressure. The differential pressure across the P.D.Pump is measured and converted to an analogue signal, either current or frequency, which is used to control the invertor driving the pressure pump. The pressure pump thus provides the total pressure required to overcome the back pressure of the system , and thus brings the P.D.Pump differentialn to zero.

At present the techniques of this invention require two pumps, one control7ing flow rate and one providing pump pressure. It is feasible to envisage 8 dual pump in which both duties are incorporated with by pass to control pressure instead of pump speed.

Claims

CLAIMS.

What we claim is:1. A system for the precision non-pulsating control of liquid flow, in which the flow rate is controlled by a Positive Displacement Pump, whilst a pressure source provides the power such that the pressure drop across the Positive Displacement Pump approaches zero.
2. A system as in 1. in which the power is supplied by air or gas pressure on the liquid storage vessels such that the air/gas pressure is controlled by the differential pressure across the Positive Displacement Pump, such that the differential pressure is caused to approach zero.
3. A system as in 1. and 2. in which the storage is in two small vessels alternating automatically between delivery as in 2. and receipt from bulk storage.
4. A system as in 1. in which the power is supplied by a pressure pump whose speed or delivered pressure is controlled by the differential pressure across the Positive Displacement Pump so as to drive that pressure towards > zero.
5. A system as in 1. and 4. in which the Positive Displacement Pump and the Pressure Pump use a single drive and/or a common housing with the pressure pump output controlled by by-pass or pressure regulation.
6. Systems as in 1. 2. 3. or 4. in which small fluctuations in back pressure are balanced by a fixed pressure power source whose mean pressure equals the mean value of the back pressure.