WO1997008517A1

WO1997008517A1 - Method and device for measuring liquid levels

Info

Publication number: WO1997008517A1
Application number: PCT/SE1996/001040
Authority: WO
Inventors: Marek Werthajm
Original assignee: Kockum Sonics Ab
Priority date: 1995-08-23
Filing date: 1996-08-21
Publication date: 1997-03-06
Also published as: SE9502919D0; JPH11511554A; SE507192C2; EP0846252A1; SE9502919L; AU6841496A

Abstract

A method of 'bubble measuring' levels in liquid storage tanks (49, Fig. 4) has been based upon supplying a constant mass flow of air through a tube (41) which has been mounted to discharge near the bottom of the tank (49). Such constant mass flow is obtained by supplying compressed air with a constant pressure to the measuring device, passing the air through a solenoid governed valve (25, Fig. 2) at the inlet to a pressure equalising volume (27, Fig. 2), measuring the pressure in said volume, transforming the pressure on both sides of the valve to electric signals, comparing said signals and using variations in the signals to govern the on-off valve (25) so as to obtain a constant mass flow. When this has been achieved the pressure after the solenoid valve is a function of the height of the liquid above the 'bubble measuring' tube discharge end.

Description

Method and device for measuring liquid levels

The present invention relates to a method of performing level measurements in a storage plant comprising a plurality of tanks for liquids.

A known and very reliable method of performing level measurements in tanks for liquids is the so called "bubble measurement". The principle in this method is that a constant mass flow of compressed air is blown through a tube the end of which is located near the bottom of the liquid containing tank. The pressure of the air flow is direct proportional to the height of the liquid above the location at which the air is blown into the liquid.

The advantage of this known method of level measurement is that the pressure drop in the air tubing - due to friction and leakage - is constant at a constant mass flow and easily may be eliminated by calibration, although the accuracy of the measurement will be reduced.

A drawback of the "bubble measurement" is that the equipment is rather expensive if there are many tanks in the storage plant. In known systems it is necessary for each tank to provide a flow control device comprising a membrane influenced by the difference in pressure between the inlet and outlet side of the flow control device. This membrane governs a valve for obtaining a pre-selected mass flow of air. The pressure after the flow control device is registered as level measurement.

The present invention has for its object to provide a method of performing a "bubble measurement" by means of an equipment which is less expensive than previously possible in storage plants having a plurality of liquid containing tanks.

This is according to the invention obtained thereby that the mass flow of air supplied to the bottom of a tank is kept constant by passing it through a pressure equalising vessel, supplied with the air through a solenoid valve governed conduit, the opening time intervals of said solenoid valve being adjusted to keep the resulting mass flow constant.

The invention also relates to a flow control device adapted to provide a gas flow having a constant delivered mass flow independent of the pressure on the delivery side and adapted for practicing the method according to the invention, said flow control device being characterised in that it comprises a transducer giving a first electric signal corresponding to the gas pressure in a supply conduit to the flow control device, a solenoid governed on-off valve downstream said supply conduit, a gas outlet conduit after said on-off valve, a constant restriction between said on-off valve and said gas outlet conduit, a transducer providing a second electric signal corresponding to the pressure in the gas outlet conduit, and a control unit comparing said first and second electπc signals and controlling the pulse width in a pulse train activating the movements of the on-off valve.

The invention furthermore relates to a device for practising the new method of measuring levels in tanks for liquids, said device being characteπsed in that it comprises a suppy conduit provided with a pressure control device - alternatively a conduit provided with a first pressure transducer - for supplying compressed air to the device, the said first transducer being adapted to produce an electric signal corresponding to the pressure in the conduit, a "bubble measuring" tube being mounted to discharge near the bottom of a storage tank, a second transducer for providing an electric signal corresponding to the pressure in said "bubble measuring" tube, a solenoid governed on-off valve arranged between the conduit and the "bubble measuring" tube governing an air flow from the supply conduit to the "bubble measuring" tube, and a central control device in which the two signals are compared and by which the solenoid valve is governed so as to maintain a constant resulting mass flow.

The invention will be described in more detail reference being made to the accompanying drawing in which

Fig. 1 shows a known equipment for providing "bubble measurement",

Fig. 2 shows a flow control device for establishing a constant mass flow of air to be used in the method of performing "bubble measurement" according to the present invention

Fig.3 shows the mass flow of air as a function of the valve opening time in a digital control system,

Fig. 4 shows an equipment for providing "bubble measurement" according to the present invention, and

Fig. 5 schematically shows a model for calculating air leakage in the measuring equipment and liquid levels in the storage tanks.

The known storage plant shown in Fig. 1 consists of three identically designed liquid containing tanks 1, 2 and 3. However, only the tank 1 and its equipment for "bubble measurement" will be described. Compressed air is supplied to the level measuring system via a conduit 4 provided with a stop valve 5 and a pressure reducing valve 6. The air is supplied to a mechanically (analogue) operating flow control device 7 having a governing membrane 8 influencing an analogue valve 9. The mass flow of air Q passing a constant restriction having the area A will depend on the pressures pi resp. p2 on the two sides of the restriction:

Q = f(pi/P2)

Here, pi is constant and at varying p2 values the restriction must be varied in order to obtain a constant delivered mass flow (about 0.5 normal litres per minute). The pressure P2 is influencing a transducer 10 transforming it to an electric signal which subsequently via a line 11 is delivered to an amplifier 12.

Via a tube 13 containing a check valve 14 the constant mass flow of air will pass to the lower part of the tank 1.

It will be understood that a separate, rather expensive flow control device 7 and a separate pressure transducer 10 should be provided for each storage tank. According to the present invention a digital flow control device of the type shown in Fig. 2 is used.

Via a gas suppy conduit 20 containing a constant pressure control device 21 a gas flow is supplied to a flow control device shown in Fig. 2. The gas flow exits the pressure control device in a conduit 22 and the pressure in said conduit 22 is measured by a first transducer 23 giving a first electric signal to a control unit 24. Via the conduit 22 the gas flow is fed to a solenoid governed on-off valve 25. The gas flow leaves the valve 25 in a conduit 26 having a constant restriction the area of which is Ai and a pressure equalising volume 27. The gas will leave said volume via an outlet conduit 28. The gas pressure in the equalising volume 27 is measured by a second transducer 29 providing a second electric signal to the control unit 24. In said control unit 24 the first and second electric signals are compared. In case the relation between the sizes of the two signals is changed the control unit 24 will influence the pulse width in a pulse train governing the opening and closing intervals of the valve 25.

The described flow control device will operate as follows:

The pressure in the conduit 22 pi is measured by the transducer 23 and transformed to a first signal to the control unit 24. The gas having the pressure pi is now passed through the solenoid valve 25 and the conduit 26 having the restriction Ai to the pressure equalising volume 27. The pressure p2 in the volume 27 is measured by the second transducer 29 and a corresponding second electric signal is also supplied to the control unit 24.

At adiabatic expansion the momentary air flow q through the solenoid valve 25 and the constant restriction having the area Ai into the pressure equalising chamber 27 will depend upon the prevailing pressures on the two sides of the restriction having the area Ay.

q = f(Pl/p2)

The momentary air flow q will vary at varying pressures pi and p2. Q is the resulting mean mass flow through the solenoid valve 25. The flow through the solenoid valve has been shown as a function of time in Fig. 3. Here q designates the momentary mass flow rate during the period in which the solenoid valve 25 is open. T is the total pulse cycle time - i.e. the time between two after each other following valve openings. This is kept constant by the control unit 24. The part of the pulse cycle time T during which the solenoid valve 25 is open has been designated by t. The control unit 24 will compare the supplied electric signals and govern the opening and the closing intervals of the solenoid valve 25.

Thus the following equations will be satisfied: Q = q ^• t/T or

Q = f(pl/p2) ^• t/T or t/T = Q/f(pι/p2)

At a certain desired Q the t/T may easily be calculated in the control unit (computer) 24. Instead of the pressure transducer on the primary side measuring the pressure pi a pressure control device 21 could be used. In such a case the pressure pi remains constant and the mass flow Q will be a function only of the pressure p2-

Compared with a conventional flow control device provided with an analogue valve the flow control device according to the present invention will only comprise a single solenoid valve and a pressure equalising volume. The costs relating to a control unit and its programming are marginal in storage plants having a greater number of flow control devices.

Fig. 4 shows a level measuring system according to the present invention based upon "bubble measuring" according to the new method.

Compressed air is supplied through a conduit 30 and passes a stop valve 31 and optionally a pressure reducing valve 32. The pressure pi in a conduit 33 is measured by a transducer 34 forwarding an electric signal to a computer 35. A number of flow control devices 36-38 according to Fig. 2 are provided in conduits branched off from the conduit 33. In the following only the branched off conduit designated by 39 and connected to the flow control device 36 will be described in detail. The outlet from the device 36 has been designated by 40 and tubes 41-44 for "bubble measurement" have been branched off from said outlet 40. Each "bubble measurement" tube has a stop valve 45-48 and has been placed with its discharge end near the bottom of the respective tank 49-52 for storage of liquid. The computer 35 is supplied with information regarding outlet pressure p2 in the flow control device 36 via a transducer 53 and a signal line 54. The computer 35 also governs the stop valves 45-48 and the solenoid valves 25 (Fig. 2) in each flow control device 36- 38. Electric signals corresponding to the pressures p2 after the flow control devices 37, 38 will also be supplied to the computer 35.

The part of the equipment in which the flow control device 36 has been provided may be used for selective level measurements in each of the storage tanks 49-52. This is obtained by influencing the stop valves 45-48. Also measurements may be carried out selectively in storage tanks connected to the remaining branched off conduits 37-38.

The valves 45-48 may be activated at a predermined sequence and the measuring results may be stored in the computer 35. Measurements for the storage tanks may be shown continuosly or in a selected tank. Updating may be performed corresponding to the sequence period. A pressure control device 32 keeping the pressure pi at a constant value may be used instead of the transducer 34.

Compared with a conventional equipment for "bubble measurement" the costs for an equipment according to the present invention will be substantially reduced. The analogue flow control devices have been replaced by a corresponding number of much cheaper solenoid governed on-off valves. The total length of tubing has been substantially reduced. The costs for the computer 35 and its programming will be marginal.

A special advantage of the method and the equipment according to the invention is that it is very easy to calibrate for air leakage in the "bubble measurement" system. This will now be explained with reference to a schematically shown pressure equalising chamber shown in Fig. 5.

A chamber has been designated by 55 in Fig. 5. It has an inlet opening 56 providing a constant restriction area Ai and it is supplied with air having the pressure pi and the flow velocity vi. Also the chamber 55 is provided with an outlet 57 having a constant restriction area A2 - the said outlet being adapted to be connected to the "bubble measuring" tube in the measuring system. At the outlet from the chamber the pressure is p₂ and the flow velocity is V2. The leakage in the system is represented by an opening 58 having a constant restriction area A3. The leakage is a mass flow having the velocity V3.

The mass of air flowing into the chamber is equal to the mass leaving the chamber i.e.:

The mass flow through a restriction hole is a function of the hole area A and the relation between the pressures on the two sides of the hole: Qm = f(A,pι/p2) At constant mass flow:

Qu-1 = fl(A2,P2/P3) + 2(A3,p2/Pa-m) At closed valve at A2 and maintained mass flow into the chamber:

Therefore, by measuring the change in pressure from pi to P2' at closed valve the restriction area A3 may be determined.

The leakage through the restriction A3 may be calculated as above and expressed as offset it may be included in the calculation of p2 (level).

This self calibration - i.e. measuring of pressure at closed valve at A2 may be performed by the computer at predetermined intervals or manually as desired.

At automatic self calibration the leakage may be compared with previously obtained or accepted values and optionally a larm signal "Leaking Tube" may be released.

Claims

1. A method of performing level measurements in a storage plant comprising a plurality of tanks (49-52, Fig.4) for liquids, according to which during a measuring period a constant mass flow of air is blown into each tank near the bottom thereof, characterised in that the mass flow of air supplied to the bottom of a tank is kept constant by passing it through a pressure equalising vessel (27, Fig.2) supplied with the air through a solenoid valve governed conduit, the opening time intervals of said solenoid valve (25) being adjusted to keep the resulting mass flow constant.

2. A flow control device adapted to provide a gas flow having a constant delivered mass flow independent of the pressure on the delivery side and adapted for practising the method according to claim 1, characterised in that it comprises a transducer (23) giving a first electric signal corresponding to the gas pressure in a supply conduit (22) to the flow control device, a solenoid governed on-off valve (25) downstream said supply conduit (22), a gas outlet conduit (28) after said on-off valve (25), a constant restriction (A ) between said on-off valve (25) and said gas outlet conduit (28), a transducer (29) providing a second electric signal corresponding to the pressure in the gas outlet conduit (28), and a control unit (24) comparing said first and second electric signals and controlling the pulse width (t) in a pulse train activating the movements of the on-off valve (25).

3. A flow control device according to claim 2, characterised in that a pressure regulator (21) has been provided in the supply conduit ensuring a constant pressure in the supplied gas.

4. A flow control device according to claim 2, characterised in that a pressure equalising volume (27) has been provided in said gas outlet conduit (28).

5. A device for performing level measurements by use of the method claimed in claim 1, characterised in that it comprises a supply conduit (30, Fig.4) provided with a pressure control device (32) - alternatively a conduit (33, Fig.4) provided with a first pressure transducer (34) - for supplying compressed air to the device, the said first transducer (34) being adapted to produce an electric signal corresponding to the pressure (pi) in the conduit (33), a "bubble measuring" tube (41) being mounted to discharge near the bottom of a storage tank, a second transducer (53) for providing an electric signal corresponding to the pressure (p2) in said "bubble measuring" tube (41), a solenoid governed on-off valve (25, Fig.2) arranged between the conduit (33) and the "bubble measuring" tube (41) governing an air flow from the supply conduit to the "bubble measuring" tube, and a central control device (35), in which the two signals are compared and by which the solenoid valve (25, Fig.2) is governed so as to maintain a constant resulting mass flow.

6. A device according to claim 2, characterised in that a pressure equalising volume (27, Fig.2) has been provided between said solenoid governed valve (25) and said "bubble measuring" tube.

7. A device according to claim 2, characterised in that a stop valve (45) has been inserted in said "bubble measuring" tube.

8. A method of calibrating a "bubble measuring" according to claim 1 in which a device according to claim 4 is used, characterised in that any leakage in the device is compensated for by closing said stop valve (45) and measuring the difference in pressure in the "bubble measuring" tube upstream the stop valve prior to and after the closing of said stop valve.