GB2286048A

GB2286048A - Flow meters

Info

Publication number: GB2286048A
Application number: GB9401481A
Authority: GB
Inventors: Ceri William Workman
Original assignee: Spirax Sarco Ltd
Current assignee: Spirax Sarco Ltd
Priority date: 1994-01-26
Filing date: 1994-01-26
Publication date: 1995-08-02
Anticipated expiration: 2014-01-26
Also published as: GB2286048B; GB9401481D0

Abstract

A flow meter comprises a measuring vessel 2 having an inlet 10 and an outlet 16. The level of fluid in the vessel 2 is measured by a capacitive level sensor comprising a central electrode 18 and the outer wall 12 of the vessel 2. The outlet 16 of the vessel 2 is provided with a controlled discharge valve 24, the discharge characteristics of which are known. During a filling operation of the vessel 2, when the discharge valve (24) is closed, the flow rate into the vessel is determined from the rate of change of the output of the capacitive level sensor. During a discharge operation, when the discharge valve 24 is opened, the flow rate into the vessel 2 is determined from the discharge characteristics of the valve 24 and a knowledge of the pressure in the inlet passage which, may be obtained from a measurement of temperature in the inlet passage. The height to which the vessel 2 is filled before opening valve 24 may be varied. The meter may be used for condensate discharged from a steam system. <IMAGE>

Description

FLOW METERS This invention relates to flow meters.

A known flow meter comprises a vessel which may be successively filled and emptied by operation of a stop valve at the outlet of the vessel. The flow can be calculated by measuring the time taken for a fixed volume of liquid to be collected in the vessel.

However, this method is only suitable where the flow rate is fairly steady. A vessel appropriately sized for either the minimum or maximum flow will be unavoidably inaccurate at the other extreme. Also, if flow varies during the set collecting period, only an average flow rate can be obtained.

According to the present invention there is provided a fluid flow meter comprising a chamber having an inlet passage and an outlet passage, one of which is closable to interrupt the flow of fluid into or out of the chamber, the meter also comprising sensing means for generating a signal representing the rate of change of fluid quantity in the chamber, and processing means, for receiving the rate of change signal and generating an output signal, representing the flow rate, determined from the rate of change signal and from a flow signal representing the flow rate through the closeable passage into or out of the chamber.

The sensing means may be responsive to the fluid level in the chamber, the rate of change of fluid quantity in the chamber being derived from the rate of change of fluid level. The sensing means may comprise a capacitive level sensor.

A flow sensor may be provided which is responsive to the flow rate through the closable passage for generating the flow signal, which is then input to the processing means. In a preferred. embodiment, however, the processing means includes flow signal generating means for generating the flow signal. The flow signal generating means may comprise a look-up table having values representing the flow rate through the closeable passage depending upon the pressure drop across the closable passage. Where the measured fluid is condensate discharged from a steam system, the upstream pressure will be the steam pressure, which will vary in accordance with the steam temperature. A temperature sensor may therefore be provided in the chamber in order to give a temperature signal which may be coverted into a system pressure value by means of a second look-up table.The processing means may also have compensating means for adjusting the flow signal in accordance with prevailing conditions, such as any back pressure present in the system.

Closure of the closeable passage is preferably by means of a valve which is controlled to open and close in response to the level of fluid in the chamber. The level of fluid in the chamber at which the valve is opened or close may be varied over time.

According to circumstances, either the inlet passage or the outlet passage will be the closable passage. If the fluid flow meter is to be used to monitor the flow rate of fluid to equipment downstream of the meter, then the inlet passage will normally be the closable passage. Alternatively, if the fluid flow meter is to be used to monitor fluid discharged from an upstream component, such as a steam trap, then the outlet passage will be closable passage.

The present invention also provides a method of measuring the flow rate of fluid in a fluid flow line, comprising the steps of: (i) directing the fluid through a chamber disposed in the flow line; (ii) interrupting the flow of fluid into or out of the chamber to maintain the fluid level within the chamber between desired limits; (iii) obtaining a flow signal representing the flow rate of the interrupted flow of fluid; (iv) measuring the rate of change of fluid quantity in the chamber; and (v) generating an output signal representing the flow rate, the output signal being determined from the flow signal and from the rate of change of fluid quantity in the chamber.

For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a fluid flow meter; Figure 2 shows a variant of the flow meter of Figure 1; and Figure 3 shows a second variant of the flow meter of Figure 1.

The flow meter shown in Figure 1 comprises a measuring vessel 2 which is disposed in a fluid flow line having an inlet passage 4 upstream of the vessel 2 and an outlet passage 6 downstream of the vessel 2. In the example shown in Figure 1, the flow meter is disposed downstream of a condensate trap 8, in order to measure the flow of condensate discharged from a steam system.

The measuring vessel 2 has an inlet opening 10 which communicates with the inlet passage 4 of the fluid flow line. The measuring vessel 2 comprises a generally cylindrical outer wall 12, which is electrically conductive. The wall 12 tapers at its upper end to define the inlet opening 10. The measuring vessel 2 has a base portion 14 which closes the base of the wall 12 and which defines an outlet opening 16 which communicates with the outlet passage 6.

An electrode 18 extends longitudinally within the measuring vessel 2 and is supported by the base portion 14. The base portion 14 is, in the embodiment shown in Figure 1, made from an insulating material.

Alternatively, an insulating collar 13 (as shown in Figure 2) may be provided around the base of the electrode 18, in order to insulate the electrode 18 from the wall 12 and from the remainder of the base 14, which need not then be made from an insulating material. The electrode 18 is spaced from the cylindrical wall 12, and extends from the base of the measuring vessel 2 to a region of the vessel 2 some distance beneath the inlet opening 10.

A splash protection baffle 20 is supported above the electrode 18 by a screen 22.

The outlet passage 6 has a discharge valve 24 which interrupts the flow from the vessel 2. A check valve 26 is disposed in the fluid flow line downstream of the discharge valve 24. The discharge valve 24 is operated by an actuator 28 in the form of a solenoid which is controlled by a processing unit 30. The processing unit 30 receives signals from the electrode 18, and sends signals to the actuator 28.

In use, the valve 24 is opened and closed by the processing unit 30 in order to maintain the level of fluid in the vessel 2 within a desired range.

Condensate expelled from the condensate trap 8 passes through the inlet passage 4 to the measuring vessel 2. The electrode 18 is protected from direct contact with the incoming flow by the protective cap 20 and the fluid passes through the screen 22 into the base portion of the vessel 2. The electrode 18 and the wall 12 of the vessel 2 function as a capacitive level sensor. The electrode and the casing are supported relatively to each other by the base 14. The capacitance between the electrode 18 and the wall 12 varies according to the level of condensate in the vessel, the dielectric constant of the condensate being different from that of the steam above it. The capacitance between the electrode 18 and the wall 12 thus provides a measure of the condensate level.

When the discharge valve 24 is shut, the flow of condensate into the vessel 2 causes the liquid level to rise. This liquid level is monitored continuously by the capacitive level sensor, and the processing unit 30 derives an instantaneous flow rate of the condensate entering the vessel 2. In order to convert the detected change in the fluid level into a mass flow rate, the processing unit has a memory system which gives a relationship between the liquid level in the vessel 2 and mass of liquid. The flow rate obtained is displayed on, for example, a digital panel, or the flow rate may be passed on to a recorder or to further control circuitry such as a building management system.

The processing unit determines when the solenoid 28 is to be operated to open the valve 24, depending upon the detected liquid level within the vessel 2.

The liquid level within the vessel 2 is kept within the range of measurement of the capacitive level sensor.

Preferably, the valve 24 is opened when the liquid level within the vessel 2 has risen by the smallest volume that can be measured to the desired accuracy.

However, the frequency of operation of the valve 24 is controlled to be sufficiently low to avoid undesirable valve chatter.

When the valve 24 has been opened, the level of liquid in the vessel 2 falls. This fall in liquid level is measured by the capacitive level sensor and is monitored by the processing unit 30. The processing unit 30 has a memory which stores data relating to the discharge characteristics of the valve 24. In particular, the memory stores information concerning the rate of flow of liquid through the valve 24 as a function of the pressure within the vessel 2. The discharge characteristics of the discharge valve 24 are stored for all possible system pressures, such that in applications where the system pressure may vary over time, the system pressure may be taken into account.

Although the head imposed by the liquid level in the vessel 2 will also influence the discharge characteristics of the valve 24, this effect will be small when compared with the effect of the system pressure on the discharge characteristics. The processing unit compares the detected rate of liquid discharge from the vessel 2 with the discharge valve discharge flow rate. Subtracting the measured rate of liquid discharge from the vessel 2 from the discharge rate through the valve 24 enables the flow rate of condensate into the vessel 2 to be determined.

Thus, the flow rate of condensate into the vessel 2 is calculated continuously during both a filling cycle and a discharge cycle of the vessel.

When the liquid level in the vessel 2 has fallen sufficiently, to a level near the base 14, the processing unit 30 causes the valve 24 to close. In this way, the flow meter is operated cyclically.

Where the flow rate of condensate into the vessel is relatively high, the valve 24 may be operated to discharge the liquid until the level of liquid in the vessel 2 is near to the base 14 of the vessel 2.

However, for relatively low flow rates, the flow meter may be operated over a fraction of the full operating range of the capacitive level sensor. The levels in the vessel 2 between which the liquid is maintained may be randomly varied over a number of cycles to avoid contamination of a portion of the level sensor by the possible deposition of dissolved solids.

The discharge characteristics of the valve 24 will depend upon the pressure drop across the valve. For known, steady system conditions, the information concerning the discharge characteristics of the valve 24 will not vary significantly over time. However, as already mentioned, for situations where the system pressure may vary, the flow meter requires the system pressure to be taken into account to enable the discharge characteristics of the valve to be determined. Determination of the system pressure is therefore required.

A modified version of the flow meter of Figure 1 is shown in Figure 2. This modification to the flow meter includes a temperature sensor 32 which is positioned above the maximum level of liquid in the vessel 2, such that the temperature sensor 32 is positioned in the steam filled top of the vessel. This sensor 32 enables the determination by the processing unit 30 of the system steam pressure by means of steam tables stored in the processing unit. The look-up table of the processing unit 30 which gives the valve discharge characteristics according to the pressure in the vessel 2 is then used to obtain the discharge characteristics of the valve, according to the system pressure.

Another temperature sensor (not shown) downstream of the discharge valve 24 could similarly provide a compensating signal proportional to any back pressure present.

A further temperature sensor 34, shown in Figure 2, which measures the temperature of the condensate being discharged enables calculation of the heat energy passing through the meter.

During normal operation, the temperature measured by the temperature sensor 32, i.e. the local steam temperature, will be fairly steady. However, the temperature at sensor 34 will vary between that of the cooler condensate during the vessel filling period and the warmer steam temperature when the vessel is emptied. The processing unit 30 monitors the temperatures at the temperature sensors 32,34 and monitors the operation of the valve 34. Any deviation from the expected cyclic pattern can be used to generate an error signal. In this way, the flow meter has self diagnostic capability.

Figure 3 shows another modification of the flow meter shown in Figure 1. The lower portion of the outer casing 12 of the measuring vessel 2 tapers inwardly. This has the effect of increasing the rate at which the level rises whilst this tapered portion is being filled. The resolution of the level sensor is thus enhanced at low condensate flow rates.

The flow meter according to the present invention provides a continuous, real time, measurement of flow rate. The meter does not give an average flow measurement which could result in the suppression of peaks or troughs in the flow rate.

Although the embodiments of the present invention described use a capacitive level sensor, other forms of level sensor may be employed.

Claims

1. A fluid flow meter comprising a chamber having an inlet passage and an outlet passage, one of which is closable to interrupt the flow of fluid into or out of the chamber, the meter also comprising sensing means for generating a signal representing the rate of change of fluid quantity in the chamber, and processing means, for receiving the rate of change signal and generating an output signal, representing the flow rate, determined from the rate of change signal and from a flow signal representing the flow rate through the closeable passage into or out of the chamber.

2. A flow meter as claimed in claim 1 in which the sensing means is responsive to the fluid level in the chamber.

3. A flow meter as claimed in claim 2 in which the sensing means comprises a capacitive level sensor.

4. A flow meter as claimed in any preceding claim in which the processing means comprises flow signal generating means for generating the flow signal.

5. A flow meter as claimed in claim 4 in which the flow signal generating means comprises a look-up table having values representing the flow rate through the closable passage depending upon the fluid pressure within the chamber.

6. A flow meter as claimed in claim 5 in which the meter further comprises means for measuring a temperature in the chamber, the temperature signal being converted into the fluid pressure value by conversion means.

7. A flow meter as claimed in claim 6 in which the conversion means comprises a second look-up table.

8. A flow meter as claimed in any preceding claim in which the processing means has compensating means for adjusting the flow signal in accordance with prevailing conditions.

9. A flow meter as claimed in any preceding claim in which the closable passage is closed by means of a valve which is controlled to open and close in response to the level of fluid in the chamber.

10. A flow meter as claimed in claim 9 in which the level of fluid in the chamber at which the valve is opened or closed is varied over time.

11. A method of measuring the flow rate of fluid in a fluid flow line, comprising the steps of: (i) directing the fluid through a chamber disposed in the flow line; (ii) interrupting the flow of fluid into or out of the chamber to maintain the fluid level within the chamber between desired limits; (iii) obtaining a flow signal representing the flow rate of the interrupted flow of fluid; (iv) measuring the rate of change of fluid quantity in the chamber; and (v) generating an output signal representing the flow rate, the output signal being determined from the flow signal and from the rate of change of fluid quantity in the chamber.

12. A method as claimed in claim 11 further comprising the step of adjusting the flow signal in accordance with prevailing conditions, the adjustment flow signal being used in the determination of the output signal.

13. A fluid flow meter substantially as described herein, with reference to, and as shown in, Figure 1, Figure 2, of Figure 3 of the accompanying drawings.

Amendments to the claims have been filed as follows (AMENDED) CLAIMS

1. A fluid flow meter comprising a chamber having an inlet passage and an outlet passage the flow meter providing an output representing the flow rate through one of the inlet passage and the outlet passage, the other passage being closable to interrupt the flow of fluid into or out of the chamber, the meter also comprising sensing means for generating a signal representing the rate of change of fluid quantity in the chamber, and processing means, for receiving the rate of change signal and generating an output signal, representing the flow rate through the said one passage, determined from the rate of change signal and from a flow signal representing the flow rate through the said other passage into or out of the chamber.

5. A flow meter as claimed in claim 4 in which the flow signal generating means comprises a look-up table having values representing the flow rate through the said other passage depending upon the fluid pressure within the chamber.

9. A flow meter as claimed in any preceding claim in which the said other passage is closed by means of a valve which is controlled to open and close in response to the level of fluid in the chamber.

11. A method of measuring the flow rate of fluid in a fluid flow line, comprising the steps of: (i) directing the fluid through a chamber disposed in the flow line the flow rate to be measured being one of the inlet flow into or the outlet from the chamber; (ii) interrupting the other flow of fluid out of or into the chamber, respectively, to maintain the fluid level within the chamber between desired limits; (iii) obtaining a flow signal representing the flow rate of the said other flow of fluid; (iv) measuring the rate of change of fluid quantity in the chamber; and (v) generating an output signal representing the flow rate, the output signal being determined from the flow signal and from the rate of change of fluid quantity in the chamber.