GB2143330A - Flow measuring devices - Google Patents

Abstract

A fluid flow measuring device includes a body 10 defining a fluid inlet 11 and an outlet 12 connected through a variable opening 17. The position of a valve member 18 is determined by a diaphragm 15 which is subjected on one side to the pressure at the outlet 12 and on its other side to a reference force generated for example by means of a constant force spring 16. As the pressure at the outlet decreases the valve member 18 is moved to increase the size of the opening and vice versa. The inlet pressure is maintained substantially constant by a similarly constructed pressure regulator 26. The position of the valve member 18 is sensed by a linear variable differential transformer 23 to provide an indication of the flow rate between the inlet and outlet. Non-linearities in the indication can be compensated by shaping opening 17 and/or member 18 or by correction with a computer. The reference force may alternatively be generated by a weight or bellows, or a variable rate spring, or a reference gas pressure; in one example a rolling diaphragm is subjected to force from a second such diaphragm exposed to a reference pressure. Compensation for gas temperature is possible using a spring or bellows. <IMAGE>

Description

SPECIFICATION Flow measuring devices This invention relates to fluid flow measuring devices more particularly but not exclusively, to flow measuring devices for measuring the flow of combustion gas to a domestic or commercial consumption unit or units.

The traditional form of gas flow meter includes a diaphragm which moves between two extreme positions as flow of gas takes place. The meter also includes valves which connect the opposite sides of the diaphragm to the inlet and outlet connections of the meter in turn and a mechanical counter which is connected to the diaphragm and which is calibrated to provide a reading of the quantity of gas which has passed through the meter.

It is required to reduce the size of the device and also to provide a device which is able to provide an electrical output to facilitate the necessary "reading" of the device at a remote point.

The object of the present invention is to provide such a device in a simple and convenient form.

According to the invention a flow measuring device for the purpose specified comprises in combination, a housing defining inlet and outlet flow connections, a valve member movable to provide a variable orifice connecting said flow connections within the housing, pressure responsive means for controlling the position of said valve member, said pressure responsive means including a surface subjected to the pressure downstream of said orifice, said pressure responsive means in response to a fall in the pressure downstream of said orifice acting to move the valve member to increase the size of the orifice thereby to restore the pressure and vice versa, and transducer means for providing a signal indicative of the position of said valve member, the position of the valve member varying as the flow of fluid through the orifice varies.

One example of a fluid flow measuring device in accordance with the invention in this case for natural gas, will now be described with reference to the accompanying drawings in which: Figure lisa sectional side elevation of one example of a device in accordance with the invention, Figure 2 is a sectional side elevation of another example of the device, and Figure 3 is a sectional side elevation of a further example of the device in experimental form.

In Figure 1 of the drawings there is shown a measuring device which comprises a housing 10 which defines an inlet flow connection 11 and an outlet flow connection 12 which in use, is connected to a domestic consumption unit 13 for example, a central heating unit.

The outlet flow connection 12 communicates with a chamber 14 one wall of which is defined in the particular example, by a pressure responsive means in the form of a flexible diaphragm 15. The diaphragm is biased by a substantially constant force spring 16 in a direction to reduce the volume of the chamber 14. In the wall of the chamber facing the diaphragm, is an opening 17 which communicates with the inlet connection 11. Movable within the opening 17 is a valve member 18. The opening and the valve member are shaped to define a variable orifice through which gas can flow from the inlet connection 11 into the chamber 14 and from the chamber 14 to the outlet connection 12. The valve member has an extension 19 which bears against the diaphragm and the valve member is biased towards the diaphragm by a light spring 20.

The valve member has a stem portion 21 which passes through an aperture in the wall of the housing, the wall of the aperture mounting an annular seal member 22 which acts to prevent the escape of gas. The portion 21 of the valve member extends into a transducer housing generally indicated at 23 and it mounts the movable component 24 of a transducer which in the particular example, includes windings 25.

In operation, assuming for the moment that there is a constant pressure at the inlet connection 11, the diaphragm and valve member will operate to maintain the pressure at the outlet connection 12 substantially constant. The actual pressure at the outlet connection will depend upon the force exerted by the spring 16 and the effective area of the diaphragm. If for example the pressure at the outlet connection 12 increases due for example to a reduction or cessation of gas consumption, the diaphragm 15 will respond to the increased pressure in the chamber 14to allow the valve member to be moved by the spring 20 to reduce the effective size of the orifice defined by the opening and the valve member. The pressure in the chamber 14 will therefore be restored to its original value.If however the consumption of gas increases the pressure in the chamber 14 will temporarily fall and the diaphragm will move the valve member to increase the size of the orifice thereby to substantially restore the pressure. The position of the valve member is therefore dependent upon the flow of gas and an electrical signal indicative of the position of the valve member is provided by the transducer 23.

Assuming that the pressure at the inlet connection is constant, the electrical output provided by the transducer will be representative of the flow taking place through the device. Non-linearities in the electrical signal and the flow of gas through the orifice can be compensated for by shaping the valve member and/or the opening 17 or by correction of the signal using a computer.

It has been mentioned above that in order for the device to function as required, the pressure in the inlet connection should be constant. The pressure can be maintained constant by providing a pressure regulator generally indicated at 26, through which the device is connected to the main gas supply. The construction of the regulator 26 can be the same as the flow measuring device with the exception that the pressure regulator does not require in the described example, a transducer. The regulator 26 may in fact be the normal regulator which is supplied by the gas undertaking.

The electrical signal provided by the transducer as has been mentioned, is representative of the flow rate of gas through the device. The signal can be supplied to an electronic circuit which can be designed to provide a volume consumed reading on an appropriate display device which can be situated at a remote point. The display device may also be arranged to give an instantaneous flow rate reading which can be used to check the consumption of an appliance or leakage in the appliance on the supply system downstream of the measuring device.

As described the diaphragm 15 is biased by a constant force spring. It may however be biased by means of a weight or a bellows. In the case of the spring or bellows, compensation can be arranged for variation in the temperature of the gas flowing through the device.

The assumption made above is that the pressure at the inlet connection 11 is substantially constant.

The pressure regulator 26 can however provide an actual pressure signal by means of a further transducer which senses the position of its diaphragm or valve member. The flow measuring device can be calibrated on a test rig at varying inlet pressures and the information stored in an electronic memory". In use, it is possible to supply the actual pressure signal and the output signal from the transducer 23, to the memory to obtain a true flow signal.

The flow measuring device as described can be utilised to measure the flow of other gases and also liquids for example, water.

Another example of flow measuring device is seen in Figure2. The device comprises a two part casing, the casing parts having the reference numerals 27, 28. The casing parts are provided with peripheral flanges between which is sandwiched the peripheral portion of a flexible diaphragm 29. The two parts of the casing can be secured together in any convenient manner. The casing part 27 defines an inlet flow connection 30 which is connected in use to a source of supply having a substantially constant pressure and the casing part 28 defines an outlet flow connection 31 for connection in use, to a consumption unit. The opposite sides of the diaphragm are therefore subjected to the inlet pressure and the outlet pressure respectively.

Located within the chamber defined by the casing part 27 is a coiled tension spring 32 one end of which is connected to the diaphragm 29 and the other end of which is mounted upon an adjustable mounting plate 33. The plate 33 is connected to an adjustment member 34 whereby the force exerted by the spring 32 upon the diaphragm can be varied.

At its centre, the diaphragm carries a sleeve-like member 35 having a central bore which permits passage of gas between the opposite sides of the diaphragm and hence between the inlet and the outlet flow connections. The sleeve member 35 is secured to the diaphragm and secured to the sleeve member is the winding assembly of a linear transducer 36 having electrical connections 37 which pass through a sealed grommet in the casing part 27. The core 38 of the transducer is mounted upon a fixed stem 39 which extends with clearance through the aperture in the sleeve 35 and is conveniently supported in an apertured mounting 40 located in the outlet connection 31. The stem 39 is contoured so that as the sleeve 35 moves with the diaphragm, the flow area between the stem and the sleeve will alter.

In operation, assuming a flow of gas between the inlet and the outlet flow connections, the diaphragm will assume a position such that the flow path for the gas between the stem and the sleeve is sufficient to maintain the forces acting on the diaphragm in balance. If for example the demand for gas increases there will be a reduction in pressure at the outlet flow connection and this will result in movement of the diaphragm in the direction towards the outlet flow connection 31 thereby increasing the area of the flow path. The diaphragm will therefore assume a new equilibrium position and the output of the transducer will reflect this position. Conversely if the flow of gas through the outlet flow connection should decrease there will be a slight increase in the pressure at the outlet flow connection and the diaphragm will move in the opposite direction.The spring adjustment enables the flow measuring device to be calibrated. The taper on the stem 39 and/or the cross-section of the bore in the sleeve are chosen to provide as linear a variation in electrical output with flow variation as possible although correction for non-linearity can be achieved by electrical means as earlier described.

Turning now to Figure 3 there is shown a flow measuring device which incorporates two diaphragms of the rolling type and with reference to the drawings the device comprises a first body part 41 which defines an open ended chamber 42 having a right cylindrical wall. Communicating with the chamber is an outlet flow connection 43 for connection to a gas consumption unit. The body also has an inlet flow connection 44 which communicates with the chamber 42 through an opening 45 in the base wall of the chamber. Extending within the opening 45 is an axially movable stem 46 which is carried by cup shaped diaphragm support 47. The support 47 is of smaller diameter than the internal surface of the wall of the chamber and the gap is occupied by a rolling diaphragm 48.The central portion of the diaphragm is secured to the support 47 while the outer portion of the diaphragm is trapped between the end of the body 41 and an annular clamping member 49.

The device also includes a second body part 50 which is of hollow right cylindrical form. The body part 50 is mounted relative to the body part 41 by means of a tubular support structure 51 such that the axes of the chamber 42 and the body part 50 are co-incident. The internal diameter of the bore in the body part 50 is less than that of the chamber 42 and there is located in the bore a further cup shaped diaphragm support 52. Afurther rolling diaphragm 53 has its central portion secured on the support and its peripheral portion is clamped between the end of the body part 50 and a further clamping member 54 which includes a right cylindrical wall and an end closure 55 in which is formed an inlet 56 for connection in use to a reference pressure.

The supports 47 and 52 mount rods 57,58 respectively which extend towards each other, the presented ends having mushroom shaped heads between which is located an end 59 of a pivotal lever. The lever passes through a slot in the support structure 51 and extends beyond its pivot point 60 to engage with the input member 61 of a linear transducer 62.

In use, the inlet 56 is connected to a reference pressure which providing a pressure regulator as previously described is employed, can be the same pressure as is supplied to the inlet flow connection 44. The operation of the device is exactly as described with reference to the example of Figure 1. In this case however the pressure at the outletconnection will be less than the pressure at the inlet connection by an amount at least in part depending upon the relative effective areas of the diaphragms 48 and 53.

The opening 45 is defined by a circular hole formed in a sleeve and the stem 46 is of cylindrical form hazing an inclined flat machined on it so that the flow area of the opening varies as the axial position of the stem within the opening varies. The variable area can be achieved in other ways for example, the stem can be provided with a groove and the sleeve with a broached non-circular hole.

The device of Figure 3 can be modified by replacing the upper diaphragm 52 together with the associated parts by a constant rate spring or by a weight or even by a spring the rate of which varies.

Moreover, as described in the example, a linear variable differential transformer type of transducer has been employed, however, cheaper forms of transducer can be used.

It will be appreciated that in describing the operation of the examples the weight of the moving parts has been ignored. The weight in general acts to oppose the force generated on the diaphragms by the pressure in the chamber 14 and 42.

If the pressure differential between the inlet and outlet is small the diaphragms 48 and 53 in the exampfé m$!e of Figure 3 will have a size which is substantially the same, even so the diaphragm 48 will be slightly larger than the diaphragm 53. If the volume of the chamber defined between the diaphragms is sufficiently large the pressure changes which occur during movement of the diaphragm can be ignored. In this case if either diaphragm is ruptured then the diaphragm assembly will be depressed until a sealing ring 63 about the stem 46 closes the opening 45 thereby to prevent the flow of gas to the outlet.

If for example the inlet pressure falls in the example of Figure 3 the weight of the diaphragm assembly can be utilized to ensure that the diaphragm assembly moves downwardly to a position such that the seal member 63 closes the opening 45 to prevent dangerously low pressures at pilot lights etc.

Claims (9)

1. Afluid flow measuring device comprising in combination, a housing defining inlet and outlet flow connections, a valve member movable to provide a variable orifice connecting said flow connections within the housing, pressure responsive means for controlling the position of said valve member, said pressure responsive means including a surface subjected to the pressure downstream of said orifice, said pressure responsive means in response to a fall in the pressure downstream of said orifice acting to move the valve member to increase the size of the orifice thereby to restore the pressure and vice versa, and transducer means for providing a signal indicative of the position of said valve member, the position of the valve member varying as the flow of fluid through the orifice varies.

2. A flow measuring device according to Claim 1 in which said pressure responsive means includes a further surface against which a reference pressure can act, the reference pressure acting on said further surface providing a force acting in opposition to the force exerted by the downstream pressure acting on said first mentioned surface.

3. Aflow measuring d,evice according to Claim 2 in which said pressure responsive means comprises a flexible diaphragm, the opposite sides of said diaphragm defining said first mentioned surface and said further surface respectively.

4. A flow measuring device according to Claim 2 in which said first mentioned surface is defined by one surface of a rolling diaphragm assembly and said further surface is defined by one surface of a further rolling diaphragm assembly said diaphragm assemblies being operatively connected to each other and to the valve member and to said transducer means.

5. Aflow measuring device according to Claim 4 in which the other surfaces of said rolling diaphragm assemblies face each other in spaced relationship.

6. A flow measuring device according to Claim 1 in which said pressure responsive means comprises a flexible diaphragm the force exerted by said downstream pressure being opposed by resilient means acting on said diaphragm.

7. A fluid flow measuring device comprising the combination and arrangdment of parts substantially as herein described with reference to Figure 1 of the accompanying drawings.

8. A fluid flow measuring device comprising the combination and arrangement of parts substantially as herein described with reference to Figure 2 of the accompanying drawings.

9. A fluid flow measuring device comprising the combination and arrangement of parts substantially as herein described with reference to Figure 3 of the accompanying drawings.