GB2177802A - Measuring fluid flow - Google Patents

Abstract

A flowmeter comprising a flexible elongate inelastic ligament 5 fixed at its upper end 7. The other end is attached to a float 1 immersed in liquid, with a near frictionless fixed roller 8 or smooth radiused edge interposed linearly between the upper and lower ends of the ligament. The fluid medium acts upon the ligament causing it to bow approximately into the shape of a catenary curve and to lift the float out of the liquid. An index mark on the base of the float is arranged to move over a fixed scale indicating velocity/volume flow or mass flow on the float chamber vertical wall 4. The shape of the float is designed to linearise the relationship between float lift and fluid velocity. The density of the float is the same as or greater than the liquid density. The ligament may measure wind speed or be inserted in a duct. <IMAGE>

Description

SPECIFICATION Flexible ligament and float type flowmeter DESCRIPTION OF THE INVENTION Technical field to which the invention relates This invention relates to the field of flow measurement. Fluids may flow in pipes, in totally or partially enclosed ducts or freely as in the open air. Flow of water in a river can be likened to flow in a partially enclosed duct of varying cross section.

The flow rate of a fluid in a pipe is the integrated velocity of the individual streamlines which make up the total velocity profile across the pipe. As the velocity profile changes with Reynolds number, the accuracy of measurement depends both on the ability of the flowmeter to effectively integrate the velocity profile and on the nature of the flowing conditions.

Because the nature, composition and condition of fluids can vary widely, numerous flow measuring techniques have been devised, each technique having particular benefits and limitations. The selection of the most suitable device for a particular application, therefore, demands not only a clear specification of the objectives and requirements, but also an understanding of the limitations of each device.

Fluid flow is generally taken to mean the flow of gases or liquids, which generally behave according to the so called Newton formulation of the Shear stress r=/t (5 V/ (5 V, but may also encompass the flow of non-Newtonian fluids such as molasses and other viscous media or of slurries composed of solid particles suspended in a liquid medium.

Background art Flowmeters can be conveniently split into the following categories: a. Those which are predominantly used for the measurement of flow volume such as positive displacement flowmeters (rotary piston, oval gear, sliding vane, reciprocating piston or flexible bellows meter) or mechanical inferential flowmeters (vane, fan or paddle rotors to turn an output shaft coupled to a mechanical counter).

b. Those which are predominantly used for the measurement of flow rate such as differential pressure devices (orifice plates, venturis, dall tubes, variable area meters, etc).

c. Those which are widely used for both flow rate and flow volume measurement.

These devices are usually flow rate devices which maintain an essentially linear output within a small error deviation over a suffici ently wide flow range to permit accurate mea surement of flow volume. Typical dual function devices are electromagnetic, turbine or Pelton wheel, vortex and ultrasonic flowmet ers. This invention would come under category c. above. Most inventions of flow indicating devices using deflection of a member, relate to the defelction of a metal cantilever in a flowing fluid with various ways of relating deflection to fluid velocity. Such devices are described in Patent Specifications 1252433, 1194187, 830211, 988822, 1099451 and 1241389.However, Patent Specification 2120792A decribes a method of measuring fluid flow using a flexible elongate inelastic ligament and it is a resisting and biassing means associated with the said ligament to which the present application referes.

Disclosure of the invention as claimed This invention relates to the indication and measurement of fluid flow, particularly, but not exclusively to the measurement of wind velocity in the open air but other uses envisaged are for the measurement of fluid velocity, mass flow or volume flow in flow passage such as ducts or pipes.

This invention relates to a flexible elongate inelastic element or ligament fixed at one end, the ligament being vertical, its other end being attached to a resisting means to exert tension on the ligament and a biassing means to linearize the scale reading on the housing means.

More particularly this invention relates to a resisting and biassing means combined in the form of a float immersed in liquid that uniquely suits the flexible ligament method of indicating and measuring flow.

The float is attached to the end of the ligament remote from the fixed end and interposed linearly between the two ends a fixed roller or smooth radiussed edge over which the ligament is free to move lengthways so that when the portion of the ligament between the fixed end and the fixed roller is subjected to a fluid flow such that it is deflected into a curve by the-dynamic pressure thereof, the deflection causes the float to the lifted out of the liquid in which it is immersed. Equilibrium occurs when the effective weight of the float exerts a tension in the ligament up to the fixed roller, equal and opposite to the tension exerted by the dynamic force of the flowing fluid as it impacts against the ligament on the other side of the fixed roller.

A mark on the float aligned with a minimum scale mark on a fixed scale on the housing means, at zero or minimum flow, will move up the scale as the fluid velocity increases, each equilibrium position of float and ligament representing a unique velocity of the fluid.

When held upright in still air the ligament is arranged to have no slack with the float rest ing with its base on the bottom of the hous ing, but when relative motion occurs between the air and the ligament (boundary cases are stationary ligament, moving air and stationary air, moving ligament) the ligament is defe lected into a curved shape approximating a catenary. The deflection is proportional, though not linearly, to the fluid (wind) velocity.

The measurement of fluid (wind) velocity relies upon the fact that the ligament is deflected, the extra ligament to feed the deflection being obtained by an appropriate upward movement of the float.

The force per unit length exerted by the flowing fluid on the ligament is approximately proportional to the square of the fluid velocity.

By applying this force to each unit of active length of ligament (i.e. that length between the fixed end and the fixed rolier) and obtaining an expression for equilibrium with the resisting means (float) it is possibie to design the shape of the float so that the vertical scale on the casing can be calibrated linearly.

A major consideration in the design of the float profile is whether the liquid is considered infinite in extent (i.e.) no change in liquid level as the float rises and falls) or whether the float is in a chamber of limited volume.

In the example of the invention described here, it is necessary to provide the fixed support for the ligament by attaching a frame to the housing means or body of the device, in order that the whole device may be self-contained but most cases where fluid velocity is required to be measured occur in pipes or ducts so that the wall of the pipe or duct may provide the support required for the fixed end of the ligament.

The advantages of this invention are as follows: a) the design integrates naturally, aesthetically and ecnomically with the flexible ligament method of flow measurement b) by suitable design of float chamber, float and density of immersing fluid, a large range of flows of liquids of different properties can be measured c) the device can be produced relatively cheaply.

d) there are a minimum of mechanical moving parts to introduce errors into the system e) the float rises and falls without friction in the casing f) the float chamber does not have to be isolated from the process fluid. For the case of gas in pipe (or wind gauge application) the float chamber is below the pipe and thus the liquid in which the float rests, being the heav ier of the two fluids, will remain in position in the float chamber and not escape into the duct except by evaporation. This loss can be made good by priming the float chamber with the requisite liquid before using the device.

For the case of liquid in pipe, the float cham ber is arranged to contain air or gas and to be on top of the horizontal pipe and the float, initially floating with its larger portion out of the liquid, is pulled down into the liquid as fluid velocity increases.

g) None of the advantages claimed for the flexible ligament invention is prejudiced by integration with the float design. In fact the float design assists in simplifying the overall design, installation and maintenance of the combined device.

h) For the pipe case, the desgin remains simple for all pressures and temperatures of process fluid and is largely self compensating for changes in process fluid pressure and temperature.

Description of Drawings The invention may be carried into practise in various ways and one embodiment will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is the rear elevation of the device, partially sectioned to show internal details.

Figure 2 shows the side elevation with dotted outline of ligament, float a water level for an intermediate position. In Fig. 1 the elongate element 5 is shown as a broad ligament or tape disposed in a straight line between the float 1 and the cylindrical drum 7 used to secure the ligament at its upper end.

When the portion of the ligament external to the housing means 2 and 6, i.e. between the drum 7 and the fixed roller 8, is acted upon by the wind, it will bow into a catenary curve, shown dotted in Fig. 2, thus causing the float to rise.

The profile of the float. its base diameter and height are calculated with respect to the diameter of the enclosing chamber 2 such that for equal increments of wind velocity, equal increments of float rise are obtained thus allowing a linear velocity scale 4 to be marked vertically on the transparent wall of the housing means.

The upper housing means 6 is a compound shaped body, easily formed in a thermo-setting plastic material, containing a screw thread or bayonnet fitting by which the lower housing means 2 can be releasably secured to it, a holding handle 11, a fixed roller 8 over which the ligament is free to slide lengthways and the struts 10 by which the remote (upper) end of the ligmanet is secured and maintained at a fixed distance from the roller.

The struts are bowed outwards in elevation forming a circle, with the ligament along the vertical diameter. The section of the struts, shown in Fig. 2, 15 is such as to cause mini mum air turbulence. Snap shoulders 13 are moulded at the top of the struts for mounting the ligament adjusting drum 7 and locking screw 14.

The lower housing means 2 is composed of transparent cylindrical body and enclosing base. The body will be releasably attached to the upper housing 6 by means of a bayonnet device or some other means. The case 3 con sisting of a cylindrical plastic mesh with top a bottom circular rings guides the float concentrically within the body.

As well as the vertical velocity scale on the transparent plastic lower housing means, horizontal circles are drawn around the casing to make it easy to hold the wind gauge in the vertical position when in use. A filling mark is also shown so that before assembly (the device being envisaged as stored in a dismantled condition although this is not mandatory) water can be introduced into the chamber and the level adjusted.

Drain holes 12 at the top level of the body casing ensure that if overfilled excess water will drain off before use.

When correctly filled, with the two parts of the housing means 2 and 6 joined together, the ligament is attached to cylindrical drum 7 and wound on until all the slack is taken up when the drum is pushed downwards into the snap shoulders 13.

Then the zero or initial reading is finally adjusted by turning drum 7 and locked by means of locking screw 14.

If seawater is used at sea, for instance, the reading of the device should be checked in still air, the locking screw loosened slack in the ligament taken up, locking screw re-tightened and the scale reading noted. In use, this reading should be substracted from scale readings obtained in order to give the true velocity.

Best Mode of Application The possible applications of this invention are so varied as to make the choice of a best mode very difficult. The embodiment described above is seen as applying specifically to the leisure industry where it could be used by Yatchsmen, hang glider pilots and others, whilst an obvious practical application would be for the measurement of wind speed at weather stations or elsewhere.

However, the most prolific field of flow measurement known to the inventor exists on chemical & petrochemical plant, natural gas pipeline networks and so on.

Figs. 3 & 4 show a typical example of a pipeline application with gas flowing in the pipe and Fig. 5 a pipeline application with liuquid flowing in the pipe.

Dealing first with the "gas in pipe" case, the clamped metal ring 21 is clamped between pipe flanges 29. The metal ring has a slightly tapered bore from each end, giving a slight venturi effect sufficient to keep foreign matter in the gas from collecting in the bottom of the pipe around the ligament.

A milled face 18 on the ring together with a bore 19 and a counterbore 20 are arranged to take the upper housing means 6 whose lower face is machined & drilled, to hold by means of nuts & bolts the lower housing means 2 which holds the float 1 & cage 3. Sealing of upper & lower housing is by means of "O" rings 16 & 17.

For low pipe pressures where the lower housing can be made of transparent material the scale can be marked directly on the housing as with the hand held wind gauge. For higher pressures the housing must be made of metal but a transparent tell tale device Fig. 4, 28 can be added below the lower housing thus reducing the bursting force to a safe level for transparent material.

On the remote side of the pipe plastic collets 27 hold the ligament and integral with the collets a tensioning device consisting of snap shoulders 26 & rotatable drum 25 with leak proof penetration of cap 23 by device 22 for externally taking up the slack in the ligament & setting the zero position or initial scale reading. Finally a cap 23 seals the collets in the housing, maintaining the pressure integrity of the pipeline by means of an "0" ring seal 24. When pipe flange bolts are at 12 o'clock & 6 o'clock transition piece 32 will be required as shown in Fig. 6. Fig. 5 shows the liquid in pipe case which requires that the float chamber 6 be mounted on the top side of the clamped ring 21. In this case instead of the ligament lifting the float out of the liquid, it now pulls a very light float 1 down into the process liquid.There is only sufficient float immersed in the zero flow case to support the float 1. Again, high pressures are catered for by means of a tell-tale, the diameter being such that the bursting forces can be withstood by transparent material.

For constant, pressure process liquids a simple float chamber can be used with float shape designed to allow for changing liquid level as the float is immersed.

However where there may be substantial variations in process liquid pressure, a sensor 36 may be employed to maintain the liquid level constant by means of pressurising or venting chamber 3 1.

It is obvious that there is no limit to the diameter of pipe or dimension of duct that can be spanned by a ligament and so a wide variety of near ambient pressure applications is envisaged, such as behind filters used in the air intakes of large centrifugal/axial compressrs, in order to sense when the filters become dirty & the velocity behind the filter therefore falls.

By correct choice of ligament material, the device can be used to measure velocity along large gas turbines exhausts, a problem hitherto unsolved, in the field.

Claims (9)

1. A solid body (called a float for sake of convenience rather than accuracy) designed to be immersed in a particular liquid and attached to a flexible ligament such that as the ligament exerts an upward force on the float, the float is lifted out of the liquid until it exerts an equal and opposite force on the ligament.

2. The combination according to claim 1 whereby the float is designed to be neutrally buoyant (i.e. to have the same density as the surrounding liquid) when it is desired to read gas velocities or flows from zero upwards.

3. The combination according to claim 1 whereby the float is designed to be denser that the surrounding liquid when it is desired that the lowest gas flow indicated i.e. flow datum, is greater than zero. Thus it is envisaged that by suitable selection of float density any chosen flow datum may be met.

4. The combination according to claim 1 whereby the float profile is designed in such a way that whatever the law of the applied forces, equal increments of flow will result in equal increments of lift of the float i.e. the effect of the appiied forces will be linearized in the read-out of vertical lift of float.

5. The combination according to claims, 1,2,3, and 4 whereby the float profile is designed to suit inmersion of the float in a chamber volume varying from infinitely large (i.e. no change in liquid level as the float is lifted out of the liquid) to a minimum volme envisaged as being a cylinder of slightly larger diameter then the largest diameter of the float.

6. The combination according to claims 1,2,3,4 and 5 whereby the float profile is designed in such a way as to indicate any desired flow range within the limitation of the ligament.

7. The comdination according to claim 6, whereby the solid body is a genuine float, floating partially submerged in the liquid whose velocity/flow is to be measured. In this case the float and chamber are above the pipe or duct carrying the process liquid and as the ligament bows under the influence of the flowing fluid it exerts a downward pull on the float, causing greater submersion of the float until upthrust on the float is in equilibrium with the force on the ligament.

8. The combination according to claims 6 & 7 except that float lift/fall is converted to an electricai signal for remote reading of the fluid property required or for use as a signal to trigger some other device.

9. The combinations substantially as herein before described with reference to the accompanying drawings.