GB2217462A - A removable fluid flow meter - Google Patents

Abstract

A fluid flow detection or measuring apparatus of the differential pressure type comprises a pipe 10 having a wall opening in which is positioned a closure 12. The closure 12 is sealed in place by sealing flange 30 and gasket 40. Suspended from closure 12 is a streamlined bullet 20 having upstream 21 and downstream 22 tapering portions. In operation of the device the pressure difference between pressure tappings 13 and 14 is recorded by differential pressure measuring device 17. After measurement, the bullet 20 may be removed through the wall opening and unconstricted flow re-established by replacing closure 12 with a plain pipe section. <IMAGE>

Description

FLOW DETECTION OR MEASURING APPARATUS This invention relates to the measurement of fluid flow in pipes and in particular to apparatus for detecting or measuring such flow.

There are several ways of measuring fluid flow within a pipe or the like. This invention is an apparatus which works on the established principle of using differential pressure measurement to determine flow rate. The flow rate is derived in volume or velocity terms from a measurement of the fluid pressure at two discrete points where the cross-sectinnal area of the pipe bore is different, by means of Bernoulli's equation: Q A2 AP Q ( ( A2 )2 ( A1 where Q is the flow rate; 1 oP is the differential pressure; A1 is the cross-sectional area of the unconstricted pipe, and A2 is the cross-sectional area of the available flow path at the point of constriction.

This relationship is valid provided the fluid flow is non-turbulent between the two pressure measurement points.

Prior art devices of the differential pressure kind include the orifice plate type and the Venturi type.

An orifice plate device incorporates a constriction in the form of a plate of thin material with a hole which can be easily installed in the pipeline by insertion between flanges. The orifice presents a sudden reduction in the fluid flow path which produces a pressure differential across the plate. The pressure difference between the upstream and downstream sides of the plate is measured using pressure taps located on either side of the device and is registered by a differential pressure recorder.

In practice, extensive empirical data are available about the flow of various fluids through an orifice, so the rate of flow can often be determined directly from the differential pressure reading without recourse to Bernoulli's equation and calculations from first principles.

The most usual configuration for an orifice plate is with a circular hole positioned concentrically with the pipe bore, but for certain applications the hole can be positioned off-centre.

Alternatively, a segmental plate can be used to generate the same effect of constriction. Whatever form the orifice plate takes, its major advantage is the short overall length which facilitates its installation between pipe flanges. However, this very geometry also constitutes its most serious drawback: The abrupt profile promotes turbulence downstream of the orifice and thereby causes irreversible pressure losses. This type of pressure differential generating device is therefore unsuitable for measuring fluid flows where maintenance of fluid pressure is important.

The Venturi type of device relies upon a constriction formed in the pipe wall. The pipe has a convergently tapering inlet cone and a downstream expansion cone. The reduced cross-section parallel-sided portion between the inlet and outlet cones is termed the Venturi throat and pressure taps are positioned at the Venturi throat and upstream of the inlet cone. The differential pressure across the Venturi throat can be recorded directly or may be translated into actual flow units by means of various types of differential pressure meters and capacity curves. A Venturi is employed wherever the permanent loss of pressure is required to be minimised or in cases where the fluids handled contain sufficient amounts of material in suspension that other devices such as orifice plates are inappropriate.

The Venturi is essentially maintenance free by virtue of its streamlined internal configuration which is in contrast to the abrupt profile of the orifice plate. These smooth contours allow laminar flow and efficient pressure recovery whilst minimising erosion and clogging.

The biggest advantage of the Venturi is its low pressure loss compared to other types of differential pressure generating devices. Its greatest drawback, however, is that it is effectively a permanent fixture in the pipeline. For constant monitoring applications the Venturi is perfectly adequate, but if spot checking along a length of pipe or initial balancing of the flow in a new system is all that is required, use of a Venturi is not convenient. Its removal involves considerable down time whilst the pipeline is breached and an unconstricted length of pipe is inserted in its place. In some safety-critical applications certification of new joints may be required.

It is an aim of this invention to avoid some of the drawbacks of these known devices by providing a differential pressure flow measuring apparatus suitable for temporarily monitoring fluid flow whereafter the differential pressure generating device can be removed quickly and easily so that fluid flow is no longer disturbed whilst minimising irreversible pressure losses in operation.

The invention is a flow measuring device of the differential pressure type to enable the detection or measurement of fluid flow within a pipe, comprising: a section of pipe having a wall opening; a bullet having a streamlined shape to minimise turbulence in longitudinal flow; support means to position the bullet spaced apart from the pipe inner surface with the longitudinal axis of the bullet substantially parallel to that of the pipe; closure means to close the pipe wall opening, and first and second pressure tappings respectively located adjacent the bullet and at a position remote therefrom, wherein the bullet and support means are removable through the pipe wall opening.

This apparatus makes use of the same principles as the conventional Venturi but it overcomes one of the previously mentioned disadvantages of the latter because the support means and bullet can be removed with comparative ease and without disturbing the integrity of the pipeline joints. The support means and bullet may be removed for cleaning or replacement if damaged, thus reducing repair costs, or they can be interchanged for others of different materials as necessary, giving the device wider applicability. The bullet can also be replaced with one of a different size if pressure readings are required for a variety of different cross-sectional areas or if the first bullet selected is too large and creates a blockage.

The major advantage of this invention, however, is that the bullet and support means can be completely removed and the pipe wall opening resealed so that unobstructed flow can take place in the pipeline after pressure measurements have been taken. A pipe having a series of wall openings along its length with strategically positioned pressure tappings can be used to make spot checks on the flow parameters throughout an entire pipe run. This is particularly useful for initially balancing the flow in a newly commissioned system.

The calculation of the rate of fluid flow is similar to that for other differential pressure flow measuring systems since Bernoulli's equation still applies. In order to simplify the calculation of flow rate it is preferable to use a bullet whose cross-sectional area is half the cross-sectional area of the pipe bore.

The fluid flow may be perturbed in the region immediately downstream of the bullet to an extent which is to some degree dependent on the configuration of the bullet. It is preferable to position that pressure tapping which is to give the fluid pressure in the unconstricted region upstream of the bullet. If this is not possible, for example when other fittings are located just upstream of the bullet, the pressure tapping may be positioned downstream at a point at least five pipe diameters from the bullet where disturbance in the fluid flow is negligible.

The streamlining of the bullet is most effective in minimising turbulence of it mimics the convergence/divergence ratios of a typical Venturi. Therefore the bullet is preferably tapered at its upstream end to a cone with an inset angle of 45" and at its downstream end with an inset angle of 30 , since this is the optimum configuration to reduce turbulence to a minimum.

Advantageously the support means is also streamlined for the same purpose.

Optimum performance will only be achieved with such a non-symmetric bullet if it is inserted in the fluid stream the right way round.

For this reason it is preferred to use a bullet which is integrally formed with a hatch cover having an array of fixing means arranged non-symmetrically around its periphery for sealing to a similarlyconfigured wall opening. In this way correct orientation of the bullet is ensured.

As with conventional differential pressure generating devices described above, the device of this invention can be readily used in conjunction with direct read-out instrumentation having high and/or low level linear alarm settings if required. This enables predetermined maximum and/or minimum flow rates to be defined so that a warning is given if the flow fluctuates to a value outside the acceptable range.

The invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a part longitudinal section through a fluid flow device constructed in accordance with the invention; Figure 2 is a section on line AA' of Figure 1, and Figure 3 is a view from above showing the non-symmetric arrangement of fixing holes in the hatch cover.

Referring to Figures 1 and 2, a section of pipe 10 has a bore 11 and is provided at its ends with flanges 15 and 16 which are secured to like flanges of adjacent pipe lengths in operation of the device. The pipe has a wall opening in which is positioned a transversely removable portion or closure 12 of arcuate form to match the internal profile of the pipe. The closure 12 is sealed in place by a sealing flange 30 and gasket 40.

A streamlined bullet 20 of circular cross-section and having a cross-sectional area half that of the pipe bore is suspended in the centre of the pipe bore from support means 25. The support means 25 is in the form of twin webs which are integrally formed with the closure 12.

A differential pressure measuring device 17, such as a manometer, is connected across first and second pressure tappings 13 and 14. The first pressure tapping 13 is positioned in the pipe wall adjacent the parallel-sided portion of the bullet and second pressure tapping 14 is positioned at a remote location upstream therefrom. It is desirable that flow upstream of pressure tapping 14 should be unperturbed by fittings such as valves or reducers. Depending on the nature of the fluid passing through the pipe system, it may be desirable to protect the differential pressure measuring device 17 by interposing filters (not shown) in the lines from the pressure tappings 13 and 14.

In operation, fluid flows along the pipe section 10 in the direction indicated in Figure 1. Bullet 20 is tapered at its upstream end 21 to a cone with an inset angle of 45" and at its downstream end 22 to a cone with an inset angle of 30O. This streamlined configuration minimises turbulence as fluid flows across the bullet in the longitudinal direction and thereby aids pressure - recovery downstream.

The fluid pressure drops at the bullet because of the increase in velocity which results from the reduction in area of the flow path. The differential pressure measuring device 17 enables the difference in pressure to be determined between the fluid flowing in the unconstricted pipe bore past pressure tapping 14 and fluid flowing past the bullet at pressure tapping 13. To calculate the flow rate the cross-sectional area of the flow path at each of the pressure tappings must also be determined.

The calculations performed to determine the flow rate will be the same as those used for a conventional Venturi.

Referring now to Figure 3, correct orientation of the bullet within the pipeline is ensured by providing the sealing flange 30 and gasket 40 with a non-symmetric array of fixing holes 31.

If the bullet 20 is inserted in the pipeline with its shorter end 21 facing downstream there is an increased risk of turbulence and pressure recovery is compromised.

The major advantage of the arrangement as described above is that the bullet 20 and support means 25 are easily removable from the pipe 10 without disturbing the integrity of other pipeline joints such as those at flanges 15 and 16. Unconstricted flow can be restored by replacing the closure 12 and its integral bullet with a plain pipe section so that the pipe has a smooth internal contour. The down time for this replacement operation is considerably less than the time that would be necessary to replace a conventional Venturi.

In a modification of the invention (not shown) the bullet may be detachable from the closure rather than integral therewith.

Thus the number of components required as stock items is reduced because the same closure can be used with or without the bullet and a single closure can be used with a variety of bullets of different sizes. The method of attachment of the bullet to the closure can take a variety of forms. For example, the bullet support means may be a single screw-threaded spigot which is receivable in a socket of complementary screw-thread located on the closure. Alternatively, the bullet support means may take the form of a pair of studs which penetrate the closure and which are secured in place on the outside by threaded nuts.

Although the invention has been described with particular reference to pipes with circular bores, it is equally applicable for pipes with other cross-sections such as square or rectangular ducting.

To minimise turbulence and aid pressure recovery, the bullet should be streamlined as before and may have a cross-sectional shape matching that of the pipe if desired.

Many other modifications may be apparent to those skilled in the art without departure from the scope of the claims which follow.

Claims (8)

1 A device of the differential pressure type to enable the detection or measurement of fluid flow within a pipe, comprising: a section of pipe having a wall opening; a bullet having a streamlined shape to minimise turbulence in longitudinal flow; support means to position the bullet (and) spaced apart from the pipe inner surface with the longitudinal axis of the bullet substantially parallel to that of the pipe.

closure means to close the pipe wall opening, and first and second pressure tappings respectively located adjacent the bullet and at a position remote therefrom, wherein the bullet and support means are removable through the pipe wall opening.

2 A device as claimed in claim 1 wherein the support means is attached to the closure means by a detachable mount.

3 A device as claimed in claim 1 or claim 2 wherein the section of pipe has a finite length and is provided with flanges at its ends.

4 A device as claimed in any preceding claim wherein the cross-sectional area of the bullet is half that of the pipe bore.

5 A device as claimed in any preceding claim wherein the pipe bore and bullet are of circular cross-section.

6 A device as claimed in claim 5 wherein the bullet is tapered at its upstream end to a cone with an inset angle of 45" and at its downstream end to a cone with an inset angle of 30C.

7 A device as claimed in any preceding claim incorporating differential pressure recording apparatus.

8 A device as claimed in claim 1 and substantially as hereinbefore described with reference to the drawings.