GB2173909A - Process and apparatus for carrying out pressure tests and measuring leakages - Google Patents

Abstract

The space 2 under test is connected to a reference space 3, the spaces 2, 3 being connected by ducting containing a constriction 1 which has a gap with well-defined, sharp edge. The gap is bridged over by a solid or liquid membrane 4 (Fig, 1) or a liquid droplet 5 pressurised to a critical state, just short of bursting or separating, respectively, The pressures in the spaces 2, 3 are equalised, whereby on any pressure difference arising due e.g. to a leakage the membrane bursts or drop separation occurs; this permits observable and measurable flow and pressure equalisation between the spaces as well as a rapid restoration of the membrane or droplet. The flow may be measured by measuring the volume of liquid displaced or by counting the droplets by means of an electric circuit connected to the constriction which is electrically bridged by a droplet. <IMAGE>

Description

SPECIFICATION Process and apparatus for carrying out pressure tests and measuring leakages The invention concerns a process and apparatus for carrying out pressure tests and measuring leakages. Pressure-tight pressure vessels monitored by certified pressure tests are used in the production, transport and storage of working media. Pressure-tightness and a leak-proof condition are fundamental conditions prescribed by the authorities in all industrial systems.

A pressure test consists of testing the strength at a pressure higher than the working pressure, followed by a leakage test and gas-tightness carried out generally at a much lower pressure.

These tests are carried out by means of non-compressible liquids or elastic gaseous media.

the application of gas mead is, in many cases, unavoidable. Gases can escape faster through even small cracks than liquids, their usage does not cause corrosion, nor problems in reducing pressure or de-aerating during filling and additional blowing out and drying.

The internal pressure of pressure chambers tested by liquid media is subjected to sudden jump-like changes if an increase of volume or leakage occurs. Prior to filling-up a vessel with an expensive, often toxic, inflammable and explosive media, generally a pressure test is carried out with water or compressed air.

During tests closed pressure chambers of different volumes are formed and after charging in the test medium the behaviour of the pressure chamber is examined. Subsequent changes in pressure-tight vessels tested in this way are evaluated from the simultaneously measured and/or recorded measurement data concerning parameters of state or condition (temperature, pressure, volume) indirectly and by way of lengthy calculations. The pressure values, the duration of and the means used in the measurements as well as the times and the maximum value of permitted leakages are prescribed by strict regulations (e.g. Hungarian Standard MSZ 11413/1-6).

The aim of the pressure tests is to detect inadmissible, detrimental permanent deformations of a pressure vessel or its parts (surfaces, seals, fittings, packings etc.) and leakages (either previously or newly formed), discontinuities, cracks, flaws in castings, faulty external and internal packings. In most cases the deformations can only be determined by visual inspection and by measuring complicated local elongations.

The characteristics of pressure tests, irrespective of the magnitude of pressure at which they are performed are controllable by one single measurement, namely, long-term fluid-tightness and constant volume. The permissible values of these characteristics are very small; therefore their measurement is difficult, mainly because their magnitude is comparable with the interfering effects (e.g. temperature).

A rapid performance of pressure tests is also technically and economically advantageous. At present, the duration of measurements is too long, partly due to the smallness of the changes that are prescribed to be measured permissible, partly due to the limited accuracy of the means and methods used to measure them. The pressure-tightness of mass-produced workpieces must be determined in a short time. The tests done by immersing the piece into water or by watching for the formation of bubbles from foamable materials cannot be automated and depend on the subjective judgment and alertness of the operator and so are not reliable. In other cases under adverse or difficult field conditions, many disturbing environmental effects are to be checked and have to be taken into account in the course of calculation of results.

Presently, although the greatest leakage is experienced in pressure tests for tensile strength, the testing of escapes of gas is carried out at low pressure. The obvious reason for this is that the sensitivity of the measuring apparatus with the wider range of measurement is not satisfactory. Such apparatuses are designed for larger loads, and so react rather sluggishly to changes.

The changes occurring in pressure-tight vessels cannot be registered at all, or only after a disadvantageously long measuring time. The highly accurate electronic measuring/computing/recording systems are not used because they are complicated. They cannot be used in the field, nor for measurements where a fire hazard or a danger of explosions exist, due to the special requirements in such circumstances. Currently, no universally applicable apparatus exists for carrying out pressure tests.

In fields in contact with a frequently changing environment, changes in state or conditions occur which deleteriously influence the measurement. The biggest problem of the evaluation of the measurement results is a simple way of distinguishing of various disturbing effects from each other, which effects are intertwined with effects of the test itself. The accurate measuring of changes in temperature, volume and pressure with traditional instruments as well as the correction of the disturbances by indirect calculations is a serious technical problem.

In order to reduce the number of variables to be considered during carrying out the pressure test certain technical measures have to be taken. It is expedient to hold constant certain variables (volume, temperature) or keep them at a known variable value (e.g. by laboratory thermostatic control) or exclude them at initio by known technical solutions, e.g. thermal insula tion, use of a non-elastic well de-aerated liquid working medium; or by thermal compensation according to Hungarian patent no. 184,165.

In the majority of the measuring methods the decrease of pressure in the tested pressure vessel is measured and/or registered. The measure of a characteristic depends on the volume of the tested pressure (which is not accurately measurable) and on the changes in temperature and atmospheric pressure occurring during measurements. Measuring methods based on the replacement of losses during measuring (e.g. feeding/supply compressors, flowmeters operating with a large pressure difference) are of complicated construction, inaccurate and can operate only under restricted conditions of measurement.

An aim of the invention is to eliminate or reduce the problems of the above-described kind and to provide a solution for a simple, quick and accurate determination of the pressure stability or pressure-tightness of pressure vessels and their components.

The aim of the invention is sought to be achieved by filling a pressure chamber with a pressurised test medium, and the change in the state of the medium is measured. According to the invention, the pressure chamber is connected with a reference chamber, pressure equalisation between the two chambers is effected sequentially, the volume of medium flowing between the two chambers is measured and from the readings the rate of escape is determined.

The apparatus according to the invention includes a reference chamber connected to the pressure chamber containing the medium to be tested, wherein a constriction terminating in an edge expediently of conical shape is located between the two pressure chambers and a tensioned or stretched membrane or liquid droplet is arranged in the constriction.

Expediently, the reference chamber is kept at constant pressure.

The membrane or diaphragm is generally a liquid membrane in a limiting position, i.e. on the verge of bursting; similarly, the applied liquid droplet has to be in a limiting state, just before flow or fall of drop.

The stability of the marginal or limiting state is assured by a potential force field (e.g.

gravitational field).

In a preferred embodiment of the apparatus according to the invention, a layer of the measuring liquid with stressed surface at a hydrostatic pressure raised to the limit, i.e. to the verge of rupture or the formation of a drop of the test liquid is positioned over the constriction.

In certain cases, the apparatus according to the invention may be connected to another identical apparatus in such a fashion that the inlet coupling of one apparatus is connected to the outlet coupling of the other apparatus.

Advantageously, the constriction has two mutually isolated conical lead rings terminating in a sharp edge and connected in the measuring circuit.

According to the invention, the task of performing pressure tests is, therefore, simply derivable from the changes of state between two pressure chambers (test and reference chambers) in such a fashion that an exactly defined constriction is formed through which flows of known volume are allowed or caused to occur, without the use of extra closing elements. The flows are periodically equalised, may be achieved with little energy input/consumption, thus creating reproducible conditions of equilibrium.

To achieve the aim of the invention a known phenomenon of physics is exploited. A boundary surface in equilibrium (liquid or solid rupturable membrane) is intentionally placed into the constriction. The membrane bulges out at small loads and on a subsequent load increase bursts, thus permanently fixing the change between the two chambers. This novel method breaks with the additional testing/measuring based on indirect computation from data of pressure/temperature changes. It is based on direct reading of volumetric measurement of the pressure test.

Leakages or escapes are compensated by a metered volume of the medium supplied from a mains or a gas bottle of constant pressure. The system according to the invention can be considered essentially as a kind of differential pressure meter wherein, in the interest of extending the limit of the range of measurement, intermittent pressure-equalisations between the space to be tested and a comparator pressure chamber are carried out.

The method according to the invention has, however, practical value only if a practical and industrially applicable apparatus can be made which ensures the possibility of continuous repetition, the required sensitivity and accuracy and in addition, also the possibility of detection of effects directly related and proportional to the changes.

In one possible embodiment of the apparatus according to the invention the basis of operation is the stressed or 'stretched' surface of liquid, i.e. a membrane, placed over the constriction.

The stressed surface over a sufficiently narrow constriction is preloaded by potential field of forces (expediently e.g. gravity). Under the effect of a load to be measured the preloaded membrane instantaneously bursts, thus equalising the pressures of the two pressure chambers by throughflow and permits a subsequent restoration of the stressed liquid surface. The number of bursts of the membrane are, e.g. optically sensed and summed, thus this method of measuring can be considered as an industrially applicable solution.

In another embodiment of the measuring apparatus according to the invention, a drop of liquid is used, which leads to a directly evaluatable result of measurement.

A stable drop of liquid is placed on the sharp edge of the constriction. The weight of the liquid drop is proportional to the third power of its diameter and is prevented from precipitating by surface-forces proportional to the circumference of the meredium. The drop is, therefore, in equilibrium but it precipitates immediately in response to the smallest relative change of state.

This can be observed in the known way by clearing the opening of the aperture and/or capturing the falling drop.

Also in the sensor of the drop precipitation, it is possible to form a liquid layer over the constriction which is thicker than the stressed liquid membrane and the hydrostatic pressure brings the otherwise automatically forming liquid drop in the constriction to just the critical state of precipitation. The change between the two pressure chambers can be measured by volumetric doses the size of which depends on the chosen diameter of the constriction. Under the effect of the smallest relative change the drop moves always into the chamber from which the escape or leakage originates and/or into the space of expanding volume and lower pressure, where the periodic separation of drops can be counted directly and/or their volume can be measured by collecting the drops.The result of the measurement is the relative change in volume related to the testing pressure during the measurement time.

The periodical volume-measuring apparatus operated with minimum energy input according to the invention can advantageously be applied to instantaneous testing for classifying purposes or to long-term tightness testing. Also, the apparatus is suitable for the observation of many phenomena which are not independently observable taking place in the pressure chamber (e.g.

porosity). The construction and operation of the apparatus according to the invention is simple and lends itself to automation. It is not sensitive to overloads or breakdowns. Contrary to measuring apparatuses based on evaluation of changes, e.g. drop in pressure, it is universally suitable for testing pressure vessels of any volume and pressure.

The result is true, can be simply and directly read independently of the volume of the tested pressure chamber. The multi-purpose apparatus according to the invention is proof against overpressurisation and explosions, can be applied with working media of any physical state to any production process or operation. Although the solutions of the described kind are applicable primarily to carry out pressure tests, they are obviously appliable to detect and measure phenomena occurring in or between the pressurised spaces and calibration of volume meters.

The apparatus according to the invention is further described, purely by way of example, with reference to embodiments illustrated in the accompanying drawings, wherein: Figures 1 to 3 are sectional illustrations of the working principle of the performance of a measurement in a constriction (liquid drops of stretched surface and precipitation thereof); Figure 4 is a diagrammatic layout of measuring the relative change of state in both senses; Figure 5 is a developed sectional view of an embodiment of the apparatus according to the invention registering the result of a pressure test in the case of protracted, long time of measurement, wherein the apparatus is fed from a pressure regulator producing constant pressure; Figure 6 shows another simpler embodiment of the apparatus according to the invention;; Figure 7 shows an embodiment of a sensor applicable to quick and automated classifying operations; Figure 8 illustrates the performance of rapid automatic pressure testing of components (valves, cocks, fittings, tubes) in the phases of a) filling b) measuring c) post-operational safeguarding the continuity of measuring.

Fig. 1 shows a membrane 4 made of liquid or solid material separating at a constriction 1 the pressure chamber 2 filled with a liquid or gas to be tested from a reference chamber 3 at the same pressure as that of chamber 2, keeping the membrane 4 in equilibrium, i.e. in its basic position and in a pre-loaded (pre-stressed) state of bulging-out at an angle 4' pushed forward to the limit of bursting under the effect of a potential field of forces 6 (e.g. gravity, electric field).

The shape of the constriction 1 is either planar with a well-defined edge or concave in top view for the purpose of forming a liquid membrane 4.

Fig. 2 shows a potential force 6 acting on a single liquid drop 5 (e.g. of mercury) placed in constriction 1 chosen to have optimum (do) diameter (increasing proportionally to the mass) and the system of forces 7 proportional with the potential forces 6 distributed on the surface in proportion to the circumference of constriction 1. The shape of the constriction 1 is a doubleconical surface expediently terminating in an edge.

The optimum diameter do can be calculated from an equation expressing the state of equilibrium of the liquid drop 5:

where a=the surface tension of the liquid employed y=the specific gravity of the liquid employed.

From the above equation:

Fig. 3 illustrates the principle of measurement in long-term pressure tests carried out by apparatus according to the invention. The diameter d of constriction 1 is smaller than the optimum diameter do but is still suitable for periodical measurement of volume. It is preselected in such a manner that it should correspond to the volumetric dimensions of the drop. A layer of the measuring liquid 9 is formed above the constriction 1. The thickness of layer 9 is increased up to the point at which the formation of a drop with the desired dimensions (volume) begins.In addition to reacting instantly even to the smallest relative changes, the layer 9 ensures the continuity of the formation of drops and hermetically sealts off (separates) the reference chamber 3 from the pressure chamber 2 filled with the liquid under test which is not necessarily identical with that in the reference pressure chamber 3. With regard to its properties the measuring liquid 9 is always separable both from the medium to be tested and that in the reference chamber 3.

the suitably chosen shape of the constriction 1 is, according to the invention, conical on its upper side with a clean sharp edge at its lower side.

Fig. 4 illustrates an embodiment of the apparatus according to the invention suiable for separate testing, measuring and registering, both as regards the sense and the magnitude of two pressure chambers which, due to their symmetry, can be connected to the apparatus according to the invention. In this particular case, there are two constrictions for the formation of two liquid drops 5 connected in parallel to the inlet-connection piece 37 and outlet-connection piece 38. The non-mixing liquid and/or gaseous test medium immiscible with the materiai of the liquid layer 9 is filled in via valve 11 while a drop-precipitating valve 12 being kept open. The initial filling-in of measuring liquid 9 and the de-aeration of the test medium is carried out by opening valve 13 as required. At the start of the measurement, valves 11 and 12 are closed.If e.g. a decrease in pressure occurs in test pressure chamber 1 (due to an escape or leakage or volume expansion) its initial pressure decreases relative to the initial pressure of chamber 14. In this case a drop is formed in the right-hand side collector chamber 16, transmitted via pipe line 15.

The change related to the number of liquid drops 5 can be determined in a known way by counting or by reading the change of volume (change of level of liquid) in the collector chamber 16. The discharge valve 18 is suitable for trueing and recovering the measuring liquid.

The embodiment according to Fig. 4 is suitable e.g. for measuring relative calorific changes (heat increases, coolings) between the two chambers with the correct sign (plus or minus).

Fig. 5 shows an embodiment of the apparatus according to the invention, wherein the measuring liquid is fed in from reference chamber 3, indicated by an arrow, in a pressure regulator giving a constant basic signal. For long-term pressure tests with long measuring time, controlled level regulation has to be provided. This is done by a float 20 ensuring a constant hydrostatic pressure supplied e.g. from a storage chamber 19. The filling-in of the measuring liquid is carried out at the 'open' position of the discharge valve 12. The liquid layer 9 is protected from reaching the critical limit of precipitation by a nut 21 with a pin 25 or by lifting a seat 23 by forward movement of a rod 22. During measurement the measuring liquid layer 9 is subsequently replaced via a pipe 24 from a storage chamber 19.

Valve 22 is an overflow keeping aperture 27 open during filling-in the measuring liquid 9 via valve 13.

Fig. 6 illustrates an embodiment of the apparatus according to the invention which is considerably simpler than the embodiment illustrated in Fig. 5 and is suitable for carrying out so-called accelerated pressure tests. The result of measuring escapes is given in recorded, immediately readable, true gas volume figures.

In this novel apparatus a swimmer or float 20 with an air tank automatically follows the changes in the level of liquid during measurement. Measuring liquid layer 9 has a high h, thus always providing a 'pre-stressed' condition assuring maximum sensitivity. Fig. 6 illustrates the state wherein the top-up of measuring liquid 9 from storage chamber 19 is taking place at the 'open' position of valves 12 and 13. Storage chamber 19 can be filled up in a funnel 40 by closing valve 13 and turning filling valve by 90". With this kind of periodical filling by opening valves 12 and 18, the result of measuring can be removed without interrupting the test, therefore the range of measurement is practically infinite. Valve 12 has to be opened at each intervening action and kept closed during measuring.This 'shunt-circuit' serves for the purpose of load relief.

Fig. 7 illustrates a possible design of the sensor of an embodiment of the apparatus according to the invention suitable for quick and automatic classification. The liquid drop 5 (e.g. mercury) having the optimum (do) diameter 10 is in its initial position in the constriction 1, located in separating pipe 28 (made preferably of clear glass) formed by the boundary-edge of conical guide rings 29 separated from one another, ensuring closure ('making') the measuring circuit 30.

The change between test chamber 2 linked to stoppers 31 and reference chamber 3 tears off liquid drop 5. The sense of measuring has to be chosen in such a manner that the precipitation of the drop should occur in the lower position 5' while the interruption of the measuring circuit 30 can be sensed. Liquid drop 5 does not disappear in case of overloading the constriction 1 (e.g. large escape) because the transverse bore 33 in trap chamber 32 channels off the sudden flow and brakes the drop.

Fig. 8 illustrates phases of quick automatic testing of components by pressure tests in a test chamber 2 of an apparatus according to the invention illustrated in Figs. 4 and 6 (but without the isolating valve 12).

Diagram 8a shows the filling-up process wherein the test medium flows unimpededly through constriction 1 from reference chamber 3, supplied from a pressure regulator indicated by an arrow, via the sensor shown in Fig. 1 inclined at an angle e.

Diagram 8b shows a phase of the measuring process wherein liquid drop 5 with optimum (do) diameter 10 automatically separates the two pressure chambers 2 and 3 by closing constriction 1 formed by the straight edge of conical guide rings 29 positioned opposite to the vertical axis.

When an escape occurs the measuring circuit 30 is interrupted by the liquid drop 5 precipitated from the conical guide rings 29.

Diagram 8c illustrates the completion of the measuring process or sensing the scape, wherein an instantaneous tilt of angle ss is shown, with the test medium being discharged in direction 36.

The liquid drop 5 ensures both precipitation (separation) and sensing, whenever any flow occurs in direction 36, the drop arriving to the side of constriction 1 which is on the side of reference chamber 3.

The apparatus according to the invention has a simple construction, lends itself to automation, is overload-proof, is suitable for quick and accurate classification and the measurement supplies true data recordal, and can therefore be used widely in industry.

The solution according to the invention has many advantages over the traditional measuring processes and apparatuses.

The process is based on correct principle of physics, renders possible to carry out testing of tensile strength and escape (leakage) at any pressure and producing true data that can be recorded, if so required. These two kinds of tests can be carried out with one single measurement substantially to shorten the time for carrying out pressure tests. The solution eliminates the disturbing effect of changes in pressure, thus making its measurement unnecessary, and does not affect the evaluation of the test.

The apparatus according to the invention has a simple construction, is accurate at high pressures and is an explosion-proof instrument, therefore is universally applicable.

List of Reference Numbers Fig. 1 1 constriction 2 test chamber 3 reference chamber 4 membrane (diaphragm) Figure 2 5 drop of liquid 6 potential field of forces 7 system of surface-stress forces Figure 3 9 measuring liquid layer Figure 4 11 valve 12 separating valve 13 valve 14 chamber 15 pipe 16 collector chamber 17 result of change of state (phase) 18 discharge valve Figure 5 19 storage chamber 20 float 21 nut 22 rod 23 seat 24 pipe 25 pin 26 valve Figure 6 28 isolating (separating) pipe 29 conical guide ring 30 measuring circuit 31 plug 32 trap chamber 33 transversal bore Figure 7 36 sense (direction) of test medium 37 inlet connection piece (nipple) 38 outlet connection piece (nipple)

Claims (14)

1. A process for carrying out pressure tests and for measuring escapes or leakages, wherein the space under test, e.g. a pressure chamber, is filled with the test medium, the test medium is pressurised, wherein the test chamber is connected to a reference chamber, at successive timeintervals pressure equalisation between the two pressure chambers is effected, the volume of the medium flowing between the chambers is measured and the rate of escape is determined from the latter.

2. Apparatus for carrying out pressure tests and measuring escapes, comprising a reference chamber connected to a chamber under test, and a membrane with a stretched surface or a drop of liquid is arranged in a constriction terminating in an edge located between the pressure chambers.

13. Apparatus according to claim 2, wherein the reference chamber is kept at constant pressure.

4. Apparatus according to claim 2 or 3, wherein the membrane is a liquid membrane.

5. Apparatus according to claim 4, wherein the membrane is in a limiting state, prior to bursting.

6. Apparatus according to claim 2 or 3, wherein the drop of liquid is in its limiting state, prior to precipitation (drop separation).

7. Apparatus according to claim 5 or 6, wherein the limiting state is brought about by a potential field of forces.

8. Apparatus according to any of claims 2 to 7, wherein the constriction is generally conical.

9. Apparatus according to any of claims 2 to 8, wherein a layer of the measuring liquid with stressed (stretched) surface is located above the constriction and the hydrostatic pressure of the liquid layer is raised to the limiting state prior to bursting of the membrane or prior to precipitation (drop separation) of the drop of liquid.

10. Apparatus according to any of claims 2 to 9, wherein it is connected to an identical apparatus in such a fashion that the inlet coupling of one apparatus is connected to the outlet coupling of the other apparatus.

11. Apparatus according to any of claims 2 to 10, wherein two mutually isolated generally conical guide rings with well defined edges are located in the constriction and are included in an electrical measuring circuit.

12. Apparatus according to any of claims 2 to 11, wherein it is provided with a drop collector chamber for storing the precipitated (separated) liquid drops.

13. A process according to claim 1 substantially as herein described with reference to and as shown in any one of Figs. 4 to 8 in combination with Figs. 1 to 3 of the accompanying drawings.

14. Apparatus according to claim 2 substantially as herein described with reference to and as shown in any one of Figs. 4 to 8 in combination with Figs. 1 to 3 of the accompanying drawings.