Discussion on the application of the hottest leak

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Discussion on the problems of leakage detection technology in production application

Abstract: This paper introduces several commonly used leakage detection methods in the production process, and discusses some factors that affect the leakage test, such as test volume, temperature, test pressure, stability time and sealing form

key words: leakage; testing; Leakage testing machine

1 overview

the traditional leakage detection method is to fill the object to be tested with water or other media, and through observation, measure the reduction of filling media in a specific time (such as by detecting the decrease of liquid level, etc.), which is a direct measurement method. Based on this method, another method is derived, that is, the object to be tested is filled with a certain pressure of gas medium (usually compressed air), and then placed in water for observation, and whether bubbles are generated around the object to be tested is taken as the standard for leakage

with the progress of technology and the improvement of detection methods, the so-called "absolutely no leakage" or "no leakage" is just a quantitative concept, which has been accepted by people. A more accurate and quantitative standard is needed to judge whether a measured object is leaking or not, especially for some occasions where small leakage needs to be measured

The emergence of the leakage detector provides a better solution to the above problems, which makes the leakage detection process more convenient and the measurement results more reliable. On the basis of using the leakage detector, the leakage testing machine, which can be used in the processing production line, can be composed of the above, blanking mechanism, automatic sealing device, electrical control, hydraulic, pneumatic system, etc. The appearance of leak test detector makes it possible to detect the leakage of parts. Using this device can meet the requirements of detecting the leakage of parts in mass production and greatly improve the quality of products

because this measurement method is different from the traditional measurement method, we often encounter some problems that need to be clarified in the actual production and application. In most cases, these problems are the reflection of the state of the test system under the measurement conditions at that time. Therefore, the correct understanding and treatment of these problems will help this measurement method to be better applied to the production line

2 there are many commonly used leakage detection methods, and their measurement principles and methods are also different. Generally speaking, under the condition of mass production, the following measurement methods are commonly used

2.1 absolute pressure measurement

the absolute pressure measurement system is shown in Figure 1, which is composed of air source, air filter, pressure gauge, charging valve, pressure sensor, etc

Figure 1 measurement principle of absolute pressure method

the measurement process is as follows:

inflation: the inflation valve is opened and the specified pressure of gas is filled into the part to be tested

stable: the charging valve is closed, and after a certain time, the charged gas reaches a stable state necessary for measurement. When the mold temperature is 50 ℃, the crystal state can be formed rapidly. The pressure sensor sets the pressure value before the end of the stabilization phase (the beginning of the measurement phase) to a measured zero point

measurement: within the specified measurement time, the detection system detects the change value of pressure Δ P. Compare with the set pressure change limit value, so as to make a qualified or unqualified judgment

exhaust: exhaust the gas inside the test piece into the atmosphere after the measurement

the typical measured pressure time relationship is shown in Figure 2

Figure 2 measurement of pressure time curve

2.2 measurement by differential pressure method

the measurement principle of differential pressure method is shown in Figure 3. The measurement process of the differential pressure method is similar to that of the absolute pressure method. The difference between the differential pressure method and the absolute pressure method is that the differential pressure method uses a reference part to be added to the measurement system, and the differential pressure sensor is used to record the pressure change between the measured part and the reference part in the measurement stage Δ P。

Figure 3 Schematic diagram of differential pressure method

the differential pressure method is similar to the absolute pressure method, which indirectly measures the leakage rate by measuring the pressure change value

2.3 flow detection method of laminar orifices

the detection system using laminar orifices is composed of parts as shown in Figure 4. During the test, first fill the test gas with the specified pressure Pt into the workpiece to be tested and the reference part. When the set pressure PT is reached, the inflation valve is closed and enters the stable stage. After the stable stage is completed, the balance valve between the workpiece to be tested and the reference part is closed. If there is leakage at the test part, compressed air flows through the laminar flow pipe to the workpiece to be tested. This measurement method belongs to the direct detection of leakage flow

Figure 4 laminar flow tube detection system figure

3 selection of leakage detection method

first compare the absolute pressure method with the differential pressure method. Generally speaking, the resolution of a pressure sensor is 1/5000; It is 10Pa for a 5bar sensor and 1pA for a 0.5bar sensor. Because the sensor of differential pressure measurement only needs to measure the change value of the system pressure relative to the measured pressure, the range of the sensor can be greatly reduced. Moreover, because the differential pressure method takes the measured pressure of the system as the test zero point, and only records the offset value between the pressure and the system pressure, such as zero drift, the influence of temperature drift is only related to the measurement range of the differential pressure sensor, and has nothing to do with the measured pressure during the test. For example, the drift of 1/1000 is 100Pa for the test pressure 1bar system; On the contrary, the pressure difference method is used to measure the drift of 1/1000. For a sensor with a range of 2000Pa, its value is 2PA

it can be seen that in terms of measurement accuracy, the measurement accuracy of differential pressure method is higher than that of absolute pressure method. However, in the actual production conditions, generally speaking, the measurement accuracy of the absolute pressure method can meet the measurement requirements, and because the absolute pressure method does not need reference parts, the measurement system is relatively simple, and the corresponding cost can be reduced to reduce the company's recycled waste copper. As for the case of large leakage, it is recommended to use the flow method for testing

4 factors that affect the accuracy of leak detection

4.1 influence of detection volume

for a specific 2014 value of leakage rate, if the detection volume increases, the speed of corresponding pressure reduction decreases, so the measurement time needs to be increased accordingly. Under some specific conditions, if we do not try to reduce the measurement volume, the sensitivity required for measurement may not be achieved

4.2 influence of test pressure

the dependence of leakage rate on test pressure is different for different measurement conditions. Generally speaking, when the porosity (such as casting bubbles and cracks) is high, the test pressure has a greater impact on the leakage rate, while when the porosity is low, the impact is small. In addition, with the increase of test pressure, it will also bring a series of problems, such as the influence of temperature, the required stabilization time, and so on. Therefore, it is suggested that leakage detection can be carried out within a certain pressure range for specific workpieces, and then a lower pressure that meets the test requirements can be selected as the final test pressure

4.3 effect of temperature

for a gas in a closed container, when the temperature rises, its internal pressure rises. Therefore, the change of temperature inevitably becomes a leakage test that affects the pressure change as the measurement object. Generally, it is estimated that the range of this effect is approximately 0.36% of the measured pressure value for the pressure change caused by every 1 ℃ change in temperature. Therefore, with the increase of test pressure, the influence of temperature will become obvious

when the differential pressure method is used for measurement, when the reference piece used has the same geometry and inner cavity volume as the measured workpiece, the temperature change effect caused by the inflation itself can be eliminated by the test system itself. However, when the temperature of the tested part is inconsistent with the ambient temperature, the influence of this temperature cannot be eliminated by using the differential pressure method. For a leak testing machine applied in the production line, its process arrangement and location in the workshop should be considered in the factors that affect the measurement results. For example, in the production line of engine cylinder block and cylinder head, the leakage test process is often arranged after the workpiece is heated and cleaned. If there is not enough time to cool the workpiece to room temperature after the cleaning process, the measurement results will inevitably be affected

the influence of temperature on the measurement results is also different for different measurement objects. The geometric shape of the workpiece, the volume of the inner cavity, the size of the surface area, the material of the workpiece and so on will become the factors that affect the temperature effect. The surface area of the workpiece is large, the volume of the inner cavity is small, and the thermal conductivity of the material is good, so the influence of temperature is more significant. Under normal measurement conditions, due to the short test time, the influence of temperature will not be very significant. However, if the influence of temperature cannot be ignored, corresponding measures should be taken. When other conditions are difficult to change, a) reduce the test pressure or use vacuum method to reduce the influence of temperature; b) Use the instrument with temperature compensation function. That is, a large number of high-temperature workpieces are tested and recorded Δ P、 Δ T value, make Δ P- Δ T curve, made according to the curve Δ Ply (non conformance limit)- Δ T curve, and then enter the value into the temperature compensation program. In order to make the temperature compensation accurate and reliable, these works must be carried out under the on-site production conditions

4.4 effect of stabilization time on measurement

when inflated, compressed air enters a closed container from a pressurized state, which will cause a series of thermodynamic dynamic changes, that is, when a certain volume of compressed air is quickly moved to a closed container, its pressure will drop. If measured at this time, this pressure change will be regarded as a pressure change caused by leakage, which will affect the accuracy of measurement results

this "flushing effect" is affected by inflation pressure, test volume and test piece material. When the inflation pressure or test volume increases, the pressure drop caused by this inflation will become obvious

one of the ways to solve this problem is to add a period of stability time between inflation and measurement to eliminate this effect. How long the stabilization time needs depends on the specific measurement object. For example, the test of a cast iron test object with an inner cavity volume of 1000cc and an inflation pressure of 3bar shows that it takes about 30s to completely eliminate this effect

in the production line, too long stabilization time will affect the production beat of the production line. At this time, the feasible measure is to use the differential pressure method for measurement. In the differential pressure measurement system, due to the introduction of the reference, this effect occurs simultaneously in the test piece circuit and the reference piece circuit, so this effect can be reduced to a great extent. The selection of reference parts can be:

1) parts exactly the same as the test parts

2) adopt a workpiece with a volume similar to that of the test piece, and adjust its volume during the measurement process to achieve a satisfactory balance effect

it should be pointed out that when the introduction of reference parts cannot meet the measurement requirements, the purpose can only be achieved by increasing the stabilization time

4.5 influence of part plugging method on measurement

the plugging of parts must ensure that its position does not change during the measurement process. The usual practice is to use blocking iron at the end of the plugging position to ensure the contact between steel parts. As shown in Figure 5. The situation in Figure 5A and figure 5B ensures that the position will not change after plugging, and the use of plugging device is correct, but the plugging in Figure 5C is inappropriate due to the elastic link of sealing material. For example, for a test with a test volume of 200cm3 and a pressure of 1bar,

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