Helium Mass Spectrometry Leak Detection Procedure

1. Purpose:

In order to ensure that the product meets technical requirements, inspection documents are specially formulated to guide on-site leak detection work.

2. Scope:

2.1 Applicable to leak detection operations for all vacuum products of the company.

3. Reference standard:

3.1 QJ 3123-2000 Helium Mass Spectrometry Vacuum Leak Detection Method

3.2 QJ 2861-96 Helium Mass Spectrometry Leak Detection Minimum Leak Detection Rate Test Method

3.3 JHFM.00-YG Nuclear Safety Vacuum Valve Project Factory Acceptance Test Outline

3.4 JHFM.00-SG002 Nuclear Safety Vacuum Valve Project Functional Test Outline

3.5 Technical Specification for LH/3S-9S-05/01 (D) - E-03-A0 Nuclear Safety Grade Vacuum Atmosphere Valve

3.6 GB/T15823-2009 Helium Leakage Detection Method

4. Definition:

4.1 Vacuum leak detection technology:

It is a technology that uses appropriate methods to determine whether a vacuum system, container, or device is leaking, and to determine the location and leakage rate of the leakage holes. The corresponding instrument is called a leak detector. In the manufacturing process of vacuum systems, containers, and devices, vacuum leak detection technology is used to determine their vacuum tightness and detect the location of leaks, in order to take measures to seal the leaks and maintain the vacuum state in the system, container, and device.

4.2 Leakage rate:

The size of the leakage rate needs to be calibrated before it can be determined. Generally, the comparative method is used, which compares the leak detection hole with the standard leak hole on the leak detector to obtain the leak rate of the leak detection hole. The method of detecting leaks in vacuum systems or their components. After pumping a certain container for a sufficient period of time, the pressure of the container no longer changes. At this time, the amount of pumping must be equal to the sum of the leakage and deflation of the container, that is, puSe=qL+q0, where pu is the ultimate pressure of the container, Se is the effective pumping rate at the exhaust port of the container, and qL and q0 are the leakage and deflation of the container, respectively. If the amount of air released is negligible, the equilibrium equation becomes puSe=qL, or pu=qL/Se. This indicates that the ultimate pressure of the container is determined by the ratio of the leakage rate to the effective pumping rate. If the pumping rate is constant, in order to obtain a low ultimate pressure, the leakage rate should be reduced, and leak detection is the key measure.

4.3 Leakage:

Leakage refers to the holes and voids in a vacuum container. The pressure difference inside and outside the container will cause gas to pass through a leak from one side of the container to the atmosphere. Leakage holes are usually very small, and the specific size of the leakage hole cannot be measured. Therefore, the size of the leakage hole is expressed by the leakage rate (the flow rate of gas passing through the leakage hole under specified conditions). There is a pressure difference on both sides of the leak, which can be detected by the effect caused by gas flow. In order to facilitate leak detection and identify the location of leaks, the detection area is generally minimized as much as possible, so the focus is on leak detection of components first. After strict leak detection, the components can be assembled to avoid air leakage.

5. Category:

5.1 Negative pressure leak detection method

Negative pressure leak detection is to connect the inspected item to the detection port of the leak detector, and use a spray gun to continuously spray tracer onto the suspicious leak hole

Gas, tracer gas enters the leak detector through a leak and is detected. The casing of general electronic devices, high-voltage switch tubes

Zinc oxide, lightning arresters, etc. should all use this method for leak detection.

Depending on the product, different sizes of fixtures or auxiliary tools need to be selected. For example, for example, the casing

leak hunting. After the leak detector works normally, the leakage rate can be calibrated using standard leak holes to perform spray leak detection on the pipe shell. First

Fix the fixture at the leak detection port, place the test tube shell on the rubber plate above the fixture, and assist the exhaust system to extract it to the pre test position

Automatically connect to the measuring system of the instrument after setting the vacuum.

Then use a spray gun to continuously spray helium gas onto the casing,

The time is usually 1-3 seconds,

When there is a leak in the casing, helium gas will enter the mass spectrometry analysis section of the instrument through the leak, and the leakage amount is shown on the leakage rate table

Directly displayed, this method can both determine the location of the leak and measure its size. The entire testing cycle is fast

5.2 Positive pressure method for leak detection

Positive pressure leak detection is the opposite of negative pressure leak detection. The suction gun is connected to the detection port of the leak detector, and the inspected object is filled with the specified pressure

The tracer gas of force, helium gas leaked from the leak hole is sucked into the leak detector by the suction gun for detection. Large containers or internal vent pipes

It is not economical to perform negative pressure leak detection on containers with large quantities, and the leak detection speed is slow. Generally, positive pressure leak detection is adopted.

Attention should be paid to this positive pressure leak detection method: first, the sensitivity of the suction gun of the instrument must be calibrated, and then the container must be filled with it

Helium gas with a pressure higher than one atmosphere; Secondly, some containers have a thin plate structure, and it is recommended to use fixtures outside the container to prevent high pressure

Deformation and damage of components; When the final suction gun moves along the weld seam, the speed should not be too fast, about 1mm away from the surface, to ensure

Inhalation sensitivity is better achieved by making the probe into a horn mouth shape.

5.3. Helium pressure leak detection method

The helium pressure leak detection method is to place the tested object under a certain pressure of tracer gas into a leak detection fixture, and then connect it to a leak detector to evacuate it. The tracer gas leaks out through the leak hole, and the total leakage amount is detected by the leak detector.

This leak detection method is generally suitable for small electronic devices. Firstly, adjust the instrument and then place the device under pressure

Inject helium gas into the tank, and helium gas enters the interior of devices with leaks. Devices without leaks only absorb helium gas on the surface. device

The pressure and time for pressurization depend on

GB2423，2328

Depending on the document, the helium absorbed on the surface of the device after being removed from the pressurized tank

Blow off the gas and then empty it in the leak detection fixture,

After the vacuum is drawn to the set value, the fixture will be automatically connected to the measuring system of the instrument.

At this point, the helium gas inside the leak hole device leaked out and was detected, and its leakage rate was displayed on the leakage rate meter.

The exclusion method is generally used for leak detection using the helium pressure method. Placing a certain number of components in the fixture resulted in a total leakage rate of this batch

Exceeding the predetermined scrap value indicates that this batch is qualified; If the total leakage rate exceeds the predetermined scrap value, half can be taken out, and the remaining one

Continue leak detection halfway, and if this half is qualified, it indicates that the leaking device is in the removed half. Repeat the leak detection process until the leaking device is detected.

6.1 Environmental Requirements

a) The ambient temperature is between 5-35 ℃, and the relative humidity is between 20% and 80%.

b) The leak detection site should be ventilated, with stable airflow, no corrosive gases, and clean air.

c) There is no strong electromagnetic field or severe vibration nearby.

6.2 Testing conditions and equipment

a) Gas source conditions: 0.5-0.7Mpa

b) Power supply voltage: A C.220×(1±10%)

c) Experimental equipment: helium mass spectrometer leak detector, helium spray gun, mechanical pump

6.3 Preparation before Testing

a) Calibration: Before leak detection, it should be confirmed that the leak detector has been calibrated and the results are qualified;

b) Preparation of fixtures/tools: Prepare the leak detection fixtures and tools, and adjust the flow rate of the helium spray gun.

c) Cleaning inspection: Inspect and clean the surface and interior of the inspected item.

d) Understanding of the inspected parts: Check the drawings and physical objects, and be familiar with the parts of the inspected parts that need to be leak checked.

e) Vacuum degree range: 1 × 10-5-105Pa

f) Leakage rate: The leakage rate of the shell, valve body, and valve seat is ≤ 5 × 10-7paL/S (equivalent to 5 × 10-10Pam3/S)

6.4 Leak testing

6.4.1 According to the process and management requirements, leak testing should be conducted on each valve housing after manufacturing.

6.4.2 According to the factory inspection and testing outline and management requirements, after each valve is assembled, the valve body and seat should be subjected to leakage rate testing.

6.4.3 During the testing process, the leakage rate inspection shall be carried out in accordance with the following steps:

1. Equipment calibration;

1.1 Equipment preheating:

Before calibrating the standard leakage holes used for verification, they should be preheated by powering on. Refer to the manufacturer's requirements for preheating time.

1.2 Calibration

According to the instrument manufacturer's operating manual, use calibrated standard leak holes to confirm whether the instrument is in * * state or has sufficient sensitivity. The minimum sensitivity of the instrument to helium is 1x10 m/s

1.3 System Calibration

Leakage standard leakage hole size

Used for system calculations. Connect the calibrated standard leak hole containing 100% helium concentration to the component

Response time:

Evacuate the components to an absolute pressure sufficient to allow the helium mass spectrometer and Ajie in the system, and open the leak hole to the system according to the leak standard for verification. The leakage standard leakage hole should be kept open until the instrument signal is stable.

The time when the leakage standard leak hole opens to the component after verification, as well as the time when the output signal increases to stability, should be recorded. The time elapsed between two degrees is the response time, and the stable instrument degree M1 should be recorded

Background reading;

The background reading M2 is determined after measuring the response time. The calibration leak standard will shut down the detection system, and when the readings reach stability, record the instrument reading M

Initial calibration

The initial system sensitivity S should be calculated according to the following formula:

S=Q/（M-M）

In the formula:

S: Initial sensitivity of the tested system. Pam/s

Q:    Calibrate the leakage rate. Pam/s

M: Calibrate the reading of the leakage hole to the system being inspected after it is opened.

M: Background reading

Inspection;

Spray helium gas onto the inspected area with a spray gun. The distance between the nozzle of the spray gun and the inspected area should be less than 6mm. The scanning speed of the spray gun should be less than 8mm/s

Leak detection scanning starts from the top of the inspected item and gradually scans downwards

The effective leak detection rate is determined by the following formula:

Qmin=( Im/（Is-Io）)Q…………………1

Q: Minimum detectable leak rate Pa m/s

I:    After opening the standard leak hole, the output scale remains stable.

I:    Background value

I:    Noise or minimum readable signal

Q:    The nominal value of standard leakage holes.

The leakage rate of the inspected item is calculated according to the formula:

Q=((I-I)/(I-I)). Q.(1/D)

Q: Leakage rate of back inspection parts Pa m/s

I: Stable output scale value of the inspected workpiece

D: Measure or estimate helium concentration

7. Attention:

7.1 Rough inspection method: During rough inspection, the nozzle opening can be adjusted to increase the coverage area of the helium gas flow. After identifying the area where the leak is located, the helium gas flow of the nozzle can be reduced to find the accurate location of the leak;

7.2 First major leaks, then minor leaks: detected major leaks must also be repaired before finding minor leaks;

7.3 Leakage point investigation: When there are two suspicious leakage points that are very close to each other, one point should be covered first, and then the other point should be sprayed;

7.4 Determination of Large Leakage Hole: When the nozzle sprays a certain point, if the leak detector indicates a change, but its rising speed is slow and the indication value is inconsistent, it indicates that there are large leakage holes in other nearby places;

Refer to other valves of the same type in the leak testing steps above 7.5, and follow the same operating procedures.

During the leak detection process, attention should be paid to adjusting the working pressure and helium peak of the instrument, regularly calibrating the instrument sensitivity, and reducing the instrument background and background noise to ensure the sensitivity of the helium mass spectrometer leak detection method.

3. Selection of leak detection methods and requirements for leak detection personnel (1) The ideal leak detection method should meet the following requirements: ① Appropriate leak detection sensitivity. Under specific leak detection conditions, the leak detection sensitivity of the selected method is usually 1-2 orders of magnitude higher than the allowable leak rate.

② Short reaction time and clearance time

③ Not only can the location of the leak be identified, but the leak rate can also be measured. The methods for identifying the leak location include spraying and suction gun method. The spray method is suitable for inspection items that can be evacuated. The high-frequency spark leak detector method, vacuum gauge method, fixed halogen meter method, and helium mass spectrometer method are classified as injection method. The suction gun method is suitable for inspected items that do not allow for evacuation, have a large amount of air release, and have complex pipelines. Bubble method, fluorescence method, ammonia leak detection method, and suction gun leak detector method can be classified as suction gun method. The method of measuring leakage rate is to measure the change in leakage rate or concentration of the leaked substance. Appropriate methods can be adopted based on the conditions.