Helium mass spectrometer leak detector for heat exchanger tube head leak detection
In recent years, with the rapid development of petrochemical industry, shell and tube heat exchanger has been in a stable position in the petrochemical industry. Because of its simple structure, convenient manufacturing and low cost, the market demand of shell and tube heat exchanger has not decreased, but increased by 30% annually. The sealing performance of tube head is the core of shell and tube heat exchanger. In the operation of the equipment, the sealing failure of the equipment is often caused by the leakage of the pipe head. Therefore, when the equipment leaves the factory, it is the core of the sealing quality of the pipe head in the inspection room of the pipe head. When the requirements for leakage of pipe head are strict, ordinary air tightness test and penetration test can not achieve the detection effect. However, helium mass spectrometry has high detection sensitivity and reliable results, which has been widely used in pressure vessel detection, and is recognized as a safe and reliable leak detection method at home and abroad.
Helium mass spectrometry leak detection method
Helium leak detection methods can be divided into positive pressure leak detection and negative pressure leak detection according to different pressure
For negative pressure leak detection, the equipment is completely pumped into a vacuum state, which often increases the test equipment (such as high and low pressure vacuum pump, vacuum valve, etc.) and equipment tooling (such as external pressure reinforcing ring) to increase the cost. For chemical equipment, it is generally positive pressure. Therefore, for the detection of tube head, the positive pressure olfactory probe is mostly used. Due to the low density of helium, the leakage detection should be carried out from the bottom weld of tube sheet layer by layer (as shown in Fig. 1), and the suction gun for helium leak detection should be less than 3mm away from the weld. However, because there are many welds on the pipe head, and the inspection area is large, there is no sealing around the tube sheet. If there are leakage points around the detection point, the helium leakage will spread around, which will affect the accuracy of detection.
(1) Helium mask method
Firstly, the tubesheet is divided into four zones. Small holes are made at the top of each zone, the initial reading is recorded, and then the small hole is sealed. Then, the equipment is connected with the gas filling device and connected. The purity is greater than 95%. In the actual test, 99.9% helium gas is added into the pressure vessel. The pressure is the pressure designed in the drawing, and the pressurization time varies with the actual volume of the container In general, the pressure in the vessel reaches the required pressure. After maintaining pressure for 12 hours (generally, the micro permeability is very small, and it takes a long time to detect), a new reading is recorded at the same position. If the leakage is found in a certain area, it shall be checked again according to the method mentioned above.
Because the temporary shield is used to isolate the space on both sides of the weld, a closed helium chamber is formed. Therefore, the tiny gap, damage and crack will make the helium in the shield diffuse to the space outside the shield, resulting in high ambient helium background, affecting the detection sensitivity and determining the leakage point. Therefore, the shield must be well sealed and easy to make on site. After many tests, 0.4mm polyethylene film meets the requirements of temporary shield. This method has the advantages of high sensitivity, high detection efficiency, and the effective minimum detectable rate is basically equal to the minimum detectable rate of the instrument, which is suitable for general inspection. Its disadvantage is that it can only detect the total leakage rate of the pipe head and can not determine the position of the pipe head leakage.
(2) Small shield detection method
According to the actual needs, a special small shield is designed for testing. The small shield will isolate the weld area to form an independent detection space. For the weld area of pipe head, it can avoid the influence of other weld seams around the inspected weld during the inspection, so as to improve the accuracy and quickly find the leakage point. The advantage of this method is that it can accurately find the location of the leak. Its disadvantage is low detection efficiency.
Because there are many welds on the tube head of heat exchanger and they are concentrated, it is suggested to use the combination of the above two methods for inspection, so as to improve the detection efficiency and accuracy.
Using the above-mentioned helium mass spectrometer leak detection operation method, the leakage point of the pipe head can be obtained timely and accurately, and the minimum leak detection rate can reach ≤ 5 × 10-11pa.m3/s, which greatly improves the reliability of equipment sealing and reduces the loss caused by equipment leakage, which has broad application prospects.