All pressure pipes are subjected to pressure tests before concealing or acceptance. Water or air may be used as test medium depending on the purpose and work of the pipe. If it is better to use water as the test medium, it is easy to leak and safe to operate. However, the actual situation is more complicated, and sometimes it is not suitable for the test medium for water, so air is required for the pressure test.
Compared with the hydraulic test, the air pressure test has the following characteristics:
(1) There is no need to set up water sources or install water diversion pipes and set up drainage facilities.
(2) Even in the winter test, there is no problem that the water in the fan coil may be drained after the hydrostatic test, so that the coil is frozen.
(3) The loss caused by the leak is small. Some special fields, such as laying pipes in the suspended ceiling, if the water pressure test is used, the ceiling will be damaged once it leaks. Caused a large loss. With the air pressure test, even if a leak occurs, the ceiling will not be damaged.
According to the characteristics of the air pressure test, it is applicable to the following three situations:
(1) Water source water, or water diversion from other places, it is difficult to test, the pipeline is large, and the drainage is difficult after the test, such as the outdoor buried pipe network with large caliber.
(2) The temperature is low. After the water is pressed, the water cannot be completely drained, and there is a risk of freezing and cracking.
(3) Pipeline pressure test for renovation and expansion projects. Since the user has already used it, if the hydraulic pressure test is performed, once it leaks, it will cause a large loss.
It is difficult to relieve pressure due to the air pressure test. In order to ensure safety, the pipeline design pressure exceeds 0.6 MPa and the air pressure test is not allowed. It is strictly forbidden to use gas for pressure test on brittle materials.
The purpose of the pressure test is to check the pressure and tightness of the pipeline and its accessories (such as valves, appliances, tubes, etc.) to ensure that the system does not break and leak under normal working conditions. The most prone to problems in the pressure test is the interface parts such as welds. According to the material mechanics principle, whether it is the internal stress of the pipe itself or the tensile stress of the weld, only the tensile stress of the test pressure household and the pipe and weld are only It is related to the size of the test pressure household and the pipeline and weld, and has nothing to do with the test medium. Therefore, it is theoretically possible to use the air pressure test and the water pressure test.
1 Pressure test process and equipment used
1.1 Process
Prepare the work with a pressure pre-test (to ensure safety, pre-test must be carried out before the formal test) a pressure test - on-site cleaning.
1.2 Machine equipment
Air compressors, pressure gauges, barometers, thermometers, brushes, kegs and other construction workers (such as electric welders, etc.).
2 Safety measures
In the air pressure test, the restricted area shall be demarcated, and unrelated personnel shall not enter. The following safety measures should be taken when using the air compressor:
(1) The gas pipeline should avoid sharp bends. Before opening the compressed air valve, the relevant personnel at the workplace must be notified in advance.
(2) The pipeline system at the outlet of the air compressor is not allowed to be used manually. The pressure gauge, safety valve and regulator should be regularly checked to keep it sensitive and effective.
(3) It is found that the indicated values ​​of the barometer, the oil pressure gauge, the thermometer and the ammeter suddenly exceed the regulation or the indication is abnormal. It is found that the water leakage, air leakage, leakage, oil leakage or coolant is suddenly interrupted, and the safety valve is not trapped or If the sound of the air compressor is not normal, and it cannot be adjusted, it should be stopped for maintenance immediately.
(4) It is strictly forbidden to wash the crankcase, filter or other air passage parts with the Central Political Bureau or kerosene.
(5) The air pressure should be reduced first when parking.
When testing the pipeline, use a pressure gauge that has passed the inspection. The operation should be stepped up slowly and the voltage can be checked after the voltage regulation. Non-operators shall not stay at the fascia, flange, weld, or wire. Repair work must be carried out after pressure relief.
3 quality requirements
The air pressure test must comply with the provisions of Chapter 7 of the "Code for Construction and Acceptance of Industrial Metal Pipeworks" (GB50235-97). If the design requires airtightness test during air pressure test, the following requirements shall be met.
3.1 Allowable pressure drop Ap (general test pressure drop â–³p <5kPa, the following standards refer to the same pipe diameter of "Urban Gas Design Code" (GB50028-93)
â–³p=6.47T/d
Where Δp-allows pressure drop, Pa;
T-air tightness test duration, h;
D-pipe inner diameter, m.
Different pipe diameters
â–³p=6.47T(d1L1+d2L2+...+dnLn)/(d12L2+d22L2+...dn2Ln)
Where d1, d2, ... dn - the inner diameter of each pipe section, m;
L1, L2, ... Ln - length of each pipe segment, m;
The rest is the same as above.
3.2 Actual pressure drop â–³p'
â–³p'=(H1+B1)-(H2+B2)(273+t1)/(273+t2)
â–³ p' - actual pressure drop, Pa;
H1, H2- pressure gauge reading at the beginning and end of the test, Pa;
B1, B2 - barometer reading at the beginning and end of the test, Pa;
T1, t2 - temperature at the end of the test, °C
If Δp' < Δp is acceptable. If the test pipe section includes pipes of different pipe diameters, the test time and allowable pressure drop are based on the maximum pipe diameter.
4 application examples
(1) The Weifang Municipal Waste Treatment Plant Project is the focus of the Weifang Municipal Government in 1999. The diameter of the buried pipe is from 500mm to 1800mm and the length is more than 2000m.
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