Hydrostatic test of
seamless steel pipe (SMLS) is a key method for testing the pressure bearing capacity, sealing and structural integrity of steel pipes. It is suitable for high-pressure pipelines in the fields of petroleum, chemical industry, boilers, machinery manufacturing, etc.
Pressure standard of hydrostatic test of seamless pipe:
There are two types of hydrostatic test of seamless steel pipes, namely conventional hydrostatic test and pressurized hydrostatic test. Among them, the conventional hydrostatic test pressure is 2.5 times the specified value of hydrostatic pressure, and the pressurized hydrostatic test pressure is 3.5 times the specified value of hydrostatic pressure.
Enterprise standards are generally stricter than national standards. Therefore, in actual production, many companies will formulate corresponding seamless pipe hydrostatic test pressure standards according to their own production requirements and quality control standards. When formulating standards, factors such as the use environment and use conditions of the pipeline are generally taken into account to ensure the safety and quality of the use of seamless pipes.
Seamless pipe hydrostatic test process:
1. Preparation before the test
Before conducting a hydrostatic test of a seamless pipe, preparations need to be made before the test, mainly including pipeline cleaning, installation of test equipment, installation of test valves, connection of test pipelines, etc.
Steel pipe inspection: ensure that there are no defects such as cracks and heavy skin on the surface (UT/RT inspection must be passed first).
Plugging installation: Seal both ends with hydraulic heads, and reserve water injection and exhaust ports.
Water injection and exhaust: slowly fill the pipe with water and exhaust air (avoid pressure fluctuations).
2. Pressurization stage
Graded pressurization: first increase to 50% test pressure and check for leaks. Then gradually pressurize to 100% test pressure (avoid pressure shock).
Pressure medium: usually clean water (corrosive media need to add corrosion inhibitor).
3. Pressure holding test
Pressure holding time:
Conventional pipe: 5~30 seconds (such as
API 5L).
Chemical pipe: more than 30 minutes (such as ASME B31.3).
Qualification standard:
No leakage: no visible water droplets or pressure drop on the pipe body and welds.
No deformation: residual deformation rate after pressure holding ≤5% (measure the change in outer diameter).
4. Pressure relief and drying
Slow pressure relief: avoid sudden pressure drop and damage to the pipe body.
Inner wall drying: use compressed air or nitrogen to purge to prevent rust.
Precautions for water pressure test of seamless pipes:
When conducting a water pressure test on seamless pipes, pay attention to the following points:
(1) When filling water, ensure that all air in the pipe is discharged to avoid bubbles or debris in the pipe that affect the test results;
(2) During the test, the pressure needs to be applied gradually to avoid accidents such as pipe burst caused by excessive pressure applied instantly;
(3) During the test, the test instrument needs to be monitored to detect abnormal conditions in time and take corresponding measures;
(4) After the test, the test equipment needs to be cleaned and maintained;
(5) Alternative test: For ultra-large diameter pipes (such as Φ>1000mm), non-destructive testing (UT+RT) can be used instead of water pressure test (subject to standard permission).
Frequently Asked Questions:
1. Why do we need to conduct a water pressure test?
Answer: Water pressure test is one of the necessary conditions to ensure the safety and reliability of pipelines. Through the water pressure test, it is possible to detect whether there are leaks, deformations and other problems in the pipeline to ensure the normal operation of the pipeline.
2. How to determine whether the pipeline is leaking?
Answer: Usually, you can determine whether the pipeline is leaking by observing whether there are water marks on the surface of the pipeline, or you can use a leak detection instrument for detection.
3. How long should the pressure holding time be?
Conventional pipelines (API 5L): 5~30 seconds, check for instantaneous leaks.
Chemical/high-pressure pipelines (ASME B31.3): ≥30 minutes, and the pressure changes need to be monitored throughout the process.
Nuclear power pipelines: may take more than 2 hours (extremely strict scenarios).
4. Can the water pressure test be replaced by gas?
Generally not recommended: Gas (such as nitrogen) is highly compressible, has a large energy storage capacity, and is prone to explosion when ruptured (the energy is more than 200 times that of the water pressure test).
Exceptions:
When it is difficult to inject water into an ultra-large diameter pipe (Φ>1000mm), a gas test can be used, but it is required to: reduce the pressure to 80% of the water pressure; and be equipped with explosion-proof isolation measures.
5. How to deal with steel pipes that fail the water pressure test?
Local defects (such as single-point leakage):
Seamless pipe: cut off the defective section and use the remaining material after re-inspection.
Welded pipe: re-test after repair welding (must comply with the welding process assessment).
Overall failure: directly scrapped, trace metallurgical or process problems.
6. How to record test data?
The test report must include:
a. Steel pipe specifications (material, outer diameter, wall thickness);
b. Test pressure, pressure holding time, ambient temperature;
c. Pressure curve (automatic recorder output);
d. Test results and operator signature.
Conclusion:
Seamless steel pipe water pressure test is one of the necessary conditions to ensure the safety and reliability of the pipeline. The correct water pressure test method and technique can effectively ensure the safety and reliability of the pipeline operation. When conducting a water pressure test, it is necessary to follow the standard procedures, pay attention to details and strictly follow the process to obtain accurate and reliable test results.
Read more: Non-destructive Testing of Seamless Pipes
