Pressure test of natural gas pipeline

For the pressure test of natural gas pipeline, the strength and tightness pressure test of the project is generally carried out in sections according to the requirements of the design drawings and the actual situation of the site. Clean water is selected as the pressure test medium. The following is a brief description of pressure test of large diameter and long-distance natural gas pipeline.

Construction procedure

Preparation ➡ Clean the pipe ➡ Measure the diameter of the pipe ➡ Sheung Shui ➡ strength test  ➡ Tightness test ➡ Sweep water ➡ end

Preparation

Preparation of pressure test scheme

  • (1) Sectional pigging and pressure test shall be carried out after the completion of pipe laying and backfilling.
  • (2) Before construction, the pigging and pressure test construction scheme shall be prepared according to the design requirements and specifications and in combination with the actual situation of the project site, and submitted to the supervisor for approval.
  • (3) The construction scheme for pipeline pigging and pressure test shall include the following contents: project overview, selection of pressure test medium, selection of pressure test equipment, division of pressure test sections, construction procedures, construction methods and key construction technical measures, quality standards, safety and environmental protection measures, schedule, labor organization, details of construction temporary facilities and materials for measures, profile diagram of test section Construction site layout plan.

Calibration of measuring instruments

  • (1) The pressure gauge, pressure balance, pressure and temperature recorder, flowmeter and other measuring instruments used for pressure test must be verified by the legal measurement department and must be used within the validity period.
  • (2) The range of the pressure gauge is about 1.5 times of the test pressure, the diameter of the dial shall not be less than 150mm, and the minimum scale shall not be greater than 0.1MPa per grid.
  • (3) At least 2 pressure gauges shall be installed at the head and end of each pressure test pipe section. An automatic pressure recorder and pressure balance shall also be installed at the head end of the pressure test pipe section. The pressure reading of the pipe section shall be subject to the pressure balance.
  • (4) The temperature recorder is installed at the head, end and middle of the pressure test pipe section respectively.

Prefabrication of pressure test head

(1) The pressure test head is composed of elliptical head, nipple, valve and connecting pipe. The schematic diagram is as follows, in which pressure test head a is the upper water end and figure B is the drainage end.

20211102235338 17052 - Pressure test of natural gas pipeline

Schematic diagram of pressure test head A
In the figure: 1 – manifold (DN200); 2 – pressure car connecting pipe (DN50); 3 – pressure gauge connecting pipe (DN15); 4 – pressure balance connecting pipe (DN15); 5 – pressure recorder nozzle (DN15); 6 – temperature recorder (DN15); 7 – elliptical head; 8-nipple; 9 – pig.

20211103003548 72827 - Pressure test of natural gas pipeline
Schematic diagram of pressure test head B
In the figure: 1 – pressure gauge connecting pipe (DN15); 2 – pressure recorder nozzle (DN15); 3 – temperature recorder nozzle (DN15); 4-jumper pipe (DN150); 5. 6 – pressure windmill nozzle (DN50); 7 – elliptical head; 8 – baffle; 9 – pig; 10 – spool.
(2) The material of the elliptical head shall be equivalent to that of the main pipe, the thickness shall meet the test pressure requirements, and the specification of the nipple shall be the same as that of the pipeline in the pressure test section.
(3) Before using the pressure test head, the strength test shall be carried out separately. The strength test pressure shall be 1.5 times of the design pressure, and it shall be qualified if it is stabilized for 2 hours without leakage and burst.

Prefabrication of transceiver

(1) Manufacture of launcher:
The launcher is composed of short pipe, large and small head, head, pressure gauge connecting pipe, air inlet pipe of pressure windmill, etc. The specification of the short pipe is the same as that of the pipeline in the pressure test section.

One is opened at the top 0.8m away from the front end of the launcher pipe φ A 50mm hole, as the first air inlet, is opened at the top of 1.8m at the front end of the spherical pipe φ 50mm hole as the second air inlet.

20211103003755 60575 - Pressure test of natural gas pipeline

In the figure: 1. Pressure gauge connector; 2. Short pipe; 3. Pig; 4. 5 pressure windmill nozzle (DN50); 6. Reducer and reducer 7. blind flange
(2) Manufacture of ball receiver

One end of a 3M long steel pipe with the same specification as the pipe in the pressure test section shall be welded with an elliptical head, and rectangular holes shall be evenly cut on the arc surface of the short pipe for exhaust during wire sweeping. Two thick wall steel pipes shall be inserted at the pipe end to prevent the pig from flying out.

20211103003938 16551 - Pressure test of natural gas pipeline

In the figure: 1. Main pipeline; 2. Pig; 3. Strip vent; 4. Thick wall steel pipe

Determination of test pressure

The pressure value during segmented hydraulic strength test shall be determined according to the given in the design drawing. The pressure stabilization time of strength test is 4h, and the pressure drop is less than 1% of the test pressure and no more than 0.1MPa; The pressure stabilization time of tightness test is 24h, and the pressure drop is less than 1% of the test pressure and no more than 0.1MPa.

Division of pressure test sections

The length of the segmented hydrostatic test pipe section shall not exceed 35km, and the natural height difference of the pressure test pipe section shall not exceed 30m. If the height difference of the pipe section exceeds 30m, the hydrostatic pressure at the low point of the pipeline shall be calculated according to the longitudinal section diagram of the section, and the circumferential stress borne by the pipeline during the low point pressure test shall be calculated. The value shall adopt the control requirement of 0.95 times the minimum yield strength of the pipe at the low point. The test pressure value shall be measured according to the pressure value measured at the highest point of the pipeline, and the pressure value at the lowest point of the pipeline shall be the sum of the test pressure and the static pressure of the liquid level difference of the pipeline.

Clean the pipeline

Before pressure test, pigging shall be carried out for the pipe section, and the pigging times shall not be less than 2 times. The pig shall be driven by compressed air, and the pig must be a polyester cup-shaped cup-shaped pig that does not damage the inner coating.
The operation speed of the pig should be controlled at 4-5km / h, the working pressure should be 0.05-0.2mpa, and the maximum shall not exceed the design pressure of the pipeline.
Before using the pig, check the change of the external size and scratch degree of the pig cup, and replace the cup with large wear.
Qualification standard for pigging and sweeping: there is no sediment, iron filings and other impurities discharged at the end of the pipeline, and the pig must be basically intact when it reaches the end.

Measure the diameter of the pipe

The caliper pig is generally a straight plate 8-piece polyester cup-shaped cup pig, which is equipped with an 8mm thick aluminum caliper disc with a diameter equal to 90% of the inner diameter of the minimum pipe section. The caliper plate is installed between the 4th and 5th polyester discs.
After the caliper pig reaches the receiver, take it out carefully and check whether the caliper plate is damaged or bent.
If the caliper plate is damaged or bent, the cause of deformation shall be analyzed and the caliper shall be measured again. If there is still deformation, the specific location of depression, bending or damage shall be determined, the damaged part shall be repaired, and the caliper shall be measured again until there is no deformation when the caliper plate is received.

Pressure test of natural gas pipeline

Installation of pressure test device

(1) After the pigging diameter measurement is qualified, remove the transceiver and install the pressure test head.

(2) When water is supplied in series, each pressure test section is connected with a jumper pipe, and the schematic diagram is as follows:

20211103004220 98393 - Pressure test of natural gas pipeline

(3) Establish a water supply point. The water supply point shall be provided with a sedimentation tank and a reservoir. The capacity of the reservoir shall meet the water supply requirements, and the inner surface of the tank must be hardened. The schematic diagram is as follows:

20211103004740 63668 - Pressure test of natural gas pipeline

In the figure: 1 – sedimentation tank; 2 – reservoir; 3 – filter cover; 4 – filter; 5 – centrifugal pump; 6 – check valve; 7-
Flowmeter; 8 – pressure test car; 9 – flowmeter; 10 – valve; 11 – inlet valve; 12 – pressure balance; 13 – automatic pressure recorder; 14 – pressure gauge; 15 – vent valve.

Water injection

  • (1) During water injection, clean water shall be injected in front of the first pig, and then the water injection valve behind the first pig shall be opened to push the first pig forward by 300m.
  • (2) Close the water injection valve behind the first pig, open the water injection valve behind the second pig, launch the second pig, accurately record the launch time, and continue to inject water to push the water injection pig until the water injection in the pressure test section is completed.
  • (3) Calculate the estimated arrival time of the water injection pig. Once the second water injection pig arrives, immediately close the drain valve or open the crossover valve to inject water into the next pressure test section.
  • (4) Monitor the water injection at any time to calculate the stroke of the two pigs.

Boosting

After filling with water for 24h, start boosting. The pressure rise speed should not be too fast, and the pressure should rise slowly. The pressure rise shall not exceed 1MPa per hour. When the pressure rises to 30% and 60% of the strength test pressure, stabilize the pressure for 15min respectively, and check whether the system is abnormal. If there is no abnormal condition, continue to boost to the strength test pressure.

Voltage stabilization

  • (1) The strength test shall be stabilized for 4h, and the stabilization time shall be calculated after the pressure at both ends of the pipe section is balanced, and shall be recorded every 30 minutes.
  • (2) During strength pressure test, if the pressure drops sharply, find the leakage point along the pipeline, organize emergency repair after pressure relief, and conduct pressure test again.
  • (3) The strength pressure test is qualified when the pressure drop is not greater than 1% of the test value and not greater than 0.1MPa.

Tightness test

  • (1) After passing the strength test, slowly open the drain valve and reduce the pipeline pressure to the tightness test pressure value at this point according to the installation position of the pressure gauge.
  • (2) Close the drain valve and start the tightness test without pressure fluctuation.
  • (3) The tightness test shall be stabilized for 24h, and the pressure value and temperature value shall be manually recorded every 1H.
  • (4) If the pressure drop is not more than 1% of the test value and not more than 0.1MPa, it is qualified.

Pressure relief

After the pressure test is accepted by the supervisor, the pressure shall be relieved at a certain rate as soon as possible.
In the whole process of pressure relief, the drain valve shall be opened and closed slowly to prevent water hammer load from damaging the assembled pipeline. The valve must not be fully opened for pressure relief.
During pressure relief, the high point must be connected with the atmosphere. In order to prevent vacuum pumping, the air compressor can be used to blow air into the pipe section at the low point.

Sweeping

After pressure relief, drive the second water injection pig and the first water injection pig back to pressure test head a with compressed air, then remove the pressure test head, and reinstall the transceiver at both ends of the pressure test section. Add a cup pig for pigging until there is no water in front of the pig when the pig reaches the receiver.
When draining with a bowl bowl pig, foam cleaner shall be used to continue cleaning until the foam cleaner is dry and does not appear to be discolouration (due to dust). Finally, the pipe shall be closed to prevent water in the air from entering.

Connection after pressure test of natural gas pipeline

After the pressure test and cleaning, the pipeline shall be connected. All short pipes for connection must be subject to hydrostatic test and qualified in advance. All welded junctions without pressure test must be subject to X-ray flaw detection, and the qualification standard is consistent with the line construction.

Source: China Natural Gas Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@ugsteelmill.com

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