What should we pay attention to when changing hydraulic test to pneumatic test
When hydrostatic test is difficult
Generally, the safety of hydrostatic test is higher than that of pneumatic test. In case of danger, the damage is less. Therefore, hydrostatic test is used most of the time.
In the capacitance regulation, also listed in some cases can do air pressure test. For example, due to structural or support reasons, or operating conditions do not allow residual test liquid pressure vessel, you can choose to use air pressure test or gas-liquid mixture test.
With the large-scale equipment, urgent schedule, many times the hydraulic test to achieve a lot of difficulties.
For example, large thin-walled equipment hydraulic test, may need to retain the support tooling inside the equipment, filling and releasing water for a long time, affecting the schedule; lack of water or high requirements for water quality, resulting in high costs of hydraulic testing; after filling the equipment is too heavy, resulting in deformation of the equipment body; in the north, the winter temperature is low, the use of production water as the pressure test medium, there is a risk of icing; or site bearing capacity can not meet the The requirements of the hydraulic test, and so on.
At this point, the manufacturing unit often wonders if the water pressure test can be changed to a pneumatic test.
Who will be responsible
There are currently three main types of tests.
Hydraulic test, pneumatic test, gas-liquid combination test.
- Among them, the hydraulic test is safe, more convenient to operate, more economical, but some restrictions, is the most common way to test.
- Pneumatic test is riskier, economic and convenient to operate. Restrictive conditions, in the hydraulic test received restrictions or conditions do not have can not be implemented, is an alternative way.
- Pneumatic-hydraulic combination test pressure stabilization difficulties, the same restrictions as the hydraulic test, the risk between the hydraulic test and pneumatic test, the general economy, the use of less.
Several pressure test methods are technically mature, risk clear, the choice of pressure test method is not essentially a matter of choice of technology, but risk preference.
Some units are more aggressive, hoping to lower costs, in a shorter schedule to complete the pressure test, and willing and able to control the risk, the degree of risk consequences, it is willing to choose the economic and efficient but risky pneumatic test.
Some units are more conservative, unwilling to take excessive risks, preferring to delay the schedule and increase costs, they are willing to use a less risky hydraulic test.
If the use of risky way to test, then who should bear the risk?
Generally speaking by the principle of reciprocity of risk and benefit.
Who benefits, who makes decisions, who takes responsibility.
The manufacturing unit is generally the largest beneficiary, reducing the cost of pressure testing, pressure testing cycle, so when the manufacturing unit proposed to change the hydraulic test to pneumatic test, it is right to assume the main responsibility.
Design institute or engineering company generally safety first, will not take the initiative to require the use of pneumatic pressure test. Water pressure to air pressure, do not enjoy any benefits, but to make decisions whether to agree to modify, for nothing to bear the risk. This is why many times, the design institute or engineering company is not willing to approve the pneumatic pressure test.
The necessary requirements that the air pressure test should meet
Gas has compressibility, compressed gas can reserve energy, the use of gas to do pressure tests, especially high pressure, large volume of gas to do pressure tests, once the leakage or rupture, no less than a super bomb, may cause disastrous consequences, the larger the volume, the greater the explosion hazard, the degree of risk is much higher than the water pressure test.
So do the air pressure test to be extra careful and cautious.
Of course, most companies apply to do air pressure test, generally speaking, should first meet the requirements of the specification, to meet the necessary basic conditions to do air pressure.
- Class A and B welded joints for all radiographic or ultrasonic testing, welded joint coefficient taken 1. (GB/T150.4 10.3.1).
- The weld surface of the container shall not have a biting edge. (GB/T 150.4 7.3.4 (g)).
- The design of the joint between the receiver tube (flange) and the shell and the joint design of the jacketed pressure vessel shall adopt a full weld-through structure. (TSG21-2016 220.127.116.11(2)).
- Conform to the provisions of GB105.4 11.4.10.
Pneumatic pressure test pressure safety distance
Because the stored energy of pneumatic pressure is relatively large, it is generally necessary to consider a certain safety distance from the equipment of pneumatic pressure during the test pressure.
In ASME PCC-2, mandatory Appendix 501-II, there is the calculation of the size of the stored energy of the pneumatic pressure test. When the air pressure medium is air or nitrogen, the energy E and equivalent TNT formula is as follows.
In the mandatory Appendix 501-III, the blast shock wave distance in accordance with the energy and TNT equivalent conversion, the distance between personnel and equipment should be greater than the shock wave distance R.
When the explosive fragments of the container affect the safety of personnel, the distance between all personnel and the container being tested is to be in accordance with Table 501-III-2-1.
As can be seen from the table above, the safety distance of the pneumatic test, starting at 50 m. In many companies, due to the limitations of the site, is not achievable.
If the above two distances are difficult to do, you can also follow the table, assess the impact and choose the appropriate Rscaled recalculate the safety distance R.
Assume a test pressure of 0.9 MPa, an absolute pressure of 1 MPa, and an equipment volume of 10 m3.
Calculated according to the formula above.
Then the gas energy during the test is 12180427 J, which is equivalent to 2.85 kg TNT.
The safety distance should be maintained at 38~50 meters.
Gas pressure test safety device
For the gas pressure test, sometimes it is dangerous to observe its gas pressure test situation at close range.
For equipment with a high PV product or a high TNT equivalent, consider using a buffer tank connected to the test vessel.
Pressurization is done by means of air pressure pumps, accumulator tanks and N2 cylinders, but these gas sources must be connected to the buffer tank so that the pressure is balanced and then passed into the test vessel.
The length of the high pressure hose connecting the buffer tank to the vessel can refer to the safety distance R of PCC-2. The test pressure personnel must stay away from the tested vessel and observe the changes of the pressure gauge installed on the buffer tank.
The site of the pneumatic test should be set in an open and unoccupied outdoor area, and the minimum distance of the isolation line from the test vessel should not be less than the safety distance R of PCC-2.
When the weight of the test vessel exceeds the lifting capacity of the open-air traveling crane, or because the outdoor temperature (to prevent brittle breakage) cannot be guaranteed, it is allowed to be carried out in the workshop, but must should meet more stringent conditions, such as a larger margin of equipment, supporting safety measures, personnel evacuation, etc.
To change the water pressure to air pressure, the following aspects should be noted:
- Clear responsibilities, identify risks and avoid risks .
- Fully comply with the requirements of the specification for the air pressure test.
- Ensure a certain safety distance when testing.
- When the risk is higher, should use the appropriate test supporting devices.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)
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