Allowable tolerance of the pressure gauge during container pressure testing
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Challenge: During the hydrostatic test, two calibrated and compliant pressure gauges were already used for the test. During the process, the technical specifications required a hydrostatic test pressure of 9.815 MPa (leaving aside the feasibility of this value). In practice, both pressure gauges showed a pressure of 10 MPa, which is 0.185 MPa higher than the specified value; this higher pressure was intentionally applied, and 10 MPa was recorded as the actual value. This has been done all along without anyone raising any objections, but now someone who was reviewing the documents objected and disagreed with it (it’s frustrating). This concept is distinct from the error of a pressure gauge. Can a concept of allowable tolerance for pressure gauges during quasi-pressure tests be established?
The normal situation is to reach the required pressure; if the pressure is too high and the material undergoes plastic deformation, is the manufacturer responsible (and must discard the product)?
Generally, the test values are consistent with the process values; the pressure in your case should be 9.82 MPa
At 10 MPa, all the data show 9.82 MPa. Three decimal places as well – it’s insane; those in charge of the manufacturing process don’t even understand the actual conditions.
The last edit to this post was made by xlxuiin on 2017-12-8 at 12:35. The accuracy requirements for pressure gauges used in endurance tests must be at least 1.6 grade. The accuracy class of a pressure gauge is indicated by the percentage of allowable error relative to the gauge’s range; it is generally divided into seven grades: 0.5, 1, 1.6, 2, 2.5, 3, and 4 (grades 3 and 4 are not used in boilers). The lower the number, the higher the accuracy. For example, for a pressure gauge with a range of 0–2.5 MPa and an accuracy class of 2.5, the allowable error between the pressure value indicated by the pointer and the actual pressure of the medium being measured shall not exceed 2.5 MPa × 2.5% = ±0.0625 MPa.
From the perspective of pressure gauge calibration, this requirement is correct; a calibration certificate is issued for each pressure gauge, and the calibration records include the absolute error at each calibration point. Corrections should be made based on these error values during pressure testing, and it is likely that the reviewer is raising this issue.
What is meant is that, assuming the process requirement is 10 MPa, it is unlikely that both gauges will read 10 MPa at the time of testing. Below 10 MPa, the test is theoretically considered failed. The only possible scenarios are those in which the gauge pressure shown on both pressure gauges is 10 MPa or higher. The question is: are there any standards or specifications specifying the maximum allowable value? What is the equivalent free tolerance?
1. Reaching 10 MPa is incorrect (overpressure). 2. Pressure gauges inherently have errors; their accuracy class represents the allowable error range for those gauges (see floor 5). 3. There is nothing like what you mentioned on floor 7 – no standards or specifications specifying the maximum allowable pressure level ”
In the owner’s case, both pressure gauges reached 10 Mpa, which confirms that the test pressure exceeded the specified value. According to GB/T150.1, if the pressure is higher than the test pressure specified in the standard, stress verification should be carried out on each pressure-bearing component prior to the pressure resistance test. Let’s talk about the accuracy issue of pressure gauges. If the readings of the two pressure gauges do not match (which is common and normal), the accuracy of the gauge used by the poster should be at least 1.6 grade. If the range of the pressure gauge is 20 MPa, then the allowable error range is ±0.32 MPa. In case the readings of the two gauges differ, as long as their accuracy is correct, the reading from the gauge with the lower value should be taken as the accurate one.
Two different issues: 1. The accuracy class and maximum allowable error when calibrating pressure gauges are governed by the JJG52-2013 standard, as mentioned by the person on floor 5. 2. Using a pressure gauge that has passed the calibration tests for conducting pressure resistance tests on-site is the issue I would like help with. Assuming the process requires 50 MPa, I use two pressure gauges with a range of 0–100 MPa and a grade of 1.0 (as specified); this allows the pressure to be adjusted to a level that is both reasonable and feasible during testing. Since no corresponding tolerance requirements could be found, I used 1% from the calibration of pressure gauges as a tolerance – is this acceptable? In other words, during the on-site pressure test, the gauge pressures shown on the two pressure gauges should be between 50 and 50.5 MPa. Are there any standards or specifications for approximate tolerances, and how do other manufacturers generally proceed?