Common Problems Using Burst Testing
Common Problems Using Burst Testing
PVEng has registered hundreds of fittings using burst testing. Based on our own experience and reports from our customers, these are the most common burst test fitting registration problems, starting with the most frequent:
1) Burst test pressure is not correct (4.5x factor used).
Many applicants have been told by a jurisdiction that they can register fittings by burst testing the parts to a 4.5x factor of safety*. Typically they are told this by phone, occasionally by email, it does not matter which. The problem starts after the burst test is complete and the fitting is being registered. Three problems can occur. i) the fitting goes to a different reviewer at the same jurisdiction who does not agree with using the 4.5x factor – the job is rejected*; ii) the registration is accepted by the first jurisdiction but rejected by another jurisdiction; or iii) The job gets registered across Canada, but as the CRN expires at the 10 year anniversary, the original burst test results are not accepted for renewal and new tests are required.
Fitting burst tests are most commonly done to the requirements of VIII-1 UG-101(m). The required test pressure is dependent on the strength of the part vs the specified minimum material strength. Additional correction factors based on operating pressure, weld or casting efficiencies, and corrosion allowances all increase the required burst pressures. It is not uncommon to require a burst test pressure 6-7x operating to meet all of the requirements of UG-101(m).
Although a burst test does not have to be taken to destruction, we recommend that the pressure be taken as high as possible to allow for all of the correction factors required. Proper safety precautions are mandatory during burst tests as large amounts of stored energy are possible, even when tested using liquids.
We have found no written guidelines that allow the 4.5x factor of safety to be used. For further information see “Registration of Plastic Fittings“.
* We at PVEng agree that a fitting is safe after being tested to 4.5x operating pressure for most service conditions; however the problem is in getting the results accepted by all review engineers.
** In one jurisdiction about 75% of the reviewers will accept the 4.5x “rule”, in another about 50%. If you use this rule, it is critical that the job goes back to the reviewer that advised you to use it. Do not expect it to be accepted Canada wide.
2) Not enough pull tests have been performed on the parts.
See UG-101(j)(2) for the rules covering the yield/tensile test coupons. Three to four test specimens are supposed to be taken from an un-yielded location of the part that has been tested. This can be difficult for small fittings, and then the tests have to be taken from another part from the same heat batch.
VIII-1 UG-101(j)(2): Yield or tensile strength so determined shall be the average from three or four specimens cut from the part tested after the test is completed. The specimens shall be cut from a location where the stress during the test has not exceeded the yield strength. The specimens shall not be flame cut because this might affect the strength of the material. If yield or tensile strength is not determined by test specimens from the pressure part tested, alternative methods are given in (l), (m), (n), and (o) below for evaluation of proof test results to establish the maximum allowable working pressure.
Some material specifications provide a maximum and minimum allowed tensile and yield range. These materials can be burst test with only 1 pull test. The trade-off is that the required burst test pressure based on the maximum strength is likely to be higher than if the three tests were taken and the results averaged.
Updated Oct 10 2014: See the above ABSA document which clarifies when more than one pull test sample is required and when the MTR can substitute.
3) The witnessing of burst test is not being accepted.
Companies not located in North America are sometimes using inspectors other than National Board certified Authorized Inspectors to witness burst tests. Although the inspectors being used are qualified for the job, at least one jurisdiction will reject burst tests not witnessed by an Authorized Inspector as defined in UG-91:
All Inspectors shall have been qualified by a written examination under the rules of any state of the United States or province of Canada which has adopted the Code.
The inspectors that meet this requirement are National Board certified. Make sure they put their National Board serial number after their signature on the burst test report.
4) Using burst tests to prove parts that could also be proven by code calculations
Proof tests are not allowed to be used to replace code rules – see UG-101(b). You might need the proof test to show the strength of part of your fitting, but if other portions can be calculated to code, then they must be. The proof test cannot be used to provide a higher operating pressure for those parts that can be calculated.
5) Not enough tests have been done to prove a complete range of parts
Typically to prove a range of parts for CRN registration by code calculations the smallest, largest and a size at mid-range are analyzed and the results are interpolated for the other sizes. Detailed geometric information is required to prove that the calculations are accurate or conservative for all the sizes not computed. This also applies if burst testing is used to justify fittings, however UG-101(d)(2) requires that 5 burst tests be done covering the range. Most reviewers will accept 3 tests to cover a range but we know of at least one that will always insist on 5 tests.
6) Burst tests used to justify materials not tested
UG-101(d)(2) specifies when one material can be used to justify another based on similarities like greater tensile strength + equal P number + same or tougher material grouping – read carefully. Typically if a product is available in multiple materials, then multiple burst tests will be required. Although it is possible to use code calculations of weak materials to prove the strength of otherwise identical fittings made from stronger materials, this is not allowed in burst testing.
Alternatives to Burst Testing – the requirements of 6 and 7 above can make it more economical to use methods such as finite element analysis instead of burst testing, even on small parts.
7) Lastly, the report is occasionally a problem.
VIII-1 UG-101(b) specifies that the report:
VIII-1 UG-101(b): The report shall include sufficient detail to describe the test, the instrumentation and the methods of calibration used, and the results obtained.
Also refer to the Alberta requirements listed above.