Pressure Vessel Engineering regularly provides burst testing services to proove products for CRN registration. We have years of experience using burst testing to register products and have the knowledge to get the job done right and as economical as possible.
Benefits of using PVEng:
For projects with several components we can identify the minimum amount of testing required and provide more economical approaches when appropriate. We also provide Finite Element Analysis for components which are not appropriate for destructive testing.
We would be happy to discuss your project with you. For more information please contact:
Jakub Luszczki, CET
The maximum allowable working pressure for vessels or vessel parts for which the strength cannot be computed with satisfactory assurance of accuracy can be established by burst testing in accordance with ASME VIII-1 UG-101(a)(1), or through Finite Element Analysis (FEA) in accordance with ASME VIII-2.
Burst testing and FEA may only be used for the purpose of establishing a maximum allowable working pressure of those elements or components for which code calculations cannot be applied. Code calculations must be completed for all elements or components which they may be applied as per ASME VIII-1 UG-101(b).
Burst testing is usually limited to "fittings"; generally a component can be classified as a fitting providing its volume is < 1.5 cu.ft. FEA is typically used for components larger than this, or where it is not economical to complete destructive testing.
In Canada fittings fall into the following categories:
Fittings can be "grouped" into the above categories and registered as a catalog to reduce registration costs. *Saskatchewan & British Columbia do not require category A, B, C & G fittings to be registered.
The MAWP for ductile metals is typically 20 - 25% of the proof test pressure, calculated in accordance with ASME VIII-1 UG-101(m).
(1) P = (B / 4) X ((Su * E) / (Suavg)) (2) P = (B / 4) X ((Su * E) / (Sur))
B = Burst test pressure, or pressure at which test was stopped E = Efficiency of welded joint as per ASME VIII-1 UW-12 (if applicable) F = Casting quality factor as per ASME VIII-1 UG-24 (if applicable) Su - Specified minimum tensile strength at test temperature Suavg = Average tensile strength at test temperature Sur = Maximum tensile strength at test temperature
When using eq (1) above some provinces require a minimum of (3) tensile test to be completed to determine Suavg. Using eq (2) eliminates this requirement; however few material specifications provided a maximum tensile value, in which case this equation may not be used.
The resulting MAWP must then be reduced by the following factors if applicable:
Elevated temperature reduction as per ASME UG-101(k)
(3) MAWP = P * (S / S2)
S = Maximum allowable material stress at design temperature S2 = Maximum allowable material stress at test temperature
Values S & S2 are listed at ASME IID Table 1A & 1B
For unlisted materials similar material allowable stresses from ASME IID may be used providing similarity is justified. Proof testing may also be completed at the elevated temperature, or tensile tests performed at elevated temperature to determine material strength reduction.
Corrosion reduction as per ASME VIII-1 UG-101(i)
(4) MAWP = P * ((t - c)n / t n)
c = corrosion allowance n = 1 for curved surfaces, 2 for flat surfaces t = nominal thickness of material at the weakest point
Casting efficiency as per applicable code, i.e. ASME VIII-1 UG-24, ASME B31.3 Table A-1A
Brittle fracture failure must also be addressed if the component is subject to operation temperature lower than proof test temperatures. Minimum temperature values are provided for ASME / ANSI listed materials in some ASME codes i.e. ASME IID, ASME B31.3. If materials are unlisted documentation must be provided to identify the brittle fracture transition temperature. If documentation is not available impact testing or a low temperature proof test may be required.
For materials other than ductile metals TSSA provides the following guidelines for minimum proof test pressure:
The above values must also be increased to account for temperature, corrosion, and casting efficiencies as listed above. These are TSSA guidelines only; higher safety factors may be required for different provinces. It is suggested for this reason that all burst tests is taken to failure.
Finite Element Analysis (FEA) can be an economical alternative to burst testing or as a means to qualify components which would not otherwise meet the above safety factors.
Burst testing must be completed for each material of construction.
The maximum allowable working pressure for geometrically similar parts may be established by a series of burst tests that uniformly cover the complete range of sizes. Interpolation between and an appropriate selection of intermediate sizes may be used to qualify all components of a range of sizes ASME VIII-1 UG-101(d)(2).
Documentation required for a CRN submission using burst testing as a qualifying means is listed below. These documents are required to be submitted to each province of registration.
Quality Control Certificate (Manufacturers - pick one of the following)
Statutory Declaration (Manufacturers)
Burst Test Report
Material specifications for non-code listed materials
Burst testing must be completed in accordance with ASME VIII-1, UG-101.
Burst testing must be witnessed and signed off by an authorized inspector.
A burst test report must be completed containing, but not limited to:
This article was written using the following documents as references:
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.
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. VIII-1 UG-101(j)(2)
For products made from plate or bar, the mill test report will typically show 1 test. More tests will be required from the same plate/batch. Multiple pull tests on plates do not often provide additional useful information but at least one reviewer insists on it.
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.
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.
5) 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.
6) 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.
7) Burst tests used to justify materials not tested. UG-101(d)(2) specifies when one material can be used to justify another. 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.
8) Lastly the report is occasionally a problem. VIII-1 UG-101(b) specifies that the report:
The report shall include sufficient detail to describe the test, the instrumentation and the methods of calibration used, and the results obtained. VIII-1 UG-101(b)
It is our experience that the variations found in the requirements for burst testing vary greatly between review engineers. As always, try to get your advice in writing, and try to get your project reviewed by the same reviewer providing the advice. Regardless of any advice that you get, the closer you stick to the requirements of bust testing as outlined by VIII-1 UG-101, the less trouble you will have now or in 10 years when it is time for renewals.