Burst Testing

General Burst Test Requirements

The maximum allowable working pressure for vessels or vessel parts for which the strength cannot be computed 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.

Determining the MAWP (maximum allowable working pressure)

Each code has a basic pressure rating calculation, for example VIII-1 uses UG-101(m):

	(1)	P = (B / 4) X ((Su * E) / (Suavg))
                         - OR - 
	(2)	P = (B / 4) X ((Su * E) / (Sur))

        Where

	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)
	Su - Specified minimum tensile strength at test temperature
	Suavg = Average tensile strength at test temperature
	Sur = Maximum tensile strength at test temperature

Burst testing must be completed for each material of construction unless they can be grouped by the same p-numbers. The burst test starts with a 4x factor on the burst pressure, but once all of the factors listed above and below are included, the factor is often 6 to 8x. Where applicable, regular code calculation methods have a lower factor of safety.

Reductions:

The calculated MAWP must be reduced by the elevated temperature reduction, corrosion reduction and casting efficiency if applicable:

Elevated temperature reduction as per ASME UG-101(k)

	(3)	MAWP = P * (S / S2)

        Where

	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)

Where information is not available, proof testing may also be completed at the elevated temperature.

Corrosion reduction as per ASME VIII-1 UG-101(i)

	(4)	MAWP = P * ((t - c)n / t n)

        Where

        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.

As mentioned, once all of the factors listed above and below are included, the test pressure is often 6 to 8x the design pressure.

Non Ductile:

TSSA provides the following guidelines for minimum proof test pressure:

• • Flexible hose assemblies 4X
  • Rubber expansion joints 4X
  • Cast Iron 6X

• Glass 10X
• Non-metallic, non-automated fabrication process 10X

The above values must also be increased to account for temperature, corrosion, and casting efficiencies. These are TSSA guidelines only, requirements vary by province.

Burst Testing Rules

There are many ASME rules covering burst testing originating in many different code books. A successful burst test requires the use of the correct code book, and the correct method within the code book. 

When dealing with the CRN system it is important to think beyond the first registration. Will the burst test report be acceptable in other provinces? Will it be acceptable in ten years when it is time for renewal? It is important to find all the differences and take a conservative approach. 

Caution on Stainless Steel: Do not strain, brittle coat or displacement test stainless materials (Except for HLW hot water heaters). ASME VIII, I, and IV refer to these as materials without a “sharp kneed” portion of the stress-strain diagram, generally a description of stainless steel. Also watch out for restrictions on the ratio of yield stress to ultimate stress being below 0.625, again a description of stainless steel. 

ASME VIII-1

This is the most commonly referenced code book for burst testing. Rules are found in UG-101. Important sections include:

Duplicate Parts: UG-101(d)(1) – This section allows results to be used on other materials with the same P number as long as it is stronger than the material tested.

Geometrically Similar Parts: UG-101(d)(2) – When other codes do not tell how many tests are required for different sized parts that are otherwise similar, this passage from VIII-1 is often used. This is usually more restrictive than the more common practice of bursting a smallest, largest and one in the middle. 

Corrosion Allowance: UG-101(i) – Your burst test was done on a new part but you have a corrosion allowance – here is how to correct the calculated rating.

Yield and Tensile Test: UG-101(j) – This section is most commonly ignored – the code requires multiple physical tests, not the 1 or 2 often performed (often from the Material Test Report). This sometimes leads to registration issues.

Elevated Temperatures: UG-101(k) – If the operating temperature will be high enough that the part is no longer as strong as during the test, this section will correct for the difference.

Brittle Coat Testing: UG-101(l) – The first of many allowed test methods. Brittle coat testing is seldom used for VIII-1 applications. See caution on stainless steel above.

Burst Testing: UG-101(m) – The most commonly used test method involving simple testing under elevated pressures. No the test does not need to go to destruction, but the test pressures are often a surprising 6-7x design pressure once all the correction factors for material strength and temperature effects are included. The pressures can be even higher if correcting for weld efficiencies and corrosion allowances.

Strain Testing: UG-101(n) – A more complex test method with lower test pressure requirements. Not used very often. See caution on stainless steel above.

Displacement Testing: UG-101(o) – Another more complex method with lower test pressures. Also not used very often. See caution on stainless steel above.

External Pressure: UG-101(p) – External pressure tests are run at 3x the external pressure, but a special test chamber is often required.

ASME IV

The HG section of the code provide 4 methods of testing:

Strain Tests: HG-502.1 – A complex method of testing using strain gauges to prove that the design did not hit the yield point. See comments on stainless above.

Displacement Tests: HG-502.2 – Another complex method of testing using dial gauges. Again to prove that the design did not hit the yield point. See comments on stainless above.

Bursting Tests: HG-502.3 – A simple method of proving that a design is safe by pressurizing it until it fails. This is more conservative than other more complex tests.

Brittle Coat Test: HG-502.4 – A procedure of coating a product with a brittle coating – usually lime, and pressurizing it until the coating flakes off. It is both simple and less conservative than the burst test making it the favorite test method. See comments on stainless above.

Duplicate Parts: HG-504 – This standard restricts the use of test results from one design on to similar parts. We commonly see manufacturers running tests on every design or material variation.

Proof Test: HLW-502 – Section HLW has very different requirements for burst testing from section HG – only the brittle coat procedure (like HG-502.4 is allowed), regardless of use of stainless material. Stainless boiler parts must burst tested once to HG-502.3 and a second time by brittle coat test to this section if the part is to be used in both HLW and H stamp boilers.

ASME I

Burst testing rules for Section I are found in A-22. Two methods are provided based on yielding (strain and displacement) and one on bursting the part:

Yield Point: A-22.5 – For a yielding based test (strain or displacement testing), the yield point must be “sharp kneed”. Our current understanding is that this precludes strain or displacement testing of stainless steel parts. See the caution on stainless steel above. Also watch out for the restrictions imposed by A-22.2.1.1 on the difference between the yield and tensile point this is often a way of describing stainless steel.

Strain Testing: A-22.6.1 – A test based on measuring the strain in a part to prove that it did not yield under test. See A-22.5.

Displacement Testing: A-22.6.2 – A test based on measuring displacement to prove that yielding did not occur. See A-22.5.

Burst Testing: A-22.6.3 – Useful for any material in ASME Section I. Refer to comments in ASME VIII-1 UG-101(m).

Parts Subject to Collapse: A-22.7 – Parts subject to external pressure can be proof tested at 3x the design pressure.

Duplicate Parts: A-22.9 – Tests on one material are not transferable to other materials. Geometrically similar parts can be proven based on “a series of tests covering the complete range of the pressure part”. Unfortunately, the number of tests, and the method of proving that the range has been certified is not provided. ASME VIII-1 UG-101(d)(2) is often used for guidance.

ASME B16.9

A piping designer assumes a B16.9 fittings has the same strength as the equivalent schedule pipe. It is up to the manufacturer to prove that the fitting actually is this strong. They have the choice of proving this through calculations or burst testing. Section 9 covers how the burst testing is to be done. Points of Interest:

Test Procedure: 9.3 – The required test results to prove that the fitting can be rated as a certain schedule pipe. Testing is done by pressurization, but does not have to proceed to destruction.

How Many Tests: 9.4 – B16.9 allows much more applicability of scaling of results from one material / size / thickness to other fittings compared with other ASME codes. 

ASME B31.1

Burst Testing: 104.7.2(C) – Burst testing done to MSS SP-97, ASME B16.9, or ASME I A-22. ASME I A-22 is most commonly used.

Related Components: 104.7.2 – “It is permissible to interpolate between sizes, wall thicknesses and pressure classes and to determine analogies among related materials.” Unfortunately how to determine material analogies and how many samples are required to interpolate sizes or thicknesses is not specified. ASME VIII-1 UG-101(d)(2) provides useful size guidelines. Likewise, VIII-1 UG-101(d)(1) limits like materials to materials with the same P number.

ASME B31.3

Proof Testing: 304.7.2(c) – Proof testing can be done under ASME B16.9, MSS SP-97, ASME VIII-1.  We prefer VIII-1.

Be cautious when choosing which standard to use. Although B16.9 allows the most lenient applicability of results from one material or size to another, it is prudent to use the more conservative VIII-1 requirements. To repeat, think beyond the first registration, think of other provinces and re-registering in the future.

Alberta Rules (Added Oct 10 2014)

Alberta has released a useful document 00370159.DOC.1 “PROOF TESTING REQUIREMENTS” which we expect to reduce the variation in requirements between reviewers, and hopefully even between provinces. This is not on the ABSA website, but you can request a copy of it. Here it is copied in full:

ABSA: 00370159.DOC.1

Proof Test Requirements

When the pressure rating of a component can’t be established by design calculations proof testing in accordance with UG-101 (m), Section VIII, Div. 1 shall be used.

The Proof Test Report shall be documented and include:

• Present complete calculations for the MAWP or Burst pressure as per UG-101(m)(2), considering the actual and specified mechanical properties;
• Identify the tested part by drawing and revision number, size or designation, and complete material specification;
• Identify the test method and equipment used, and provide the test gauges # and last calibration date;
• Identify the pressure at which the test was stopped and specify the reasons and observations;
• The Test Report shall be signed by: Manufacturer’s representative and by a Provincial Authorised Inspector (if test done in Canada) or a National Board commissioned Authorized Inspector (third party).
• A copy of the A.I’s National Board card (both faces) shall be provided;

Other required documentation to support the Proof Test Report:

• Calculations, as required, to account for usage at higher temperature and corrosion allowance, see UG-101(i) and (k);
• The drawing of the tested part, must identify the part# and complete material specification;
• Where the tested part is too small to permit obtaining 3 tensile specimen samples, then the Material Test Report (MTR) data may be used;
• A copy of the Material Test Report of the tested part. This document must identify the Heat/Cast/Lot# and the part/drawing# of the part).

NOTE: to achieve the required proof test pressure the proof tested assembly may require modifications or adaptations to avoid leakage. Please consult ABSA’s Design Survey with respect to the proof test assembly and any modifications or adaptations and obtain the approval, before starting the testing.

Our comments: Very few reports include information on the Authorized Inspector other than endorsement number and signature. Copying both sides of the N.B. card is not usually done.

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).

This is the most common problem people have CRN registering parts in Canada by Burst test. 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. Four problems can occur:

• i) The company doing the burst test did not ask or did not understand that the 4.5x factor does not include other required reductions like temperature, corrosion or casting or weld efficiencies.
• ii) 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.
• iii) the registration is accepted by the first jurisdiction but rejected by another jurisdiction which would prefer rules from the code books or:
• iv) 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 or reports are required.

It is not uncommon to require a burst test pressure 6-7x operating to meet all of the requirements of UG-101(m). This 4.5x factor is still out there – be careful!

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.

Equation UG-101(m)(2)(a)(#2)

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.

The ABSA document in the previous section does a lot to clarify this issue.

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. VIII-1 UG-91 specifies who the qualified inspector is:

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. See the new Alberta guide in the previous section for more on the 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.

Also, 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

Often, 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.

This is often inadequate. Detailed geometric information is required to prove that the calculations are accurate or conservative for all the sizes not computed. VIII-1 UG-101(d)(2) requires that at least 5 burst tests be done covering the range with good spacing of the data points. Even if VIII-1 is not your code of construction, do not be surprised if you are required to comply with this.

It takes a lot of geometric data to prove that the design scales uniformly between the smallest and the largest. It can be more practical just to test all sizes.

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 useful Alberta requirements form the previous section.