11.3 B31.1 Mandatory NDE Examination, Acceptance Standards & Pressure Leak Tests

11.3 B31.1 Mandatory NDE Examination, Acceptance Standards & Pressure Leak Tests

Chapter VI of ASME B31.1 Power Piping establishes the framework for quality assurance, nondestructive examination (NDE), and pressure testing of completed power piping. Paragraph 136 covers inspection and examination; Paragraph 137 covers pressure testing. Together they supply a large share of the 26 B31.1 items on the endorsement exam, and nearly all of those items are table lookups that reward tabbing over memorization.

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Inspection vs. Examination Under ASME B31.1

B31.1 states the distinction explicitly in Para 136.1.1: this Code distinguishes between "examination" and "inspection."

• Inspection (Para 136.1): Inspection is the responsibility of the Owner and may be performed by employees of the Owner or a party authorized by the Owner, except for the inspections required by Para 136.2. Before initial operation, a piping installation shall be inspected to confirm compliance with the engineering design and with the Code's material, fabrication, assembly, examination, and test requirements.
• Verification by an Authorized Inspector (Para 136.1.2): Compliance shall be verified by an Authorized Inspector when a Code stamp is required by Section I of the ASME BPVC — that is, for Boiler External Piping. The AI's duties are those defined in Section I PG-90.
• Owner's Inspector Qualification (Para 136.1.4): The Owner's Inspector is designated by the Owner and shall be an employee of the Owner, of an engineering or scientific organization, or of a recognized insurance or inspection company acting as the Owner's agent — and shall not represent or be employed by the piping manufacturer, fabricator, or erector unless the Owner is also that party. The Inspector shall have not less than 10 years of experience in the design, manufacture, erection, fabrication, or inspection of power piping, with each year of satisfactorily completed work toward an accredited engineering degree counting as 1 year, up to 5 years total.
• Examination (Para 136.3.1): Examination denotes the functions performed by the manufacturer, fabricator, erector, or a party authorized by the Owner, and includes the nondestructive examinations themselves — visual, radiographic, ultrasonic, eddy current, liquid penetrant, and magnetic particle. Para 136.3.2 requires NDE personnel to be qualified and certified for each examination method under an established program.

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|                         INSPECTION vs. EXAMINATION IN ASME B31.1                        |
+-----------------------------------------------------------------------------------------+
|  ENTITY                ROLE & FUNCTION                              PARAGRAPH           |
|  -------------------   ------------------------------------------   ------------------- |
|  Owner / Owner's Insp. Verifies compliance before initial operation  136.1.1 / 136.1.4   |
|  Authorized Inspector  Verifies compliance where Sect. I stamping    136.1.2 / 136.2     |
|                        is required (Boiler External Piping)                              |
|  Manufacturer/Erector  Performs VT, MT, PT, RT, UT (the examination) 136.3.1             |
|  Qualified NDE Pers.   Certified for each method under a program     136.3.2             |
+-----------------------------------------------------------------------------------------+

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Table 136.4: Mandatory Minimum Nondestructive Examinations

Para 136.4.1 frames the table: the types and extent of mandatory examinations for pressure welds and welds to pressure-retaining components are specified in Table 136.4, and for welds other than those covered by Table 136.4, only visual examination is required. Two operational rules travel with it:

• Sequencing: NDE for P-No. 3, 4, 5A, and 5B material welds shall be performed after postweld heat treatment unless the engineering design directs otherwise. For all other materials, NDE may be performed before or after PWHT.
• Default acceptance: Welds not requiring RT, UT, MT, or PT by the Code or engineering design are judged acceptable if they meet the Para 136.4.2 visual requirements and the Para 137 pressure test.

Table 136.4 is indexed by piping design conditions across three columns and by type of weld down three rows. Reading the correct column is where candidates lose points.

Notes that change answers:

• General Note (a): All welds receive a visual examination in addition to whatever specific NDE the table requires. Volumetric examination never displaces VT.
• Note (1): The thickness of a butt weld is the thicker of the two abutting ends after end preparation.
• Note (2): RT may be used as an alternative to PT or MT when performed per Para 136.4.5.
• Note (3): RT or UT of branch welds shall be performed before any nonintegral reinforcing material is applied.
• Note (4): In lieu of the required volumetric examination of welded branch connections, surface examination (PT or MT) is acceptable, performed at the lesser of one-half the weld thickness or each 1/2 in. (12.5 mm) of weld thickness, plus all accessible final weld surfaces.
• Note (5): Fillet welds not exceeding 1/4 in. (6 mm) throat thickness used for permanent attachment of nonpressure-retaining parts are exempt from the PT or MT requirement.

[!IMPORTANT] Two structural facts about Table 136.4 that distractors exploit. First, the pressure trigger of 1,025 psig does not stand alone — it applies only inside the 350°F to 750°F temperature band. Above 750°F the table applies at all pressures; below 350°F, ordinary piping falls in "all others" and needs only VT. Second, the table never asks for "100% volumetric of all girth welds." Even in the harshest column, butt welds NPS 2 and less are examined by MT or PT, not RT or UT.

Boiler External Piping

BEP carries Section I administrative requirements on top of B31.1's technical rules. For pressure testing, Para 137.3.1 is explicit: boiler external piping shall be hydrostatically tested in accordance with PG-99 of Section I, and the test shall be conducted in the presence of the Authorized Inspector. Non-boiler external piping is tested under Para 137.4 (or, when the owner specifies, by the alternatives in Paras 137.5, 137.6, or 137.7).

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Visual Examination Acceptance Standards (Para 136.4.2)

Visual examination is performed in accordance with Section V, Article 9. Under Para 136.4.2(A), the following indications are unacceptable:

1. Cracks — external surface. Zero tolerance.
2. Undercut on the surface greater than 1/32 in. (1.0 mm) deep.
3. Weld reinforcement greater than specified in Table 127.4.2.
4. Lack of fusion on the surface.
5. Incomplete penetration — this criterion applies only when the inside surface is readily accessible.
6. Any other linear indication greater than 3/16 in. (5.0 mm) long.
7. Surface porosity with rounded indications having dimensions greater than 3/16 in. (5.0 mm), or four or more rounded indications separated by 1/16 in. (2.0 mm) or less edge to edge in any direction. Rounded indications are circular or elliptical with length less than three times the width.

[!NOTE] The undercut number is unconditional. B31.1 rejects surface undercut deeper than 1/32 in. (1.0 mm) — there is no companion percentage-of-wall allowance in Para 136.4.2(A.2). Options that offer "1/32 in. or 10% of wall, whichever is less" are importing a criterion from another code.

Weld Reinforcement Limits (Table 127.4.2)

Para 136.4.2(A.3) rejects reinforcement exceeding Table 127.4.2, so the visual criterion is a lookup, not a single number. Know the table's structure so you can read it in seconds:

• Rows are brackets of thickness of base metal, running from "up to 1/8 in." through "over 2 in."
• Columns are design temperature bands: over 750°F (400°C), 350°F to 750°F (175°C to 400°C), and under 350°F (175°C). Hotter service allows less reinforcement.
• General Note (c): the thickness of weld reinforcement is based on the thinner of the materials being joined.
• General Note (a) and (b): for double-welded butt joints the limit applies separately to both the inside and the outside surface; for single-welded butt joints it applies to the outside surface only.
• Over 2 in. base metal: the limit is the greater of 1/4 in. (6 mm) or 1/8 times the width of the weld.
• General Note (e): weld reinforcement may be removed if desired.

Two related profile rules sit in Para 127.4.2: when welding pipe to pipe, the surface of the weld shall as a minimum be flush with the outer surface of the pipe; and for welds made without the addition of filler metal, concavity is limited to 1/32 in. (1 mm) below the outside surface and shall not encroach on the minimum required thickness.

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Radiographic Acceptance Standards (Para 136.4.5)

Radiography is performed in accordance with Section V, Article 2, with T-285 used as a guide but not as grounds for rejecting a radiograph unless geometric unsharpness exceeds 0.07 in. (2.0 mm). Under Para 136.4.5(A), welds shown by radiography to have any of the following are unacceptable:

1. Planar Discontinuities

Any type of crack, or any zone of incomplete fusion or penetration. There is no length allowance — this is absolute.

2. Elongated Indications

Any other elongated indication with a length greater than:

Where the weld joins two members of different thickness, $t$ is the thinner of the two.

3. Aligned Groups

Any group of indications in line with an aggregate length greater than $t$ in a length of $12t$, except where the distance between successive indications exceeds $6L$, with $L$ the longest indication in the group.

4. Porosity and Root Concavity

Porosity in excess of that shown as acceptable in Appendix A-250 of Section I; and root concavity when there is an abrupt change in density as indicated on the radiograph.

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Pressure Leak Testing: ASME B31.1 Para 137

Para 137.1.5 fixes the sequence: pressure testing is performed after completion of the PWHT required by Para 132, the nondestructive examinations required by Table 136.4, and all other fabrication, assembly, and erection activities. Para 137.1.2 requires the test medium temperature to be that of the available source unless the Owner specifies otherwise, prohibits applying test pressure until system and medium are at approximately the same temperature, and requires that the possibility of brittle fracture shall be considered when testing at low metal temperatures — B31.1 sets no numeric minimum test temperature. Para 137.1.4 forbids stressing any part of the system beyond the limit of Para 102.3.3(B) at any time during the test. Para 137.2 requires all joints, including welds not previously tested, to be left uninsulated and exposed for examination during the test.

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|                       ASME B31.1 LEAK TESTING OPTIONS (PARA 137)                        |
+-----------------------------------------------------------------------------------------+
| 1. Hydrostatic Test (Para 137.4)      --> Default test; water; not less than 1.5 x P    |
| 2. Pneumatic Test (Para 137.5)        --> Owner-specified/permitted; 1.2 to 1.5 x P     |
| 3. Mass-Spectrometer / Halide (137.6) --> Owner-specified where greater sensitivity is  |
|                                           required than hydro or pneumatic can give     |
| 4. Initial Service Test (Para 137.7)  --> Owner-specified; NOT applicable to BEP        |
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1. Hydrostatic Leak Test (Para 137.4)

Para 137.4.5 is short and should be quoted, not paraphrased: the hydrostatic test pressure at any point in the piping system shall not be less than 1.5 times the design pressure, but shall not exceed the maximum allowable test pressure of any nonisolated component (vessels, pumps, valves) nor the limits imposed by Para 102.3.3(B).

$P_T \ge 1.5 \times P$

[!WARNING] B31.1 does not use a stress ratio in this formula. The $1.5 P S_T/S$ temperature-correction form belongs to B31.3 Para 345.4.2, where it applies when the design temperature exceeds the test temperature. Carrying the ratio into a B31.1 item — or carrying B31.1's flat 1.5× into a B31.3 item — is one of the most reliably exploited confusions on this exam, precisely because both codes are open in front of you.

Procedure under Para 137.4:

1. Para 137.4.2 — Vent the high points. Vents shall be provided at all high points of the system in the test position to purge air pockets while filling. Loosening flanges with at least four bolts, or using equipment vents, is acceptable venting.
2. Para 137.4.3 — Test medium. Water shall normally be used unless the Owner specifies otherwise, and shall be clean and of a quality that minimizes corrosion of the system materials. Appendix IV, Para IV-3.4 carries added precautions for austenitic (300 series) and ferritic (400 series) stainless steels — this is where chloride control on stainless systems comes from.
3. Para 137.4.4 — Check the test equipment before applying pressure; disconnect or isolate low-pressure fill lines and anything not subject to the test pressure.
4. Para 137.4.5 — Hold and examine. Maintain the pressure continuously for a minimum of 10 minutes, then it may be reduced to the design pressure and held as long as necessary to examine all joints and connections. Except for possible localized instances at pump or valve packing, the system shall show no visual evidence of weeping or leaking.

Para 137.2 also recommends a pressure relief device during the test, set at 1-1/3 times the test pressure, provided the limits of Paras 137.1.4, 137.4.5, and 137.5.5 are not exceeded.

2. Pneumatic Leak Test (Para 137.5)

Compressed gas stores large amounts of energy, so Para 137.5.1 allows pneumatic testing only when the Owner specifies it or permits it as an alternative, and recommends it only when the system cannot be filled with water or when traces of the test medium cannot be tolerated in service. Para 137.5.2 requires the gas to be nonflammable and nontoxic.

Para 137.5.4 — Preliminary test. A preliminary pneumatic test not to exceed 25 psig [175 kPa (gage)] may be applied before other leak testing as a means of locating major leaks.

Para 137.5.5 — Required pressure and staging:

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|                B31.1 PNEUMATIC TEST SEQUENCE (PARA 137.5.4 AND 137.5.5)                 |
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| Test pressure: NOT LESS THAN 1.2 x P and NOT MORE THAN 1.5 x P                          |
|                (and never above the max allowable test pressure of any nonisolated      |
|                 component such as a vessel, pump, or valve)                             |
|                                                                                         |
| Step 1: Optional preliminary pneumatic test, maximum 25 psig, to find major leaks.      |
| Step 2: Raise pressure gradually to NOT MORE THAN ONE-HALF of the test pressure.        |
| Step 3: Continue in steps of approximately ONE-TENTH of the test pressure until the     |
|         required test pressure is reached.                                              |
| Step 4: Hold continuously at test pressure for a MINIMUM OF 10 MINUTES.                 |
| Step 5: Reduce to the LESSER OF design pressure or 100 psig [700 kPa (gage)] and hold   |
|         while examining all joints by soap bubble or an equivalent method.              |
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3. Mass-Spectrometer and Halide Testing (Para 137.6)

When the Owner specifies it, systems whose operating and design conditions require greater sensitivity than a hydrostatic or pneumatic test can provide shall be tested by a method such as a helium mass-spectrometer test or halide test, conducted in accordance with the test equipment manufacturer's instructions.

4. Initial Service Testing (Para 137.7)

Para 137.7.1 permits an initial service test and examination when specified by the Owner, and only where other types of tests are not practical or where leak tightness is demonstrable from the nature of the service — for example, a line with no available shut-off valves for isolation, or a system that is exercised during pump or compressor check-out. An initial service test is not applicable to boiler external piping. Under Para 137.7.2 the system is gradually brought up to normal operating pressure and held continuously for a minimum of 10 minutes, with all joints and connections examined for leakage; except for localized instances at pump or valve packing, no visual weeping or leaking is permitted.

[!WARNING] B31.1's initial service test is not keyed to a pressure threshold. There is no "150 psig steam and water / 15 psig other fluids" rule in Para 137.7 — that framing comes from other codes. B31.1 gates the test on practicality, Owner specification, and the BEP exclusion.

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Real-World Field Inspection Scenario & Exam Traps

Field Scenario: The Premature Hydro Inspection

A contractor is hydrostatically testing a 14 in. main steam header designed for 900 psig at 700°F. The calculated test pressure is $1.5 \times 900 = 1{,}350\text{ psig}$. The contractor pumps to 1,350 psig and immediately sends the welding inspector up the scaffold to put their face within inches of the uninsulated girth welds looking for weeping.

The CWI's Refusal: Under Para 137.4.5, the line is held at 1,350 psig for at least 10 minutes to demonstrate structural capacity, after which the pressure may be reduced to the design pressure of 900 psig for the leakage examination. Close personal examination at full test pressure is both unnecessary under the Code and unsafe. Note also that this header's design temperature of 700°F places it in the 350°F to 750°F column of Table 136.4, so whether its girth welds required RT or UT in the first place depends on both the design pressure exceeding 1,025 psig and the weld thickness exceeding 3/4 in. — at 900 psig design pressure, this header falls in the "all others" column and needs only visual examination unless the engineering design says more.

Common Exam Traps to Avoid

• The Standalone-1,025-psig Trap: The 1,025 psig trigger only operates within the 350°F to 750°F band. A 900 psig / 700°F line is "all others"; a 100 psig / 800°F line is in the over-750°F column.
• The "100% RT" Overreach: Even in the over-750°F column, butt welds NPS 2 and less take MT or PT, and branch connections NPS 4 and less take MT or PT.
• The Cross-Code Hydro Formula: B31.1 = 1.5 × design pressure, flat. B31.3 = 1.5 P $S_T/S$ when design temperature exceeds test temperature. ASME Section VIII Div 1 uses 1.3 × MAWP with its own stress ratio.
• The Pneumatic Staging Trap: The first step is to not more than one-half the test pressure, then increments of approximately one-tenth. "50% steps to full pressure" is wrong.
• The Relief-Device Setting: B31.1 Para 137.2 recommends a relief device at 1-1/3 times the test pressure. The "test pressure plus the lesser of 50 psi or 10%" setting is B31.3 Para 345.5.2.
• The Preliminary Check Pressure: The B31.1 preliminary pneumatic test is capped at 25 psig. Distractors offer 10, 50, and 100 psig.
• The RT Incomplete Penetration Trap: Under Para 136.4.5(A.1), any crack or zone of incomplete fusion or penetration shown by radiography is unacceptable with no length allowance.