How to Verify Ball Valve Wall Thickness and Corrosion Allowance | Drawing Review, Material Loss, Design Margin

ASME B16.34 covers pressure-temperature ratings, materials, dimensions, tolerances, examination, testing and marking for new cast, forged and fabricated valves.^[1] API Specification 6D may also apply to ball valves used in petroleum and natural-gas pipeline or piping systems.^[2]

For valves installed in process piping, ASME B31.3 may control the connected piping system, including its materials, components, design, fabrication, examination, inspection and testing.^[3] In-service valves within an API 570 piping circuit should be inspected under the applicable API 570 and API RP 574 requirements.^[4]

Set the Design Basis

Do not start by comparing an ultrasonic reading with a value taken from an uncontrolled table. First confirm the exact valve and service conditions.

A pressure class is not one fixed pressure. The allowable pressure changes with material and temperature. Use the material, design temperature and design pressure to confirm that the selected class is suitable. Then determine the wall requirement for each pressure-containing location.

The valve structure also changes the review. A forged floating ball valve normally requires close checks behind both seats and around the stem bore. A forged trunnion-mounted ball valve also has upper and lower trunnion bores, bearing pockets, drain passages and seat-injection passages.

If essential records are missing:

• Do not assume a standard corrosion allowance such as 3 mm.
• Do not automatically use the newest standard edition.
• Do not use a catalogue image as the manufacturing drawing.
• Do not treat nominal drawing thickness as an original measurement.
• Do not calculate corrosion rate from readings taken at different positions.

Define the Thickness Values

A ball valve can have different requirements at the seat pocket, stem bore, drain passage and end connector. Each value should therefore be linked to a specific location. The letter i below represents one defined location.

When the project requires corrosion allowance to be added outside the pressure-design minimum:

t_required,i = t_standard,i + CA_i

This formula applies only when the purchase specification clearly requires the allowance as additional metal. Confirm that it applies to the component and surface being reviewed and that it has not already been included in the manufacturer’s minimum wall.

Example:

• Pressure-design minimum: 18.0 mm
• Specified corrosion allowance: 3.0 mm
• Required finished wall: 21.0 mm

A finished wall of 20.6 mm remains 2.6 mm above the pressure minimum, but it is 0.4 mm below the full project requirement.

Corrosion allowance is useful for predictable wall loss. It should not be treated as protection against stress-corrosion cracking, hydrogen damage, fatigue, deep pitting or severe cavitation. For H₂S-containing oil and gas production and natural-gas sweetening environments within its scope, ISO 15156 covers the selection of cracking-resistant metallic materials.^[5]

Review the Drawing

Use the approved sectional manufacturing drawing. Confirm its number, revision, valve model, size, class, material, bore type and purchase-order item.

Mark every pressure-containing part:

• Main body
• End connector or closure
• Stem housing
• Lower trunnion housing
• Drain, vent and injection bosses
• Body joints and pressure-containing welds
• Flanged, threaded or butt-welded ends

Then locate the shortest metal path between the pressure surface and the outside surface. The main centerline section is rarely enough.

Critical sections normally include:

• Behind the upstream and downstream seat pockets
• Around the stem bore
• Around the lower trunnion bore
• Between the body cavity and drain passage
• Between the body cavity and vent passage
• Below sealant-injection passages
• Between seal grooves and the pressure cavity
• Near bolt holes close to the cavity
• At end-piece necks and weld-end transitions
• At repaired or ground areas

An angled drain or injection hole can create a diagonal minimum ligament that does not appear on the main section. Normal-beam ultrasonic testing measures along the sound path and may not find the shortest diagonal distance. Use controlled drawing geometry, coordinate measurement, radiography, computed tomography or another qualified method when required.

For threaded ports, check the largest material-removal diameter created by the internal thread root, counterbore, thread relief, entry chamfer and tapered profile. The nominal opening alone may overstate the remaining metal.

The body cavity may also be larger than the ball port. A full-bore valve should not be reviewed only from the pipeline bore. Check the flow port, ball cavity and seat-pocket diameter separately. The main structural differences are also discussed in this high-pressure API 6D forged ball valve overview.

Check the Drawing Margin

Convert nominal drawing dimensions into the lowest permitted finished wall.

t_{finished-min,i} = t_drawing,i − permitted negative tolerance − additional machining

This is a simplified one-dimensional calculation. Core shift, forging eccentricity and forming reduction may need separate measurements or geometric checks.

The following example shows how a drawing that appears to have a large nominal margin can have little usable margin after production deductions.

These values are an illustrative engineering example, not a standard wall requirement for every NPS 8 Class 600 valve.

The nominal wall appears to be 3.0 mm above the 22.0 mm requirement, but the permitted negative tolerance and machining reduce the actual drawing margin to 0.7 mm.

Do not subtract the same machining allowance twice. Confirm whether the drawing dimension represents raw material, rough machining or the final-machined condition.

Check Core Shift

An off-center casting core can leave one side too thin and the opposite side too thick while the separate inside and outside diameters remain within tolerance.

The 3.4 mm difference between the left and right readings does not prove core shift by itself, but it justifies a denser scan and a check of the cavity center against the drawing datums.

Useful verification methods include:

• Opposite-side ultrasonic readings
• Cavity-center measurement from external datums
• Coordinate measuring machine inspection
• Radiography
• Three-dimensional scanning

Check the Body Type

For forged bodies, review the forging envelope and all final machining dimensions. The forged ball valve manufacturing process shows where material checks, machining and dimensional inspection fit into production.

For butt-welded valves, the end preparation can remove additional metal. ASME B16.25 covers welding bevels and the internal and external shaping of heavy-wall components.^[6] The local transition should also match the connected pipe and project requirements. See the related valve end-connection guide.

Verify the Material

A wall calculation is valid only for the material used in that calculation. Confirm that the purchase order, drawing, design calculation, body marking and material test report agree on:

• Material specification and grade
• Cast, forged or fabricated product form
• Heat number
• Chemical composition
• Mechanical properties
• Heat-treatment condition
• Impact and hardness requirements where applicable

Do not accept a forged-body calculation for a cast body simply because both are carbon steel. Product form affects properties, tolerances, repairs and ultrasonic response.

Show weld overlay, cladding, rubber lining, PTFE lining and paint separately from the structural base metal. Do not count them as pressure wall unless the approved design method gives them structural credit.

Material also affects the expected damage:

• Carbon steel may develop broad internal or external thinning.
• Stainless steel may retain a high average wall while developing deep pits.
• Slurry service may create directional erosion.
• Wet sour service may create cracking rather than simple wall loss.

For a practical comparison, see carbon steel, stainless steel and duplex ball valves.

Measure the Wall

Manual contact pulse-echo ultrasonic testing is widely used when only one side of the body is accessible. ASTM E797/E797M gives guidance for this method at temperatures not exceeding 93°C or 200°F.^[7]

Testing personnel should be qualified under the project’s approved NDT program. ASNT publishes standards and recommended practices for NDT personnel qualification and certification.^[8]

Before measuring, confirm:

• The correct sound velocity for the body material
• A calibration range that covers the expected wall
• A probe suitable for the local curvature
• Clean and stable probe contact
• The effect of paint, lining or overlay
• Pre-test and post-test calibration checks

A coating may cause a false-high reading, but the error is not equal to the coating’s physical thickness. It depends on coating sound velocity, base-metal velocity, thickness, instrument mode and echo selection. Through-coating or echo-to-echo modes should only be used after validation for the actual coating system.

Create a marked measurement grid. Each point should identify:

• Valve tag and serial number
• Upstream or downstream side
• Clock position
• Distance from a flange or body joint
• Measurement-point number
• Raw and repeated readings

For an illustrative shop inspection, the body may first be screened on a 50 mm grid. If a low area is found, spacing may be reduced to 10 mm around that location. The actual grid should be selected for the valve size, geometry and expected damage.

Confirm Low Readings

Do not reject a valve from one unstable number. Repeat the reading and define the surrounding area.

The repeated values between 20.5 and 20.7 mm support a confirmed local minimum of about 20.6 mm. The 21.4 mm adjacent reading shows that the thin area is local rather than uniform.

When a low value appears:

1. Repeat the reading.
2. Reapply couplant.
3. Rotate or change the probe.
4. Review the A-scan waveform.
5. Measure the surrounding area.
6. Confirm calibration and sound velocity.
7. Compare the signal with the internal geometry.
8. Use another qualified method if the result remains uncertain.

A report that states only “UT acceptable” is not enough. Raw readings and exact locations are needed for future comparison.

Identify the Damage

For an in-service valve, use actual earlier measurements whenever possible.

Classify the damage before selecting a calculation:

• General loss: Broad thinning across a larger area.
• Local loss: A limited thin area near a seat leak, passage, weld or coating defect.
• Pitting: Isolated or connected cavities with a deep local minimum.
• Erosion: Smooth, directional loss caused by solids, high velocity, flashing or cavitation.
• Cracking: A separate defect that cannot be evaluated as normal corrosion allowance loss.

Check Local Pitting

AMPP notes that pitting is harder to detect and predict than uniform corrosion and that a narrow pit can cause failure even when total metal loss is small.^[9]

The surrounding wall is close to 17 mm, but the 13.6 mm pit controls the local review. Using the surrounding average would hide more than 3 mm of local penetration.

Spot UT may miss the deepest point of a narrow pit because the sound beam covers a larger area. Use pit gauges, surface profiling, scanning UT or another suitable method when the pit shape cannot be defined by spot readings.

API 579-1/ASME FFS-1 contains different assessment methods for general metal loss, local metal loss, pitting, hydrogen damage and crack-like flaws.^[10] Confirm that the valve geometry, damage type and inspection data are within the limits of the selected method.

For abrasive or erosive service, a forged metal-seated ball valve may be more suitable than a soft-seated design. More wall alone will not correct severe cavitation, seat leakage or unsuitable throttling.

Calculate Corrosion Rate

Only compare measurements taken at the same or reliably matched point.

Long-term corrosion rate = (original thickness − current thickness) ÷ total service time

(19.2 − 17.0) ÷ 10 = 0.22 mm/year

Short-term corrosion rate = (previous thickness − current thickness) ÷ inspection interval

(17.9 − 17.0) ÷ 3 = 0.30 mm/year

The short-term rate is higher. Check whether water content, temperature, solids, chemistry, coating condition, seat leakage or valve operating position changed during the latest inspection period.

If a later reading is higher than an earlier reading, review point location, calibration, probe, surface preparation and echo selection. Do not report a negative corrosion rate without checking the data quality.

Calculate Remaining Life

Remaining life = (current thickness − retirement thickness) ÷ selected corrosion rate

Using the same example:

• Current minimum: 17.0 mm
• Approved retirement thickness: 13.5 mm
• Remaining margin: 3.5 mm
• Selected corrosion rate: 0.30 mm/year
• Estimated remaining life: 11.7 years

The retirement thickness must come from an approved design basis, manufacturer calculation, owner-user procedure, rerating calculation or fitness-for-service assessment.

The inspection interval should be shorter than the calculated remaining life. A simple future-thickness check is:

Predicted future thickness = current thickness − corrosion rate × time

After four years:

17.0 − 0.30 × 4 = 15.8 mm

The predicted value remains 2.3 mm above the 13.5 mm retirement thickness, but the final inspection interval should also consider damage shape, measurement uncertainty, process changes and failure consequence.

Decide the Result

The following sample acceptance table shows why every location must be compared with its own requirement.

A hydrostatic pressure test does not prove that the full corrosion allowance exists, that every local wall is acceptable or that the valve has a stated remaining life. Pressure testing and dimensional inspection answer different questions. See the related API 6D ball valve testing guide.

Request the Evidence

A purchase or inspection package should contain:

• Approved sectional drawing and revision
• Applicable standard and contract edition
• Pressure-temperature rating basis
• Required wall for each critical location
• Corrosion-allowance statement
• Casting, forging and machining tolerance review
• Final critical-location measurement map
• Raw thickness readings and calibration records
• Material certificates and heat-number traceability
• Repair, grinding and weld records
• Nonconformance and concession records
• Approved retirement thickness for in-service assessment

The specification should state whether dimensions are nominal, minimum as-manufactured or actual measured values. It should also require the supplier to identify the minimum finished wall behind seat pockets, around stem and trunnion bores, near drain and vent passages, at body joints and at end connections.

Final Check

A reliable review compares local requirements with local measurements. In the worked example, a 25.0 mm nominal wall falls to 22.7 mm after a 1.5 mm negative tolerance and 0.8 mm machining allowance, leaving only 0.7 mm above the 22.0 mm project requirement. A confirmed 20.6 mm drain-passage reading is still above an 18.0 mm pressure minimum but is 0.4 mm short of the required corrosion allowance. For an installed valve, use matching measurement points, compare short- and long-term corrosion rates, and assess pits, erosion grooves and cracks separately before setting the next inspection date.