API 510 Pressure Testing After Repairs and Alterations

Short answer: API 510 does not treat a pressure test as part of routine inspection. Under the current eleventh edition (October 2022), a pressure test is normally required after an alteration or a major repair. After any other repair, a test is applied when the inspector judges it necessary and writes it into the repair plan. If a test is impractical after an alteration or major repair, appropriate NDE can stand in for it, but only once both the pressure vessel engineer and the inspector have approved the substitution.

That rule is short, but applying it on a real vessel raises the questions this guide answers. Is the work a repair, a major repair or an alteration? What should the test pressure be? Which valves and gauges come off first? How warm does the metal need to be? And when the test is impractical, which NDE will the inspector and engineer accept in its place? Everything below is described in our own words with attribution to API 510, eleventh edition, including Errata 1 (2023) and Errata 2 (2025), as listed on API's API 510 announcement page. API codes are copyrighted, so we paraphrase. Before you build a procedure on any number here, check it against your licensed copy and your jurisdiction's rules.

What API 510 requires: when a pressure test is expected

API 510 links the pressure test to the type of work done, not to the inspection calendar. Routine internal, on-stream and external inspections do not call for one. The test comes into play when the pressure boundary has been changed or restored, and how strongly the code expects it depends on how much of the boundary was involved.

The eleventh edition sets three levels. After an alteration or a major repair, a pressure test is the normal expectation. After a repair that is not major, there is no automatic requirement. The inspector decides whether a test is needed and, if so, specifies it in the repair plan. Section 5.8 of the code also points to a set of pressure-testing alternatives, which are covered further down.

The code also expects a test normally to be applied to the whole vessel. Where it is practical, an individual component or section can be tested instead, a new nozzle being the usual example. The code says a pressure vessel engineer should be consulted before a component or sectional test is used, to confirm it actually proves what it is meant to prove. That consultation matters. A sectional test that leaves out the new attachment weld, or that loads the joint differently from service, can generate a test record without testing the joint that changed.

API 510 also separates a code hydrostatic test from a noncode leak or tightness test. A code test demonstrates pressure integrity at a pressure set by the construction code rules. A tightness test after a repair can be run at whatever pressure the owner-operator chooses, but the code is clear that it is generally not a proof of the repair's strength. Repair records sometimes describe a low-pressure leak check as "pressure tested", and an auditor or the next inspector will read that as a strength test. Record which kind of test was done.

Repair, major repair or alteration: why the category decides the test

The category of the work drives both the test expectation and who must approve it, so it has to be settled before anything else. Here is how API 510's definitions work, in plain terms:

• Repair is work that restores a vessel to a condition suitable for safe operation at its design conditions. Welding, cutting or grinding on a pressure-containing part that is not specifically an alteration counts as a repair.
• Major repair is non-alteration work that removes and replaces a major part of the pressure boundary other than a nozzle. Replacing part of the shell or a head is the typical example.
• Alteration is a physical change with design implications that affect the vessel's pressure-containing capability beyond what the existing data reports describe. The code lists items that are not alterations: a comparable or duplicate replacement, a reinforced nozzle no larger than existing reinforced nozzles, and a nozzle that needs no reinforcement.
• Rerating is a change to the design temperature, the minimum design metal temperature (MDMT) or the maximum allowable working pressure (MAWP). Repair work that changes any of these becomes an alteration and has to meet the rerating requirements as well.

The roles are set out in full in our companion guide, API 510 repairs, alterations and rerating: who authorizes what. For testing, the main point is that once work is a major repair or an alteration, the inspector cannot waive the test alone. Any NDE substitute needs the engineer's agreement as well.

Test pressure: what basis API 510 uses

When a code hydrostatic test is required, API 510 says the minimum test pressure should follow the rules of the applicable construction code. For ASME Section VIII, Division 1 vessels, the code then gives the basis that applies after a rerating. Which allowable stresses the vessel was rated on decides the multiplier:

• Vessels rerated using Division 1 design allowable stresses from the 1999 addendum or later, or under Code Case 2290 or Code Case 2278, have a minimum test pressure of 130% of MAWP, corrected for temperature.
• Vessels rerated using Division 1 allowable stresses from before the 1999 addendum have a minimum test pressure of 150% of MAWP, corrected for temperature.

"Corrected for temperature" means the multiplier is scaled by the ratio of the material's allowable stress at test temperature to its allowable stress at design temperature. A vessel designed for high temperature, where the allowable stress is lower, gets a higher test pressure than the bare multiplier suggests. The engineer works out the number for each vessel from the stress tables in the edition of the code that governs it. Never copy it from another vessel.

Three practical points follow from that.

• Know the vessel's design basis. A vessel built before the 1999 addendum and rerated under the newer stresses moves from the 150% basis to the 130% basis. The test pressure, the nameplate and the calculations all have to agree.
• Add static head. A tall vessel hydrotested in the vertical position puts more pressure at the bottom than at the top gauge. A horizontal shop test and a vertical field test on the same vessel are different events.
• Check that the weakest part can take the test. Flanges, gauge glasses, instruments and anything else not designed for the test pressure must be blinded off or removed. The code says this directly.

Division 2 vessels, vessels built to other construction codes and noncode vessels follow their own rules. For these, the engineer sets the test pressure, and API 510's statement that it should follow the applicable construction code is where to start.

Preparing the vessel: relief devices, foundations and water quality

API 510's preparation guidance is mostly about safety, and in the field it is often where a pressure test goes wrong. In summary, the code expects the following:

• A safety review before pressurising. This matters most for in-service vessels that may have metallurgical degradation or cracking, and even more for pneumatic tests, where far more energy is stored. Nobody does a close visual inspection while the vessel is above MAWP. Close examination waits until pressure is at or below MAWP.
• Relief devices. If the test pressure will exceed the set pressure of the pressure-relief devices, they should be removed. Test clamps that hold the valve disks down are an accepted alternative. Turning the compression screw to add spring load is prohibited. Gauge glasses, pressure gauges and rupture disks that cannot take the test pressure come off or are blanked. Everything removed or disabled must be reinstalled or reactivated afterwards, and that step needs a sign-off, not an assumption.
• Foundations and supports. A vessel full of water can be much heavier than it ever is in service, especially a gas or vapour vessel. The code says the supports and foundation design should be reviewed for the hydrostatic load before the test.
• Water quality. Equipment with Type 300 series stainless steel parts should be tested with potable water, steam condensate or another fluid below 50 ppm chloride. Afterwards the vessel should be fully drained and dried with the high-point vents open. The inspector should confirm both the water quality and the drying. If draining and drying cannot be done promptly, the code suggests very low-chloride water, higher pH and inhibitor (including biocide) to limit pitting, chloride stress corrosion cracking and microbiologically influenced corrosion. Where sensitized austenitic stainless steel is at risk of polythionic acid stress corrosion cracking, it points to an alkaline-water test solution and NACE SP0170.
• Pneumatic and hydropneumatic tests. These are allowed when a hydrotest is impracticable, for example because of a limited foundation, refractory linings or process reasons. An engineer must weigh the personnel and property risks first, at minimum the ASME Code precautions apply, and the code says the engineer should write the procedure following ASME PCC-2 Article 501.

For hazard controls during a test, such as exclusion zones, staged pressurisation, gauges and communication, see our guide on pressure system safety during hydrostatic testing.

Test temperature and brittle fracture

Carbon, low-alloy and other ferritic steels can fail in a brittle way at ambient temperature. API 510 notes that most brittle fractures happen on the first application of high stress, which is often the first hydrotest or overload. The potential for brittle failure therefore has to be evaluated before any hydrostatic test, and especially before a pneumatic one.

The eleventh edition gives a metal-temperature margin to reduce that risk. In summary:

• for vessels more than 2 in. (5 cm) thick, keep the metal at least 30°F (17°C) above the MDMT or minimum allowable temperature (MAT);
• for vessels 2 in. (5 cm) thick or less, keep it at least 10°F (6°C) above the MDMT or MAT;
• the test temperature does not need to exceed 120°F (50°C) unless information on the material's brittle behaviour calls for a higher temperature;
• when hydrotesting solid weld-overlaid or clad austenitic stainless steel, keep the water at or below 120°F (50°C) to avoid chloride stress corrosion cracking.

The code points to low-alloy steels, especially 2¼Cr-1Mo because of temper embrittlement, to any material prone to embrittlement under the mechanisms in API RP 571, and to high triaxial stress from thickness or geometry. On a winter turnaround in the northern United States or Canada, a thick-walled reactor in one of these materials may need heated test water or a heated enclosure. If that cannot be done safely, the inspector and engineer may decide an NDE alternative carries less risk than the test. API 510's brittle-fracture guidance after welded repairs (Section 8.8) supports this: it recommends MT and other effective surface methods, chosen to find the critical flaw sizes, because a weld toe flaw can start a brittle fracture during the test itself.

NDE in lieu of a pressure test: how the substitution works

This is usually the main question. Process isolation, refractory, foundations, cold weather or the time a turnaround allows can all make a hydrotest impractical. API 510 provides a defined alternative, with conditions.

Approval. NDE can replace the pressure test after an alteration or major repair only after both the engineer and the inspector have approved it. The examination crew cannot decide this, and neither can the repair contractor.

Specified NDE. When a test is not done after a major repair or alteration, the code says appropriate NDE must be specified and carried out. It names RT, UT, PT and MT as examples. "Appropriate" means chosen for the flaws the new weld could contain and for the vessel's damage mechanisms.

Acceptance criteria. The code says it is advisable to carry out a fitness-for-service (FFS) assessment in these cases to find the critical flaw sizes, which then set the acceptance criteria for the NDE. That FFS work is an engineering assessment the owner commissions under API 579-1/ASME FFS-1. It sits with the engineer, not the examiner. The examination then has to be able to detect and size flaws at or below those critical sizes.

Guidance document. API 510 points to ASME PCC-2, Article 502, on nondestructive examination in lieu of pressure testing for repairs and alterations. Our overview of ASME PCC-2 explains how that standard is organised.

Examiner qualification. When manual UT is used on welds in place of a pressure test, the owner-operator must specify industry-qualified UT angle beam examiners. Where UT replaces RT, the code calls for ASME Code Case 2235 or ASME Section VIII, Division 2, paragraph 7.5.5. Separately, Section 8.7 requires new repair or alteration welds that the construction code originally required to be radiographed to get RT. Where RT is impractical, the accessible surfaces get full UT in lieu of RT following ASME Section V's requirements for UT used with fracture-mechanics-based acceptance criteria. If another technique is used, the joint efficiency should be reduced to the value for no radiography.

A substitution package that holds up to audit usually contains the engineer's and inspector's signed approval, the critical flaw sizes and their source, the NDE procedure and scan plan showing coverage of the full new weld volume, examiner certifications, the encoded data files or radiographs, and the evaluation against the agreed acceptance criteria. For guidance on matching methods to damage types beyond the new weld, see NDE method selection by damage mechanism.

A worked example, described qualitatively

Take a carbon steel overhead accumulator built to Section VIII, Division 1. During a turnaround, the inspection finds a corroded nozzle that has to be replaced with a larger, reinforced nozzle to suit a process change. A reinforced nozzle larger than any existing reinforced nozzle falls outside the code's list of non-alterations, so the work is an alteration.

The inspector authorises the work only after the engineer has also authorised it. The engineer checks the reinforcement design against the construction code and approves the nozzle installation, which API 510 requires for every nozzle installation. The repair plan sets hold points: surface NDE of the cut-out and weld preparation, fit-up, the root pass, the completed weld, and the final pressure test or its alternative.

A full hydrotest would mean blinding the overhead line, checking the support structure for a full water load, and draining and drying a vessel that will be back in wet hydrocarbon service within days. The engineer and inspector review this and agree to replace the test with NDE. The engineer sets acceptance criteria based on critical flaw sizes. The plan calls for MT of the excavation and finished weld and encoded PAUT of the nozzle-to-shell weld, using a procedure demonstrated on a representative mock-up, plus TOFD where through-wall sizing is needed. The examination company performs the NDE and reports results. The inspector evaluates and accepts them under API 510 and signs off the final hold point. The records go into the vessel file with the alteration documentation, and any registration the jurisdiction requires is completed.

If the change had also raised the vessel's MAWP, it would be a rerating as well, and the test-pressure basis and nameplate requirements would apply too.

Jurisdiction overlay: NBIC, state rules, OSHA PSM and Canada

API 510 is an owner-operator code. Whether it applies, and what else applies alongside it, depends on where the vessel is.

• US states and cities with boiler and pressure vessel laws. Many jurisdictions adopt the National Board Inspection Code (NBIC). Part 3 of the NBIC covers repairs and alterations, and in jurisdictions that require it, welded repairs and alterations are done by holders of a National Board "R" Certificate of Authorization and documented on National Board report forms. Some jurisdictions accept an owner-user inspection programme run to API 510 for process vessels. See our page on NBIC NB-23 and confirm with your jurisdiction which route applies to your vessel.
• OSHA Process Safety Management (29 CFR 1910.119). For covered processes, the mechanical integrity element requires inspection and testing that follows recognised and generally accepted good engineering practice, and documentation of it. API 510 is widely used as that practice for process vessels, so a substitution that is approved and documented under the code is also the record a PSM auditor will expect to see.
• Canada. Pressure equipment is regulated by each province. In Alberta, ABSA sets repair and alteration requirements in its AB-513 document, which is separate from its AB-506 owner-user inspection requirements. Other provinces have their own regulators and adopt CSA B51. Confirm testing and registration requirements with the provincial authority before the outage.

Common mistakes

• Classifying an alteration as a repair to avoid the engineer's approval and the test expectation. Nozzle additions and anything that changes MAWP, design temperature or MDMT are the usual cases.
• Calling a tightness test a pressure test. A low-pressure leak check does not prove strength, and the record should say what was actually done.
• Gagging relief valves with the compression screw instead of removing them or using test clamps, then forgetting to reinstate them.
• Testing cold. Not checking metal temperature against the MDMT or MAT on thick ferritic or temper-embrittled vessels.
• Leaving chloride water in stainless steel, or not confirming water quality and drying.
• Substituting NDE without the paperwork. No documented engineer and inspector approval, no stated acceptance criteria, or a manual UT examiner without the industry qualification the owner was required to specify.
• Coverage gaps. A scan plan that does not cover the full new weld volume, or no surface examination at weld toes where brittle fracture is a concern.

How Atlantis supports this

Atlantis NDT performs the examinations that API 510 repair plans call for: MT and wet fluorescent MT of excavations and finished welds, PT on austenitic materials, encoded phased array UT, TOFD sizing, manual angle beam UT, and radiography by crews licensed where the work is. Our technicians are ASNT-certified and work under ASNT NDT Level III oversight. Atlantis has delivered 1,500+ inspection activities. We do the NDE and deliver results to your API-certified Authorized Inspector, who stays inspector of record. Atlantis does not authorise repairs, approve substitutions, set acceptance criteria or perform FFS assessments. Those decisions stay with your inspector and engineer. See pressure vessel inspection services and API 510 inspector support services. We send quotes within 24 hours. Request a quote for repair NDE.

Frequently asked questions

Is a pressure test required after a repair under API 510?

Not automatically. After an alteration or major repair a test is normally required. After other repairs the inspector decides and, if one is needed, writes it into the repair plan.

Is a hydrotest required after a weld repair on a pressure vessel?

For a routine weld repair such as overlay of local corrosion, often not. The inspector may give prior general authorisation for that kind of repair when no pressure test will be needed. A weld repair that replaces a major part of the pressure boundary is a major repair, and a test is normally expected.

Is a pressure test required after an alteration under API 510?

Normally yes. The alternative is specified NDE, approved by both the engineer and the inspector, with ASME PCC-2 Article 502 as guidance.

Can NDE be used in lieu of a pressure test under API 510?

Yes, after an alteration or major repair, provided the engineer and inspector both approve it. The code says an FFS assessment to find the critical flaw sizes is advisable, so the acceptance criteria rest on engineering rather than habit.

What is the API 510 hydrostatic test pressure, 1.3 or 1.5 times MAWP?

For Division 1 vessels rerated using allowable stresses from the 1999 addendum or later, or Code Cases 2290 or 2278, the minimum is 130% of MAWP corrected for temperature. For earlier allowable stresses it is 150%, also corrected for temperature.

Do relief valves have to be removed for a hydrotest?

If the test pressure will exceed their set pressure, they should be removed, or the disks held with test clamps. Adding spring load with the compression screw is prohibited. Everything must be reinstated after the test.

What metal temperature is needed during the test?

At least 30°F above the MDMT or MAT for vessels over 2 in. thick, and 10°F above for 2 in. or less. It need not exceed 120°F unless the material's brittle behaviour calls for more.

Who qualifies the UT examiner when UT replaces the test?

The owner-operator must specify industry-qualified UT angle beam examiners. The examiner's employer keeps the certification records, and the inspector confirms examiners are qualified for the work.

Is a pneumatic test allowed instead of a hydrotest?

Yes, when hydrotesting is impracticable. An engineer must first consider the personnel and property risks and should write the procedure following ASME PCC-2 Article 501.

Does a rerating always need a new pressure test?

No. A test is not needed if the vessel was previously tested at or above the pressure the construction code would require, or if special NDE techniques confirm its integrity.

Planning repair or alteration work? Ask us to scope NDE in lieu of a pressure test, or request PAUT and TOFD for repair welds.