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API 570 General Practical Knowledge: Terms People Mix Up

By Sandeep Bandyopadhyay, Senior aerospace engineer · PMP, MBA, Six Sigma

TL;DR

Most lost marks in API 570 Practical Knowledge General come from word pairs, not hard physics. A repair is not an alteration, a WPS is not a PQR, an essential variable is not a nonessential one, pressure design thickness is not minimum required thickness, and a pneumatic test is not a hydrostatic one. Learn what separates each pair, which document decides it, and what the exam does with the difference.

Original questions, not exam dumps. Every CertCrafter question is written from the published body of knowledge and the referenced codes. None is copied from a live exam, and CertCrafter is independent: not affiliated with or endorsed by any awarding body.

A petrochemical plant with process towers and pipe racks seen from across open ground
Photo: Russia, Irkutsk, Angarsk - Ангарская нефтехимическая компания (Ангарск) WMID4616211 by Kupchinskiy_A_B (Wikimedia Commons), CC BY-SA 3.0, via Wikimedia Commons.

Two words that sound alike on a plant can send you to different paragraphs of the code. Practical Knowledge General is full of them.

This post is a short glossary of the pairs that cost candidates marks. Each one gets the difference, the document that settles it, and the way the question is usually framed.

Which term pairs do API 570 candidates confuse most?

Here are the ones worth fixing in your head first. The right-hand column says where the answer lives, using the references API’s Body of Knowledge names.

Pair What separates them Where to look
Repair and alteration Whether a pressure test is normally required, or NDE may stand in API 570, ASME PCC-2 Article 502
WPS and PQR The procedure written down, and the test record behind it ASME Section IX
Essential and nonessential variable Whether a change forces new test coupons ASME Section IX
Pressure design and minimum required thickness The allowances added on top ASME B31.3 304.1
Pneumatic and hydrostatic leak test The multiplier on design pressure ASME B31.3 345.4 and 345.5
Weld joint and casting quality factor Ej for a longitudinal weld, Ec for a casting ASME B31.3 302.3.3 and 302.3.4
Closed book and open book What must live in your head The Body of Knowledge

None of these asks you to be clever. They ask you to know which word belongs to which rule.

Video: API 570 — Three of these four answers look right. Only one survives the code. API 570 — Three of these four answers look right. Only one survives the code. Watch on YouTube · CertCrafter
Image: video thumbnail, source: CertCrafter on YouTube (@certcrafters).

If you like seeing this pressure in action, our channel has a short on the same habit: Three of these four answers look right. Only one survives the code. Wrong options on this exam are rarely absurd. They are real rules applied to the wrong word.

What is the difference between a repair and an alteration?

On a plant, people say “we fixed the line” for both. The code does not.

API 570 treats an alteration as work that normally calls for a pressure test afterward. For a repair, appropriate nondestructive examination can be substituted where a test is not practical, and the inspector has to accept the substitution. The Body of Knowledge points at ASME PCC-2 Article 502, nondestructive examination in lieu of pressure testing, and lists it among the PCC-2 articles that are examinable.

One of our free sample questions shows the trap. A leaking weld on a process line is cut out and rewelded during the run, and the unit superintendent wants the line back without a hydrotest. The correct reading is that rewelding a leak is a repair, so a route to omitting the test exists, and it runs through documented NDE, not through the schedule. The wrong options are the absolutes: every weld needs a hydrotest, or no test is needed after any repair.

On a real job the same mix-up shows up in paperwork. A work pack labelled “repair” that quietly changes the pipe’s rating or configuration has stopped being a repair, and the person signing it needs to notice.

Four steps from pressure design thickness, adding thread depth and corrosion allowance, to minimum required thickness
Pressure design thickness and minimum required thickness are different numbers. Illustration: CertCrafter, from API 570 Body of Knowledge, February 2027 and June 2027.

Is pressure design thickness the same as minimum required thickness?

No, and this is the quiet killer of calculation questions.

The Body of Knowledge asks the inspector to find the minimum required thickness of straight pipe for internal pressure using ASME B31.3, 304.1.1 and 304.1.2. It adds that the inspector should compensate for the corrosion allowance, adding or subtracting as the problem requires. Read the last clause twice. The exam is telling you the allowance is a moving part.

Worked example: a threaded connection

Take a threaded small-bore connection on a process line. The numbers change from question to question, the order of the steps does not.

  1. Pressure design thickness: pressure times outside diameter, divided by twice the sum of the allowable stress times E times W, plus pressure times Y. This is the number for pressure alone.
  2. Add the thread depth. It counts as a mechanical allowance because threading removes metal from the wall.
  3. Add the corrosion allowance, or subtract it if the problem says so.
  4. The result is the minimum required thickness, before mill tolerance.

One of our free sample questions runs exactly this case. The distractors map to the term mix-up: one option forgets the corrosion allowance, one forgets the thread depth, and a third divides by a mill tolerance factor, which is a step used afterward to pick a nominal wall to order, so it answers a different question. Read the last line of the stem, the one that says “before mill tolerance”, and stop where it stops.

What is the difference between a WPS, a PQR and a WPQ?

Three documents, three jobs, and the Body of Knowledge expects you to review all three.

The PQR records the qualification test. The WPS is the written procedure supported by that test. The WPQ shows a welder is qualified to make a production weld according to the WPS. For the exam, the weld procedure review is limited on purpose: no more than one process on a single WPS, PQR or WPQ, a WPS supported by a single PQR, and base metals limited to P-No. 1, 3, 4, 5 and 8. Dissimilar metal joints and overlays are excluded.

One detail is worth memorizing because it is so easy to forget. The Body of Knowledge says the PQR and WPQ must be signed and dated. Editorial items such as the company name, WPS number and testing lab are not tested.

Diagram showing a PQR supporting a WPS, a welder's WPQ following the WPS, and the difference between an essential variable change needing a new PQR and a nonessential change needing only a reissued WPS
Who supports whom, and what each kind of change triggers. Illustration: CertCrafter, from API 570 Body of Knowledge, February 2027 and June 2027.

How do essential and nonessential variables change the answer?

The Body of Knowledge asks whether all required essential and non-essential variables have been properly addressed. The difference is what a change costs you.

A change in an essential variable means the procedure has to be requalified with new test coupons and a new PQR. A nonessential variable is revised on the WPS without retesting. A sample question has a contractor cutting the minimum preheat on P-No. 4 material because of schedule pressure. The inspector cannot accept that on a signature. The decrease is far beyond the margin Section IX allows, so it needs a new PQR.

The wrong options each borrow a real rule: a welding engineer’s signature (that handles nonessential changes), impact testing (tied to supplementary essential variables, which the exam leaves out of the WPS and PQR), or requalifying welders (which fixes nothing about the procedure).

Pipework and process equipment at a petrochemical site, seen from close range
Photo: Russia, Irkutsk, Angarsk - Ангарская нефтехимическая компания (Ангарск) WMID4616212 by Kupchinskiy_A_B (Wikimedia Commons), CC BY-SA 3.0, via Wikimedia Commons.

What is the difference between a pneumatic and a hydrostatic leak test?

Both are leak tests in ASME B31.3, hydrostatic in 345.4 and pneumatic in 345.5, and the Body of Knowledge expects you to calculate the pressure for each.

The pneumatic multiplier is 110 percent of design pressure. The hydrostatic case uses a larger factor. A compressed gas stores far more energy than water, so a failure during a pneumatic test is far more dangerous, which is why the multiplier is lower and why the code wants the pneumatic route justified.

The sample case is a nitrogen header that must stay dry, so a hydrotest is out. The answer is 1.10 times the design pressure. One wrong option applies the hydrostatic factor to the wrong test, and another tests at design pressure, which proves nothing beyond normal operation.

In the field, the term pair changes who needs to be standing where. A pneumatic test needs a larger clear zone and a low-pressure preliminary check, a point the question explanation also makes.

Which quality factor goes with which product form?

This one is a quick win. The Body of Knowledge says the inspector should determine the casting quality factor, Ec, from ASME B31.3 302.3.3(b) and Table A-1A, and the weld joint quality factor, Ej, from 302.3.4 and Table A-1B. It adds that supplemental NDE can increase the joint factor, and that the joint factor sits inside the thickness and pressure formulas.

So a question that gives you a longitudinal weld wants Ej. A casting wants Ec. A candidate who reaches for the wrong table can do everything else right and still miss.

How do closed book, open book and the two Practical Knowledge sections differ?

The exam has two parts. The closed-book part tests working knowledge of API 570 and the reference documents. The open-book part uses more detailed information that you should be able to find but would not normally memorize. According to API’s published figures, the closed-book part is 110 questions in 2 hours 45 minutes, followed by 60 open-book questions in 3 hours 45 minutes.

Practical Knowledge General (section IV) is the list of nineteen topics. Practical Knowledge Specific (section V) says which parts of each reference may be tested. API 570, RP 574, RP 577 and RP 578 are subject to testing in their entirety. RP 571 and RP 576 are limited to named sections.

A last pair worth separating is the Body of Knowledge and the effectivity sheet. The first says what is covered, the second says which edition is effective for your window. Your plant may run an older edition, but the exam is written to the sheet.

How should you drill the term pairs?

Make a two-column card for each pair in the table, then write a one-line trigger for each: “alteration, test normally required”, “essential, new PQR”, “pneumatic, 1.10”. Say them aloud on the drive home.

Then check yourself against the free ten-question sample, where several of these pairs appear. Our General Practical Knowledge guide covers the thickness, life and interval chain, and the common mistakes post covers how candidates trip on editions and units.

What to do next

Open the Body of Knowledge from API’s API 570 page and underline every “and” and “or” in sections I and II. Each one hides a pair.

Then read the certification guide, browse the other API 570 posts and work through the practice tests.

Try these free questions

  1. [Open book] A threaded NPS 2 connection, outside diameter 2.375 in., runs at 1,000 psig. The allowable stress is 20,000 psi, E and W are 1.0, Y is 0.4, the corrosion allowance is 0.050 in. and the thread depth to be included as a mechanical allowance is 0.07 in. Using ASME B31.3, what minimum required thickness must the pipe provide before mill tolerance is considered?

    1. About 0.128 in., the pressure design thickness plus the thread depth only.
    2. About 0.108 in., the pressure design thickness plus the corrosion allowance only.
    3. About 0.178 in., the pressure design thickness plus the thread depth and corrosion allowance.
    4. About 0.204 in., the required thickness divided by 0.875 for the mill tolerance.
    Show the answer and explanation

    Answer: C. ASME B31.3 builds the minimum required thickness as the pressure design thickness plus the sum of the mechanical, corrosion and erosion allowances, and thread depth is a mechanical allowance because threading removes metal from the wall. The pressure design thickness is P D divided by twice the quantity S E W plus P Y: 1,000 times 2.375 is 2,375, and twice 20,400 is 40,800, giving 0.058 in. Adding the 0.07 in. thread depth and the 0.050 in. corrosion allowance gives 0.178 in. Option one forgets the corrosion allowance and option two forgets the thread depth, the commonest slip on threaded pipe. Option four divides by 0.875, which is the step used afterwards to select a nominal wall to order, and the question asked for the requirement before mill tolerance. Schedule 80 at 0.218 in. nominal gives 0.191 in. after under-tolerance, which would satisfy the requirement.

  2. [Open book] A contractor submits a WPS revision dropping the minimum preheat on P-No. 4 material from 300 degrees F (149 degrees C) to 150 degrees F (66 degrees C), citing schedule pressure. Under ASME Section IX, what should the inspector require?

    1. Acceptance once the contractor's welding engineer signs the revision, since minimum preheat is a nonessential variable on this procedure
    2. Rejection until a new procedure qualification record supports the lower preheat, since that decrease is an essential variable
    3. Acceptance provided impact testing is added to the qualification, since preheat is a supplementary essential variable only
    4. Acceptance once each welder assigned to the job requalifies at the reduced preheat temperature
    Show the answer and explanation

    Answer: B. ASME Section IX treats a decrease in preheat temperature of more than a set margin below the qualified value as an essential variable for the common arc processes, and an essential variable change means the procedure has to be requalified with new test coupons and a new procedure qualification record. Dropping the minimum from 300 degrees F to 150 degrees F on P-No. 4 low alloy material is a decrease far beyond that margin, and P-No. 4 is exactly the material group where preheat controls the cooling rate and the risk of hydrogen assisted cracking. The inspector therefore cannot accept the revision on paper. A welding engineer's signature handles nonessential variables, which are revised on the procedure without retesting, and preheat in this direction is not one of them. Maximum interpass temperature is the variable tied to supplementary essential status and impact testing. Requalifying welders does nothing about the procedure.

  3. A leaking weld on 4"-P-2208-C2 was excavated and rewelded during the run, and the unit superintendent wants the line back in service without a hydrotest. What governs whether the pressure test can be omitted?

    1. Any weld made on in-service piping requires a hydrostatic test before the line is returned to service
    2. An alteration normally requires a test, while for a repair the inspector may accept suitable NDE instead
    3. A pressure test is required only where the repair involves a full encirclement welded sleeve over the defect
    4. No pressure test is needed after any repair or alteration provided the line passed its most recent scheduled test
    Show the answer and explanation

    Answer: B. API 570 separates repairs from alterations when it comes to pressure testing. An alteration normally calls for a pressure test, while for a repair the code allows appropriate nondestructive examination to be substituted where a test is not practical, with that substitution accepted by the inspector and, in the alteration case, by the piping engineer. Cutting out and rewelding a leaking weld is a repair, so a route to omitting the hydrotest exists, and it turns on suitable NDE being performed and documented rather than on the superintendent's schedule. The claim that every weld on in-service piping demands a hydrostatic test before restart would make most on-stream repair work impossible and is not what the code says. Tying the requirement to full encirclement sleeves picks one repair method arbitrarily. Relying on a previous successful test ignores that the earlier test says nothing about the weld deposited afterwards.

  4. [Open book] A new nitrogen header on the hydrotreater has a design pressure of 285 psig (1965 kPa) and cannot be hydrotested because the system must stay dry for service. The owner elects a pneumatic leak test under ASME B31.3. What test pressure applies?

    1. 314 psig
    2. 428 psig
    3. 285 psig
    4. 570 psig
    Show the answer and explanation

    Answer: A. ASME B31.3 sets the pneumatic leak test pressure at 110 percent of the design pressure, a lower multiplier than the hydrostatic case because the stored energy in a compressed gas makes the consequence of a failure during the test far more severe. The code also calls for a preliminary check at low pressure before the full test pressure is approached, and for the pneumatic route to be justified because a hydrostatic test is impractical, which the dry service requirement supplies here. The arithmetic is 1.10 times 285, which is 313.5 psig, reported as 314 psig. The 428 psig option applies the 1.5 factor belonging to hydrostatic testing and would put an unacceptable amount of energy into the header. Testing at 285 psig simply holds the line at design pressure, which proves nothing beyond normal operation. Doubling the design pressure to 570 psig comes from an old shop rule and has no standing in the code.

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Frequently asked questions

What is the difference between a repair and an alteration in API 570?

The difference matters most for pressure testing. An alteration normally calls for a pressure test. For a repair, suitable nondestructive examination can be substituted where a test is not practical, accepted by the inspector. API's Body of Knowledge also lists ASME PCC-2 Article 502 on NDE in lieu of pressure testing.

What is the difference between a WPS and a PQR?

The WPS is the written welding procedure. The PQR is the record of the qualification test that supports it. API's Body of Knowledge says the WPS under review will be supported by a single PQR, and that the PQR and the welder's WPQ must be signed and dated.

Is the minimum required thickness the same as the pressure design thickness?

No. In ASME B31.3 the minimum required thickness is the pressure design thickness plus the mechanical, corrosion and erosion allowances. Thread depth counts as a mechanical allowance. The Body of Knowledge tells candidates to add or subtract the corrosion allowance as the problem requires.

Official sources

  1. API 570 Piping Inspector, exam information — accessed 22 October 2026
  2. API ICP Schedules and Fees — accessed 22 October 2026
  3. API 570 Publications Effectivity Sheet, February 2027 and June 2027 — accessed 22 October 2026
  4. API 570 Body of Knowledge, February 2027 and June 2027 — accessed 22 October 2026

Exam rules change. Check the awarding body's official page before you apply. CertCrafter is independent and is not affiliated with or endorsed by any awarding body.

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