API 570 Practice Questions (Free, With Explanations)

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1. [Open book] On the hydrocracker, a level control valve takes a large pressure drop and the liquid flashes as it passes the trim. The inspector is laying out a thickness survey of the valve and the piping around it. Following API RP 574, where should the readings be concentrated?

   1. On the inlet piping and the upstream flange, where the velocity of the fluid through the line reaches its highest value on the approach to the valve seat.
   2. On the bonnet and the packing area, on the reasoning that this is the thinnest part of a cast valve body in service.
   3. On the body wall directly under the seat ring, where the trim wears first and metal loss therefore starts earliest.
   4. On the outlet side of the valve body and the downstream piping including the first elbow, where the flashing two phase stream impinges.
   Show the answer and explanation

   Answer: D. Flashing across a control valve turns liquid into a high velocity two phase stream on the downstream side, and the resulting erosion and erosion-corrosion attacks the outlet half of the valve body and the piping just beyond it, concentrating wherever the stream changes direction. API RP 574 points the inspector at valve body thickness measurement in erosive service and at the piping downstream of pressure letdown, so the survey belongs on the outlet body wall, the downstream spool and the first elbow or tee. The upstream side sees liquid at a much lower velocity and rarely thins first. The bonnet and packing area is where leaks to atmosphere show up, but it is not where the flow does mechanical damage, and a cast bonnet is normally heavy. The seat ring is trim, and trim wear is a maintenance matter rather than a pressure boundary matter; a worn seat leaks through the valve while the body wall that actually contains pressure is untouched.

2. [Open book] A repair crew leaves an opened can of E7018 electrodes on the scaffold through a wet night shift and wants to burn them in the morning. What does API RP 577 indicate about those electrodes?

   1. They may be used once the exposed rods have been wiped down and the coating on each one looks and feels completely dry to the welder who will burn them.
   2. They may be used for the fill and cap passes but should be kept out of the root, where restraint is highest.
   3. They must be reconditioned as the manufacturer directs or discarded, because moisture in a low hydrogen coating raises the risk of hydrogen cracking.
   4. They may be used if the preheat on the joint is raised, which drives the absorbed moisture off ahead of the arc.
   Show the answer and explanation

   Answer: C. The whole point of a low hydrogen electrode is the low moisture content of its coating, since moisture dissociates in the arc and puts diffusible hydrogen into the weld and the heat affected zone, where it combines with residual stress and a hard microstructure to produce delayed cracking. API RP 577 covers consumable control and describes storage in heated ovens with limits on atmospheric exposure, together with reconditioning by baking to the manufacturer's instruction or scrapping once those limits are passed. An overnight exposure in wet weather puts the can outside any reasonable exposure limit. A visual check is worthless here, because the moisture that matters is absorbed into the flux and cannot be seen or felt. Restricting the rods to fill and cap still puts hydrogen into the joint, and the cap is where transverse cracking is often found. Raised preheat does help hydrogen diffuse out, but it is a supplement to consumable control rather than a licence to weld with wet flux.

3. [Open book] A single-wall radiograph of a butt weld in 6"-P-1204-A1A was made with an Ir-192 source, and the densitometer reads 1.9 through the body of the image beside the IQI. Site power is unavailable, so an X-ray source is not an option. What should the inspector do?

   1. Reject the film and reshoot with greater exposure, since a gamma radiograph must reach 2.0 minimum density
   2. Accept the film, because 1.9 meets the 1.8 minimum that the code sets for radiographs of this kind
   3. Accept the film if the required IQI hole is visible, since IQI sensitivity governs over film density
   4. Accept the film after re-reading it on a brighter viewer, since density limits apply only to composite viewing
   Show the answer and explanation

   Answer: A. ASME Section V sets the minimum transmitted film density through the body of the image at 1.8 for radiographs made with an X-ray source and 2.0 for those made with a gamma source, with a maximum of 4.0. This shot was made with Ir-192, so the 2.0 figure applies and 1.9 falls short. With no site power the X-ray option is closed, so the practical remedy is to increase exposure, by time or by geometry, and reshoot. Accepting 1.9 by quoting the 1.8 X-ray minimum is the trap the constraint is built around, since the source rather than the convenience of the number decides which minimum applies. IQI sensitivity and film density are separate acceptance requirements, and satisfying one does not excuse the other, so a visible hole does not rescue an underexposed film. Re-reading on a brighter viewer changes the viewing conditions rather than the density of the film.

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