Pipefitter Valves, Fittings, and Pipe Materials 2

Question 1

What is a 'double block and bleed' (DBB) valve arrangement, and when is it required?

Accepted Answer

Two valves in series with a bleed valve between them; required for positive isolation to verify both block valves seal and allow draining the cavity between them to confirm zero pressure

Answer

A double block and bleed arrangement consists of two isolation valves in series with a vent or drain valve (bleed) between them. It provides positive isolation by closing both block valves and opening the bleed to confirm both have sealed and to verify zero pressure in the cavity before line break work.

Question 2

What is the function of 'allowable stress' (S) in ASME pressure piping design, and where does the value come from?

Accepted Answer

Allowable stress is a code-assigned conservative value based on tensile and yield strength at temperature, ensuring the pipe does not yield or creep under design conditions with a safety factor

Answer

ASME B31.3 Appendix A tabulates allowable stress (S) values for pipe materials at various temperatures. The value is the lowest of: 1/3 ultimate tensile strength, 2/3 yield strength, or 100 percent of creep/rupture limits, providing a safety factor against failure modes.

Question 3

What is the key functional difference between a swing check valve and a lift check valve?

Accepted Answer

Swing checks use a disc that rotates on a hinge pin; lift checks use a piston that moves perpendicular to flow. Swing has lower pressure drop; lift handles high velocity and frequent flow reversal better.

Answer

Swing check valves use a disc hinged at the top that swings open with forward flow and closes by gravity/backflow. Lift check valves use a piston or disc that lifts off a seat vertically. Swing checks have lower pressure drop in full-flow conditions; lift checks handle high velocity, pulsating, and frequent flow reversal conditions better.

Question 4

What is a 'union' fitting and in what situations is it preferred over a standard coupling?

Accepted Answer

A union is a three-piece fitting (two end pieces and a nut) that allows a pipe connection to be made and broken without disturbing the rest of the piping, enabling easy equipment removal

Answer

A union consists of two pipe-end fittings and a center nut; tightening the nut pulls the two ends together to seal. Loosening the nut disconnects the joint without cutting pipe or removing other fittings, making equipment removal and reinstallation quick.

Question 5

What does the pipe material specification 'ASTM A53 Grade B Type S' indicate?

Accepted Answer

ASTM A53 covers welded and seamless black and galvanized steel pipe for general service; Grade B is the higher strength version; Type S designates Seamless manufacture

Answer

ASTM A53 covers both welded and seamless carbon steel pipe. Grade B is the higher tensile strength option (min 60 ksi tensile, 35 ksi yield). Type S designates Seamless; Type E designates Electric Resistance Welded; Type F is furnace-butt welded.

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Pipefitter Valves, Fittings, and Pipe Materials 2