Pipefitter Valves, Fittings, and Pipe Materials 2 — Questions and Answers
Question 1: What is a 'double block and bleed' (DBB) valve arrangement, and when is it required?
- 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 (Correct answer)
- A single valve with two seats and a built-in drain; used for standard flow control
- Two valves in parallel for redundancy; the bleed equalizes pressure across both
- A double-seat check valve with a drain port; required on all steam systems
Correct 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
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.
Double block and bleed (DBB) is a safety isolation method required in high-hazard process plants before maintenance work on instrumentation, equipment, or piping connected to a live system. Operation: close upstream block valve, close downstream block valve, open bleed valve between them. If both block valves are holding, the bleed shows no pressure/flow. If upstream block valve leaks, bleed will show pressure, prompting additional isolation. Per OSHA PSM and many company safety standards, DBB is required for LOTO isolation of hazardous energy in process lines where a single valve might leak through. DBB valves can be integrally manufactured (a single valve body with two seats and a bleed port) or made up of separate valves in a spool.
Question 2: What is the function of 'allowable stress' (S) in ASME pressure piping design, and where does the value come from?
- Allowable stress is the material's yield strength at room temperature
- 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 (Correct answer)
- It is the maximum stress the pipe can sustain before rupture with no safety factor included
- It is determined by the pipe schedule number divided by the design pressure
Correct 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
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.
ASME pressure piping codes (B31.1, B31.3, B31.8) tabulate allowable stresses for each listed material at temperature increments from ambient to the material's limit. The allowable stress S is determined as the minimum of: S_ut/3 (one-third of ultimate tensile strength, prevents brittle fracture); 2S_y/3 (two-thirds yield strength, prevents gross plastic deformation); and time-dependent creep/rupture values at elevated temperature. At temperatures where creep governs, S decreases rapidly, limiting the design conditions where carbon steel (above approximately 750 degrees F) or alloy materials can be used. These tabulated values are available in ASME B31.3 Appendix A and are used directly in wall thickness calculations.
Question 3: What is the key functional difference between a swing check valve and a lift check valve?
- 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. (Correct answer)
- Swing checks only work on horizontal lines; lift checks work on vertical lines only
- Swing checks prevent backflow on gas lines only; lift checks prevent backflow on liquid lines only
- They are functionally identical. The difference is only in physical size.
Correct 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.
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.
Swing check valve: disc is hinged on a pin at the top of the valve body and swings through a large arc to open. When flow stops or reverses, the disc swings back to close against the seat. Advantages: low pressure drop when fully open; handles large solids in flow; available in large sizes. Disadvantages: slow closure increases water hammer risk on pump trip; cannot be used in vertical downflow. Lift check valve: disc (or ball or piston) lifts vertically off a seat as flow pushes it up; backflow or gravity closes it. Advantages: fast closure reduces water hammer; can be installed in any orientation including vertical upflow. Disadvantages: higher pressure drop due to direction change; requires higher minimum velocity to stay open.
Question 4: What is a 'union' fitting and in what situations is it preferred over a standard coupling?
- A union is a permanent weld fitting stronger than a coupling
- 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 (Correct answer)
- A union is a reducing fitting connecting two different pipe sizes
- A union is a threaded coupling used only on plastic pipe
Correct 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
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.
Threaded unions (and the welded equivalent, weld nipple unions) are used in small-bore piping wherever equipment (pumps, strainers, control valves, instruments) may need to be removed for maintenance without cutting and re-welding the pipe. A standard coupling, once threaded or welded, requires cutting the pipe to remove it. The union's three-piece design allows the nut to be loosened, the male and female ends pulled apart, and the equipment removed. Common applications: small pump connections (allow pump pull without pipe work), instrument root valve blocks, chemical injection points, and safety relief valve piping. Unions are generally limited to low-pressure applications in small bore (2 inches or less) piping. For larger lines, flanges serve the same disconnect function.
Question 5: What does the pipe material specification 'ASTM A53 Grade B Type S' indicate?
- Type S means schedule and Grade B is the pressure rating
- 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 (Correct answer)
- Grade B indicates the pipe is for below-grade (underground) installation
- Type S is a surface treatment code for galvanized coating
Correct 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
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.
ASTM A53 (Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless) covers three types: Type F (furnace-butt welded, Grade A only, limited to NPS 2 or less, low-pressure utility use); Type E (electric-resistance welded, Grades A and B); Type S (seamless, Grades A and B). Grade A: min 48 ksi tensile, 30 ksi yield; Grade B: min 60 ksi tensile, 35 ksi yield. A53 Type S Grade B is nearly identical in composition and mechanical properties to ASTM A106 Grade B and is often listed as an acceptable substitute in piping material specifications for standard-service carbon steel.
Question 6: What is a 'weldolet,' and when is it the preferred branch connection method over a standard tee fitting?
- A weldolet is a branch fitting that butt-welds to the header pipe and the branch pipe; preferred when the branch is significantly smaller than the header, no tee fitting is available for the header size, or when reinforced branch connections are required by code (Correct answer)
- A weldolet is a threaded branch fitting for use in low-pressure service
- A weldolet replaces a flange when bolted connections are not permitted
- A weldolet is used only on plastic or fiberglass pipe systems
Correct answer: A weldolet is a branch fitting that butt-welds to the header pipe and the branch pipe; preferred when the branch is significantly smaller than the header, no tee fitting is available for the header size, or when reinforced branch connections are required by code
A weldolet is an integrally-reinforced butt-welding branch fitting (per MSS SP-97) that is contour-cut and welded to the OD of the header pipe; preferred when branch-to-run ratios require reinforcement, when header sizes exceed available tee fittings, or when full-penetration welds are required by specification.
Branch connections in process piping: standard tee (ASME B16.9) is preferred when the branch is equal to or close to the header size and the size/schedule is covered by available tee fittings. For large headers (greater than 24 inches) or unusual schedule/alloy combinations where tees are not manufactured, branch connections are made using weldolets (butt-weld integrally-reinforced fitting for same-schedule branch connections), sockolets (socket-weld version for small-bore branches up to 2 inches), threadolets (threaded version for small-bore branches), or sweepolets (contoured version for reduced stress concentration in severe cyclic service). All olets are manufactured to MSS SP-97, are self-reinforcing (built-in excess material replaces the metal removed from the header for the branch opening), and provide a full-penetration weld joint. The code (ASME B31.3 para. 304.3) requires branch connections to have adequate reinforcement.
Pipefitter Valves, Fittings, and Pipe Materials 2
What is a 'double block and bleed' (DBB) valve arrangement, and when is it required?