Question 1

Packed towers with Raschig rings: liquid redistribution spacing should be every 6 m or approximately how many column diameters (whichever is lower)?

Accepted Answer

Introduction / Context: Liquid maldistribution in packed towers degrades mass-transfer efficiency and can lead to channeling, flooding onset, and off-spec separation. Raschig rings, being relatively simple random packings, are more sensitive to distribution quality than modern structured packings. Designers therefore specify periodic liquid redistributors along the tower height. Given Data / Assumptions: Random packing: Raschig rings. Nominal recommendation: redistribute every 6 m vertical height or a specified multiple of column diameter, whichever is smaller. Goal: maintain uniform wetting and minimize channeling. Concept / Approach: As liquid flows downward, maldistribution increases due to packing void variability and wall effects. Installing redistributors resets the liquid pattern. Common practice for rings is a spacing of roughly 5–7.5 column diameters. Taller spacing risks significant efficiency loss; closer spacing adds cost and pressure drop but may be justified for difficult services or very large diameters. Step-by-Step Solution: Identify typical vertical spacing: about 6 m.Convert to a diameter-based rule: around 5–7.5 D supports good wetting uniformity.Choose the lower of the two criteria to be conservative.Thus, the correct diameter range is 5–7.5. Verification / Alternative check: Packed-tower design guidelines commonly present height or diameter-based rules; both generally point to comparable distances for Raschig rings under average services. Why Other Options Are Wrong: 2.5–3.0 D is overly conservative for many services and increases capital unnecessarily. 10–12.5 D and above risk maldistribution and reduced efficiency before redistribution. Common Pitfalls: Ignoring fluid property effects (viscosity/foaming); using distributor designs with too few drip points; not leveling internals. Final Answer: 5–7.5

Question 2

Tray columns: under which flow pattern is the maximum plate (stage) efficiency generally achieved?

Accepted Answer

Introduction / Context: Plate (Murphree) efficiency in tray columns reflects the degree of approach to equilibrium on a given tray. Hydraulics and contacting patterns strongly influence efficiency. Different tray layouts—cross, reverse, or split flow—change liquid path lengths and vapor–liquid contacting times, impacting mass transfer and backmixing. Given Data / Assumptions: Conventional tray internals with outlet weirs and downcomers. Comparable vapor and liquid loadings for fair comparison. Interest in relative efficiency potential of common patterns. Concept / Approach: Reverse-flow trays route liquid across nearly the full tray active area, often yielding longer liquid path and improved contacting with rising vapor. This tends to increase tray efficiency compared to simpler cross-flow arrangements that may have shorter paths or more non-ideal zones. Split-flow trays divide liquid into parallel paths; they can reduce path length and may lower efficiency relative to well-designed reverse flow (though they can reduce tray pressure drop and handle higher capacities). Step-by-Step Solution: Consider liquid path length: longer path often improves mass transfer.Reverse flow creates extended flow across the tray and better utilization of area.Cross/split flow may be preferred for other reasons (capacity, pressure drop) but not for maximum efficiency.Hence, reverse flow commonly achieves the highest plate efficiency. Verification / Alternative check: Design literature indicates reverse-flow trays can reach higher efficiencies given comparable hydraulics, though exact values depend on system properties and tray details. Why Other Options Are Wrong: Cross flow and split flow often trade some efficiency for capacity/operability. “Cascade” is not a standard tray-flow category for general efficiency comparison. Common Pitfalls: Ignoring weeping, entrainment, and froth behavior; assuming efficiency gains without verifying hydraulics and pressure drop. Final Answer: Reverse flow

Question 3

Rotary dryers (countercurrent): the typical slope (fall) provided is approximately what range in metre per metre of length?

Accepted Answer

Introduction / Context: Rotary dryers rely on slight inclination to promote axial movement of solids while the drum rotates. The slope must be large enough to convey material yet small enough to provide adequate residence time for heat and mass transfer. In countercurrent operation, gas and solids flow in opposite directions, and fine control of slope is essential to achieve target outlet moisture and temperature profiles. Given Data / Assumptions: Countercurrent rotary dryer handling granular or friable solids. Standard rotational speeds and typical flighting arrangement. Industrial practice ranges for slope expressed as metre per metre (m/m). Concept / Approach: Experience shows that only a small geometric incline is required. Typical designs use slopes up to a few centimeters per meter of length, which translates to about 0 to 0.08 m/m. Larger slopes accelerate solids excessively, reducing residence time and risking insufficient drying or wide residence-time distributions. Very small or zero slope may be used for very fine or sticky solids where holdup must be increased. Step-by-Step Solution: Define axial transport drivers: drum inclination and rotation speed.Choose a conservative slope range that maintains target residence time.Industry practice converges near 0–0.08 m/m for countercurrent units.Therefore, select 0 to 0.08 m/m as the appropriate range. Verification / Alternative check: Vendor design charts and scale-up rules corroborate small inclinations; adjustments are complemented by changes in rpm and flighting. Why Other Options Are Wrong: 0–0.8 m/m and larger values are unrealistically steep and would dump solids too quickly. Even higher ranges further shorten residence time and cause poor control. Common Pitfalls: Ignoring particle size effects; failing to adjust rpm/flights along with slope; not accounting for shrinkage or stickiness as moisture falls. Final Answer: 0 to 0.08

Question 4

Gas–liquid contacting for highly corrosive services: which contacting device is generally the most suitable?

Accepted Answer

Introduction / Context: Handling corrosive gas–liquid systems (e.g., acid gases with aggressive solvents) demands contacting equipment that can be economically built from corrosion-resistant materials. The choice affects maintenance, uptime, and total cost of ownership. While trays can be fabricated in alloys, packed towers offer unique flexibility in material selection and surface area per unit volume. Given Data / Assumptions: Highly corrosive fluids requiring nonmetallics or ceramic options. Desire for broad material choices and easy replacement of internals. Normal pressure and hydraulic ranges for absorption/stripping services. Concept / Approach: Packed towers can use ceramic, graphite, or plastic (e.g., PTFE, PVDF, PP) packings and internals, limiting metal exposure. This reduces corrosion and permits operation where alloy trays would be prohibitively expensive. High surface area of packing promotes mass transfer at moderate pressure drop. Trayed columns are workable but often require costly exotic alloys for decks, weirs, downcomers, and support structures. Step-by-Step Solution: Identify corrosion challenge: need materials compatible with fluids.Note that packed towers accept ceramics/plastics that resist many corrosives.Acknowledge that pressure drop and efficiency targets remain achievable with proper packing choice and distribution.Therefore, packed towers are generally preferred for highly corrosive duties. Verification / Alternative check: Industry experience and vendor offerings show wide use of ceramic/plastic packings in acid gas absorbers and scrubbing systems. Why Other Options Are Wrong: Sieve and bubble-cap trays can work but typically force expensive alloy construction and more complex maintenance. “None” is incorrect given the strong case for packed towers. Common Pitfalls: Poor liquid distribution causing channeling; choosing packing unsupported by appropriate corrosion-resistant internals; neglecting thermal effects on plastics. Final Answer: Packed tower

Question 5

Fabrication rule of thumb: for plain carbon steel vessels, butt-welded joints are typically used when the shell plate thickness is less than or equal to approximately how many centimeters?

Accepted Answer

Introduction / Context: Pressure vessel construction uses different joint types depending on plate thickness, fabrication practicality, and code requirements. For thin to moderate shell plates in plain carbon steel, butt-welded joints are common and economical, providing good strength and inspectability. As thickness increases, bevel geometry, weld volume, preheat, and distortion control become more demanding. Given Data / Assumptions: Plain carbon steel shell plates for typical process equipment. Interest in a practical threshold where butt-welded joints are commonly applied. Rule-of-thumb guidance rather than a mandatory code limit (actual projects must follow code and WPS). Concept / Approach: Thin plate sections are routinely joined by full-penetration butt welds with manageable heat input and distortion. Around a centimeter of thickness represents a convenient upper bound for “easy” butt-weld fabrication on many shop floors before special procedures and increased weld volume raise cost and complexity. The referenced choice reflects an indicative threshold used in many instructional contexts. Step-by-Step Solution: Relate joint selection to thickness: thinner plates favor butt welds.Identify a commonly cited cutoff in teaching examples: about 1.2 cm.Conclude that for ≤ 1.2 cm, butt-welded joints are typically used for plain carbon steel shells. Verification / Alternative check: Shop practices and educational problem data often use ~12 mm as a practical benchmark; exact limits vary by code, service, and fabricator capability. Why Other Options Are Wrong: 0.5 cm is overly restrictive; butt welds are common above this. 3.8 cm and 6.8 cm enter heavy-plate territory requiring substantial weld prep and may favor different joint details or segmental construction. Common Pitfalls: Confusing rules of thumb with code requirements; ignoring impact toughness, PWHT, and NDE that also influence joint selection. Final Answer: 1.2

Question 6

Shell-and-tube exchangers: a practical minimum tube pitch is commonly taken as approximately how many times the outside diameter of the tube?

Accepted Answer

Introduction / Context: Tube pitch (center-to-center spacing) in shell-and-tube heat exchangers governs mechanical clearance, tube-to-tube vibration risk, shell-side flow distribution, and cleaning access. Designers typically set a minimum pitch relative to the tube outside diameter to balance compactness with manufacturability and performance. Given Data / Assumptions: Circular tubes arranged on standard layouts (triangular or square). Mechanical cleaning may not be required for the tightest triangular patterns. Normal industry rules of thumb for minimum pitch. Concept / Approach: A common lower bound for triangular pitch is about 1.25 times the tube OD, providing just enough ligament for drilling the tubesheet and for tube-to-tube clearance. Larger pitches are used for square layouts, mechanical cleaning lanes, or to reduce shell-side pressure drop. Tighter than 1.25 can compromise drilling integrity or increase risk of damage during expansion/rolling. Step-by-Step Solution: Select a practical minimum that preserves tubesheet strength and manufacturability.Recall the widely cited figure: pitch ≈ 1.25 × OD for very compact layouts (typically triangular pitch).Therefore, choose 1.25 as the practical minimum. Verification / Alternative check: TEMA and vendor practices echo ~1.25 OD as a lower limit for close pitch without mechanical cleaning lanes. Why Other Options Are Wrong: 1.75, 2.5, 3.5 are not “minimums” but looser pitches used for other objectives (cleaning, lower pressure drop). 1.00 OD gives zero ligament—impractical. Common Pitfalls: Choosing minimum pitch where mechanical rodding is later required; ignoring fouling tendency that would argue for wider spacing. Final Answer: 1.25

Question 7

Distillation trays: how are bubble caps typically secured in place on the tray?

Accepted Answer

Introduction / Context: Bubble-cap trays use risers and caps with slots to disperse vapor into the liquid on the tray. Mechanical attachment must resist vibration, thermal cycling, and two-phase flow forces, while allowing maintenance and replacement. Understanding the typical fastening method helps during inspection and turnaround planning. Given Data / Assumptions: Conventional bubble-cap hardware (risers, caps, and decks). Industrial towers with access for bolting and bar installation. Need for secure yet removable retention. Concept / Approach: A common method is to use a hold-down bar spanning across the top of caps or engaging cap lugs, fastened by bolts to the tray deck or through brackets. This prevents caps from lifting or rotating under vapor thrust. The bolted hold-down system also allows disassembly for cleaning or replacement. Permanent methods such as welding to the deck are generally avoided due to maintenance implications and potential deck distortion. Step-by-Step Solution: Identify the need: retain bubble caps securely yet maintain serviceability.Use hold-down bars to clamp caps in place.Tighten with bolts to ensure positive retention and easy removal.Therefore, both hold-down bars and bolts are used together in typical practice. Verification / Alternative check: Tray hardware catalogs depict bolted hold-down bars as standard accessories for bubble-cap trays. Why Other Options Are Wrong: Rivets and spot welding hinder maintenance and are not standard for removable caps. Hold-down bar without bolts is incomplete; bolts provide the clamping force. Common Pitfalls: Improper torque leading to loose hardware; mixing cap models with incompatible hold-down systems. Final Answer: Both (b) and (c)

Question 8

Water flow in plant piping: what is the typical “normal” range of average velocity selected for design?

Accepted Answer

Introduction / Context: When designing plant water lines (cooling water, service water), engineers balance pressure drop, pipe size, erosion potential, and noise. A widely used rule-of-thumb velocity range provides a good compromise between capital and operating costs while avoiding cavitation and excessive vibration. Given Data / Assumptions: Clean water service in carbon-steel or similar piping. Economic pipe sizing based on standard friction criteria. Aim to avoid erosion and keep pump head reasonable. Concept / Approach: Average velocities of roughly 1–2 m/s are commonly selected for water distribution in plants. Below this, pipe diameters become large and capital cost rises; above this, friction losses escalate and risk of erosion, noise, and water hammer increases. Local velocities can be higher at fittings, but the line sizing target remains in this band for most services. Step-by-Step Solution: Define acceptable pressure drop per length consistent with pump power.Select a velocity band that balances OPEX and CAPEX for water.Industry practice points to ~1–2 m/s as the “normal” design range.Confirm that this range avoids common erosion/cavitation issues for clean water. Verification / Alternative check: Pipe-sizing charts used in utilities engineering consistently map economically optimal diameters for water to velocities near 1–2 m/s for a wide variety of flows. Why Other Options Are Wrong: 0.1–0.5 m/s oversizes lines and is uneconomical except in special cases. 10–50 m/s and 15–30 m/s are far above practical levels, causing severe head loss and erosion risk. Common Pitfalls: Ignoring water quality (solids can lower allowable velocity); not checking noise limits in building services; neglecting transients that can spike velocities. Final Answer: 1–2 m/s

Question 9

In piping design, the corrosion allowance assigned specifically for small fittings such as couplings and unions should be considered __________ under normal service selection and thickness calculations.

Accepted Answer

Introduction / Context: While sizing and specifying piping components, engineers add a corrosion allowance to compensate for expected wall loss over the service life. However, not all items in a line class are treated equally. Very small fittings like couplings and unions are typically purchased to standardized schedules and replaced as consumables. This question probes practical specification practice for such fittings. Given Data / Assumptions: Standard carbon-steel or alloy steel piping in general service. Small screwed or socket-weld couplings and unions, not large fabricated nozzles. Normal plant corrosion rates; no severe corrosion service requiring special design. Concept / Approach: Corrosion allowance is primarily ensured on pressure parts whose wall thickness is directly sized by the designer (e.g., straight pipe, large nozzles). Commercial couplings and unions are catalog items with fixed wall thickness and short replacement cycles. Hence their life is not extended by adding “design allowance”; instead they are replaced on condition. Therefore, they are generally treated as having negligible design corrosion allowance. Step-by-Step Solution: Identify component category: small, standardized fittings (couplings, unions).Check specification practice: these are bought to standard wall schedules and replaced as needed.Conclusion: treat design corrosion allowance as negligible. Verification / Alternative check: Typical piping class notes specify corrosion allowance for pipe, flanges, and large branch connections; small couplings/unions are exempt and controlled by minimum schedule. Why Other Options Are Wrong: 60 / 200 / 350: These have no unit context and do not represent a rational “allowance” entry; corrosion allowance is normally a thickness (e.g., 1.5 mm) not such figures. Common Pitfalls: Assuming the same allowance applies to every catalog fitting; ignoring that unions and couplings are inexpensive and routinely replaced. Final Answer: negligible

Question 10

Shell-and-tube heat exchanger design: which of the following baffle shapes is not a transverse baffle used to increase shell-side turbulence?

Accepted Answer

Introduction / Context: Baffles in shell-and-tube exchangers direct shell-side flow to enhance heat transfer and support tubes. Most common designs use transverse baffles that force crossflow across the tube bundle, raising turbulence and film coefficients. A contrasting family creates longitudinal flow with a helical path. Given Data / Assumptions: Conventional single-shell-pass exchangers. Transverse baffles promote crossflow; longitudinal/helical promote helical/axial flow. Goal: identify the non-transverse option. Concept / Approach: Segmental (single, double), disk-and-donut, and flat-plate baffles are classic transverse styles. They repeatedly redirect flow across the bundle. Helical baffles, by contrast, use a spiral baffle arrangement to guide near-plug, longitudinal flow with lower pressure drop and reduced fouling—these are not transverse baffles. Step-by-Step Solution: List transverse types: segmental, flat plates, disk-type (disk-and-donut).Recognize helical design produces axial/helical channeling.Therefore, the non-transverse choice is the helical type baffle. Verification / Alternative check: Vendor literature for helical-baffle exchangers highlights axial flow and lower shell-side pressure drop compared to segmental crossflow designs. Why Other Options Are Wrong: Segmental / flat plate / disk type all interrupt flow transversely, promoting crossflow. Common Pitfalls: Confusing “disk-type” with donut rings in disk-and-donut (still transverse overall); assuming helical equals segmental because of baffle count. Final Answer: Helical type baffle

Question 11

Thin-walled cylindrical pressure vessel: what is the longitudinal stress (axial membrane stress) under internal pressure p, diameter d, and wall thickness t?

Accepted Answer

Introduction / Context: Design of thin-walled cylinders relies on two primary stresses: circumferential (hoop) and longitudinal (axial). Correct formulas determine required thickness and verify code compliance. Given Data / Assumptions: Thin-wall assumption: t ≪ d. Uniform internal pressure p; flat or hemispherical heads integrated so axial force is carried by shell. Elastic, isotropic behavior; no external loads. Concept / Approach: From static equilibrium of a free body cut normal to the axis, the pressure force on the projected area equals the membrane force in the shell wall. Using diameter d instead of radius r simplifies the standard relationship. Step-by-Step Solution: Hoop stress for reference: σh = p d / (2 t).Longitudinal stress: balance pressure on end area (π d^2 / 4) with shell force (σl * π d t) → σl = p d / (4 t).Select the matching option: p d / 4 t. Verification / Alternative check: Using radius r = d/2 yields σl = p r / (2 t); substituting r gives p d / 4 t, consistent with standard texts. Why Other Options Are Wrong: p d / 2 t: this is the hoop stress, not longitudinal.p d / t and p d / 8 t: over- and under-estimations not supported by equilibrium. Common Pitfalls: Mixing up hoop and longitudinal stresses; applying thin-wall formulas to thick shells where stress distributions are not uniform. Final Answer: p d / 4 t

Question 12

Packed column design: for saddle-type packings (e.g., Berl or Intalox), the maximum individual packing size should not exceed what fraction of the column diameter?

Accepted Answer

Introduction / Context: Choosing packing size relative to tower diameter is vital for good distribution and low wall effects. Oversized packings in a small column lead to maldistribution and poor performance, while undersized packings may cause high pressure drop and flooding. Given Data / Assumptions: Saddle-type random packing (Berl/Intalox). Conventional liquid distributors; typical gas/liquid loads. Objective: a practical upper limit as a fraction of column diameter. Concept / Approach: A common rule is that the packing size should be much smaller than the column diameter to ensure many pieces across the cross-section, minimizing edge effects. For many random packings, a conservative guideline is that the maximum packing size be around 1/10–1/15 of the tower diameter; for saddles, the tighter 1/15 rule is often applied to preserve distribution quality. Step-by-Step Solution: Relate performance to the count of packing pieces across diameter.Select the most conservative common limit in choices: 1/15.Conclude that 1/15 of the column diameter is a suitable maximum size for saddles. Verification / Alternative check: Vendor handbooks often tabulate recommended minimum column diameters for each packing size, which correspond to size/diameter ratios near this range. Why Other Options Are Wrong: 1/5: too large; causes maldistribution and channeling.1/30 or 1/50: overly small packings increase pressure drop and cost unnecessarily. Common Pitfalls: Ignoring distributor design; using the same rule for structured packing (different criteria apply). Final Answer: 1/15 of column diameter

Question 13

When one process fluid is highly corrosive and tends to foul, how should it be routed in a shell-and-tube exchanger for easier maintenance and reliability?

Accepted Answer

Introduction / Context: Fluid allocation to tube or shell side affects maintenance, pressure limits, and heat-transfer performance. For problematic fluids, correct routing can greatly reduce downtime and cost. Given Data / Assumptions: One fluid is corrosive and fouling. Conventional removable-bundle design available. Objective is easier cleaning and component replacement. Concept / Approach: Routing corrosive or fouling fluids inside tubes is preferred because tubes can be mechanically or chemically cleaned, plugged individually if necessary, and made of a different (corrosion-resistant) alloy without making the entire shell of that alloy. Tube-side also confines hazardous inventory in smaller passages, reducing risk. Step-by-Step Solution: Identify the problem fluid: corrosive/fouling.Select routing that eases maintenance: tube-side for rodding/cleaning or replacement.Confirm design flexibility: tubes can use higher-alloy materials at lower cost than an alloy shell. Verification / Alternative check: Heat-exchanger selection guides list “corrosive/fouling fluid to tube side” among standard allocation rules, alongside high-pressure fluid to tube side. Why Other Options Are Wrong: Shell side: harder to clean mechanically; may demand costly alloy shell.Very low velocity: can worsen fouling by reducing wall shear; not a routing solution.Direction-dependent rule (outside if countercurrent…): not a recognized criterion. Common Pitfalls: Ignoring allowable pressure drop and velocity criteria; underestimating fouling factors in rating calculations. Final Answer: Preferably inside the tubes for easier internal cleaning

Question 14

McCabe–Thiele analysis: what is the slope of the operating line in the rectifying section (assume constant molal overflow and total condenser)?

Accepted Answer

Introduction / Context: Binary distillation under the constant-molal-overflow assumption yields straight operating lines on the y–x diagram. The rectifying section line characterizes mass balances above the feed tray and is central to stage-count estimation. Given Data / Assumptions: Total condenser; no side draws. Constant molal overflow; negligible heat of mixing. Reflux ratio R = L/V. Concept / Approach: The rectifying line can be written as y = (R/(R+1)) x + xD/(R+1). The slope is L/V = R/(R+1), which is always less than 1 for finite reflux (R > 0). Only in the limiting case of infinite reflux does the slope approach 1, not exceed it. Step-by-Step Solution: Write the rectifying operating line: y = (L/V) x + xD (1 − L/V).Recognize slope = L/V = R/(R+1).Since R > 0, R/(R+1) < 1 → choose “< 1.” Verification / Alternative check: At minimum reflux, the slope is relatively small; as R increases, the slope approaches 1 from below, consistent with textbook diagrams. Why Other Options Are Wrong: 0: would imply no internal liquid flow (not rectifying).∞: vertical line—nonsensical for operating lines.> 1: impossible under constant molal overflow without side draws. Common Pitfalls: Mixing rectifying and stripping lines; forgetting the intercept is xD/(R+1). Final Answer: <1

Question 15

Cross-flow tray hydraulics: the typical total length of straight rectangular weirs on a tray is about what fraction of the distillation column diameter?

Accepted Answer

Introduction / Context: On cross-flow trays, outlet weirs retain a liquid head, generate froth, and set residence time. The total straight weir length relative to column diameter affects capacity and efficiency. Given Data / Assumptions: Sieve/valve/bubble-cap trays in cross-flow configuration. Conventional downcomers with segmental or straight weirs. Typical industrial practice ranges. Concept / Approach: For good capacity and controllable weir loading, designers often select total straight weir length of roughly 60–80% of the tower diameter. This balances froth depth, downcomer backup, and active area. Very long weirs reduce vapor area; very short weirs increase crest velocity and entrainment risk. Step-by-Step Solution: Relate weir length to hydraulic parameters (weir load, crest, tray active area).Select a practical range used in design handbooks: 0.6–0.8 D.Choose the option matching that range. Verification / Alternative check: Vendor guidelines for cross-flow trays commonly quote total weir length in this proportion, adjusted for specific vapor/liquid loads. Why Other Options Are Wrong: Equal to or twice D: typically excessive; sacrifices vapor area.Fixed 2 ft: ignores scale; unsuitable beyond small columns. Common Pitfalls: Ignoring foaming tendency; neglecting approach to flooding where backup across weir governs. Final Answer: 0.6–0.8 times the column diameter

Question 16

Riveted joint patterns: in which joint does the number of rivets reduce progressively from the inner row to the outer row?

Accepted Answer

Introduction / Context: Before widespread welding, boilers and pressure parts were joined using riveted joints with different patterns to distribute load and reduce stress concentration. Recognizing geometric patterns is a core drafting and design skill. Given Data / Assumptions: Lapped or butt riveted joints in plate work. Comparison among chain, zig-zag, and diamond arrangements. Focus: relative count of rivets per row from inner to outer rows. Concept / Approach: In the diamond pattern, rivets are arranged so the number per row diminishes toward the edges, forming a diamond outline. This can smooth load transfer and limit edge stresses. Chain and zig-zag patterns keep a regular count of rivets per row or simply offset rivets between adjacent rows rather than tapering the number. Step-by-Step Solution: Identify pattern with tapered count of rivets: diamond.Eliminate chain (rows aligned) and zig-zag (staggered but constant count).Select “Diamond pattern.” Verification / Alternative check: Engineering drawing manuals illustrate the diamond layout with fewer rivets in the outer rows, clearly distinct from chain or zig-zag. Why Other Options Are Wrong: Chain: equal rivet count per row, aligned vertically.Zig-zag: staggered but non-tapered distribution.None of these: incorrect because diamond exists and fits the description. Common Pitfalls: Confusing “diamond” with “staggered” in general; overlooking that the defining feature is tapering of count. Final Answer: Diamond pattern

Question 17

Storage tanks: why must any shell opening larger than about 5 cm be reinforced around the nozzle location?

Accepted Answer

Introduction / Context: Nozzles and manways cut into thin shells remove load-bearing metal and create stress concentrations. Codes require reinforcement to replace the metal removed and maintain shell strength under internal pressure, wind, and other loads. Given Data / Assumptions: Atmospheric or low-pressure storage tanks. Shell openings greater than about 50 mm diameter. Standard nozzle neck plus reinforcement pad or integrally reinforced design. Concept / Approach: The primary reason for reinforcement is structural: removal of shell material reduces the area that carries membrane stresses. A properly sized reinforcement pad (or added neck thickness) compensates for this loss and spreads stresses to limit peak values around the opening. While good geometry and sealing matter, they are secondary to restoring strength. Step-by-Step Solution: Recognize that a hole removes load-bearing shell metal.Apply code principle: replace the lost area with reinforcement to prevent local overstress.Select the option aligning with structural intent. Verification / Alternative check: Design standards (e.g., storage tank codes) present area-replacement calculations and limits on reinforcement placement to ensure adequate strength. Why Other Options Are Wrong: Reducing discontinuity helps, but the core requirement is strength, not merely shape smoothing.Leak-proofing is achieved by welding and gaskets; reinforcement is not primarily for sealing. Common Pitfalls: Undersizing the pad by ignoring corrosion allowance; omitting tell-tale holes for leak detection between shell and pad. Final Answer: To prevent local overstressing of the shell around the opening

Question 18

At equal mass velocities, how do typical convective film heat-transfer coefficients for gases compare with those for liquids?

Accepted Answer

Introduction / Context: Convective film coefficients depend on fluid properties and flow regime. Understanding the relative magnitude for gases versus liquids guides exchanger area estimates and selection of compact geometries when dealing with gas streams. Given Data / Assumptions: Comparison at the same mass velocity (kg/m^2·s) and similar hydraulic diameter. Fully developed turbulent region assumed for both. Typical ambient-pressure gases and water-like liquids. Concept / Approach: Dimensionless correlations (e.g., Dittus–Boelter type) show h ∝ k * Nu / L, with Nu depending on Re and Pr. Gases generally have much lower thermal conductivity k and higher kinematic viscosity, with Prandtl numbers near 0.7, while liquids like water have higher k and Pr of order 2–10. As a result, for similar Re at the same mass velocity, liquid-side coefficients are typically an order of magnitude larger than gas-side coefficients. Step-by-Step Solution: Compare properties: k_liquid ≫ k_gas; Pr_liquid > Pr_gas.Use Nu correlations to infer h_liquid ≫ h_gas for equal mass flux.Conclude gas-side h is lower than liquid-side under comparable conditions. Verification / Alternative check: Typical exchanger design tables list gas film coefficients in the tens–hundreds W/m^2·K, versus liquids in the hundreds–thousands W/m^2·K for similar geometry. Why Other Options Are Wrong: Higher or same: contradicts property-based scaling and common data.Unpredictable: while exact values require calculation, the trend is well established. Common Pitfalls: Comparing at equal volumetric flow instead of mass velocity; overlooking large property changes at high pressure (supercritical gases may deviate). Final Answer: Lower

Question 19

In mechanical manufacturing, how are seamless pipes produced? Select the method that correctly describes true seamless pipe manufacture, as used for pressure service and high-integrity applications.

Accepted Answer

Introduction / Context: Seamless pipe is specified where weld integrity and isotropic properties are critical, such as high pressure, cyclic service, or corrosive duty. Understanding how seamless pipe is truly manufactured helps designers choose correct materials and interpret data sheets and code requirements. Given Data / Assumptions: Seamless pipe implies no longitudinal weld seam. Conventional industrial methods include rotary piercing (Mannesmann process), extrusion, rolling, and drawing. Welded pipe (from plate or strip) is not seamless, even if heat treated. Concept / Approach: In the most common route, a solid billet is heated, then pierced using a rotating piercer over a mandrel. The hollow shell is elongated and sized by rotary rolling and/or plug-mandrel mills. Final dimensions and properties are achieved by hot or cold drawing. This produces a pipe with no weld seam and the microstructure of a wrought product. Step-by-Step Solution: Identify which choices yield a pipe with no weld seam and a wrought microstructure.Rolled strip + seam welding creates a longitudinal weld → not seamless.Casting (static or centrifugal) produces cast tubes, not wrought seamless pipe for pressure service.Rotary piercing + rolling/drawing yields true seamless pipe. Verification / Alternative check: Standards (e.g., ASTM A106/A53 Type S) describe seamless manufacture by hot working a steel billet, usually with piercing and rolling, optionally followed by cold finishing. Why Other Options Are Wrong: (a) is welded; (b) produces cast tubes lacking wrought properties; (d) includes an incorrect method; (e) unnecessary because a correct method exists. Common Pitfalls: Confusing centrifugal cast pipe (used for some services) with seamless; assuming post-weld heat treatment makes welded pipe “seamless.” Final Answer: By forging a solid round, piercing it while rotating over a mandrel, then reducing by rolling and drawing

Question 20

In an agitated tank with vertical baffles, what is the recommended minimum baffle height relative to tank size to control swirling and improve mixing?

Accepted Answer

Introduction / Context: Baffles suppress vortexing, increase turbulence, and enhance top-to-bottom mixing in stirred tanks. Their height is a key geometric parameter that determines liquid-surface stability and circulation patterns for a wide range of viscosities and impeller types. Given Data / Assumptions: Cylindrical, baffled tank with a standard impeller (e.g., Rushton or pitched-blade turbine). Goal is to reduce swirling and improve mixing uniformity. Typical design practice for general chemical service. Concept / Approach: Best practice extends baffles close to the liquid surface and down toward the bottom to intercept tangential flow. Heights significantly shorter than the liquid depth allow a strong surface vortex; excessively long baffles can be impractical or interfere with internal fittings. A commonly adopted guideline is a baffle height of roughly 70–80% of the liquid height (≈ 3/4 of tank height). Step-by-Step Solution: Choose a baffle height that reaches most of the liquid column to damp swirl.Adopt 3/4 of tank height as a robust rule of thumb balancing performance and fabrication.Verify clearances near the bottom and freeboard near the top to avoid solids build-up or gas entrainment. Verification / Alternative check: Textbook and vendor guidelines (with baffle width ≈ 1/12 to 1/10 of tank diameter) pair well with 3/4 height for general liquids, delivering predictable power draw and flow patterns. Why Other Options Are Wrong: (a) and (b) tie height to impeller size rather than liquid depth; (c) is unrealistic; (e) is incorrect—most axial impellers still benefit from baffles. Common Pitfalls: Stopping baffles far below the surface; ignoring freeboard and bottom clearances; assuming gas dispersion duties use the same geometry without checks. Final Answer: About three-quarters (3/4) of the tank height

Process Equipment and Plant Design Questions