PIPE SIZING VERIFIED EXAM QUESTIONS AND ANSWERS -
LATEST VERSION 2026/2027
Q1. What is pipe sizing?
ANSWER Pipe sizing is the engineering process of selecting the correct pipe
diameter, wall thickness, and material to safely and efficiently convey a fluid or
gas at a given flow rate, pressure, and temperature.
Q2. What is nominal pipe size (NPS)?
ANSWER Nominal Pipe Size (NPS) is a North American standard for pipe
dimensions. It is a dimensionless number that approximates the pipe's inner
diameter in inches but does not directly correspond to actual measurements
for pipes above NPS 14.
Q3. What is DN (Diamètre Nominal)?
ANSWER DN is the metric equivalent of NPS, used in international
standards. For example, NPS 2 corresponds roughly to DN 50. DN values are
in millimeters.
Q4. What is the difference between pipe ID and OD?
ANSWER ID (Inner Diameter) is the clear internal bore of the pipe, which
determines flow area. OD (Outer Diameter) is the external measurement. The
OD is fixed for a given NPS regardless of schedule, while the ID changes as
wall thickness increases.
Q5. What is pipe schedule?
ANSWER Pipe schedule is a standardized system defining wall thickness.
Higher schedule numbers indicate thicker walls. Common schedules include
SCH 40, SCH 80, SCH 160, and XXS (Double Extra Strong).
Q6. How is flow velocity related to pipe sizing?
ANSWER Flow velocity is the speed of the fluid through the pipe cross-
section. A larger pipe diameter reduces velocity for the same flow rate.
Velocity is calculated as V = Q/A, where Q is volumetric flow rate and A is
cross-sectional area.
,Q7. What is volumetric flow rate?
ANSWER Volumetric flow rate (Q) is the volume of fluid passing a cross-
section per unit time, typically expressed in gallons per minute (GPM), liters
per second (L/s), or cubic meters per hour (m³/h).
Q8. What is mass flow rate?
ANSWER Mass flow rate (ṁ) is the mass of fluid passing a point per unit
time, expressed in kg/s or lb/hr. It equals volumetric flow rate multiplied by
fluid density: ṁ = ρQ.
Q9. What is Reynolds number and why is it important in pipe sizing?
ANSWER Reynolds number (Re) is a dimensionless number indicating
whether flow is laminar or turbulent. Re = ρVD/μ. Below ~2300 is laminar,
above ~4000 is turbulent. It directly affects friction factor and pressure drop
calculations.
Q10. What is the Darcy-Weisbach equation?
ANSWER The Darcy-Weisbach equation calculates pressure drop due to
friction: ΔP = f × (L/D) × (ρV²/2), where f is the Darcy friction factor, L is pipe
length, D is diameter, ρ is density, and V is velocity.
2. Pressure Drop
Q11. What is pressure drop in a pipe?
ANSWER Pressure drop is the decrease in fluid pressure as it travels
through a pipe due to friction, fittings, elevation changes, and other
resistances. Minimizing pressure drop is a key goal of pipe sizing.
Q12. What causes pressure drop in pipes?
ANSWER Pressure drop is caused by fluid friction against pipe walls (major
losses), turbulence through fittings and valves (minor losses), elevation
changes, and changes in fluid velocity.
Q13. What are major losses in a piping system?
ANSWER Major losses (also called friction losses) are pressure drops
caused by viscous friction along the straight length of a pipe. They are
calculated using the Darcy-Weisbach or Hazen-Williams equation.
Q14. What are minor losses in a piping system?
ANSWER Minor losses are pressure drops due to valves, fittings, elbows,
tees, reducers, and other flow disturbances. They are expressed as equivalent
pipe length or loss coefficients (K-values).
Q15. What is the Hazen-Williams equation?
, ANSWER The Hazen-Williams equation estimates head loss in water pipes:
V = 0.849 × C × R^0.63 × S^0.54, where C is the roughness coefficient, R is
the hydraulic radius, and S is the hydraulic gradient. It is widely used in water
supply design.
Q16. What is the Darcy friction factor?
ANSWER The Darcy friction factor (f) is a dimensionless coefficient relating
wall shear stress to kinetic pressure. For laminar flow, f = 64/Re. For turbulent
flow, it is determined from the Moody diagram or Colebrook equation.
Q17. What is the Moody diagram?
ANSWER The Moody diagram is a graph used to find the Darcy friction
factor based on Reynolds number and relative pipe roughness (ε/D). It covers
laminar, transitional, and turbulent flow regimes.
Q18. What is the Colebrook-White equation?
ANSWER The Colebrook-White equation implicitly defines the friction factor
for turbulent flow: 1/√f = -2 log(ε/(3.7D) + 2.51/(Re√f)). It is considered the
most accurate formula for turbulent pipe flow friction.
Q19. What is the Swamee-Jain equation?
ANSWER The Swamee-Jain equation is an explicit approximation of the
Colebrook equation: f = 0.25 / [log(ε/(3.7D) + 5.74/Re^0.9)]². It avoids iterative
calculation.
Q20. What is hydraulic gradient?
ANSWER Hydraulic gradient is the slope of the hydraulic grade line,
representing the pressure head loss per unit length of pipe. It is used in gravity
flow and drainage pipe sizing.
3. Flow Regimes
Q21. What is laminar flow?
ANSWER Laminar flow is smooth, orderly fluid motion in parallel layers with
no lateral mixing. It occurs at low velocities (Re < 2300) and has a parabolic
velocity profile. Friction losses are proportional to velocity.
Q22. What is turbulent flow?
ANSWER Turbulent flow is chaotic, irregular fluid motion with significant
mixing between layers. It occurs at high velocities (Re > 4000) and is the most
common regime in industrial piping. Friction losses increase with velocity
squared.
Q23. What is transitional flow?
, ANSWER Transitional flow occurs between laminar and turbulent regimes
(Re between ~2300 and ~4000). Flow behavior is unpredictable and can shift
between regimes. This zone is generally avoided in pipe design.
Q24. How does flow regime affect pressure drop calculation?
ANSWER In laminar flow, friction factor f = 64/Re, making ΔP proportional to
velocity. In turbulent flow, the friction factor depends on both Re and
roughness, and ΔP is roughly proportional to V².
Q25. What is a fully developed flow profile?
ANSWER A fully developed flow profile occurs far from pipe entry points
where the velocity distribution across the cross-section becomes constant
along the pipe length. For laminar flow this is parabolic; for turbulent flow it is
flatter.
Q26. What is the hydraulic diameter?
ANSWER Hydraulic diameter (Dh) is used for non-circular cross-sections:
Dh = 4A/P, where A is cross-sectional area and P is the wetted perimeter. For
a circular pipe flowing full, Dh equals the actual diameter.
Q27. What is critical velocity in pipe flow?
ANSWER Critical velocity is the flow speed at which the transition from
laminar to turbulent flow begins. Below it, flow is laminar; above it, flow
transitions to turbulence. It corresponds approximately to Re = 2300.
Q28. What is slug flow?
ANSWER Slug flow is a two-phase flow pattern in which alternating slugs of
liquid and gas travel through the pipe. It causes pressure surges and vibration
and must be considered when sizing two-phase piping.
Q29. What is annular flow in two-phase systems?
ANSWER Annular flow is a two-phase flow regime where liquid flows as a
thin film on the pipe wall and gas flows in the core. It occurs at high gas
velocities and is common in steam and gas condensate lines.
Q30. How does fluid viscosity affect pipe sizing?
ANSWER Higher viscosity increases resistance to flow, raising the friction
factor and pressure drop. Viscous fluids typically require larger pipe diameters
or shorter runs to maintain acceptable pressure drop.
4. Water & Plumbing Systems
Q31. What is the recommended water velocity range for supply pipes?
LATEST VERSION 2026/2027
Q1. What is pipe sizing?
ANSWER Pipe sizing is the engineering process of selecting the correct pipe
diameter, wall thickness, and material to safely and efficiently convey a fluid or
gas at a given flow rate, pressure, and temperature.
Q2. What is nominal pipe size (NPS)?
ANSWER Nominal Pipe Size (NPS) is a North American standard for pipe
dimensions. It is a dimensionless number that approximates the pipe's inner
diameter in inches but does not directly correspond to actual measurements
for pipes above NPS 14.
Q3. What is DN (Diamètre Nominal)?
ANSWER DN is the metric equivalent of NPS, used in international
standards. For example, NPS 2 corresponds roughly to DN 50. DN values are
in millimeters.
Q4. What is the difference between pipe ID and OD?
ANSWER ID (Inner Diameter) is the clear internal bore of the pipe, which
determines flow area. OD (Outer Diameter) is the external measurement. The
OD is fixed for a given NPS regardless of schedule, while the ID changes as
wall thickness increases.
Q5. What is pipe schedule?
ANSWER Pipe schedule is a standardized system defining wall thickness.
Higher schedule numbers indicate thicker walls. Common schedules include
SCH 40, SCH 80, SCH 160, and XXS (Double Extra Strong).
Q6. How is flow velocity related to pipe sizing?
ANSWER Flow velocity is the speed of the fluid through the pipe cross-
section. A larger pipe diameter reduces velocity for the same flow rate.
Velocity is calculated as V = Q/A, where Q is volumetric flow rate and A is
cross-sectional area.
,Q7. What is volumetric flow rate?
ANSWER Volumetric flow rate (Q) is the volume of fluid passing a cross-
section per unit time, typically expressed in gallons per minute (GPM), liters
per second (L/s), or cubic meters per hour (m³/h).
Q8. What is mass flow rate?
ANSWER Mass flow rate (ṁ) is the mass of fluid passing a point per unit
time, expressed in kg/s or lb/hr. It equals volumetric flow rate multiplied by
fluid density: ṁ = ρQ.
Q9. What is Reynolds number and why is it important in pipe sizing?
ANSWER Reynolds number (Re) is a dimensionless number indicating
whether flow is laminar or turbulent. Re = ρVD/μ. Below ~2300 is laminar,
above ~4000 is turbulent. It directly affects friction factor and pressure drop
calculations.
Q10. What is the Darcy-Weisbach equation?
ANSWER The Darcy-Weisbach equation calculates pressure drop due to
friction: ΔP = f × (L/D) × (ρV²/2), where f is the Darcy friction factor, L is pipe
length, D is diameter, ρ is density, and V is velocity.
2. Pressure Drop
Q11. What is pressure drop in a pipe?
ANSWER Pressure drop is the decrease in fluid pressure as it travels
through a pipe due to friction, fittings, elevation changes, and other
resistances. Minimizing pressure drop is a key goal of pipe sizing.
Q12. What causes pressure drop in pipes?
ANSWER Pressure drop is caused by fluid friction against pipe walls (major
losses), turbulence through fittings and valves (minor losses), elevation
changes, and changes in fluid velocity.
Q13. What are major losses in a piping system?
ANSWER Major losses (also called friction losses) are pressure drops
caused by viscous friction along the straight length of a pipe. They are
calculated using the Darcy-Weisbach or Hazen-Williams equation.
Q14. What are minor losses in a piping system?
ANSWER Minor losses are pressure drops due to valves, fittings, elbows,
tees, reducers, and other flow disturbances. They are expressed as equivalent
pipe length or loss coefficients (K-values).
Q15. What is the Hazen-Williams equation?
, ANSWER The Hazen-Williams equation estimates head loss in water pipes:
V = 0.849 × C × R^0.63 × S^0.54, where C is the roughness coefficient, R is
the hydraulic radius, and S is the hydraulic gradient. It is widely used in water
supply design.
Q16. What is the Darcy friction factor?
ANSWER The Darcy friction factor (f) is a dimensionless coefficient relating
wall shear stress to kinetic pressure. For laminar flow, f = 64/Re. For turbulent
flow, it is determined from the Moody diagram or Colebrook equation.
Q17. What is the Moody diagram?
ANSWER The Moody diagram is a graph used to find the Darcy friction
factor based on Reynolds number and relative pipe roughness (ε/D). It covers
laminar, transitional, and turbulent flow regimes.
Q18. What is the Colebrook-White equation?
ANSWER The Colebrook-White equation implicitly defines the friction factor
for turbulent flow: 1/√f = -2 log(ε/(3.7D) + 2.51/(Re√f)). It is considered the
most accurate formula for turbulent pipe flow friction.
Q19. What is the Swamee-Jain equation?
ANSWER The Swamee-Jain equation is an explicit approximation of the
Colebrook equation: f = 0.25 / [log(ε/(3.7D) + 5.74/Re^0.9)]². It avoids iterative
calculation.
Q20. What is hydraulic gradient?
ANSWER Hydraulic gradient is the slope of the hydraulic grade line,
representing the pressure head loss per unit length of pipe. It is used in gravity
flow and drainage pipe sizing.
3. Flow Regimes
Q21. What is laminar flow?
ANSWER Laminar flow is smooth, orderly fluid motion in parallel layers with
no lateral mixing. It occurs at low velocities (Re < 2300) and has a parabolic
velocity profile. Friction losses are proportional to velocity.
Q22. What is turbulent flow?
ANSWER Turbulent flow is chaotic, irregular fluid motion with significant
mixing between layers. It occurs at high velocities (Re > 4000) and is the most
common regime in industrial piping. Friction losses increase with velocity
squared.
Q23. What is transitional flow?
, ANSWER Transitional flow occurs between laminar and turbulent regimes
(Re between ~2300 and ~4000). Flow behavior is unpredictable and can shift
between regimes. This zone is generally avoided in pipe design.
Q24. How does flow regime affect pressure drop calculation?
ANSWER In laminar flow, friction factor f = 64/Re, making ΔP proportional to
velocity. In turbulent flow, the friction factor depends on both Re and
roughness, and ΔP is roughly proportional to V².
Q25. What is a fully developed flow profile?
ANSWER A fully developed flow profile occurs far from pipe entry points
where the velocity distribution across the cross-section becomes constant
along the pipe length. For laminar flow this is parabolic; for turbulent flow it is
flatter.
Q26. What is the hydraulic diameter?
ANSWER Hydraulic diameter (Dh) is used for non-circular cross-sections:
Dh = 4A/P, where A is cross-sectional area and P is the wetted perimeter. For
a circular pipe flowing full, Dh equals the actual diameter.
Q27. What is critical velocity in pipe flow?
ANSWER Critical velocity is the flow speed at which the transition from
laminar to turbulent flow begins. Below it, flow is laminar; above it, flow
transitions to turbulence. It corresponds approximately to Re = 2300.
Q28. What is slug flow?
ANSWER Slug flow is a two-phase flow pattern in which alternating slugs of
liquid and gas travel through the pipe. It causes pressure surges and vibration
and must be considered when sizing two-phase piping.
Q29. What is annular flow in two-phase systems?
ANSWER Annular flow is a two-phase flow regime where liquid flows as a
thin film on the pipe wall and gas flows in the core. It occurs at high gas
velocities and is common in steam and gas condensate lines.
Q30. How does fluid viscosity affect pipe sizing?
ANSWER Higher viscosity increases resistance to flow, raising the friction
factor and pressure drop. Viscous fluids typically require larger pipe diameters
or shorter runs to maintain acceptable pressure drop.
4. Water & Plumbing Systems
Q31. What is the recommended water velocity range for supply pipes?
