Pipe Diameter, Length, and Flow Rate Affect Friction Losses

Introduction 

Fluid friction plays a crucial role in determining the efficiency and energy losses in any piping network. Whether in chemical plants, powder systems, or laboratory-scale experiments, understanding frictional pressure losses helps engineers design safer and more efficient systems. Process simulation software is crucial for engineers to visualise, calculate, and optimise fluid flow behaviour—including friction losses in pipes and fittings—without extensive manual computation. In this article, we explore how pipe fittings can be used to model friction and pressure drops in complex piping networks. 

When a fluid flows through a pipe, part of it is lost because of friction between the fluid and the pipe's internal surface. This energy loss is called head loss. 

• Pressure drop across the pipeline. 
• Increased pumping power requirements. 
• Reduced flow efficiency.

In real systems, friction losses occur in two main forms: 

Major Losses—because of friction along the length of the pipe. 

Minor Losses—because of valves, fittings, and expansion/contractions. 

Understanding and quantifying both are essential for accurate process design. 

Modelling Fluid Friction    

DWSIM allows us to understand the model, both simple pipelines and complex ones, by interconnected networks using built-in unit operations like: 

• Pipe Segment
• Pump 
• Mixer/Splitter 
• Valve 
• Tee Junction 

Each Pipe Segment unit in DWSIM can simulate pressure drop because of friction using correlations such as: 

• Darcy-Weisbach equation 
• Haaland Correlation for friction factor 
• Colebrook-White equation (for turbulent flow)

In real piping networking, frictional resistance also rises from elbows, Bends, and tees, Valves (globe, gate, ball), reducers and expanders. These fittings are essential for changing the direction of flow and also reducing the size of the pipes with each fitting step. Stainless steel pipes are excellent for resistance to corrosion, high strength and durability under both high temperature and pressure conditions. It depends on the piping length, Thickness that is driven by the Schedule Numbers (SCN). Good SCN means better compatibility and utilisation for high-pressure ranges. 

Figure 1: Piping Network and Fluid Friction

Applications

1. Chemical Process Industries: In Chemical Process Industries, the piping network is essential for fluid transfer. The estimation of data or evaluation of flow rate, pressure drop, and fluid velocity at various cross sections is contingent upon the flow regimes in fluid dynamics, with fittings also exhibiting variability. This can improve operational conditions to reduce losses in piping systems.

2. Wastewater Treatment Plants: In WWT plants, pipes and fittings are essential for transporting raw sewage, process water, sludge, and treated effluent between different units. The choice of pipe type, materials, and fittings depends on pressure, chemical exposure, temperature, and different flow characteristics. Pipes are made up of ductile iron, HDPE, or PVC. Diameter size around (100-200 mm) for the main inflow. Fittings like elbows, reducers, and tees for directing flow. 

Materials of Fittings used in Different Units: 

• Primary Treatment - Butterfly valves, gate valves, bends, T-joints. 
• Biological Stage - Diffuser pipe connections, check valves, reducers, and manifolds. 
• Tertiary Treatment and Filtration - Tee and elbow joints, reducers, valves and union fittings for chemical dosing lines. 
• Sludge Treatment - Flanged elbows, tees, reducers and pressure-rated valves. 
• Effluent Discharge - bends, reducers, tees and non-return valves. 

Materials of Pipes used in Different Units at WWT: 

• Primary Treatment - Ductile iron, HDPE or PVC. 
• Biological Stage - Inlet/Outlet pipes to sedimentation basins (PVC or HDPE).  
• Tertiary Treatment and Filtration - PVC, HDPE for filtered water and chlorine dosing lines. 
• Sludge Treatment - Cast Iron, HDPE, or Carbon Steel. 
• Effluent Discharge - PVC, HDPE or concrete (for gravity flow).

3. Oil and Gas Transportation: In oil and gas transportation, pipes and fittings are the backbone of the transportation system, carrying crude oil, natural gas, refined products, and process fluids safely over long distances. In the oil and gas industries, various applications span extraction, transportation, refining, and distribution. 

Typical Pipe and Fittings Materials (Extraction site): 

Pipe material: Carbon steel (API 5L, ASTM A106, A53), SS316, SS304, Duplex/Super Double Steel. 

Fittings Materials: Elbows (45°, 90°), Tees and crosses, Reducers, Flanges and unions, Couplings, Nipples, caps and plugs. 

Typical Pipe and Fittings Materials (Transportation and Storage):

 

Pipe material: Carbon steel (API 5L Gr.B, X42-X70), coated steel, HDPE/GRE).  

Fittings Materials: Butt-welded fittings, Insulated fittings, Ball/Gate valves, Pig Launchers/receivers.

 

Typical Pipe and Fittings Materials (Refining and Petrochemical Processing):

Pipe material: SS304, SS316, Alloy steel (P11, P22, Inconel), PVC, CPVC, PP. 

Fittings Materials: Socket-welded and threaded fittings, Butt weld fittings, instrumentation fittings, and expansion joints. 

Fluid Friction Effects in Chemical Process Industries: 

In chemical engineering plants, an installed piping network facilitates the transportation of fluids and gases, effectively delivering a sufficient mass of materials to the desired reaction in the reactor at a constant flow rate, thereby enabling further chemical reactions. To reduce the corrosion effect on metallic pipelines, most of the industries use SS type of materials in piping design, which not only serves for transportation but also enhances the efficiency of the plant working process without cracking on the surface of the metallic layer. To reduce thermal load, stability on metals by coating the pipes. That directly affects the minimal losses and also covers the surface from damage. According to Mathews 1981, most metals affect shoes' ductility behaviour with certain thermal limits. 

The fluid friction in the piping network means quantifying resistance in the pipelines because of a wide range of pressures in the industrial piping network. Those experiences in the pipes, ducts, packed beds, reactors, and heat exchangers. It is s critical factor in design, energy efficiency, and safety. 

Frictional losses: The drops in pressure as fluids move due to viscous shear and form drag. It affects pump sizing, energy consumption, and process controllability. 

Viscosity: A fluid property that directly influences friction, with higher viscosity with higher frictional losses. 

Reynolds number: a dimensionless parameter that characterises flow regimes (laminar, transitional, turbulent) and helps select correlations for friction factors. It is a dimensionless coefficient used in head/pressure drop correlations like Darcy-Weisbach.  

L = length of the pipe

D = pipe diameter

ρ = fluid density

v = average fluid velocity

f = friction factor (function of Re and roughness)

Moodsy chart / Colebrook-White equation:

f = f (Re, ε/D) for turbulent flow in smooth/rough pipes.

For laminar flow (Re < ~2000),  f = 64 / Re.

Equivalent roughness or empirical adjustments may be applied to ducts and some fittings.

 

Components and sources of friction loss in processes

Straight pipe segments: primary contributor to pressure drop via viscous shear, build tiny losses (K-values) that build up to big losses in fittings, valves, bends, expansions, and contractions.

Reactions in packed or structured beds: flow distribution and bypassing affect pressure drop.

Heat exchangers and surface roughness: fouling increases effective roughness over time, increasing friction.

Multiphase flow: gas-liquid or liquid-solid mixtures complicate friction; slip, emulsions, and phase distribution change effective viscosity.

Design considerations

Pipe sizing: balance between minimum sufficient diameter to keep Re in the desired regime and material/cost constraints.

Pump selection: ensure net positive suction head (NPSH) availability and energy efficiency; consider variable-speed drives for part-load efficiency.

Fouling management: anticipate fouling factors, clean-in-place (CIP) intervals, and design for ease of cleaning.

Temperature and viscosity changes: viscosity typically decreases with temperature; account for process heat integration and viscosity variation in friction calculations.

Hydrodynamic modelling: use computational fluid dynamics (CFD) or one-/two-dimensional models for complex internals; otherwise, rely on standard correlations for pipes and ducts.

Common equipment where fluid friction matters

Pipelines and distribution networks

Heat exchangers (shell-and-tube, plate): pressure drop across tubes/plates and fouling resistance

Reactors with internal coils or baffles

Packed-bed reactors and distillation columns (radial/axial flow packs, random packings)

Absorbers/strippers with gas-liquid contactors

Slurry pipelines and slurry transport lines

Practical steps for engineers

Collect fluid properties: density, viscosity (and viscosity as a function of temperature), phase fractions.

Determine the flow regime by computing Re and anticipating possible transitions due to temperature or composition changes.

Select appropriate friction factor correlations: Laminar (f = 64/Re) or Turbulent (Colebrook-White, Haaland, Swamee-Jain, etc.).

Add minor losses: use K-values for fittings, valves, expansions, contractions; sum with major losses.

Account for fouling: include a fouling factor (e.g., in heat transfer and friction calculations) and plan maintenance.

Validate with plant data: compare predicted ΔP with measured pressure drops; recalibrate correlations if needed.

Quick example (pipe flow)

Given: 2-inch pipe (D = 0.1524 m), length L = 50 m, fluid: water, 25°C, ρ ≈ 997 kg/m³, μ ≈ 1.0 mPa·s.

Compute velocity from volumetric flow rate Q (if provided).

Re = ρvD / μ.

If Re < 2000, f = 64/Re; else use Colebrook-White with ε/D (pipe roughness for commercial steel ~0.045 mm).

ΔP = f (L/D) (ρ v² / 2)

Fouling and maintenance notes

Over time, fouling increases the friction factor. Monitor the pressure difference between heat exchangers and reactors.

Establish CIP and pigging schedules for pipelines to mitigate buildup.

Use coatings or smoother internal components where feasible to reduce surface roughness.