Correctly designed discharge pipework is essential for ensuring a pump operates at its intended duty point. Poor design can result in excessive pressure losses, wasted energy, vibration, and even system failure. Unlike suction lines, discharge lines operate under positive pressure, giving more flexibility in layout — but careful calculation of friction losses and proper sizing remain critical.
Design Impacts
System pressure requirements — affects Total Dynamic Head (TDH)
Pump efficiency — energy wasted if friction is too high
Longevity — excess velocity increases erosion and vibration
Operational stability — poor layouts cause water hammer or surges
Key Factors
Pipe Diameter and Flow Velocity
Typical recommended velocity ranges:
Clean Water Systems
1.5 – 3.0 m/sWastewater/Slurry
≤ 2.0 m/sto limit abrasion
Long Pipelines
≤ 1.5 m/sto reduce friction losses
Smaller diameters reduce material cost but increase friction loss. Oversized diameters lower losses but raise installation costs. A balance must be achieved.
Pipe Material
Steel
Strong and durable, suited for high pressures; prone to corrosion if unprotected.
Ductile Iron
High strength and impact resistance, commonly used in municipal systems.
PVC/HDPE
Lightweight, corrosion-resistant, smooth internal surfaces; ideal for irrigation and temporary pipelines.
Rubber Hose
Portable, flexible, but limited in pressure rating; common in dewatering and construction.
Fittings & Valves
Every bend, tee, valve, or reducer introduces minor losses. These are expressed as an equivalent length of straight pipe or a loss coefficient (K).
Elbows & Tees: Increase turbulence and friction
Check Valves: Prevent backflow, but introduce head loss
Gate/Butterfly Valves: Used for isolation and control
Non-Return Valves (NRVs): Essential in vertical discharge lines
Pressure Loss Calculations
Friction Loss (Hf)
Calculated using Darcy-Weisbach equation:
Friction factor
Pipe length (m)
Pipe diameter (m)
Velocity (m/s)
Gravity (9.81 m/s²)
Minor Losses (Hml)
Each fitting or valve contributes additional head loss:
Where K is the loss coefficient (dimensionless):
90° Elbow
K ≈ 0.3 – 0.9Swing Check Valve
K ≈ 2.0 – 5.0Fully Open Gate Valve
K ≈ 0.2Total Discharge Losses
These values are added to static discharge head and suction lift to determine Total Dynamic Head (TDH).
Example Calculation
Application: Irrigation Pump Discharge Line
Flow Rate
60 L/s (216 m³/h)Pipe Length
120 mPipe Diameter
200 mm PVCStep 1: Calculate velocity
Step 2: Calculate friction head loss (f ≈ 0.018 for PVC)
Step 3: Calculate minor losses (2 elbows K=0.5 each, 1 gate valve K=0.2)
Total Discharge Losses:
Htotal = 2.0 + 0.18 = 2.18 mThis value is added to static lift and required pressure head for full TDH.
Best Practices
Keep flow velocities within recommended ranges to avoid erosion and excessive friction
Minimise the number of fittings; use long-radius bends instead of sharp elbows
Provide air release valves on high points of pipelines to prevent air lock
Install non-return valves close to the pump to protect against reverse flow
Ensure adequate anchoring at bends and fittings to withstand thrust forces
In long pipelines, consider surge analysis to protect against water hammer
Special Considerations
Variable Speed Pumps
Discharge losses change with flow rate; pipe sizing must suit the maximum expected duty.
High-Pressure Systems
Pipe material and joint rating must exceed pump shut-off head.
Slurry & Wastewater
Use abrasion-resistant materials and keep velocity high enough (≥ 1.2 m/s) to avoid solids settling.
Temporary Installations
Flexible hoses are common but must be supported to prevent sagging and excessive strain on pump outlets.
Summary
Discharge pipework design directly affects pump performance and efficiency. Correct sizing, material choice, and accurate calculation of both friction and minor losses allow pumps to operate at their intended duty point with reduced wear and energy consumption. By following best practices and checking calculations against system requirements, engineers can achieve safe, reliable, and efficient pumping systems.